3D mapping of compositional gradients of core-shell structures in AgIn_xGa_1-xS_2 quantum dots by atom probe tomography

Domain: biology
Published: 2026-04-03
Editorial outcome: major_revision

Abstract
Colloidal quantum dots (QDs), which exhibit tunable band gaps depending on their size and composition, are widely studied for light-emitting and optoelectronic applications. AgIn Ga S -based QDs are particularly promising due to their pure green emission, high blue absorption, and environmental friendliness. However, a comprehensive understanding of these quaternary QDs remains challenging because of the difficulty in examining their complex compositional structure. Here, we three-dimensionally characterize quaternary QDs (AgIn Ga S -based heterostructured QDs with core/shell and core/shell/shell structures) on the atomic scale using atom probe tomography. We reveal that both the AgIn Ga S /AgGaS QDs with and without an outer ZnS shell have compositional gradients at their interfaces and elemental inhomogeneity among their cores. Furthermore, an Ag-deficient AgIn G a S layer is identified on the outer surface of the AgGaS shell, where the stoichiometric fractions satisfy x >> y, arising from differences in the precursor reactivity. Meanwhile, in the AgIn Ga S /AgGaS /ZnS QDs, the outer ZnS shell evolves into Zn Ga S through a cation exchange process, ensuring structural and chemical compatibility with the inner shell. Our findings uncover the internal architecture and nanoscale elemental distributions of quaternary QDs, providing guidance for the future development of QDs.

Introduction
Over the past several decades, colloidal semiconductor quantum dots (QDs) have garnered significant attention owing to their versatility in applications including optoelectronic and biomedical devices.1-11 QDs are particularly attractive because of their high photoluminescence (PL) performance, size-tunable optoelectronic characteristics, and cost-effectiveness.1-14 The introduction of a core/shell structure, where the core is encapsulated by a shell, stabilizes QDs and increases their PL efficiency.1,6,9-11,14-17 Recently, multinary I–III– S VI-based QDs, such as AgInS 2 and CuInS 2, have emerged as promising environmentally friendly alternatives to conventional Cd- or In-based QDs for next-generation applications because their high compositional flexibility enables band-gap tuning.1,7,14-16,18-24 Notably, the incorporation of Ga atoms into AgInS 2 cores has produced green-emitting AgIn Ga S -based QDs with enhanced color purity and strong blue absorption.18-24

Despite these advantages, AgIn Ga S -based QDs still face critical challenges, including an inherently lower PL quantum yield (QY) and poor photochemical stability.20,21,24 These issues may stem from factors such as the internal structure, compositional distribution, defects, elemental diffusion, and residual impurities. However, little is known about the atomistic mechanisms, such as the incorporation and diffusion of elements within AgIn Ga S -based QDs. Several key questions remain unanswered: Is the QD structure a core/shell configuration? What is the three-dimensional (3D) compositional distribution within each layer? How do elements diffuse in heterostructured QDs? Are the interfaces and layers compositionally discrete or gradual? How do these elemental distributions affect the properties of QDs? What happens during shell growth on these complex structures?

Although the research on nanometer-sized core/shell QDs has advanced significantly through techniques such as transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared (FT-IR) spectroscopy, these questions remain unresolved.25-30 In particular, directly mapping the 3D distribution of constituent elements is a significant challenge. This difficulty might be due to the restricted spatial resolution and sensitivity of these instruments, as well as their inherent inability to analyze the 3D distribution in such complex structures. Atom probe tomography (APT) provides a potential solution by enabling direct 3D atomic imaging with a spatial resolution on the sub-nanometer scale.31-35 In APT, an electric or laser pulse directed at a needle-shaped specimen (<100 nm apex) causes individual ions to evaporate and travel to a position-sensitive detector, where their time-of-flight yields the mass-to-charge ratio, enabling 3D elemental mapping.31-35 However, applying APT to nanostructures, especially colloidal QDs, remains technically challenging because of their porosity, which complicates the preparation of needle-shaped specimens and often leads to structural instability.

In this study, we employ APT analysis to achieve the direct 3D mapping of two types of quaternary AgIn Ga S -based heterostructured QDs: (1) AgIn Ga S core/AgGaS shell QDs and (2) AgIn Ga S core/AgGaS inner shell/ZnS outer shell QDs. The direct 3D observation of individual atoms enables us to uncover the complex internal structure of these QDs. We reveal that both types of QDs exhibit a compositional gradient across the layers and interfaces, rather than discrete boundaries. For the AgIn Ga S /AgGaS QDs, we identify the formation of an Ag-deficient shell on the surface of the AgGaS shell during synthesis because of differences in the precursor reactivity. This Ag-deficient shell may sufficiently confine charge carriers while maintaining the crystalline structure. Interestingly, during the ZnS precursor step to form an outer shell on the AgIn Ga S /AgGaS QDs, a Zn Ga S outer shell spontaneously forms instead, driven by Ag+ and Ga3+ to Zn2+ cation exchange. This Zn Ga S shell exhibits structural and chemical compatibility with the AgGaS inner shell and is expected to induce a Type-I band alignment. Additionally, APT analysis directly demonstrates noticeable compositional inhomogeneity among different AgIn Ga S cores, with off-stoichiometry in the core material. In addition to the effects of the size distribution, this nonuniform composition may also contribute to the inhomogeneous spectral broadening of the ultraviolet–visible (UV–Vis) absorption and PL band. These findings highlight the potential of APT to deepen our understanding of complex nanostructures and guide improvements in their performance.

Results
Structural and optical characteristics

Figure 1a,d schematically illustrates the internal structure, band diagram, UV–Vis absorption, and PL characteristics of the AgIn Ga S /AgGaS and AgIn Ga S /AgGaS /ZnS QDs, respectively. The AgIn Ga S /AgGaS QDs were synthesized by modifying a method described in refs.18-20 Both types of QDs exhibit bright green luminescence, with emission peaks at ≈530 nm. Notably, the QY increases from 85% to 92% after the ZnS precursor step. Furthermore, the stability of QD–acrylate composite films under ambient conditions was evaluated in terms of the change in the QY after 48 h with respect to the initial QY. As shown in Supplementary Fig. 1, the ZnS shell significantly improves this normalized QY from 66% to 97%. The enhanced optical properties can be attributed to the effective confinement of electrons and holes provided by the wide-bandgap ZnS encapsulating the AgIn x Ga 1-x S 2 /AgGaS 2. In addition to the dominant green emission, the AgIn Ga S /AgGaS /ZnS QDs exhibit a long-wavelength emission tail in the 600–700 nm range (Fig. 1d). The area ratio of this long-wavelength emission, defined as the fraction of the total PL intensity in the long-wavelength region (λ > λ + 50 nm), increases from ≈3.7% in AgIn Ga S /AgGaS QDs to ≈14.9% in AgIn Ga S /AgGaS /ZnS QDs. Such long-wavelength emission is generally attributed to defect- or trap-assisted recombination rather than band-edge excitonic transitions in AgInS 2 -based QDs and related I–III–VI 2 -based QDs.36-40

The synthesized AgIn Ga S /AgGaS core/shell and AgIn Ga S /AgGaS /ZnS core/shell/shell structured QDs were characterized using high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM). Figure 1b,e displays the HAADF–STEM images of the AgIn Ga S /AgGaS QDs before and after the formation of the ZnS outer shell, respectively. The ZnS shell, which facilitates the formation of Type-I structures and improves the PL QY and stability, was synthesized through thermal decomposition.1,4,5,11,15,21 The AgIn Ga S /AgGaS QDs primarily exhibit a round-cornered polyhedral or spherical shape, with an average diameter of 6.14 ± 0.76 nm. Interestingly, after the ZnS precursor step for the outer shell formation, the QDs grow only negligibly, despite the addition of more precursors, and remain similar in both size and shape, with an average diameter of 6.11 ± 1.03 nm (Supplementary Fig. 2; see also Supplementary Fig. 3 for HAADF–STEM images of the core-only AgIn Ga S QDs). Moreover, the high-resolution STEM results reveal that the QDs are structurally similar before and after the ZnS precursor step, with all crystal planes showing a lattice spacing of ≈0.29 nm, which almost corresponds to the interplanar spacing of the (200) planes of chalcopyrite-structured AgGaS 2. The lattice fringes are clearly evident not only inside the AgIn Ga S -based QDs but also at the edges, indicating the high crystallinity across the entire QD structure (Fig. 1b,e). X-ray diffraction (XRD) measurements were further performed on the core-only AgIn Ga S , AgIn Ga S /AgGaS , and AgIn Ga S /AgGaS /ZnS QDs to corroborate the structural similarity before and after shell formation. All XRD patterns are consistent with a chalcopyrite-based tetragonal structure, and the overall diffraction features remain essentially unchanged, even after the Zn precursor step (Supplementary Fig. 4). This comparative analysis indicates that the crystal structure is preserved throughout the shell formation process.

Meanwhile, HAADF–STEM imaging did not provide sufficient contrast to clearly distinguish the core from the shell. This limitation arises from the lack of significant mass differences between the constituent elements, as well as the similar crystallinity extending from the core to the shell. Specifically, for AgIn Ga S -based QDs (x ≈ 0.5) synthesized for green emission in this study, the limited Z-contrast makes the core visually indistinguishable from the shell. The STEM–energy-dispersive X-ray spectroscopy (STEM–EDS) mapping showed that Zn atoms are distributed over a broader region than Ag and In after the Zn precursor step (Fig. 1c,f and Supplementary Fig. 5). However, the current STEM results remain insufficient for resolving the detailed internal and local compositional information of such 3D nanostructures. To identify the factors affecting the optical properties of the QDs and to further improve their performance, their internal structure must be more comprehensively understood.

3D compositional distribution of AgIn Ga S /AgGaS QDs

To characterize and quantify the 3D atomic distribution of the AgIn Ga S -based core/shell structured QDs, APT was employed. For the APT analysis in this work, dried QDs were prepared in powder form and analyzed following a method of ref.41. In that study, we demonstrated that APT could resolve core/shell architectures and 3D compositional distributions in commercially available CdSe/ZnS core/shell QDs, in good agreement with TEM observations. Specimen preparation strategies, potential APT artifacts, and reconstruction considerations specific to CdSe/ZnS core/shell QDs were systematically examined, thereby establishing the applicability of APT to colloidal QD nanostructures. More broadly, the feasibility of APT in the analysis of metallic nanoparticles has been demonstrated in numerous prior studies, including ligand-resolved analyses and combined experiment–simulation approaches.42-47 On this basis, we report a 3D atomic-scale characterization of AgIn Ga S -based QDs using APT, providing detailed insight into their internal compositional structure.

Figure 2a shows the 3D reconstructed APT atom maps of the AgIn Ga S /AgGaS QDs, representing a hemispherical APT specimen containing five QDs (see Supplementary Fig. 6 for dashed guides indicating the approximate outlines of individual QDs). The QDs appear as slightly ellipsoidal features in the APT reconstruction, and a slice-view image along the x–z plane is presented in Fig. 2b to visualize their internal elemental distribution. We note that APT analyses of nanostructures may exhibit reconstruction artifacts, such as apparent elongation along the analysis (z) direction and interfacial broadening, primarily arising from local magnification and trajectory aberrations.42-52 Accordingly, the slightly ellipsoidal morphology observed here is attributed to such effects rather than the intrinsic particle shape. However, compositional distributions along the z-direction have been shown to provide more reliable quantitative information than lateral directions because the reduced trajectory overlap suppresses artificial intermixing.42-44,51,52 Under these conditions, compositional distributions and interfacial characteristics closer to the intrinsic distributions can be more evaluated.

APT shows that the detected Ga atoms are more broadly distributed than the Ag and In atoms in the QDs, and S atoms appear in both the core and shell regions of the QDs. Because the QDs are closely packed in a non-collinear configuration within the APT specimen and may therefore partially overlap in projection views, 3D reconstructions with C- and H-related ions removed are also provided in Supplementary Fig. 7 to better visualize the individual QDs. The 3D concentration profile obtained from an iso-concentration surface (proxigram) for Ag atoms reveals the average compositional trends across the five QDs (Fig. 2c). This profile more clearly indicates a higher concentration of Ag and In atoms toward the QD core, whereas Ga atoms increase toward the outer shell, confirming the formation of an AgIn Ga S core/AgGaS shell heterostructure.

To more precisely analyze the atomic-scale elemental distribution within individual QDs, two representative AgIn Ga S /AgGaS QDs were extracted from the 3D reconstruction map. Figure 3a,d shows the 3D atom maps and the cross-sectional regions cropped by the green boxes for each QD. The cross-sectional maps of individual elements are presented in Fig. 3b,e, providing detailed insights into the internal structure. The 2D compositional profiles along two different directions (Fig. 3c,f) further confirm the formation of the AgIn Ga S core/AgGaS shell heterostructure with a compositional gradient. From the extracted profiles, we primarily focus on z-direction compositional analyses, cross-validated by proxigrams and lateral profiles. Ag and In atoms are highly concentrated at the center of the QD, at more than 20 and 10 at.%, respectively, but their concentrations gradually decrease toward the shell. Conversely, Ga atoms are less concentrated in the QD center but significantly more present in the outer shell. In addition, trace amounts of In atoms remain present in the shell region, leading to a shell composition closer to AgIn Ga S , where x denotes the fraction of In in the core and y represents the residual In fraction in the outer layer, with x >> y, rather than a pure AgGaS shell. This is likely due to the outward diffusion of In atoms from the AgIn Ga S core to vacant sites in AgGaS , driven by the concentration gradient or thermal budget during synthesis. Nevertheless, the resulting AgIn Ga S shell is expected to effectively confine charges within the AgIn Ga S core owing to its higher band gap, consistent with a Type-I heterostructure, which promotes radiative recombination.

Another notable feature of the AgIn Ga S /AgGaS QDs revealed by the APT analysis is the Ag-deficient outermost shell on the AgGaS shell, which, strictly speaking, corresponds to the AgIn Ga S shell. Comparing the cross-sectional atomic maps of Ag and Ga (Fig. 3b,e), Ga shows a broader distribution than Ag. The 2D compositional profiles also indicate a higher concentration of Ga atoms in the outermost region of the QDs, although distinguishing individual layers is challenging because of the compositional gradient (Fig. 3c,f). The unexpected formation of the Ag-deficient AgIn Ga S outermost shell is associated with the relatively lower reactivity of Ga3+ compared with that of Ag+ during the AgGaS shell synthesis. The lower reactivity between S2- and group 13 elements, such as Ga3+, compared with that of group 11 elements, e.g., Ag+, has been observed when synthesizing group 11, 13, and 16 semiconductor QDs; hence, the reaction conditions must be precisely controlled to achieve the desired shell formation.53 To compensate for the limited incorporation of Ga caused by its lower reactivity, we introduced an excess of Ga precursors during shell growth. Given that Ag+ is more reactive than Ga3+, Ag preferentially reacts with S during the initial stage of AgGaS shell growth. The excess Ga, which may not be readily incorporated during the early stage of the reaction, contributes to forming the Ag-deficient AgIn Ga S shell at a later stage. The resulting Ag-deficient outermost shell is expected to effectively passivate surface defects without introducing additional defect levels into the band gap, despite its Ag deficiency, thereby preserving the crystalline structure and stabilizing the QDs.

To demonstrate the role of differential precursor reactivity, we performed controlled syntheses in which the Ag precursor amount was increased at a fixed Ga level, a condition under which increasing the Ag precursor amount directly promotes the further growth of the AgGaS shell. STEM–EELS analyses showed the pronounced thickening of the AgGaS shell and consistently increased Ag incorporation, supporting the role of precursor reactivity in the formation of the shell (Supplementary Fig. 8 and Supplementary Table 1). These changes were accompanied by an increased contribution from trap-related emission and a decreased photoluminescence QY, indicating that Ag-driven shell growth directly impacts the optical properties of the QDs.

3D compositional distribution of AgIn Ga S /AgGaS /ZnS QDs

Following the ZnS precursor step to form the outermost shell, APT was conducted to investigate the compositional evolution of AgIn Ga S /AgGaS /ZnS QDs. Figure 4a shows a reconstructed APT 3D atom map containing five nearly spherical AgIn Ga S /AgGaS /ZnS QDs (see Supplementary Fig. 9 for dashed guides indicating the approximate outlines of individual QDs). To further clarify the presence of these five QDs within the reconstructed volume and their internal structures, slice-view images along the x–z planes are provided in Fig. 4b. Although the 3D distribution of individual elements varies slightly among QDs depending on the cross-section position, all QDs consistently exhibit the AgIn Ga S core/AgGaS inner shell/Zn Ga S outer shell structure, as shown in Fig. 4b. Further, the 3D atom maps show that Zn atoms, primarily located in the shell regions, are more broadly distributed than Ag and In. In APT, some Zn and S isotopes overlap in the mass spectrum. However, distinct peaks, such as S+, allow imaging with minimal interference from Zn isotopes (Supplementary Fig. 10). Interestingly, the Ga atoms, which are expected to reside primarily in the core and inner shell, are more broadly distributed throughout the QD than initially anticipated (Supplementary Fig. 11).

The proxigram from an iso-concentration surface of Ag atoms, which describes the five QDs, reveals that In atoms are more narrowly distributed than Ag atoms (Fig. 4c and Supplementary Fig. 12). Zn atoms are predominantly present in the outer shell, as evidenced by their increasing concentration toward the exterior. Ga atoms are primarily abundant in the AgGaS inner shell but are also present in significant amounts within the ZnS outer shell. The 2D concentration profile along the analysis direction offers additional insights into the internal structure of each QD (Fig. 4d). Although the elemental concentrations vary slightly depending on the profiling path owing to the spatial offset of individual QDs in the APT specimen, the 2D profile confirms the significant presence of Ga atoms in the outer shell, albeit at lower concentrations than Zn atoms. C-related signals, likely originating from ligands, were also observed on the outside of the QDs (Supplementary Fig. 13). Nevertheless, because of the difficulty in quantifying these signals, our discussion focuses on the distribution of the main elements (see Supplementary Fig. 14 for the 3D reconstructions with the C- and H-related ions removed for clarity). Notably, this work provides the first 3D atomic-scale insights into heterostructured quaternary QDs with a core/shell/shell geometry using APT.

To validate the internal structure and elemental distribution of AgIn Ga S /AgGaS /ZnS QDs, two representative QDs were extracted from the 3D reconstruction map (Fig. 5). Figure 5a,d shows the extracted 3D atom maps and the cross-sectional regions cropped by the green boxes for each QD. According to the z-direction compositional profiles cross-validated by proxigrams and lateral profiles, Ag atoms are more widely distributed than In atoms in the 3D atom maps (Fig. 5b,e). However, both Ag and In atoms are at their highest concentrations in the central region of the QDs, with values exceeding 20 and 10 at.%, respectively. The concentration of In atoms remains lower than that of Ag atoms outside the core, substantiating the formation of an inner shell closer to AgIn Ga S or AgGaS (Fig. 5c,f). Similar to those of the AgIn Ga S /AgGaS QDs, the shell regions of the AgIn Ga S /AgGaS /ZnS QDs also exhibit trace amounts of In atoms, likely due to diffusion during synthesis. Zn atoms show the highest concentrations in the outer shell surrounding the AgIn Ga S /AgGaS core/inner shell. Interestingly, the compositional distributions of Ag, In, and Ga are gradual rather than discrete across the layers. From a structural perspective, this compositional gradient is expected to offer advantages such as minimizing lattice mismatch between the core and inner shell and effectively reducing the likelihood of internal defect formation. Such structural features—often referred to as compositional-gradient shells or gradient alloy shells—have been widely reported in InP- and CdSe-based QDs to contribute to enhancing both the QY and stability.54-58 As observed in the reconstructed maps and proxigram, Ga atoms are also present not only in the AgIn Ga S core and AgGaS inner shell, but also in the ZnS outer shell, thus forming a Zn Ga S shell (Fig. 5). The Ga content is highest in the AgGaS inner shell and lower than the Zn content in the outer shell.

Because APT inherently probes only a limited number of individual QDs and may therefore suffer from limited statistical representativeness, the compositional features identified by APT were further examined using complementary techniques. XPS analyses performed on the same samples reveal consistently higher Ga-to-Ag ratios after the Zn precursor step, evaluated from multiple core-level regions, including Ag MNN Auger transition (where MNN denotes an Auger transition involving the M and N shells) vs. Ga 2p and Ag 4d vs. Ga 3d. This systematic increase in the Ga-to-Ag ratios supports the formation of Ga-containing outer shells and is consistent with the Zn 1- Ga S shell formation revealed by APT (Supplementary Fig. 15 and Supplementary Table 2). Inductively coupled plasma analyses performed on the same samples show overall elemental compositions consistent with the APT results (Supplementary Tables 3 and 4). Taken together, these results indicate that the structural and compositional features identified by APT are not limited to a small number of individual QDs but are representative of overall sample.

Interestingly, directly comparing two individual QDs reveals compositional differences, even among co-synthesized QDs. These differences are particularly pronounced in the core region, with noticeable variations in the Ag, In, and Ga contents; some QDs exhibit cores with a very low Ga content (Figs. 4d, 5c). Similar results were observed in the AgIn Ga S /AgGaS QDs, albeit to a lesser extent. The pronounced inhomogeneity in the cores likely originates from the intrinsic complexity of the quaternary Ag–In–Ga–S system. The synthesis of the core simultaneously involves multiple cation precursors with different reactivities, and the cores are formed under a continuous heating process. Previous studies have also shown that AgIn Ga S core formation proceeds through intermediate phases and cation-exchange processes rather than a simple single-step growth process.18,53,59 More specifically, AgGaS or AgInS seeds form initially, followed by the incorporation of In and Ga to generate the chalcopyrite AgIn Ga S lattice, which is likely to promote local compositional fluctuations within the core. Furthermore, the subsequent ZnS precursor step introduces an additional thermal treatment, which can promote cation diffusion and redistribution, thereby amplifying pre-existing compositional fluctuations within the cores. Consistent with the APT observations, STEM–EDS line profiles collected from multiple QDs reveal noticeable particle-to-particle variations in the relative Ag, In, and Ga distributions (Supplementary Figs. 16, 17). For each QD, several independent line profiles were extracted and compared, all of which show dot-to-dot compositional fluctuations. These results indicate that the observed compositional inhomogeneity is an intrinsic feature of the synthesized quaternary QDs, thereby supporting the features revealed by APT. A deviation from the ideal stoichiometry in the core can change the emission wavelengths. Thus, such compositional inhomogeneity in the quaternary semiconductor cores may contribute to the inhomogeneous spectral broadening of the UV–Vis absorption and PL bands for both QDs, along with other factors such as the size distribution. Our APT analysis directly shows compositional inhomogeneity among the cores and highlights the importance of controlling such variations, along with defects, to improve the color purity of the green emission.

Our analytical findings have thus far proved that the outer shell in AgIn Ga S /AgGaS /ZnS QDs is indeed a Zn Ga S shell rather than a ZnS shell. The unintended formation of this Zn Ga S shell can be ascribed to cation exchange between Zn2+ and both Ag+ and Ga3+, with Zn2+ ions partially replacing both Ag+ and Ga3+ ions in the AgGaS shell during the ZnS precursor step (Supplementary Fig. 18). During the initial stages of this process, Zn2+ ions approach the AgGaS surface and replace Ag+ and Ga3+ ions, which occupy tetrahedral coordination sites in the AgGaS lattice. These two ions are progressively substituted by Zn2+ ions while maintaining charge neutrality, leading to a gradual transition in the composition of the outer shell into Zn Ga S. This process forms a compositionally mixed Zn Ga S outer shell with a chalcopyrite structure, which is the overall crystallographic framework of the AgGaS inner shell.18,53,60-65 Fortuitously, the formation of Zn Ga S with a chalcopyrite-like structure, which is the same as that of AgGaS , is crystallographically preferable over zincblende-structured ZnS. This structural compatibility between Zn Ga S and AgGaS is expected to minimize lattice mismatch, thus reducing lattice distortion and residual strain. Furthermore, this explains why the STEM–HAADF analysis revealed that the overall size of the QDs remained nearly unchanged after the ZnS precursor step, while their crystallinity was preserved throughout the QDs, thus verifying the formation of the Zn Ga S shell, which is structurally similar to ZnGaS , via cation exchange.

The emergence of the Zn Ga S outer shell via cation exchange offers a plausible origin for the long-wavelength emission tail observed after the ZnS precursor step. In our system, the substitution of Ag+ and Ga3+ by Zn2+ during shell formation inherently introduces cation vacancies and local charge-compensating defects. Consistent with our APT observations, the inward diffusion of Zn2+ into the interior of the QDs further induces cation exchange and promotes the formation of such defects. When these defects form in the vicinity of In-rich regions, where carriers are preferentially confined, they can introduce intragap trap states that locally perturb the confinement potential and promote trap-assisted recombination. These defect-related states are commonly associated with donor–acceptor-pair-like recombination pathways that give rise to long-wavelength emission in I–III–VI semiconductors.23,38-40,66-68

In terms of the band gap energy, the Zn Ga S shell typically exhibits a slightly lower band gap than ZnS (≈3.5 eV), even at the nanoscale.62,65,69-72 The formation of the compositionally mixed Zn Ga S layer is therefore expected to adequately preserve the quantum efficiency, with negligible weakening of charge confinement, consistent with Type-I-like band alignment. Moreover, the spontaneous formation of the Zn Ga S shell could significantly reduce crystallographic or interfacial defects that might arise from the structural differences between the inner and outer shells. By creating a more seamless interface with the AgGaS inner shell, the Zn Ga S shell may enhance the stability of the QDs under ambient conditions and contribute to a more stable and efficient green luminescence. This optical stability is particularly advantageous for optoelectronic applications where long-term emission reliability is critical.

Trace detection of residues in AgIn Ga S /AgGaS /ZnS QDs

To investigate the possibility of trace elements remaining in the nanoscale QDs, we evaluated the distribution of Cl atoms. As shown in Fig. 6 and Supplementary Fig. 19, Cl atoms are detected only in the AgIn Ga S /AgGaS /ZnS QDs and are localized in the outer shell region in only trace amounts. The Cl atoms, derived from the ZnClN precursor used in the ZnS precursor step, show a strong spatial correlation with Zn atoms in the APT analysis, whereas no comparable correlation is observed with Ag, In, or Ga atoms. XPS measurement further confirms that Cl is absent in AgIn Ga S cores and nearly absent in AgIn Ga S /AgGaS QDs, whereas the Cl signal is pronounced in the AgIn Ga S /AgGaS /ZnS QDs, exhibiting metal–Cl chemical states attributable to Zn–Cl-related species (Supplementary Fig. 20 and Supplementary Table 2). These Cl species can be both detrimental and beneficial to the AgIn Ga S /AgGaS /ZnS QDs. As for the detrimental effects, residual Cl may introduce lattice defects and surface trap states, which can serve as non-radiative recombination pathways, ultimately reducing the QY and optical stability of the QDs.73,74 Conversely, in the absence of such detrimental effects, Cl can play a beneficial role by acting as a surface passivation agent or inorganic ligand.75-77 Given these opposing effects, the incorporation of residual elements such as Cl must be carefully controlled by monitoring both their amounts and spatial distribution. Achieving an optimal balance in the incorporation of residues can maximize their passivating and stabilizing effects while minimizing potential adverse impacts on QD performance.

Although our study only demonstrated the 3D distribution of trace elements remaining after synthesis, APT is expected to be readily applicable to a broad range of nanostructures, particularly those with complex heterostructures. Such applications will enable detailed elucidation of the spatial distribution, segregation behavior, and interfacial incorporation of trace elements or dopants, thereby providing critical insights and design guidelines for the structural and electronic optimization of advanced nanomaterials.

In this study, we elucidated the internal structure and 3D elemental distribution of AgIn Ga S -based QDs, which have a heterostructure in core/shell geometry, via APT analysis. Both AgIn Ga S /AgGaS and AgIn Ga S /AgGaS /ZnS QDs exhibited compositional gradients across layers and interfaces rather than discrete boundaries. Such gradient interfaces minimize lattice mismatch between adjacent layers, thereby reducing potential internal defects and enabling the realization of highly luminescent AgIn x Ga 1-x S 2 -based QDs. For AgIn x Ga 1-x S 2 /AgGaS 2 core/shell structured QDs, we identified an Ag-deficient shell on the surface of the AgGaS shell, which could be attributed to the differences in the precursor reactivity during synthesis. For the AgIn Ga S /AgGaS /ZnS core/shell/shell structured QDs, we revealed the presence of compositional inhomogeneity among AgIn Ga S cores. Additionally, a Zn Ga S outer shell was found to form spontaneously through cation exchange during the ZnS precursor step for outer shell formation, and this shell effectively confined charge carriers within the core with its Type-I band alignment. These findings provide insights into the atomic-scale incorporation and diffusion behavior in QDs, offering guidance for optimizing the design and performance for advanced optoelectronic applications.

Methods
Materials

Gallium(III) acetylacetonate (Ga(acac) , 99.99%, Sigma-Aldrich), gallium(III) chloride (GaCl , 98%, Allegra), silver acetate (AgOAc, 99.99% trace metal basis, Sigma-Aldrich), 1-octadecene (ODE, 90%, Sigma-Aldrich), sulfur powder (99.98%, Sigma-Aldrich), n-trioctylphosphine (TOP, 90%, Strem), oleylamine (OAm, ≥98%, Allgre), toluene (anhydrous, 99.8%, Sigma-Aldrich), N,N'-dimethylthiourea (DMTU, 97% Tokche), indium acetate (In(OAc) , 99.99%, Sigma-Aldrich), and ethanol (99.5%, Samchun Chemical) were used as purchased.

Precursor preparation

A 0.06 M Ag/OAm stock solution was prepared by dissolving AgOAc (6 mmol) in 100 mL of OAm, and the mixture was evacuated at 50 °C. In addition, a 1 M S/OAm stock solution was prepared by dissolving S powder (0.1 mol) in 100 mL of OAm under vacuum. Finally, 1 M GaCl /TOP and 0.5 M ZnCl /TOP stock solutions were prepared by separately dissolving GaCl (0.1 mol) and ZnCl (0.05 mol) powders in 100 mL of TOP, respectively.

Synthesis of the AgIn Ga S cores and AgIn Ga S /AgGaS QDs

AgIn Ga S -core QDs were synthesized by injecting a S precursor into a mixture of Ag, In, and Ga precursors, following a procedure similar to methods reported in refs.18-20 First, AgOAc, In(OAc) , and Ga(acac) were mixed in 100 mL of OAm in a reaction flask, and the mixture was evacuated at room temperature for 10 min. The flask was then filled with N gas, and 40 mL of the 1 M S/OAm stock solution was swiftly injected into the flask. Next, the flask was heated to 240 °C, and the temperature was maintained until the desired particle size was achieved, while monitoring the reaction progress using UV–Vis absorption and PL spectrometry. The resulting AgIn Ga S cores were centrifuged with acetone and re-dispersed in toluene. The molar ratio of Ga to In precursors was adjusted between 1.0 and 1.4 to tune the PL peak wavelength to ≈530 nm.

Subsequently, to coat the cores with the AgGaS shell, a flask containing 2.3 g of DMTU and 200 mL of an OAm/ODE (1:1) solvent mixture was evacuated at 120 °C for 10 min. Then, it was filled with N gas and heated to 260 °C. When it reached this temperature, the 1 M GaCl /TOP solution, the 0.06 M Ag/OAm solution, and the AgIn Ga S core QDs in toluene (≈5 mL) were injected into the flask, and the temperature was maintained for 100 min. The final AgIn Ga S /AgGaS QDs were centrifuged at 3760× g with the addition of excess ethanol and re-dispersed in toluene for subsequent coating with ZnS. The Ag/Ga and Ga/S precursor ratios were 0.04 and 0.8, respectively.

Synthesis of the AgIn Ga S /AgGaS /ZnS QDs

First, 0.25 g of DMTU and 80 mL of OAm were loaded into a reaction flask, and the mixture was evacuated at 120 °C and heated to 210 °C under N flow. Then, the 0.5 M ZnCl /TOP solution and the AgIn Ga S /AgGaS core/shell QDs in toluene (≈10 mL) were quickly injected into the flask, and the temperature was maintained for 100 min. The Zn and S precursors were injected in equal molar amounts.

Optical property characterization

UV–Vis and PL measurements were conducted with a UV-2600 (Shimadzu) and F-7100 (HITACHI), respectively. The absolute PL QYs of the QDs dispersed in toluene and QD–acrylate composite films were measured using a quantum efficiency measurement system (Otsuka, QE-2100) equipped with an integrating half sphere with excitation at 450 nm.

STEM observations

Colloidal AgIn Ga S /AgGaS and AgIn Ga S /AgGaS /ZnS QDs were dispersed in hexane and deposited on TEM grids with a carbon support film (Tedpella, P/N 01824). Spherical aberration (Cs)-corrected STEM (FEI-Titan Cubed) was performed in the HAADF mode at 300 kV. High-resolution STEM images were acquired using an 80 pA current probe. STEM–EDS elemental maps were obtained using a 130 pA current probe over a total acquisition time of 10 min. The average particle size was determined by measuring the diameters of more than 300 randomly selected QDs from HAADF–STEM images using ImageJ 1.53 software.

APT specimen preparation

Needle-shaped specimens for APT analysis were prepared from dried QDs in powder form using the focused ion beam (FIB) lift-out method (Helios5 HX, Thermo Fisher Scientific). Initially, a 100 nm-thick Pt layer was deposited using an electron beam, followed by depositing a 1 μm-thick Pt layer with a Ga-ion beam to passivate a region of interest measuring 12 μm × 1.7 μm on the sample surface. The target region was then extracted and mounted onto a sharpened W tip. The specimens on the W tip were further shaped into a needle-like geometry using an annular milling pattern (30 kV, 80 pA). To refine the samples and reduce damage induced by the Ga-ion beam, the region of interest was thinned again with an annular pattern at 5 kV and 8 pA. The morphology and dispersion of the QDs prepared by this procedure were confirmed by HAADF–STEM (Supplementary Fig. 21).

APT analysis

APT analysis was conducted using a CAMECA atom probe (Invizo 6000) operating in the deep-UV laser mode (λ = 257.5 nm) with a pulse repetition rate of 200 kHz and a base temperature of 50 K. A laser pulse energy of 100 pJ was applied, achieving a detection rate of 0.01 atoms pulse-1. The acquired data were reconstructed and analyzed using CAMECA AP Suite 6.3 software. All elemental species constituting the AgIn Ga S -based QDs, as well as C-related ions, were identified from the mass spectra and assigned before reconstruction. The reconstructions were performed using a shank-angle-based protocol. SEM and HAADF–STEM images were utilized to guide the reconstruction and constrain the specimen geometry, thereby enhancing the accuracy of the APT reconstruction process. No artificial geometric constraints were imposed (e.g., forcing spherical particle shapes) during the reconstruction process. The slightly ellipsoidal morphologies observed in the APT datasets were therefore preserved as reconstructed rather than being artificially forced into an idealized geometry and are attributed to known trajectory aberration and local magnification effects.

Data availability

The data that support the findings of this study are available from the corresponding authors upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents an impressive and highly novel application of atom probe tomography (APT) to elucidate the 3D atomic structure of complex AgInxGa1-xS2-based quaternary quantum dots. The use of APT to map these materials at this scale is a significant technical achievement, and the findings—particularly regarding compositional gradients, precursor reactivity effects, and the spontaneous formation of a Zn(1-3/2x)GaxS shell via cation exchange—are of considerable interest to the nanomaterials and optoelectronics communities.\nThe work is thorough and the data is compelling. However, there are several key issues regarding data interpretation, visualization, and the substantiation of claims that require major revision before this manuscript can be considered for publication.\nBelow are specific points that must be addressed:\n1. The APT technique is fundamentally destructive, \"disassembling\" the specimen atom-by-atom via high-field evaporation. A critical and unaddressed point is how the authors can be certain that the resulting 3D reconstruction faithfully represents the original, pre-analysis structure of the QDs. High electric fields and laser pulses can, in some cases, induce artifacts such as atomic diffusion or re-ordering during the analysis. The authors must include a discussion on the potential for such artifacts and provide a justification (e.g., by referencing reconstruction parameters, evaporation behavior, or literature on similar materials) for why the presented 3D maps are considered a reliable representation of the as-synthesized QD structure.\n2. The manuscript's optical analysis (e.g., Figure 1) focuses on the bright green emission peak at ~530 nm. However, the provided PL spectra appear to show a correspondingly enhanced fluorescence in the long-wavelength region (approx. 600-700 nm) after the ZnS/cation exchange step. This feature is not discussed. The authors must provide a clear hypothesis or explanation for the origin of this long-wavelength emission. Is this attributable to unpassivated defect states, newly formed interfacial trap states, or a specific structural component? A complete optical analysis requires addressing all significant features in the spectra.\n3. The 3D atomic map in Figure 2a is intended to show the five QDs analyzed, but the presence of C atoms (presumably from ligands or the matrix) makes it very difficult to discern the specific shapes or interfaces of the QDs themselves. To improve image clarity, it is strongly recommended to remove the C atoms (and any other extraneous matrix-related atoms) from the visualization in Figure 2a (and Figure 4a). This would provide a much clearer view of the actual QD structures being analyzed.\n4. The authors provide a convincing chemical argument that the Ag-deficient AgInyGa1-yS2 outermost shell is associated with the relatively lower reactivity of Ga3+ compared to Ag+. This is a key finding. To strengthen this claim, have the authors attempted (or can they propose) an experiment to confirm this mechanism? For example, using a low-activity Ag precursor should, in principle, lead to a more uniform elemental distribution. A discussion of this point, or ideally further experimental evidence, is needed to more robustly validate the proposed formation mechanism.\n5. A close inspection of the 2D compositional profile in Figure 4d reveals an anomaly. For QD#4, Zn atoms (blue squares) are shown to be present within the QD core, not just in the outer shell. This observation is not discussed by the authors, whose analysis focuses on cation exchange at the shell interface. The authors must address this finding.\n6. There is a fundamental contradiction in the manuscript's description of the final shell. The text frequently refers to \"synthesizing the ZnS shell\", \"ZnS shell coating\", or \"shell growth\". However, two key pieces of the authors' own data refute this:\n• TEM Data: The authors state the QDs \"grow only negligibly\" and remain \"similar in both size and shape\" after the ZnS precursor step. This is inconsistent with the epitaxial growth of a new shell.\n• APT Data: The authors' own excellent APT analysis proves the process is cation exchange, forming a Zn(1-3/2x)GaxS shell.\nThe authors must revise the manuscript to consistently describe this process as cation exchange, as their own evidence strongly contradicts the narrative of simple \"shell growth.\"\n7. The authors claim that the final Zn(1-3/2x)GaxS shell \"maintains the charge carrier confinement with a Type-I band alignment\". This is a critical conclusion for the QD's performance, but it is not experimentally proven. This claim appears to be an inference based on the band gaps of related bulk materials. Given the novel, complex, and compositionally graded structure revealed by APT, such an assumption is not sufficient. The authors must provide spectroscopic evidence (e.g., time-resolved PL or other measurements) to conclusively demonstrate the Type-I alignment. If such evidence is unavailable, this claim must be significantly toned down and presented as a well-reasoned hypothesis, not a confirmed fact.", "reviewer_2": "The present manuscript simply reported the measurement of AgInGaS QDs with atom probe tomography (APT). Although atom resolution images were obtained, no relationship between the nanostructure of QDs and their photochemical properties were reported. Thus, no novel results enough to publish in highly ranked journals were contained in the present manuscript.\nEspecially, following points were very weak.\n1. As mentioned in the introduction, the authors claimed that they three-dimensionally characterize quaternary QDs (AgInxGa1-xS2-based heterostructured QDs with core/shell and core/shell/shell structures in this case) on the atomic scale for the first time using APT. If so, the authors must show at first the reliability of the APT method for measurements of QD nanostructures, using well known QDs, such as CdSe QDs and CdSe/ZnS core-shell structured QDs.\n2. No relationship between the nanostructure of QDs and their photochemical properties were reported.\n3. No definition of the x and y values were indicated. No explanation of x >> y was made, because each formula only contained x or y.\n4. The charge neutrality in single QDs and its spatial modulation should be discussed.\n5. AgInxGa1-xS2/AgGaS2 had quite Ag-deficient surface layer. What is the crystal structure of this layer? Ag-doped Ga2S3??\n6. Also, what is the crystal structure of the surface of AgInxGa1-xS2/AgGaS2/ZnS? The authors assumed the ZnGaS alloy layer was formed on the surface. However, no difference of lattice fringes between core and shell in the corresponding HAADF-STEM images (Fig. 1) was observed.\nThus, the present manuscript does not contain novel results. The manuscript may not be suitable for the publication in the preset journal.", "reviewer_3": "The authors present an important study on the atomistic level for Quantum Dots using APT. The interface design and the intermixing of the components is still a great uncertainty for quantum dots, therefore, a knowledge of the distribution of the components is very important. APT is very promising for obtain more important and exciting results about the intermixing which can use for a better tuning of the synthesis of QDs. All in all, the manuscript is very important contribution in this field.\nBut there are some points which should be clarified before publication:\nIn Figs. 2, 4, and 6 different particles are shown in (a), in Figs. 2 and 4 there are five. It is unclear if all particles of them were further evaluated or only one of them was evaluated. In Figs. 4 and 6 the five partices are not homogeneous, in QD 4 and QD 5 (Fig.4) Ag seems be not so present, in Fig. 6 In was inhomegenously distributed. Was the Cl correlated with some of the cations? I am missing a statement about this question?\nThe general problem of all microscopic studies are the statistical relevance. Some comments to this issue would be very helfpul for the readership of this manuscript.\nIt would be helpful, to mark the single particles in Fig. 2, 4 and 6, then the reader can see the borders of the particles.", "reviewer_4": "The paper highlights an additional pathway to uncover the complex internal structure of quaternary AgInGaS core-shelled quantum dots using atom probe tomography (APT). It is a well-written paper, providing elemental compositional profiles across nanocrystals to obtain detailed information about he compositional variations of core-shell and core-shell-shell nanocrystals. Furthermore, insights are provided about relative precursor reactivity and potential substitution reactions occurring, as to why and how some of these layers were formed. I recommend publication in nature communications, with minor revisions.\n--The authors make broad claims about how the compositional gradient would improve basically every QD property (quantum yield, stability, longevity, etc). The authors should provide some literature references that have shown this or indicate that this should be the case.\n--A prominent tail is seen in the PL spectra after the addition of a ZnS shell (or alloyed ZnGaS layer as the authors later conclude). Could the others discuss the origin of this tail? Is it related to the compositional gradients observed in the QDs.\n--In Figures 2c and 4c: is the iso-surface proxigram shown just for one quantum dot? How does it different between the 5 QDs identified?\n--What happens if quantum dots aggregate? Is it possible to differentiate using the APT techniqe aggregated regions of QDs from individual qds that have clearly defined domains?\n--For the core-shell-shell structure (with ZnS), is the first shell still Ag-deficient like with the core-shell structure (as mentioned in line 76)?\n--Since the nanocrystal size does not change after ZnS shelling, does this mean the core-size decreases after addition of the ZnS shell? Or does the thickness of the first shell (AgGaS2 shell) decrease instead? Is it possible to estimate the diameter of the core and the thickness of two shells using this technique?\n--As mentioned in line 235 to 245, why was the compositional inhomogeneity more pronounced in the core versus the shell for the samples presented? Also, why does this inhomogeneity get more noticeable/severe after shelling with ZnS?\n--X-ray diffraction pattern not included. Can the XRD of the QDs before and after ZnS shelling be included as supporting information? This would further corroborate the high-resolution STEM imaging showing that the particles are structurally similar before and after coating with ZnS (lines 114 to 119).\n--Do the authors have elemental analysis (such as ICP-OES or ICP-MS) of the sample to compare/confirm the elemental composition obtained from atomic probe tomography (ATP)?\n--Method section (line 330) indicates that core-only AgInGaS were synthesized as well. How does the size, morphology, crystal structure and elemental composition between the core-only and core-shell (and core-shell-shell) nanocrystals differ? Was APT conducted on the core-only QDs? If so, are the elemental composition trends seen for the core-only, similar to the cores of the shelled QDs? Is inhomogeneity between different nanocrystals seen also for the core-only samples? Or is it a by-product of shelling?"}, "author_response": "NCOMMS-25-77544-T\nRESPONSE TO REVIEWERS’ COMMENTS\nReviewer #1 (Remarks to the Author):\nGeneral Comment: This manuscript presents an impressive and highly novel application of atom probe\ntomography (APT) to elucidate the 3D atomic structure of complex AgInxGa1-xS2-based quaternary\nquantum dots. The use of APT to map these materials at this scale is a significant technical achievement,\nand the findings—particularly regarding compositional gradients, precursor reactivity effects, and the\nspontaneous formation of a Zn(1-3/2x)GaxS shell via cation exchange—are of considerable interest to\nthe nanomaterials and optoelectronics communities. The work is thorough and the data is compelling.\nHowever, there are several key issues regarding data interpretation, visualization, and the\nsubstantiation of claims that require major revision before this manuscript can be considered for\npublication.\nAnswer to General Comment: We sincerely thank the reviewer for the careful evaluation of our\nmanuscript and for recognizing the novelty and technical significance of applying atom probe\ntomography to elucidate the 3D atomic structure of complex AgIn Ga S -based quaternary quantum\nx 1-x 2\ndots. We particularly appreciate the reviewer’s positive and constructive assessment of our findings. We\nhave carefully addressed all raised concerns and revised the manuscript to improve data interpretation\nand visualization, as well as the substantiation of our key claims, as detailed below.\nBelow are specific points that must be addressed:\nComment 1. The APT technique is fundamentally destructive, \"disassembling\" the specimen atom-by-\natom via high-field evaporation. A critical and unaddressed point is how the authors can be certain that\nthe resulting 3D reconstruction faithfully represents the original, pre-analysis structure of the QDs.\nHigh electric fields and laser pulses can, in some cases, induce artifacts such as atomic diffusion or re-\nordering during the analysis. The authors must include a discussion on the potential for such artifacts\nand provide a justification (e.g., by referencing reconstruction parameters, evaporation behavior, or\nliterature on similar materials) for why the presented 3D maps are considered a reliable representation\nof the as-synthesized QD structure.\nAnswer to Comment 1: We appreciate the reviewer’s important point regarding the inherently\ndestructive nature of APT and the possibility that high electric fields and laser pulses may induce\nartifacts such as atomic diffusion or reconstruction distortion during analysis. We acknowledge that\nsuch effects must be carefully considered, particularly when APT is applied to nanoscale systems such\nas quantum dots (QDs).\nThe applicability and reliability of APT for 3D compositional analysis of nanoparticles have been\ndemonstrated in multiple prior studies. In particular, our previous work on CdSe-based core/shell QDs\n(Chae et al. ACS Nano, 2018), while focusing on a relatively simple binary QD system, directly showed\nthat APT can reliably resolve core/shell architectures in colloidal QDs, with results consistent with\ncomplementary transmission electron microscopy (TEM) observations. In that work, we discussed the\napplication of APT to commercial CdSe/ZnS core/shell QDs, the representativeness and reliability of\nthe resulting datasets, potential artifacts and reconstruction considerations. More broadly, numerous\nAPT studies on metallic and semiconductor nanostructures have established APT as a viable technique\nfor nanoscale structural and compositional characterization.\nWe also recognize that specific artifacts have been reported in APT analyses of nanostructures and QDs,\nsuch as apparent geometric distortion along the analysis (z) direction, artificially broadened interfaces\nat nominally sharp interfaces, and apparent intermixing between adjacent regions. These effects are\nprimarily associated with trajectory aberrations arising from the local magnification effect and cannot\nbe entirely eliminated in APT analyses of nanostructures. In the revised manuscript we explicitly\nacknowledge these potential artifacts and cite relevant prior studies that have investigated their origin\nand impact.\nImportantly, in APT analyses of nanostructures, the reconstruction fidelity depends on the analysis\ndirection. Compositional results obtained along the direction perpendicular to the interfaces—\ncorresponding to the z-direction in APT— have been reported to provide more reliable quantitative\ninformation in resolving the distributions of segregating atoms at grain boundaries as well as constituent\natoms within nanoparticles or precipitates. The improved reliability of z-direction analyses originates\nfrom the reduced trajectory overlap, which effectively suppresses artificial intermixing and preserves\ncompositional distributions and interfacial characteristics. In contrast, lateral (x/y) analyses are more\nsusceptible to local magnification effects and thus tend to overestimate interfacial broadening.\nIn the present study, we therefore place particular emphasis on z-direction compositional analyses,\nwhile also cross-validating the observed trends using proxigrams and lateral (x/y) profiles. Although\nminor variations in profile broadening are observed depending on the analysis direction, all datasets\nconsistently reveal the same qualitative compositional tendencies. The absence of severe trajectory\noverlap observed in the reconstructed datasets further suggests that intermixing artifacts are minimized\nin our datasets.\nFurthermore, the compositional features revealed by APT were corroborated using complementary\ncharacterization techniques. TEM observations show compositional trends consistent with the APT\nresults at the spatial resolution accessible by electron microscopy. In addition, XPS analyses support\nthe presence of the key phenomena identified by APT, such as Zn Ga S formation and Cl\n1-3/2x x\ndistribution at the outer shell, further corroborating the validity of our interpretations.\nWe also have clarified the APT acquisition and reconstruction procedures in the Methods section. All\nelemental species were identified from the mass spectra prior to reconstruction, and the datasets were\nreconstructed using a standard shank-angle–based protocol. SEM images of the APT specimens and\nHAADF–STEM images of the QDs were used to guide the reconstruction and constrain the specimen\ngeometry. This approach improves reconstruction fidelity without artificially modifying the particle\nshape, and these details have now been added to the revised manuscript. We have revised the manuscript\nto include a dedicated discussion of these limitations and justifications, clarifying why the presented\n3D compositional maps can reasonably be regarded as representative of the original QD architecture.\nThe manuscript has been revised as follows:\nPlease see Page #7,\n“To characterize and quantify the 3D atomic distribution of the AgIn Ga S -based core/shell structured\nx 1-x 2\nQDs, APT was employed. For the APT analysis in this work, dried QDs were prepared in powder form\nand analyzed following our previously described method.41 In that study, we demonstrated that APT\ncould faithfully resolve core/shell architectures and 3D compositional distributions in commercially\navailable CdSe/ZnS core/shell QDs, in good agreement with TEM observations. Specimen preparation\nstrategies, potential APT artifacts, and reconstruction considerations specific to CdSe/ZnS core/shell\nQDs were systematically examined, thereby establishing the applicability of APT to colloidal QD\nnanostructures. More broadly, the feasibility of APT in the analysis of metallic nanoparticles has been\ndemonstrated in numerous prior studies, including ligand-resolved analyses and combined experiment–\nsimulation approaches.42-47 On this basis, we report a 3D atomic-scale characterization of AgIn Ga S -\nx 1-x 2\nbased QDs using APT, which, to the best of our knowledge, represents the first such analysis of\nquaternary QDs.”\nPlease see Page #8,\n“Figure 2a shows the 3D reconstructed APT atom maps of the AgIn Ga S /AgGaS QDs, representing\nx 1-x 2 2\na hemispherical APT specimen containing five QDs (see Supplementary Fig. 6 for dashed guides\nindicating the approximate outlines of individual QDs). The QDs appear as slightly ellipsoidal features\nin the APT reconstruction, and a slice-view image along the x–z plane is presented in Fig. 2b to visualize\ntheir internal elemental distribution. We note that APT analyses of nanostructures may exhibit\nreconstruction artifacts, such as apparent elongation along the analysis (z) direction and interfacial\nbroadening, primarily arising from local magnification and trajectory aberrations.42-52 Accordingly, the\nslightly ellipsoidal morphology observed here is attributed to such effects rather than the intrinsic\nparticle shape. However, compositional distributions along the z-direction have been shown to provide\nmore reliable quantitative information than lateral directions because the reduced trajectory overlap\nsuppresses artificial intermixing.42-44,51,52 Under these conditions, compositional distributions and\ninterfacial characteristics closer to the intrinsic distributions can be more faithfully evaluated.”\nPlease see Page #9,\n\"The 2D compositional profiles along two different directions (Figs. 3c and 3f) further confirm the\nformation of the AgIn Ga S core/AgGaS shell heterostructure with a compositional gradient. From\nx 1-x 2 2\nthe extracted profiles, we primarily focus on z-direction compositional analyses, cross-validated by\nproxigrams and lateral profiles. Ag and In atoms are highly concentrated at the center of the QD, at\nmore than 20 and 10 at.%, respectively, but their concentrations gradually decrease toward the shell.”\nPlease see Page #12,\n“Figures 5a and 5d show the extracted 3D atom maps and the cross-sectional regions cropped by the\ngreen boxes for each QD. According to the z-direction compositional profiles cross-validated by\nproxigrams and lateral profiles, Ag atoms are more widely distributed than In atoms in the 3D atom\nmaps (Figs. 5b and 5e). However, both Ag and In atoms are at their highest concentrations in the central\nregion of the QDs, with values exceeding 20 and 10 at.%, respectively.”\nPlease see Page #13,\n“Because APT inherently probes only a limited number of individual QDs and may therefore suffer\nfrom limited statistical representativeness, the compositional features identified by APT were further\nexamined using complementary techniques. XPS analyses performed on the same samples reveal\nconsistently higher Ga-to-Ag ratios after the Zn precursor step, evaluated from multiple core-level\nregions, including Ag MNN vs Ga 2p and Ag 4d vs Ga 3d. This systematic increase in the Ga-to-Ag\nratios supports the formation of Ga-containing outer shells and is consistent with the Zn Ga S shell\n1-3/2x x\nformation revealed by APT (Supplementary Fig. 15 and Supplementary Table 2). Inductively coupled\nplasma analyses performed on the same samples show overall elemental compositions consistent with\nthe APT results (Supplementary Tables 3 and 4). Taken together, these results indicate that the structural\nand compositional features identified by APT are not limited to a small number of individual QDs but\nare representative of overall sample.”\nPlease see Page #21,\n“APT analysis\nAPT analysis was conducted using a CAMECA atom probe (Invizo 6000) operating in the deep-UV\nlaser mode (λ = 257.5 nm) with a pulse repetition rate of 200 kHz and a base temperature of 50 K. A\nlaser pulse energy of 100 pJ was applied, achieving a detection rate of 0.01 atoms pulse-1. The acquired\ndata were reconstructed and analyzed using CAMECA AP Suite 6.3 software. All elemental species\nconstituting the AgIn Ga S -based QDs, as well as C-related ions, were identified from the mass\nx 1-x 2\nspectra and assigned before reconstruction. The reconstructions were performed using a shank-angle-\nbased protocol. SEM and HAADF–STEM images were utilized to guide the reconstruction and\nconstrain the specimen geometry, thereby enhancing the accuracy of the APT reconstruction process.\nNo artificial geometric constraints were imposed (e.g., forcing spherical particle shapes) during the\nreconstruction process. The slightly ellipsoidal morphologies observed in the APT datasets were\ntherefore preserved as reconstructed rather than being artificially forced into an idealized geometry and\nare attributed to known trajectory aberration and local magnification effects.”\nPlease see Supplementary Fig. 15,\nSupplementary Fig. 15 XPS analysis results of AgIn Ga S -based QDs. XPS spectra of a Ga 2p, b\nx 1-x 2\nZn 2p , c In 3d, d Ag 3d, and e S 2s core levels acquired from AgIn Ga S (AIGS), AgIn Ga\n2/3 x 1-x 2 x 1-\nS /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs using an Al K-alpha\nx 2 2 x 1-x 2 2\nX-ray source. All QDs contain Ag–S, In–S, Ga–S, and Zn–S chemical bonds, as designed without\ndetectable oxidation. f Valence band spectra of AIGS, AIGS/AGS, and AIGS/AGS/ZnS QDs. The Ga-\nto-Ag ratio is consistently higher in the AIGS/AGS/ZnS QDs than in the AIGS/AGS QDs. The detailed\ncompositions are provided in Supplementary Table 2. These results further support the presence of a\nGa-containing outer shell, consistent with the Zn Ga S shell identified by APT after the ZnS\n1-3/2x x\nprecursor step.\nPlease see Supplementary Fig. 20,\nSupplementary Fig. 20 XPS Cl 2p core-level spectra of AgIn Ga S -based QDs. XPS spectra of Cl\nx 1-x 2\n2p core-level acquired from AgIn Ga S (AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga\nx 1-x 2 x 1-x 2 2 x 1-\nS /AgGaS /ZnS (AIGS/AGS/ZnS) QDs. Cl-related signals are observed only in the AIGS/AGS/ZnS\nx 2 2\nQDs, indicating the presence of metal–Cl chemical states associated with the Zn-based shell introduced\nduring the ZnS precursor step.\nPlease see Supplementary Table 2,\nAtomic composition (%) (XPS)\nSpecimen\nCl Zn Ga Ag In\nC O F S\n(in Cl (in Zn (in Ga (in Ag (in In\n(in C 1s) (in O 1s) (in F 1s) (in S 2s)\n2p) 2p ) 2p ) 3d) 3d )\n3/2 3/2 5/2\nAIGS 76.29 1.91 0 11.54 0 0 1.1 4.46 4.7\nAIGS/AGS 69.37 2.83 0 14.69 0.23 0 5.16 5.9 1.83\nAIGS/AGS/ZnS 62.92 6.58 0 13.93 2.07 6.07 4.54 2.85 1.06\nAtomic composition (%) (XPS)\nSpecimen\nS Ga Ag In\n(in S 2s) (in Ga 2p ) (in Ag 3d) (in In 3d )\n3/2 5/2\nAIGS 52.92 5.07 20.44 21.57\nAIGS/AGS 53.25 18.71 21.41 6.63\nAIGS/AGS/ZnS 62.28 20.28 12.73 4.72\nAtomic composition (%) (XPS)\nSpecimen\nAg Zn In Ga\n(in Ag 4d) (in Zn 3d) (in In 4d) (in Ga 3d)\nAIGS 36.1 0 37.7 26.2\nAIGS/AGS 37.1 0 10.0 52.9\nAIGS/AGS/ZnS 21.3 30.5 5.1 43.1\nSupplementary Table 2. Atomic compositions of AgIn Ga S /AgGaS (AIGS/AGS) and\nx 1-x 2 2\nAgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs measured by XPS. The Ga-to-Ag ratio is\nx 1-x 2 2\nconsistently higher in the AIGS/AGS/ZnS QDs than in the AIGS/AGS QDs, suggesting the formation\nof a Ga-containing outer shell. This trend is observed both when comparing Ag MNN and Ga 2p and\nwhen comparing Ag 4d and Ga 3d, all of which lie in similar binding energy ranges\nPlease see Supplementary Table 3,\nAtomic ratio (ICP–OES)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.40 0.12 0.44 0\nAIGS/AGS/ZnS 0.13 0.08 0.35 0.46\nSupplementary Table 3. Atomic ratios measured of AgIn Ga S /AgGaS (AIGS/AGS) and\nx 1-x 2 2\nAgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) determined by ICP–OES.\nx 1-x 2 2\nPlease see Supplementary Table 4,\nAtomic ratio (APT)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.43 0.13 0.39 0\nAIGS/AGS/ZnS 0.15 0.08 0.30 0.48\nSupplementary Table 4. Atomic ratios of AgIn Ga S /AgGaS (AIGS/AGS) and AgIn Ga\nx 1-x 2 2 x 1-\nS /AgGaS /ZnS (AIGS/AGS/ZnS) determined by APT.\nx 2 2\nReferences have been added as follows:\n41. Chae, B. G. et al. Direct three-dimensional observation of core/shell-structured quantum dots\nwith a composition-competitive gradient. ACS Nano 12, 12109-12117 (2018).\n10.1021/acsnano.8b05379.\n10. Jang, K. et al. Three-dimensional atomic mapping of ligands on palladium nanoparticles by\natom probe tomography. Nat. Commun. 12, 4301 (2021). 10.1038/s41467-021-24620-9.\n43. Kim, S. H. et al. Characterization of Pd and Pd@ Au core-shell nanoparticles using atom probe\ntomography and field evaporation simulation. J. Alloys Compd. 831, 154721 (2020).\n10.1016/j.jallcom.2020.154721.\n44. Tedsree, K. et al. Hydrogen production from formic acid decomposition at room temperature\nusing a Ag–Pd core–shell nanocatalyst. Nat. Nanotechnol. 6, 302-307 (2011). 10.1038/nnano.2011.42.\n45. Grenier, A. et al. 3D analysis of advanced nano-devices using electron and atom probe\ntomography. Ultramicroscopy 136, 185-192 (2014). 10.1016/j.ultramic.2013.10.001.\n46. Khan, M. A., Ringer, S. P. & Zheng, R. Atom probe tomography on semiconductor\ndevices. Adv. Mater. Interfaces 3, 1500713 (2016). 10.1002/admi.201500713.\n47. Hatzoglou, C., Radiguet, B. & Pareige, P. Experimental artefacts occurring during atom probe\ntomography analysis of oxide nanoparticles in metallic matrix: quantification and correction. J. Nucl.\nMater. 492, 279-291 (2017). 10.1016/j.jnucmat.2017.05.008.\n48. Philippe, T., Gruber, M., Vurpillot, F. & Blavette, D. Clustering and local magnification effects\nin atom probe tomography: a statistical approach. Microsc. Microanal. 16, 643-648 (2010).\n10.1017/S1431927610000449.\n49. Lawitzki, R., Stender, P. & Schmitz, G. Compensating local magnifications in atom probe\ntomography for accurate analysis of nano-sized precipitates. Microsc. Microanal. 27, 1–12 (2021).\n10.1017/S1431927621000180.\n50. Beinke, D., Oberdorfer, C. & Schmitz, G. Towards an accurate volume reconstruction in atom\nprobe tomography. Ultramicroscopy 165, 34-41 (2016). 10.1016/j.ultramic.2016.03.008.\n51. Takahashi, J. & Kawakami, K. Position artifacts in 3D reconstruction of plate‐shaped\nprecipitates in steels depending on the analysis direction of atom probe tomography. Surf. Interface\nAnal. 53, 982-995 (2021). 10.1002/sia.7001.\n52. Maruyama, N., Smith, G. D. W. & Cerezo, A. Interaction of the solute niobium or molybdenum\nwith grain boundaries in α-iron. Mater. Sci. Eng. A 353, 126-132 (2003). 10.1016/S0921-\n5093(02)00678-0.\nComment 2. The manuscript's optical analysis (e.g., Figure 1) focuses on the bright green emission\npeak at ~530 nm. However, the provided PL spectra appear to show a correspondingly enhanced\nfluorescence in the long-wavelength region (approx. 600-700 nm) after the ZnS/cation exchange step.\nThis feature is not discussed. The authors must provide a clear hypothesis or explanation for the origin\nof this long-wavelength emission. Is this attributable to unpassivated defect states, newly formed\ninterfacial trap states, or a specific structural component? A complete optical analysis requires\naddressing all significant features in the spectra.\nAnswer to Comment 2: We thank the reviewer for pointing out the enhanced long-wavelength\nemission (approximately 600–700 nm) observed after the ZnS/cation exchange process. We agree that\nall major spectral features should be addressed, and accordingly, we have revised the manuscript to\nexplicitly discuss the origin of the long-wavelength emission.\nLong-wavelength emission following ZnS shell coating has been widely observed in AgInS - and\n2\nrelated I–III–VI -based quantum dots (QDs) and is generally attributed to defect- or trap-related\n2\nrecombination pathways rather than band-edge excitonic transitions. As the reviewer correctly notes,\nseveral mechanisms may contribute to such emission, including unpassivated defect states, newly\nformed interfacial trap states, and defects introduced during cation exchange. To date, however, the\nprecise origin of this long-wavelength emission has not yet been fully established.\nIn the present system, we attribute the emergence of the long-wavelength (tail) emission primarily to\ndefect formation during the ZnS precursor step and the associated cation exchange process. Our\ntransmission electron microscopy and X-ray diffraction analyses indicate that the shell retains a\nchalcopyrite-related tetragonal structure rather than forming an incoherent zinc-blende-type ZnS phase,\nsuggesting that Zn Ga S is formed via cation exchange through substitution of Ag+ and Ga3+ by Zn2+.\n1-3/2x x\nThis intrinsically non-charge-neutral process is expected to generate a substantial density of cation\nvacancies, which act as intragap trap states in I–III–VI semiconductors. Such defect-related intragap\n2\nstates are commonly associated with donor–acceptor-pair–like recombination, which has been proposed\nas a major origin of long-wavelength emission. Similar trap-assisted long-wavelength emissions have\nbeen reported in AgInS /ZnS and Zn-alloyed Ag–In–S QDs, where they are commonly attributed to\n2\ndefect-mediated donor–acceptor-pair–like recombination rather than band-edge transitions. In addition,\nmultiple studies have reported the emergence of tail emission after ZnS shelling of AgInS -based QDs,\n2\nand have shown that when Zn2+ is present in the core or inner shell regions, its presence is associated\nwith defect formation and the emergence of trap states.\nConsistent with our APT observations, the inward migration of Zn2+ species into the particle interior\ncan trigger the further formation of cation vacancies and associated defect complexes. When these\ndefects are introduced in the vicinity of In-rich domains, where charge carriers are preferentially\nlocalized, they can give rise to intragap trap states that locally perturb the confinement and facilitate\ntrap-assisted recombination. We attempted to further elucidate the origin of this emission through time-\nresolved photoluminescence (TR-PL) measurements. However, the long-wavelength feature could not\nbe reliably decomposed into distinct lifetime components, suggesting that it arises from a distribution\nof energetically broad and spatially heterogeneous trap states. Accordingly, the revised manuscript now\nexplicitly discusses that the observed long-wavelength emission is consistent with defect-related\nrecombination processes associated with the ZnS precursor step.\nThe manuscript has been revised as follows:\nPlease see Page #5,\n“In addition to the dominant green emission, the AgIn Ga S /AgGaS /ZnS QDs exhibit a long-\nx 1-x 2 2\nwavelength emission tail in the 600–700 nm range (Fig. 1d). The area ratio of this long-wavelength\nemission, defined as the fraction of the total PL intensity in the long-wavelength region (λ > λ +\nPL max\n50 nm), increases from ~3.7% in AgIn Ga S /AgGaS QDs to ~14.9% in AgIn Ga S /AgGaS /ZnS\nx 1-x 2 2 x 1-x 2 2\nQDs. Such long-wavelength emission is generally attributed to defect- or trap-assisted recombination\nrather than band-edge excitonic transitions in AgInS -based QDs and related I–III–VI -based QDs.36-40”\n2 2\nPlease see Page #6,\n“X-ray diffraction (XRD) measurements were further performed on the core-only AgIn Ga S ,\nx 1-x 2\nAgIn Ga S /AgGaS , and AgIn Ga S /AgGaS /ZnS QDs to corroborate the structural similarity\nx 1-x 2 2 x 1-x 2 2\nbefore and after shell formation. All XRD patterns are consistent with a chalcopyrite-based tetragonal\nstructure, and the overall diffraction features remain essentially unchanged, even after the Zn precursor\nstep (Supplementary Fig. 4). This comparative analysis indicates that the crystal structure is preserved\nthroughout the shell formation process.”\nPlease see Page #15,\n“The emergence of the Zn Ga S outer shell via cation exchange offers a plausible origin for the long-\n1-3/2x x\nwavelength emission tail observed after the ZnS precursor step. In our system, the substitution of Ag+\nand Ga3+ by Zn2+ during shell formation inherently introduces cation vacancies and local charge-\ncompensating defects. Consistent with our APT observations, the inward diffusion of Zn2+ into the\ninterior of the QDs further induces cation exchange and promotes the formation of such defects. When\nthese defects form in the vicinity of In-rich regions, where carriers are preferentially confined, they can\nintroduce intragap trap states that locally perturb the confinement potential and promote trap-assisted\nrecombination. These defect-related states are commonly associated with donor–acceptor-pair-like\nrecombination pathways that give rise to long-wavelength emission in I–III–VI semiconductors.23,38-\n2\n40,66-68”\nPlease see Fig. 1,\n“Fig. 1 Comparison of AgIn Ga S /AgGaS QDs and AgIn Ga S /AgGaS /ZnS QDs. a, d\nx 1-x 2 2 x 1-x 2 2\nSchematic representation of the internal structure, band diagrams, and UV–vis absorption, and PL\ncharacteristics. Both QDs exhibit bright green luminescence at ~530 nm, with the QY increasing from\n85% to 92% after the ZnS precursor step to form the outer shell. In addition, AgIn Ga S /AgGaS /ZnS\nx 1-x 2 2\nQDs exhibit a long-wavelength emission tail in the ~600–700 nm range.”\nPlease see Supplementary Fig. 4,\nSupplementary Fig. 4 XRD patterns of AgIn Ga S -based QDs. XRD patterns of AgIn Ga S\nx 1-x 2 x 1-x 2\n(AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs.\nx 1-x 2 2 x 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, with only minor peak\nshifts attributable to slight lattice parameter variations.\nReferences have been added as follows:\n23. Kim, J. H. et al. Synthesis of widely emission-tunable Ag–Ga–S and its quaternary derivative\nquantum dots. Chem. Eng. J. 347, 791-797 (2018). 10.1016/j.cej.2018.04.167.\n36. Dai, M. et al. Tunable photoluminescence from the visible to near-infrared wavelength region\nof non-stoichiometric AgInS nanoparticles. J. Mater. Chem. 22, 12851-12858 (2012).\n2\n10.1039/C2JM31463K.\n37. Chen, Y. et al. Green and facile synthesis of high-quality water-soluble Ag-In-S/ZnS core/shell\nquantum dots with obvious bandgap and sub-bandgap excitations. J. Alloys Compd. 753, 364-370\n(2018). 10.1016/j.jallcom.2018.04.242.\n38. Park, S. M. et al. Red Ag-based I–III–VI quantum dots as competitive alternatives to InP\nemitters. ACS Energy Lett. 10, 3005-3013 (2025). 10.1021/acsenergylett.5c00962.\n39. Farid, A. et al. One-pot synthesis of luminescent Ag−Cu−Ga−S/ZnS quantum dots bridging\nthe cyan gap for ultrahigh-color-rendering white-light-emitting diodes. ACS Appl. Nano Mater. 8,\n14703-14712 (2025). 10.1021/acsanm.5c02386.\n40. Park, S. et al. Suppressing tail emission from AgIn Ga S /AgGaS quantum dots by GaI –\n1−x x 2 2 3\nassisted interface reinforcement. ACS Nano 19, 26831-26842 (2025). 10.1021/acsnano.5c07418.\n66. Shen, F. et al. Photophysics and photovoltaic properties of Zn-alloyed Ag-In-S quantum dots\nsensitized solar cells. J. Alloys Compd. 922, 166296 (2022). 10.1016/j.jallcom.2022.166296.\n67. Rivaux, C. et al. Continuous flow aqueous synthesis of highly luminescent AgInS and\n2\n68. Kameyama, T. et al. Controlling the electronic energy structure of ZnS–AgInS solid solution\n2\nnanocrystals for photoluminescence and photocatalytic hydrogen evolution. J. Phys. Chem. C 119,\n24740-24749 (2015). 10.1021/acs.jpcc.5b07994.\nComment 3. The 3D atomic map in Figure 2a is intended to show the five QDs analyzed, but the\npresence of C atoms (presumably from ligands or the matrix) makes it very difficult to discern the\nspecific shapes or interfaces of the QDs themselves. To improve image clarity, it is strongly\nrecommended to remove the C atoms (and any other extraneous matrix-related atoms) from the\nvisualization in Figure 2a (and Figure 4a). This would provide a much clearer view of the actual QD\nstructures being analyzed.\nAnswer to Comment 3: We thank the reviewer for the helpful suggestion regarding the visualization\nof the 3D atomic maps. We agree that C atoms originating from organic ligands can obscure the clarity\nof the quantum dot (QD) morphology in Figures 2a and 4a. Following the reviewer’s recommendation,\nwe revised the visualization by excluding C- and H-related ions from the 3D atom maps. The exclusion\nof these species does not alter the overall QD morphology or compositional trends but improves the\nvisual clarity of the QD shapes and interfaces. We note that even after removing C- and H-related ions,\nsome degree of apparent overlap between neighboring QDs may remain depending on the viewing\ndirection. In addition, because the QDs are not arranged in a perfectly linear manner but instead adopt\na partially close-packed, non-collinear configuration within the APT specimen, individual particles may\nnot always appear completely isolated in a single projection view. The corresponding 3D\nreconstructions excluding C- and H-related ions are provided in the Supplementary Information.\nThe manuscript has been revised as follows:\nPlease see Page #8,\n“Because the QDs are closely packed in a non-collinear configuration within the APT specimen and\nmay therefore partially overlap in projection views, 3D reconstructions with C- and H-related ions\nremoved are also provided in Supplementary Fig. 7 to better visualize the individual QDs.”\nPlease see Page #12,\n“C-related signals, likely originating from ligands, were also observed on the outside of the QDs\n(Supplementary Fig. 13). Nevertheless, because of the difficulty in quantifying these signals, our\ndiscussion focuses on the distribution of the main elements (see Supplementary Fig. 14 for the 3D\nreconstructions with the C- and H-related ions removed for clarity).”\nPlease see Supplementary Fig. 7,\nSupplementary Fig. 7 APT 3D reconstructions of AgIn Ga S /AgGaS QDs after the removal of\nx 1-x 2 2\nC- and H-related species. C- and H-related ions were excluded from the datasets for visual clarity.\nBoth the entire reconstructed volume (11 × 11 × 22 nm3) and a sliced 3D ion map (6 × 11 × 22 nm3) are\nshown, with dashed guides indicating the approximate boundaries of individual QDs. Because the QDs\nare arranged in a close-packed, non-collinear configuration within the APT specimen, some particles\nmay appear partially overlapped in the projection views. (Scale bars: 2 nm.)\nPlease see Supplementary Fig. 14,\nSupplementary Fig. 14 APT 3D reconstructions of AgIn Ga S /AgGaS /ZnS QDs after the\nx 1-x 2 2\nremoval of C- and H-related species. C- and H-related ions were excluded from the datasets for visual\nclarity. For better visualization, the entire 3D ion map (8.5 × 8.5 × 31 nm3) and a sliced 3D ion map (6\n× 8.5 × 31 nm3) are both shown, with dashed guides indicating the approximate boundaries of individual\nQDs. Because the QDs are closely packed within the APT specimen, some particles may appear partially\noverlapped in the projection views. (Scale bars: 2 nm.)\nComment 4. The authors provide a convincing chemical argument that the Ag-deficient AgInyGa1-yS2\noutermost shell is associated with the relatively lower reactivity of Ga3+ compared to Ag+. This is a\nkey finding. To strengthen this claim, have the authors attempted (or can they propose) an experiment\nto confirm this mechanism? For example, using a low-activity Ag precursor should, in principle, lead\nto a more uniform elemental distribution. A discussion of this point, or ideally further experimental\nevidence, is needed to more robustly validate the proposed formation mechanism.\nAnswer to Comment 4: We thank the reviewer for this constructive comment and for highlighting\nthe importance of further validating the proposed formation mechanism.\nWe agree that directly varying the chemical activity of the precursors would be an ideal way to probe\nthis mechanism. In practice, however, replacing the Ag and Ga precursors with alternative precursors\nof different reactivity is not straightforward under our current synthetic conditions. Instead, to\nexperimentally assess the role of relative precursor reactivity, we performed a series of controlled\nsyntheses in which the Ga precursor amount was fixed while the Ag precursor amount was varied.\nWhen the Ga precursor amount was fixed and the Ag precursor amount was increased, TEM\nmeasurements revealed a clear and systematic increase in the overall QD size, most notably in the\nthickness of the AgGaS shell. In particular, owing to the relatively low reactivity of the Ga precursor,\n2\nincreasing the Ag precursor under Ga-rich conditions preferentially promoted further growth of the\nAgGaS shell. STEM–EELS analyses consistently showed increased Ag incorporation and thickening\n2\nof the AgGaS shell with increasing Ag precursor amount. At the same time, the trap-related emission\n2\ncontribution increased and the photoluminescence quantum yield decreased, suggesting that Ag-driven\nshell growth could be accompanied by enhanced defect formation. Importantly, in contrast, when the\nAg precursor amount was fixed and the Ga precursor amount was increased, no significant increase in\nQD size or AgGaS shell thickness was observed. This asymmetric response provides compelling\n2\nexperimental support for our interpretation that the outer Ag-deficient AgIn Ga S shell originates\ny 1-y 2\nfrom the relatively lower reactivity of the Ga precursor compared to Ag, leading to preferential Ag\nincorporation and shell growth when additional Ag is supplied, rather than a homogeneous\nredistribution of cations. These additional results and discussions have now been incorporated into the\nrevised manuscript and Supplementary Information.\nThe manuscript has been revised as follows:\nPlease see Page #10,\n“To demonstrate the role of differential precursor reactivity, we performed controlled syntheses in which\nthe Ag precursor amount was increased at a fixed Ga level, a condition under which increasing the Ag\nprecursor amount directly promotes the further growth of the AgGaS shell. STEM–EELS analyses\n2\nshowed the pronounced thickening of the AgGaS shell and consistently increased Ag incorporation,\n2\nsupporting the role of precursor reactivity in the formation of the shell (Supplementary Fig. 8 and\nSupplementary Table 1). These changes were accompanied by an increased contribution from trap-\nrelated emission and a decreased photoluminescence QY, indicating that Ag-driven shell growth directly\nimpacts the optical properties of the QDs.”\nPlease see Supplementary Fig. 8,\nSupplementary Fig. 8 STEM–EELS analysis of AgIn Ga S /AgGaS QDs as a function of the\nx 1-x 2 2\nGa/Ag precursor ratio. (a) Representative STEM–EELS results for high-Ga/Ag-ratio (Ag-poor)\nAgIn Ga S /AgGaS QDs and low-Ga/Ag-ratio (Ag-rich) AgIn Ga S /AgGaS QDs. As the Ag\nx 1-x 2 2 x 1-x 2 2\nprecursor amount increases, the overall diameter of the QDs increases, particularly because of the\nincreasing thickness of the AgGaS shell. Owing to the relatively low reactivity of the Ga precursor,\n2\nincreasing the Ag precursor under Ga-rich conditions mainly promotes the growth of the AgGaS shell,\n2\naccompanied by increased Ag incorporation. (Scale bars: 5 nm.)\nPlease see Supplementary Table 1,\nQD size\nAg Ga/Ag Trap\nλ FWHM measured by\nprecursor (precursor PL max emission QY\n(band edge) (nm) TEM\namount ratio) area ratio\n(nm)\nLow Ag 29 524 35 5.1 77 6.0\n10 520 37 7.8 68 6.8\n↓\n6 517 37 12.9 57 6.9\nHigh Ag 4 516 38 21.3 37 7.9\nSupplementary Table 1. Dependence of the optical properties and size of AgIn Ga S /AgGaS\nx 1-x 2 2\nQDs on the Ga/Ag precursor ratio. As the Ag precursor amount increases, the average particle size\nof the AgIn Ga S /AgGaS QDs as measured by TEM increases. Meanwhile, the trap emission\nx 1-x 2 2\nincreases, and the QY decreases. Owing to the relatively low reactivity of the Ga precursor, increasing\nthe Ag precursor under Ga-rich conditions primarily leads to further growth of the AgGaS shell. The\n2\ntrap emission area ratio was defined as the fraction of the integrated PL intensity in the long-wavelength\nregion (λ > λ + 50 nm) to the total integrated emission.\nPL max\nComment 5. A close inspection of the 2D compositional profile in Figure 4d reveals an anomaly. For\nQD#4, Zn atoms (blue squares) are shown to be present within the QD core, not just in the outer shell.\nThis observation is not discussed by the authors, whose analysis focuses on cation exchange at the shell\ninterface. The authors must address this finding.\nAnswer to Comment 5: We thank the reviewer for the careful inspection of Fig. 4d. The apparent\npresence of Zn within the core region of QD #4 should not be interpreted as intrinsic Zn incorporation\ninto the core, but rather arises from the way the 2D compositional profile was constructed.\nThe 2D compositional profiles in Fig. 4d were extracted uniformly along the analysis (z) direction using\na fixed cylindrical region, corresponding to a sampling volume of 3 × 3 × 21 nm3. In the APT specimen,\nhowever, individual quantum dots (QDs) do not share an identical center along the z-axis but are\nspatially arranged in a close-packed, non-collinear manner. While QD#3 and QD#5 are located at\nsimilar z positions, QD#4 is laterally and vertically offset within the APT specimen. As a result, the z-\ndirectional 2D profile for QD#4 partially intersects the shell region, rather than exclusively sampling\nthe core, resulting in the apparent Zn signal within the “core” region in Fig. 4d. This interpretation is\nconsistent with the iso-surface visualization in the same figure, which resolves the core/shell/shell\narchitecture and, in particular, shows that Zn atoms are predominantly located at the outer shell of QD#4\nrather than within the core. Furthermore, because a fixed-diameter cylindrical profile was used,\nvariations in QD size and spatial offset inevitably lead to differences in whether the extracted volume\ncaptures only the core or includes portions of the shell.\nFor this reason, the individual compositional profiles extracted from individual QD volumes (Fig. 5c\nand 5f) provide a more direct representation of the true core/shell distribution and do not show\nsignificant Zn incorporation into the core. Nevertheless, Fig. 4d is included to demonstrate the overall\nconsistency and reliability of the APT dataset across multiple QDs. We have clarified this point in the\nrevised manuscript to avoid potential misinterpretation.\nThe manuscript has been revised as follows:\nPlease see Page #8,\n“Because the QDs are closely packed in a non-collinear configuration within the APT specimen and\nmay therefore partially overlap in projection views, 3D reconstructions with C- and H-related ions\nremoved are also provided in Supplementary Fig. 7 to better visualize the individual QDs.”\nPlease see Page #12,\n“The 2D concentration profile along the analysis direction offers additional insights into the internal\nstructure of each QD (Fig. 4d). Although the elemental concentrations vary slightly depending on the\nprofiling path owing to the spatial offset of individual QDs in the APT specimen, the 2D profile confirms\nthe significant presence of Ga atoms in the outer shell, albeit at lower concentrations than Zn atoms.”\nPlease see Fig. 4,\nFig. 4 3D reconstruction and compositional analysis of AgIn Ga S /AgGaS /ZnS QDs using APT.\nx 1-x 2 2\na Reconstructed 3D atom map of the entire analyzed volume, showing five slightly ellipsoidal QDs. b\nSlice-view atom maps of Ag, In, Ga, Zn, and S, illustrating their internal spatial distributions. Although\nmost Ag and In atoms are in the core regions, a significant amount of Ga is distributed over the shell\nregion. Zn atoms predominantly exist in the outer shell. c Ag and Zn iso-surface and proxigram obtained\nfrom Ag iso-surfaces; the proxigram was obtained from five QDs. The Ag and In contents increase\ntoward the QD core, while the content of Zn increases toward the outer shell of the QD. A significant\namount of Ga also appears in the outer shell. d 2D compositional profile extracted from a cylindrical\nvolume (3 × 3 × 21 nm3) through the 3D reconstruction, confirming the core/shell/shell structure. (Scale\nbars: 2 nm in a–d). The slight variations in the compositional distributions from QD to QD arise from\nthe spatial offsets of individual QDs within the APT specimen. Error bars in c, d indicate one-sigma\ncounting statistics.\nComment 6. There is a fundamental contradiction in the manuscript's description of the final shell. The\ntext frequently refers to \"synthesizing the ZnS shell\", \"ZnS shell coating\", or \"shell growth\". However,\ntwo key pieces of the authors' own data refute this:\n• TEM Data: The authors state the QDs \"grow only negligibly\" and remain \"similar in both size and\nshape\" after the ZnS precursor step. This is inconsistent with the epitaxial growth of a new shell.\n• APT Data: The authors' own excellent APT analysis proves the process is cation exchange, forming a\nZn(1-3/2x)GaxS shell.\nThe authors must revise the manuscript to consistently describe this process as cation exchange, as\ntheir own evidence strongly contradicts the narrative of simple \"shell growth.\"\nAnswer to Comment 6: We agree with the reviewer that our experimental evidence clearly and\nconsistently supports cation exchange as the dominant mechanism for shell formation rather than the\nepitaxial growth of a new ZnS shell. As correctly pointed out, the negligible change in quantum dot size\nand shape observed by TEM, together with the APT results demonstrating the formation of a Zn\n1-\nGa S shell, is inconsistent with conventional epitaxial shell growth. Accordingly, we have revised\n3/2x x\nthe manuscript to consistently describe this process as cation exchange throughout the text and have\nremoved or rephrased terminology implying simple ZnS shell growth or coating.\nThe manuscript has been revised as follows:\n“synthesizing the ZnS shell”, “ZnS shell coating”, “shell growth”\n “ZnS precursor step”, “during the ZnS precursor step to form an outer shell”, “after the ZnS precursor\nstep for the outer shell formation”\nComment 7. The authors claim that the final Zn(1-3/2x)GaxS shell \"maintains the charge carrier\nconfinement with a Type-I band alignment\". This is a critical conclusion for the QD's performance, but\nit is not experimentally proven. This claim appears to be an inference based on the band gaps of related\nbulk materials. Given the novel, complex, and compositionally graded structure revealed by APT,\nsuch an assumption is not sufficient. The authors must provide spectroscopic evidence (e.g., time-resolved\nPL or other measurements) to conclusively demonstrate the Type-I alignment. If such evidence is\nunavailable, this claim must be significantly toned down and presented as a well-reasoned hypothesis,\nnot a confirmed fact.\nAnswer to Comment 7: We thank the reviewer for this important comment regarding the band\nalignment of the Zn Ga S shell. We agree that the original wording may have overstated the level\n1-3/2x x\nof experimental certainty, and we have revised the manuscript to present the Type-I-like carrier\nconfinement as a well-reasoned interpretation rather than a conclusively demonstrated fact.\nDirect spectroscopic determination of the band alignment or bandgap of the compositionally graded\nZn Ga S shell itself is experimentally challenging in the present system. While time-resolved\n1-3/2x x\nphotoluminescence (TR-PL) measurements were performed, the decay dynamics did not allow shell-\nrelated decay components to be clearly distinguished. Nevertheless, our interpretation can be informed\nby indirect experimental observations as well as prior studies on Zn–Ga–S compounds and quantum\ndots (QDs). Although there are no reports directly measuring the bandgap of a Zn Ga S shell layer,\n1-3/2x x\nprevious studies on Zn Ga S QDs and related compounds indicate that the bandgap energy of this\n1-3/2x x\nsystem lies between that of Ga S (~2.5 eV) and ZnS (~3.5 eV), depending on the cation ratio. In\n2 3\nparticular, ZnGa S exhibits a bandgap of approximately ~3.18 eV, and literature reports show that\n2 4\nZnGa S /ZnS heterostructured QDs form a Type-I band alignment, whereas ZnGa S /Ga S QDs do\n2 4 2 4 2 3\nnot. In addition, although not in the form of QDs, previous studies have reported that nanocrystalline\nZnGa S films exhibit a wide bandgap in the range of approximately 3.75–4.11 eV, classifying them as\n2 4\nwide-bandgap semiconductors.\nBased on prior studies showing that Zn–Ga–S–based materials generally possess wider bandgaps than\nAgInS -based cores, the Zn Ga S shell formed in the present work is expected to provide Type-I-\n2 1-3/2x x\nlike carrier confinement. This interpretation is further supported by several experimental observations:\n(i) the quantum yield (QY) remains high (80–90%) after cation exchange and increases to ~92%\nfollowing the ZnS precursor step, and (ii) the QDs exhibit markedly enhanced ambient-condition\nstability in QD–acrylate composite films at room temperature. In contrast, a Type-II alignment would\ntypically be associated with reduced radiative efficiency, pronounced red-shifts, and QY degradation\nupon shell formation, none of which are observed in the present system. Accordingly, we now describe\nthe Zn Ga S shell as consistent with or suggestive of Type-I carrier confinement, rather than\n1-3/2x x\nclaiming definitive experimental proof. The revised manuscript has been carefully edited to reflect this\ninterpretation and to avoid overstating the certainty of the band alignment.\nThe manuscript has been revised as follows:\nPlease see Page #4,\n“Interestingly, during the ZnS precursor step to form an outer shell on the AgIn Ga S /AgGaS QDs,\nx 1-x 2 2\na Zn Ga S outer shell spontaneously forms instead, driven by Ag+ and Ga3+ to Zn2+ cation exchange.\n1-3/2x x\nThis Zn Ga S shell exhibits excellent structural and chemical compatibility with the AgGaS inner\n1-3/2x x 2\nshell and is expected to induce a Type-I band alignment.”\nPlease see Page #9,\n“Nevertheless, the resulting AgIn Ga S shell is expected to effectively confine charges within the\ny 1-y 2\nAgIn Ga S core owing to its higher band gap, consistent with a Type-I heterostructure, which\nx 1-x 2\npromotes radiative recombination.”\nPlease see Page #16,\n“In terms of the band gap energy, the Zn Ga S shell typically exhibits a slightly lower band gap than\n1-3/2x x\nZnS (~3.5 eV), even at the nanoscale.62,65,69-72 The formation of the compositionally mixed Zn Ga S\n1-3/2x x\nlayer is therefore expected to adequately preserve the quantum efficiency, with negligible weakening of\ncharge confinement, consistent with Type-I-like band alignment. Moreover, the spontaneous formation\nof the Zn Ga S shell could significantly reduce crystallographic or interfacial defects that might arise\n1-3/2x x\nfrom the structural differences between the inner and outer shells. By creating a more seamless interface\nwith the AgGaS inner shell, the Zn Ga S shell may enhance the stability of the QDs under ambient\n2 1-3/2x x\nconditions and contribute to a more stable and efficient green luminescence. This optical stability is\nparticularly advantageous for optoelectronic applications where long-term emission reliability is\ncritical.”\nReferences have been added as follows:\n71. Hu, T., Zhu, K., Cheng, H., Teng, Y. & Pan, Z. Core–shell energy band engineering of cyan\nlight-emitting ternary ZnGa S @ZnS quantum dots toward anti-counterfeiting and bioimaging\n2 4\napplications. J. Mater. Chem. C 13, 21797-21811 (2025). 10.1039/D5TC02797G.\n72. Yadav, A. N. & Singh, K. Investigation of photophysical properties of ternary Zn–Ga–S\nquantum dots: band gap versus sub-band-gap excitations and emissions. ACS Omega 4, 18327-18333\n(2019). 10.1021/acsomega.9b02546.\nReviewer #2 (Remarks to the Author):\nGeneral Comment: The present manuscript simply reported the measurement of AgInGaS QDs with\natom probe tomography (APT). Although atom resolution images were obtained, no relationship\nbetween the nanostructure of QDs and their photochemical properties were reported. Thus, no novel\nresults enough to publish in highly ranked journals were contained in the present manuscript.\nEspecially, following points were very weak.\nAnswer to General Comment: We thank the reviewer for the constructive comment. We\nacknowledge that, in the original version of the manuscript, the broader significance of this study and\nits connection to photochemical properties were not sufficiently articulated.\nThe primary novelty of this work lies in the first direct, atomically resolved analysis of 3D nanoscale\ncompositional distributions in quaternary AgIn Ga S quantum dots (QDs) using APT. Our APT\nx 1-x 2\nanalysis reveals previously inaccessible structural features arising during synthesis, including the\nspontaneous formation of a Zn Ga S outer shell via cation exchange, the development of Ag-\n1-3/2x x\ndeficient shell regions governed by precursor reactivity, and the presence of compositional gradients\nand nanoscale inhomogeneities within individual QDs.\nIn response to the reviewer’s comment, we have revised the manuscript to more explicitly establish the\nrelationship between these nanoscale structural features and the photochemical properties of the QDs.\nSpecifically, we have added a detailed discussion on the evolution of quantum yield and stability\nassociated with the formation of the Zn Ga S shell, as well as the origin of the observed long-\n1-3/2x x\nwavelength emission. In addition, new experimental evidence has been included to demonstrate how\nvariations in the Ag/Ga precursor ratio affect shell formation behavior and associated optical properties,\nand to experimentally substantiate the compositional inhomogeneity among individual QDs.\nFurthermore, relevant literature has been incorporated to support the reliability and validity of our\nexperimental approach and APT analysis. We therefore believe that the revised manuscript goes beyond\na simple structural characterization study and provides new insight into how nanoscale compositional\nevolution during synthesis influences the optical behavior of complex quaternary QDs, which is directly\nrelevant to their rational design and optimization.\nComment 1. As mentioned in the introduction, the authors claimed that they three-dimensionally\ncharacterize quaternary QDs (AgIn Ga S -based heterostructured QDs with core/shell and\nx 1-x 2\ncore/shell/shell structures in this case) on the atomic scale for the first time using APT. If so, the authors\nmust show at first the reliability of the APT method for measurements of QD nanostructures, using well\nknown QDs, such as CdSe QDs and CdSe/ZnS core-shell structured QDs.\nAnswer to Comment 1: We thank the reviewer for the comment regarding the reliability of APT for\nquantum dot (QD) nanostructures.\nThe applicability and reliability of APT for the 3D compositional analysis of colloidal QDs have been\npreviously demonstrated using well-known CdSe-based systems in our previous work (Chae et al. ACS\nNano, 2018). In that study, we reported APT analyses of commercial CdSe/ZnS core/shell QDs and\nsystematically addressed specimen preparation strategies, reconstruction considerations, and potential\nartifacts relevant to APT analysis of colloidal QDs. We further discussed the representativeness and\nreliability of the resulting datasets. The elemental distributions obtained by APT were directly compared\nwith transmission electron microscopy (TEM) results, demonstrating that APT can reliably resolve\ncore/shell architectures and provide compositional information that is complementary to, and in some\naspects beyond, what is accessible by TEM. Based on this prior validation using CdSe/ZnS core/shell\nQDs, we consider the methodological reliability of APT for QD nanostructures to be well supported. In\nthe revised manuscript, we now explicitly reference this previous work and discuss known APT-related\nartifacts relevant to nanostructured materials, together with justification for the reliability of the present\nmeasurements.\nIt is well recognized that APT analyses of nanostructures can be affected by artifacts such as local\nmagnification effects and trajectory aberrations, which may lead to artificially broadened interfaces and\napparent intermixing. These effects have been systematically investigated in prior studies. In particular,\ncompositional profiles extracted parallel to nominal interfaces often appear artificially diffuse due to\ntrajectory overlap, whereas compositional analyses performed perpendicular to the interface—\ncorresponding to the analysis (z) direction in APT—have been shown to provide superior reliability in\nresolving compositional distributions. This behavior has been widely reported for metallic and\nsemiconductor nanostructures, where reduced trajectory overlap preserves more reliable compositional\ndistributions.\nIn the present study, compositional trends were evaluated using multiple APT analysis approaches,\nincluding proxigrams and compositional profiles extracted along different spatial directions. While\ncompositional profiles obtained along the analysis (z) direction—generally considered to provide more\nreliable compositional distribution in APT—serve as an important reference, the trends observed in\nlateral (x/y) profiles are qualitatively consistent. Although minor variations in profile broadening are\nobserved depending on the analysis direction, all datasets reveal the same overall compositional\ntendencies.\nTo further support the reliability of the APT analysis, we performed additional complementary\ncharacterization beyond APT. X-ray photoelectron spectroscopy (XPS) measurements further\ncorroborated the formation of a Ga-containing outer shell after the ZnS precursor step. Although XPS\ndoes not provide 3D spatial information, we compared the relative Ag and Ga signals before and after\nthe ZnS shelling process using multiple core-level regions, including Ag MNN vs Ga 2p and Ag 4d vs\nGa 3d, each of which occupy equivalent binding energy ranges. In all cases, the Ga-to-Ag ratios were\nconsistently higher in the AgIn Ga S /AgGaS /ZnS QDs than in the AgIn Ga S /AgGaS QDs,\nx 1-x 2 2 x 1-x 2 2\nsuggesting the presence of substantial Ga-related components in the outermost shell region after the\nZnS precursor step. These XPS results are consistent with the Zn Ga S shell formation revealed by\n1-3/2x x\nAPT. In addition, Cl-related signals were detected only after the ZnS precursor step, implying the\nincorporation of Zn–Cl–related species. TEM further revealed structural and compositional features\nconsistent with the APT observations. Our TEM results show both the overall similarity in elemental\ndistribution trends and the presence of particle-to-particle variability observed by APT. These results\nprovide additional confidence that the APT reconstructions are consistent with the underlying\nnanostructure. Corresponding clarifications and references have been added to the revised manuscript.\nThe manuscript has been revised as follows:\nPlease see Page #7,\n“To characterize and quantify the 3D atomic distribution of the AgIn Ga S -based core/shell structured\nx 1-x 2\nQDs, APT was employed. For the APT analysis in this work, dried QDs were prepared in powder form\nand analyzed following our previously described method.41 In that study, we demonstrated that APT\ncould faithfully resolve core/shell architectures and 3D compositional distributions in commercially\navailable CdSe/ZnS core/shell QDs, in good agreement with TEM observations. Specimen preparation\nstrategies, potential APT artifacts, and reconstruction considerations specific to CdSe/ZnS core/shell\nQDs were systematically examined, thereby establishing the applicability of APT to colloidal QD\nnanostructures. More broadly, the feasibility of APT in the analysis of metallic nanoparticles has been\ndemonstrated in numerous prior studies, including ligand-resolved analyses and combined experiment–\nsimulation approaches.42-47 On this basis, we report a 3D atomic-scale characterization of AgIn Ga S -\nx 1-x 2\nbased QDs using APT, which, to the best of our knowledge, represents the first such analysis of\nquaternary QDs.”\nPlease see Page #8,\n“Figure 2a shows the 3D reconstructed APT atom maps of the AgIn Ga S /AgGaS QDs, representing\nx 1-x 2 2\na hemispherical APT specimen containing five QDs (see Supplementary Fig. 6 for dashed guides\nindicating the approximate outlines of individual QDs). The QDs appear as slightly ellipsoidal features\nin the APT reconstruction, and a slice-view image along the x–z plane is presented in Fig. 2b to visualize\ntheir internal elemental distribution. We note that APT analyses of nanostructures may exhibit\nreconstruction artifacts, such as apparent elongation along the analysis (z) direction and interfacial\nbroadening, primarily arising from local magnification and trajectory aberrations.42-52 Accordingly, the\nslightly ellipsoidal morphology observed here is attributed to such effects rather than the intrinsic\nparticle shape. However, compositional distributions along the z-direction have been shown to provide\nmore reliable quantitative information than lateral directions because the reduced trajectory overlap\nsuppresses artificial intermixing.42-44,51,52 Under these conditions, compositional distributions and\ninterfacial characteristics closer to the intrinsic distributions can be more faithfully evaluated.”\nPlease see Page #9,\n\"The 2D compositional profiles along two different directions (Figs. 3c and 3f) further confirm the\nformation of the AgIn Ga S core/AgGaS shell heterostructure with a compositional gradient. From\nx 1-x 2 2\nthe extracted profiles, we primarily focus on z-direction compositional analyses, cross-validated by\nproxigrams and lateral profiles. Ag and In atoms are highly concentrated at the center of the QD, at\nmore than 20 and 10 at.%, respectively, but their concentrations gradually decrease toward the shell.”\nPlease see Page #12,\n“Figures 5a and 5d show the extracted 3D atom maps and the cross-sectional regions cropped by the\ngreen boxes for each QD. According to the z-direction compositional profiles cross-validated by\nproxigrams and lateral profiles, Ag atoms are more widely distributed than In atoms in the 3D atom\nmaps (Figs. 5b and 5e). However, both Ag and In atoms are at their highest concentrations in the central\nregion of the QDs, with values exceeding 20 and 10 at.%, respectively.”\nPlease see Page #13,\n“Because APT inherently probes only a limited number of individual QDs and may therefore suffer\nfrom limited statistical representativeness, the compositional features identified by APT were further\nexamined using complementary techniques. XPS analyses performed on the same samples reveal\nconsistently higher Ga-to-Ag ratios after the Zn precursor step, evaluated from multiple core-level\nregions, including Ag MNN vs Ga 2p and Ag 4d vs Ga 3d. This systematic increase in the Ga-to-Ag\nratios supports the formation of Ga-containing outer shells and is consistent with the Zn Ga S shell\n1-3/2x x\nformation revealed by APT (Supplementary Fig. 15 and Supplementary Table 2). Inductively coupled\nplasma analyses performed on the same samples show overall elemental compositions consistent with\nthe APT results (Supplementary Tables 3 and 4). Taken together, these results indicate that the structural\nand compositional features identified by APT are not limited to a small number of individual QDs but\nare representative of overall sample.”\nPlease see Page #14,\n“Consistent with the APT observations, STEM–EDS line profiles collected from multiple QDs reveal\nnoticeable particle-to-particle variations in the relative Ag, In, and Ga distributions (Supplementary Figs.\n16 and 17). For each QD, several independent line profiles were extracted and compared, all of which\nshow dot-to-dot compositional fluctuations. These results indicate that the observed compositional\ninhomogeneity is an intrinsic feature of the synthesized quaternary QDs, thereby supporting the features\nrevealed by APT.”\nPlease see Page #21,\n“APT analysis\nAPT analysis was conducted using a CAMECA atom probe (Invizo 6000) operating in the deep-UV\nlaser mode (λ = 257.5 nm) with a pulse repetition rate of 200 kHz and a base temperature of 50 K. A\nlaser pulse energy of 100 pJ was applied, achieving a detection rate of 0.01 atoms pulse-1. The acquired\ndata were reconstructed and analyzed using CAMECA AP Suite 6.3 software. All elemental species\nconstituting the AgIn Ga S -based QDs, as well as C-related ions, were identified from the mass\nx 1-x 2\nspectra and assigned before reconstruction. The reconstructions were performed using a shank-angle-\nbased protocol. SEM and HAADF–STEM images were utilized to guide the reconstruction and\nconstrain the specimen geometry, thereby enhancing the accuracy of the APT reconstruction process.\nNo artificial geometric constraints were imposed (e.g., forcing spherical particle shapes) during the\nreconstruction process. The slightly ellipsoidal morphologies observed in the APT datasets were\ntherefore preserved as reconstructed rather than being artificially forced into an idealized geometry and\nare attributed to known trajectory aberration and local magnification effects.”\nPlease see Supplementary Fig. 15,\nSupplementary Fig. 15 XPS analysis results of AgIn Ga S -based QDs. XPS spectra of a Ga 2p, b\nx 1-x 2\nZn 2p , c In 3d, d Ag 3d, and e S 2s core levels acquired from AgIn Ga S (AIGS), AgIn Ga\n2/3 x 1-x 2 x 1-\nS /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs using an Al K-alpha\nx 2 2 x 1-x 2 2\nX-ray source. All QDs contain Ag–S, In–S, Ga–S, and Zn–S chemical bonds, as designed without\ndetectable oxidation. f Valence band spectra of AIGS, AIGS/AGS, and AIGS/AGS/ZnS QDs. The Ga-\nto-Ag ratio is consistently higher in the AIGS/AGS/ZnS QDs than in the AIGS/AGS QDs. The detailed\ncompositions are provided in Supplementary Table 2. These results further support the presence of a\nGa-containing outer shell, consistent with the Zn Ga S shell identified by APT after the ZnS\n1-3/2x x\nprecursor step.\nPlease see Supplementary Fig. 16,\nSupplementary Fig. 16 Detailed elemental distributions of AgIn Ga S /AgGaS QDs measured\nx 1-x 2 2\nby STEM–EDS. STEM image and corresponding EDS line profiles of individual AgIn Ga S /AgGaS\nx 1-x 2 2\nQDs. Although the QDs show similar compositional trends, the elemental compositions vary from\nparticle to particle. Owing to the projected nature and limited spatial resolution of STEM–EDS, these\ndata provide qualitative compositional trends rather than a definitive 3D core/shell/shell assignment.\n(Scale bars: 7 nm.)\nPlease see Supplementary Fig. 17,\nSupplementary Fig. 17 Detailed elemental distributions of AgIn Ga S /AgGaS /ZnS QDs\nx 1-x 2 2\nmeasured by STEM–EDS. STEM image and corresponding EDS line profiles of individual AgIn Ga\nx 1-\nS /AgGaS /ZnS QDs. Zn is distributed over a broader region than Ag and In, indicating the formation\nx 2 2\nof a Zn-containing outer region. Although the QDs show similar overall compositional tendencies, the\nelemental compositions vary among individual QDs, indicating particle-to-particle variability. Owing\nto the projected nature and limited spatial resolution of STEM–EDS, these data provide qualitative\ncompositional trends rather than a definitive 3D core/shell/shell assignment. (Scale bars: 7 nm.)\nPlease see Supplementary Table 2,\nAtomic composition (%) (XPS)\nSpecimen\nCl Zn Ga Ag In\nC O F S\n(in Cl (in Zn (in Ga (in Ag (in In\n(in C 1s) (in O 1s) (in F 1s) (in S 2s)\n2p) 2p ) 2p ) 3d) 3d )\n3/2 3/2 5/2\nAIGS 76.29 1.91 0 11.54 0 0 1.1 4.46 4.7\nAIGS/AGS 69.37 2.83 0 14.69 0.23 0 5.16 5.9 1.83\nAIGS/AGS/ZnS 62.92 6.58 0 13.93 2.07 6.07 4.54 2.85 1.06\nAtomic composition (%) (XPS)\nSpecimen\nS Ga Ag In\n(in S 2s) (in Ga 2p ) (in Ag 3d) (in In 3d )\n3/2 5/2\nAIGS 52.92 5.07 20.44 21.57\nAIGS/AGS 53.25 18.71 21.41 6.63\nAIGS/AGS/ZnS 62.28 20.28 12.73 4.72\nAtomic composition (%) (XPS)\nSpecimen\nAg Zn In Ga\n(in Ag 4d) (in Zn 3d) (in In 4d) (in Ga 3d)\nAIGS 36.1 0 37.7 26.2\nAIGS/AGS 37.1 0 10.0 52.9\nAIGS/AGS/ZnS 21.3 30.5 5.1 43.1\nSupplementary Table 2. Atomic compositions of AgIn Ga S /AgGaS (AIGS/AGS) and\nx 1-x 2 2\nAgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs measured by XPS. The Ga-to-Ag ratio is\nx 1-x 2 2\nconsistently higher in the AIGS/AGS/ZnS QDs than in the AIGS/AGS QDs, suggesting the formation\nof a Ga-containing outer shell. This trend is observed both when comparing Ag MNN and Ga 2p and\nwhen comparing Ag 4d and Ga 3d, all of which lie in similar binding energy ranges.\nReferences have been added as follows:\n41. Chae, B. G. et al. Direct three-dimensional observation of core/shell-structured quantum dots\nwith a composition-competitive gradient. ACS Nano 12, 12109-12117 (2018).\n10.1021/acsnano.8b05379.\n42. Jang, K. et al. Three-dimensional atomic mapping of ligands on palladium nanoparticles by\natom probe tomography. Nat. Commun. 12, 4301 (2021). 10.1038/s41467-021-24620-9.\n43. Kim, S. H. et al. Characterization of Pd and Pd@ Au core-shell nanoparticles using atom probe\ntomography and field evaporation simulation. J. Alloys Compd. 831, 154721 (2020).\n10.1016/j.jallcom.2020.154721.\n44. Tedsree, K. et al. Hydrogen production from formic acid decomposition at room temperature\nusing a Ag–Pd core–shell nanocatalyst. Nat. Nanotechnol. 6, 302-307 (2011). 10.1038/nnano.2011.42.\n45. Grenier, A. et al. 3D analysis of advanced nano-devices using electron and atom probe\ntomography. Ultramicroscopy 136, 185-192 (2014). 10.1016/j.ultramic.2013.10.001.\n46. Khan, M. A., Ringer, S. P. & Zheng, R. Atom probe tomography on semiconductor\ndevices. Adv. Mater. Interfaces 3, 1500713 (2016). 10.1002/admi.201500713.\n47. Hatzoglou, C., Radiguet, B. & Pareige, P. Experimental artefacts occurring during atom probe\ntomography analysis of oxide nanoparticles in metallic matrix: quantification and correction. J. Nucl.\nMater. 492, 279-291 (2017). 10.1016/j.jnucmat.2017.05.008.\n48. Philippe, T., Gruber, M., Vurpillot, F. & Blavette, D. Clustering and local magnification effects\nin atom probe tomography: a statistical approach. Microsc. Microanal. 16, 643-648 (2010).\n10.1017/S1431927610000449.\n49. Lawitzki, R., Stender, P. & Schmitz, G. Compensating local magnifications in atom probe\ntomography for accurate analysis of nano-sized precipitates. Microsc. Microanal. 27, 1–12 (2021).\n10.1017/S1431927621000180.\n50. Beinke, D., Oberdorfer, C. & Schmitz, G. Towards an accurate volume reconstruction in atom\nprobe tomography. Ultramicroscopy 165, 34-41 (2016). 10.1016/j.ultramic.2016.03.008.\n51. Takahashi, J. & Kawakami, K. Position artifacts in 3D reconstruction of plate‐shaped\nprecipitates in steels depending on the analysis direction of atom probe tomography. Surf. Interface\nAnal. 53, 982-995 (2021). 10.1002/sia.7001.\n52. Maruyama, N., Smith, G. D. W. & Cerezo, A. Interaction of the solute niobium or molybdenum\nwith grain boundaries in α-iron. Mater. Sci. Eng. A 353, 126-132 (2003). 10.1016/S0921-\n5093(02)00678-0.”\nComment 2. No relationship between the nanostructure of QDs and their photochemical properties\nwere reported.\nAnswer to Comment 2: We thank the reviewer for this important comment. We agree that\nestablishing clear correlations between the nanostructure of the QDs and their photochemical properties\nis essential, and we have made substantial efforts to strengthen this aspect in the revised manuscript.\nIn the original version, our primary focus was on directly resolving, at the 3D atomic scale, several\npreviously inaccessible structural features in this quaternary system, including the unexpected\nformation of a Zn Ga S outer shell via cation exchange, particle-to-particle compositional variations,\n1-3/2x x\ncompositional gradients, and the retention of Cl-related species originating from the ZnS precursor step.\nWe also briefly discussed how such nanoscale features could influence QD properties. This represents\nan important methodological and conceptual advance, given the analytical challenges and limited prior\nreports on quaternary I–III–VI QDs. Nevertheless, we agree that it is critical to explicitly correlate\n2\nthese nanostructural characteristics with the photochemical behavior of the QDs. Accordingly, in\nresponse to the reviewer’s comment, we have now significantly expanded both the experimental\nevidence and the discussion linking the resolved nanostructures to the observed photochemical\nproperties.\nFirst, we clarify that despite the formation of a Zn Ga S shell instead of an ideal ZnS shell, the QDs\n1-3/2x x\nexhibit both an increased photoluminescence quantum yield (QY) from 85% to 92% after the ZnS\nprecursor step and improved ambient-condition stability in QD–acrylate composite films (Fig. 1 and\nSupplementary Fig. 1). Based on prior studies on Zn–Ga–S compounds and quantum dots (QDs),\nwhich indicate that Zn–Ga–S systems possess bandgap energies between those of Ga S and ZnS depending on composition\n2 3\nand can form Type-I heterostructures (e.g., ZnGa S /ZnS), the Zn Ga S shell formed here is\n2 4 1-3/2x x\nexpected to have a wider bandgap than the AgIn Ga S core or AgGaS shell, making Type-I-like\nx 1-x 2 2\ncarrier confinement energetically plausible. Consistent with this interpretation, our QDs retain high\nQYs (80–90%) and show a further increase to ~92% following the ZnS precursor step. Moreover, they\nexhibit markedly enhanced ambient-condition stability, as evidenced by the significantly improved\nretention of QY after 48 h relative to the initial value. In contrast, a Type-II alignment would typically\nbe associated with reduced radiative efficiency and QY degradation, which are not observed in the\npresent system. These results and their interpretation have now been explicitly incorporated into the\nrevised manuscript.\nSecond, we substantially expanded the discussion of the long-wavelength tail emission that emerges\nafter the ZnS precursor step. PL characteristics show that the trap-related emission contribution\nincreases from ~3.7% in AgIn Ga S /AgGaS QDs to ~14.9% in AgIn Ga S /AgGaS /ZnS QDs (the\nx 1-x 2 2 x 1-x 2 2\ntrap emission area ratio was defined as the fraction of the integrated photoluminescence intensity in the\nlong-wavelength region (λ > λ + 50 nm) to the total integrated emission.). Importantly, XRD and\nPL max\nTEM confirm that the shell retains a chalcopyrite-related tetragonal structure rather than forming a new\nzinc-blende-type ZnS heterophase. This demonstrates that the observed tail emission does not originate\nfrom the formation of a structurally distinct shell, but is instead associated with defect formation.\nConsistent with this interpretation, our APT results directly reveal a complex, four-layered\nnanostructure consisting of an Ag- or In-rich AgIn Ga S core/Ga-rich AgGaS inner shell/Ag-\nx 1-x 2 2\ndeficient AgGaS intermediate shell/Zn- and Ga-rich Zn Ga S outer shell. After the ZnS precursor\n2 1-3/2x x\nstep, APT further shows inward diffusion of Zn2+ into the QDs. Because excitons in I–III–VI QDs are\n2\npreferentially confined to In-rich regions, Zn diffusion and cation exchange in the vicinity of these\nregions are expected to generate a high density of cation vacancies and related complexes. Such\nintrinsically non-charge-neutral substitution processes are well known to introduce intragap trap states\nresponsible for donor–acceptor-pair-like recombination, leading to the long-wavelength emission in I–\nIII–VI QDs. We therefore attribute the enhanced tail emission to ~14.9% after the ZnS precursor step\n2\nprimarily to defect formation induced by Zn inward diffusion and cation exchange, rather than to the\nemergence of a new heterostructural phase. Relevant literature on Ag–In–S and Zn-alloyed Ag–In–S\nQDs reporting similar trap-assisted long-wavelength emission and our explanation has now been\nincorporated into the revised manuscript.\nThird, to further link nanostructure formation to optical properties, we performed additional controlled\nsyntheses in which the Ag and Ga precursor amounts were systematically varied. When the Ga\nprecursor amount was fixed and the Ag precursor amount was increased, STEM–EELS revealed\npronounced thickening of the AgGaS shell and increased Ag incorporation. At the same time, the trap-\n2\nrelated emission contribution increased from 5.1% to 21.3% as the Ag precursor amount was increased\nup to sixfold, while the photoluminescence QY decreased from 77% to 37%, indicating that Ag-driven\nshell growth could be accompanied by enhanced defect formation (Supplementary Fig. 8 and\nSupplementary Table. 2). In contrast, when the Ag precursor amount was fixed and the Ga precursor\namount was increased, no significant increase in QD size or shell thickness was observed. This\nasymmetric behavior provides compelling experimental support for our interpretation that the formation\nof the Ag-deficient outer shell originates from the relatively lower reactivity of the Ga precursor and\ndirectly impacts the defect and optical response of the QDs. This interpretation and its implications for\nthe optical properties have now been incorporated into the revised manuscript.\nTaken together, these new experiments and expanded discussions explicitly connect the 3D\nnanostructure revealed by APT to the observed photochemical and optical properties, including QY,\nstability, and trap-related emission. The corresponding data, analyses, and discussions have been\nincorporated into the revised manuscript and Supplementary Information.\nThe manuscript has been revised as follows:\nPlease see Page #5,\n“Notably, the QY increases from 85% to 92% after the ZnS precursor step. Furthermore, the stability\nof QD–acrylate composite films under ambient conditions was evaluated in terms of the change in the\nQY after 48 h with respect to the initial QY. As shown in Supplementary Fig. 1, the ZnS shell\nsignificantly improves this normalized QY from 66% to 97%. The enhanced optical properties can be\nattributed to the effective confinement of electrons and holes provided by the wide-band-gap ZnS\nencapsulating the AgIn\nx\nGa\n1-x\nS\n2\n/AgGaS\n2\n. In addition to the dominant green emission, the AgIn\nx\nGa\n1-\nS /AgGaS /ZnS QDs exhibit a long-wavelength emission tail in the 600–700 nm range (Fig. 1d). The\nx 2 2\narea ratio of this long-wavelength emission, defined as the fraction of the total PL intensity in the long-\nwavelength region (λ > λ + 50 nm), increases from ~3.7% in AgIn Ga S /AgGaS QDs to ~14.9%\nPL max x 1-x 2 2\nin AgIn Ga S /AgGaS /ZnS QDs. Such long-wavelength emission is generally attributed to defect- or\nx 1-x 2 2\ntrap-assisted recombination rather than band-edge excitonic transitions in AgInS -based QDs and\n2\nrelated I–III–VI -based QDs.36-40”\n“To demonstrate the role of differential precursor reactivity, we performed controlled syntheses in which\nthe Ag precursor amount was increased at a fixed Ga level, a condition under which increasing the Ag\nprecursor amount directly promotes the further growth of the AgGaS shell. STEM–EELS analyses\n2\nshowed the pronounced thickening of the AgGaS shell and consistently increased Ag incorporation,\n2\nsupporting the role of precursor reactivity in the formation of the shell (Supplementary Fig. 8 and\nSupplementary Table 1). These changes were accompanied by an increased contribution from trap-\nrelated emission and a decreased photoluminescence QY, indicating that Ag-driven shell growth directly\nimpacts the optical properties of the QDs.”\nPlease see Page #16,\n“The emergence of the Zn Ga S outer shell via cation exchange offers a plausible origin for the long-\n1-3/2x x\nwavelength emission tail observed after the ZnS precursor step. In our system, the substitution of Ag+\nand Ga3+ by Zn2+ during shell formation inherently introduces cation vacancies and local charge-\ncompensating defects. Consistent with our APT observations, the inward diffusion of Zn2+ into the\ninterior of the QDs further induces cation exchange and promotes the formation of such defects. When\nthese defects form in the vicinity of In-rich regions, where carriers are preferentially confined, they can\nintroduce intragap trap states that locally perturb the confinement potential and promote trap-assisted\nrecombination. These defect-related states are commonly associated with donor–acceptor-pair-like\nrecombination pathways that give rise to long-wavelength emission in I–III–VI semiconductors.23,38-\n2\n40,66-68”\nPlease see Page #16,\n“In terms of the band gap energy, the Zn Ga S shell typically exhibits a slightly lower band gap than\n1-3/2x x\nZnS (~3.5 eV), even at the nanoscale.62,65,69-72 The formation of the compositionally mixed Zn Ga S\n1-3/2x x\nlayer is therefore expected to adequately preserve the quantum efficiency, with negligible weakening of\ncharge confinement, consistent with Type-I-like band alignment.”\nPlease see Fig. 1,\nFig. 1 Comparison of AgIn Ga S /AgGaS QDs and AgIn Ga S /AgGaS /ZnS QDs. a, d\nx 1-x 2 2 x 1-x 2 2\nSchematic representation of the internal structure, band diagrams, and UV–vis absorption, and PL\ncharacteristics. Both QDs exhibit bright green luminescence at ~530 nm, with the QY increasing from\n85% to 92% after the ZnS precursor step to form the outer shell. In addition, AgIn Ga S /AgGaS /ZnS\nx 1-x 2 2\nQDs exhibit a long-wavelength emission tail in the ~600–700 nm range. b, e HAADF–STEM image of\nAgIn Ga S -based QDs before and after the ZnS precursor step, which negligibly changed the size of\nx 1-x 2\nthe QDs, with average diameters of 6.14 ± 0.76 and 6.11 ± 1.03 nm, respectively. Inset: high-\nmagnification HAADF–STEM image. c, f STEM–EDS elemental maps of AgIn Ga S -based QDs\nx 1-x 2\nbefore and after the ZnS precursor step.\nPlease see Supplementary Fig.8,\nSupplementary Fig. 8 STEM–EELS analysis of AgIn Ga S /AgGaS QDs as a function of the\nx 1-x 2 2\nGa/Ag precursor ratio. (a) Representative STEM–EELS results for high-Ga/Ag-ratio (Ag-poor)\nAgIn Ga S /AgGaS QDs and low-Ga/Ag-ratio (Ag-rich) AgIn Ga S /AgGaS QDs. As the Ag\nx 1-x 2 2 x 1-x 2 2\nprecursor amount increases, the overall diameter of the QDs increases, particularly because of the\nincreasing thickness of the AgGaS shell. Owing to the relatively low reactivity of the Ga precursor,\n2\nincreasing the Ag precursor under Ga-rich conditions mainly promotes the growth of the AgGaS shell,\n2\naccompanied by increased Ag incorporation. (Scale bars: 5 nm.)\nPlease see Supplementary Table 1,\nQD size\nAg Ga/Ag Trap\nλ FWHM measured by\nprecursor (precursor PL max emission QY\n(band edge) (nm) TEM\namount ratio) area ratio\n(nm)\nLow Ag 29 524 35 5.1 77 6.0\n10 520 37 7.8 68 6.8\n↓\n6 517 37 12.9 57 6.9\nHigh Ag 4 516 38 21.3 37 7.9\nSupplementary Table 1. Dependence of the optical properties and size of AgIn Ga S /AgGaS\nx 1-x 2 2\nQDs on the Ga/Ag precursor ratio. As the Ag precursor amount increases, the average particle size\nof the AgIn Ga S /AgGaS QDs as measured by TEM increases. Meanwhile, the trap emission\nx 1-x 2 2\nincreases, and the QY decreases. Owing to the relatively low reactivity of the Ga precursor, increasing\nthe Ag precursor under Ga-rich conditions primarily leads to further growth of the AgGaS shell. The\n2\ntrap emission area ratio was defined as the fraction of the integrated PL intensity in the long-wavelength\nregion (λ > λ + 50 nm) to the total integrated emission.\nPL max\nReferences have been added as follows:\n36. Dai, M. et al. Tunable photoluminescence from the visible to near-infrared wavelength region\nof non-stoichiometric AgInS nanoparticles. J. Mater. Chem. 22, 12851-12858 (2012).\n2\n10.1039/C2JM31463K.\n37. Chen, Y. et al. Green and facile synthesis of high-quality water-soluble Ag-In-S/ZnS core/shell\nquantum dots with obvious bandgap and sub-bandgap excitations. J. Alloys Compd. 753, 364-370\n(2018). 10.1016/j.jallcom.2018.04.242.\n38. Park, S. M. et al. Red Ag-based I–III–VI quantum dots as competitive alternatives to InP\nemitters. ACS Energy Lett. 10, 3005-3013 (2025). 10.1021/acsenergylett.5c00962.\n39. Farid, A. et al. One-pot synthesis of luminescent Ag−Cu−Ga−S/ZnS quantum dots bridging\nthe cyan gap for ultrahigh-color-rendering white-light-emitting diodes. ACS Appl. Nano Mater. 8,\n14703-14712 (2025). 10.1021/acsanm.5c02386.\n40. Park, S. et al. Suppressing tail emission from AgIn Ga S /AgGaS quantum dots by GaI –\n1−x x 2 2 3\nassisted interface reinforcement. ACS Nano 19, 26831-26842 (2025). 10.1021/acsnano.5c07418.\n71. Hu, T., Zhu, K., Cheng, H., Teng, Y. & Pan, Z. Core–shell energy band engineering of cyan\nlight-emitting ternary ZnGa S @ZnS quantum dots toward anti-counterfeiting and bioimaging\n2 4\napplications. J. Mater. Chem. C 13, 21797-21811 (2025). 10.1039/D5TC02797G.\n72. Yadav, A. N. & Singh, K. Investigation of photophysical properties of ternary Zn–Ga–S\nquantum dots: band gap versus sub-band-gap excitations and emissions. ACS Omega 4, 18327-18333\n(2019). 10.1021/acsomega.9b02546.\nComment 3. No definition of the x and y values were indicated. No explanation of x >> y was made,\nbecause each formula only contained x or y.\nAnswer to Comment 3: We thank the reviewer for pointing out the lack of clarity regarding the\ndefinitions of the variables x and y and the meaning of the relation x >> y. We apologize for this\nambiguity in the original manuscript.\nThe notation x >> y is used to emphasize that In is not completely absent from the outer Ag-deficient\nshell, but is present at a significantly lower concentration than in the core. This notation therefore serves\nas a qualitative compositional description highlighting the strong depletion of In in the outer shell region.\nIn the revised manuscript, we now explicitly define x and y as the In fractions in different regions of the\nheterostructured QDs. Specifically, x denotes the In fraction in the AgIn Ga S core, whereas y\nx 1-x 2\nrepresents the residual In fraction in the Ag-deficient layer formed at the outer surface of the AgGaS\n2\nshell. Here, the notation x >> y is intended as a qualitative description rather than a strict mathematical\ninequality.\nThe manuscript has been revised as follows:\nPlease see Page #9,\n“In addition, trace amounts of In atoms remain present in the shell region, leading to a shell composition\ncloser to AgIn Ga S , where x denotes the fraction of In in the core and y represents the residual In\ny 1-y 2\nfraction in the outer layer, with x >> y, rather than a pure AgGaS shell. This is likely due to the outward\n2\ndiffusion of In atoms from the AgIn Ga S core to vacant sites in AgGaS , driven by the concentration\nx 1-x 2 2\ngradient or thermal budget during synthesis. Nevertheless, the resulting AgIn Ga S shell is expected\ny 1-y 2\nto effectively confine charges within the AgIn Ga S core owing to its higher band gap, consistent with\nx 1-x 2\na Type-I heterostructure, which promotes radiative recombination.”\nComment 4. The charge neutrality in single QDs and its spatial modulation should be discussed.\nAnswer to Comment 4: We thank the reviewer for raising the important point regarding charge\nneutrality in single quantum dots (QDs) and its possible spatial modulation.\nIn the present study, the AgIn Ga S -based QDs are capped with oleylamine ligands, which act as\nx 1-x 2\nelectrically neutral L-type ligands. Accordingly, each individual QD is expected to maintain overall\ncharge neutrality. This is further supported by bulk elemental analysis (e.g., Inductively Coupled\nPlasma Optical Emission Spectrometry), which confirms that the overall cation-to-anion balance of the\nsynthesized QDs is consistent with charge neutrality.\nFrom a spatial perspective, the APT results are also consistent with local charge balance within the QDs.\nIn the AgIn Ga S core, S is distributed relatively uniformly, while variations in cation composition\nx 1-x 2\noccur in a compensatory manner, such that regions enriched in Ag correspond to reduced In and Ga\nfractions, and vice versa. Similarly, in the shell regions, S remains approximately constant while\nenrichment of one cation species (e.g., Ga in the Zn Ga S shell) is accompanied by a corresponding\n1-3/2x x\nreduction of other cations. These observations indicate that, despite compositional variations, local\nelectroneutrality may be largely preserved through redistribution of cation species rather than through\nnet charge accumulation. In addition, in AgIn Ga S -based QDs (e.g., AIGS, AIS/ZnS, and AIZS),\nthe\nx 1-x 2\nformation of the core and shell is known to involve cation exchange processes. During these processes,\ncation vacancies can be generated as part of the charge-compensation mechanism required to maintain\nlocal charge neutrality within the lattice. This behavior has been widely discussed in the context of I–\nIII–VI QDs, where such vacancies are generally regarded as intrinsic structural defects associated with\n2\nthe synthesis and shell-formation process, rather than as sources of net charging of the QDs. That is,\nthe QDs are globally charge-neutral, and local compositional variations and defects formed during\ncation exchange can lead to spatial modulation of defect states while maintaining overall charge\nneutrality.\nThe manuscript has been revised as follows:\nPlease see Page # 14,\n“The pronounced inhomogeneity in the cores likely originates from the intrinsic complexity of the\nquaternary Ag–In–Ga–S system. The synthesis of the core simultaneously involves multiple cation\nprecursors with different reactivities, and the cores are formed under a continuous heating process.\nPrevious studies have also shown that AgIn Ga S core formation proceeds through intermediate\nx 1-x 2\nphases and cation-exchange processes rather than a simple single-step growth process.18,53,59 More\nspecifically, AgGaS or AgInS seeds form initially, followed by the incorporation of In and Ga to\n2 2\ngenerate the chalcopyrite AgIn Ga S lattice, which is likely to promote local compositional\nx 1-x 2\nfluctuations within the core.”\nPlease see Supplementary Table 3,\nAtomic ratio (ICP–OES)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.40 0.12 0.44 0\nAIGS/AGS/ZnS 0.13 0.08 0.35 0.46\nSupplementary Table 3. Atomic ratios measured of AgIn Ga S /AgGaS (AIGS/AGS) and\nx 1-x 2 2\nAgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) determined by ICP–OES.\nx 1-x 2 2\nPlease see Supplementary Table 4,\nAtomic ratio (APT)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.43 0.13 0.39 0\nAIGS/AGS/ZnS 0.15 0.08 0.30 0.48\nSupplementary Table 4. Atomic ratios of AgIn Ga S /AgGaS (AIGS/AGS) and AgIn Ga\nx 1-x 2 2 x 1-\nS /AgGaS /ZnS (AIGS/AGS/ZnS) determined by APT.\nx 2 2\nReferences have been added as follows:\n18. Lee, H. J. et al. Coherent heteroepitaxial growth of I-III-VI Ag(In,Ga)S colloidal\n2 2\nnanocrystals with near-unity quantum yield for use in luminescent solar concentrators. Nat. Commun.\n14, 3779 (2023). 10.1038/s41467-023-39509-y.\n53. Uematsu, T., Tepakidareekul, M., Hirano, T., Torimoto, T. & Kuwabata, S. Facile high-yield\nsynthesis of Ag–In–Ga–S quaternary quantum dots and coating with gallium sulfide shells for narrow\nband-edge emission. Chem. Mater. 35, 1094-1106 (2023). 10.1021/acs.chemmater.2c03023.\n54. Lee, S. H. et al. The effects of discrete and gradient mid-shell structures on the\nphotoluminescence of single InP quantum dots. Nanoscale 11, 23251-23258 (2019).\n10.1039/C9NR06847C.\n55. Lim, J. et al. InP@ ZnSeS, core@ composition gradient shell quantum dots with enhanced\nstability. Chem. Mater. 23, 4459-4463 (2011). 10.1021/cm201550w.\n56. Bae, W. K., Char, K., Hur, H. & Lee, S. Single-step synthesis of quantum dots with chemical\ncomposition gradients. Chem. Mater. 20, 531-539 (2008). 10.1021/cm070754d.\n57. Duan, X. et al. InP quantum dots with a strain-engineered gradient shell for enhanced optical\nperformance and stability. Nano Lett. 25, 13539-13548 (2025). 10.1021/acs.nanolett.5c03042.\n58. Yu, P. et al. Highly efficient green InP-based quantum dot light-emitting diodes regulated by\ninner alloyed shell component. Light Sci. Appl. 11, 162 (2022). 10.1038/s41377-022-00855-z.\n59. Xie, X. et al. Narrow-bandwidth I–III–VI semiconductor nanocrystals: synthesis,\nluminescence and applications in quantum-dot light-emitting diodes. Adv. Phys. Res. 3, 2400071 (2024).\n10.1002/apxr.202400071.\nComment 5. AgInxGa1-xS2/AgGaS2 had quite Ag-deficient surface layer. What is the crystal structure\nof this layer? Ag-doped Ga2S3??\nAnswer to Comment 5: We thank the reviewer for this insightful question regarding the crystal\nstructure of the Ag-deficient surface layer in the AgIn Ga S /AgGaS quantum dots (QDs).\nx 1-x 2 2\nBoth the AgIn Ga S core and the AgGaS shell are well known to crystallize in a chalcopyrite\nx 1-x 2 2\nstructure with tetragonal symmetry. Our X-ray diffraction (XRD) analysis confirms that the core-only\nAgIn Ga S samples and the AgIn Ga S /AgGaS samples exhibit essentially identical diffraction\nx 1-x 2 x 1-x 2 2\npatterns, consistent with a chalcopyrite-type tetragonal structure. This XRD observation indicates that\nthe overall crystal structure is preserved even after shell formation. Consistently, AgInS and AgGaS\n2 2\nQDs reported in the literature are also known to adopt the chalcopyrite structure, and AgIn Ga S QDs\nx 1-x 2\ncan be regarded as chalcopyrite AgInS or AgGaS lattices in which Ga or In partially substitutes the\n2 2\ngroup-III cation sites.\nWe acknowledge that the presence of an Ag-deficient surface layer may raise the question of whether\na different crystalline phase, such as Ga S , is formed. However, complete removal of Ag from the\n2 3\nlattice is unlikely, and as long as the S sublattice is preserved, Ag deficiency alone does not necessarily\nlead to a collapse or reconstruction into a fundamentally different crystal structure. Instead, Ag\nvacancies are expected to introduce defects while maintaining the underlying chalcopyrite framework.\nTherefore, we interpret the Ag-deficient surface region as retaining a tetragonal chalcopyrite-based\nstructure with a substantial density of Ag vacancies, which can be more appropriately described as a\nchalcopyrite-like or defective chalcopyrite phase rather than as a distinct Ga S -type phase.\n2 3\nConsequently, our structural and diffraction data support the assignment of the Ag-deficient surface\nlayer to a chalcopyrite-derived structure with Ag vacancies.\nThe manuscript has been revised as follows:\nPlease see Page # 6,\n“Moreover, the high-resolution STEM results reveal that the QDs are structurally similar before and\nafter the ZnS precursor step, with all crystal planes showing a lattice spacing of ~0.29 nm, which almost\ncorresponds to the interplanar spacing of the (200) planes of chalcopyrite-structured AgGaS . The lattice\n2\nfringes are clearly evident not only inside the AgIn Ga S -based QDs but also at the edges, indicating\nx 1-x 2\nthe high crystallinity across the entire QD structure. (Figs. 1b and 1e). X-ray diffraction (XRD)\nmeasurements were further performed on the core-only AgIn Ga S , AgIn Ga S /AgGaS , and\nx 1-x 2 x 1-x 2 2\nAgIn Ga S /AgGaS /ZnS QDs to corroborate the structural similarity before and after shell formation.\nx 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, and the overall\ndiffraction features remain essentially unchanged, even after the Zn precursor step (Supplementary Fig.\n4). This comparative analysis indicates that the crystal structure is preserved throughout the shell\nformation process.”\nPlease see Supplementary Fig. 4,\nSupplementary Fig. 4 XRD patterns of AgIn Ga S -based QDs. XRD patterns of AgIn Ga S\nx 1-x 2 x 1-x 2\n(AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs.\nx 1-x 2 2 x 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, with only minor peak\nshifts attributable to slight lattice parameter variations.\nComment 6. Also, what is the crystal structure of the surface of AgInxGa1-xS2/AgGaS2/ZnS? The\nauthors assumed the ZnGaS alloy layer was formed on the surface. However, no difference of lattice\nfringes between core and shell in the corresponding HAADF-STEM images (Fig. 1) was observed.\nAnswer to Comment 6: We thank the reviewer for the question regarding the crystal structure of the\nsurface region in the AgIn Ga S /AgGaS /ZnS quantum dots (QDs) and the absence of discernible\nx 1-x 2 2\nlattice-fringe differences between the core and shell in the HAADF-STEM images.\nThe AgIn Ga S core is well known to crystallize in a chalcopyrite structure with tetragonal symmetry,\nx 1-x 2\nand AgGaS QDs reported in the literature also adopt the same chalcopyrite structure. In the present\n2\nwork, no obvious difference in lattice fringes is observed between the core and shell regions in the\nHAADF-STEM images (Fig. 1). Although resolving subtle structural differences in nanometer-scale\nQDs is inherently challenging, this observation is consistent with our interpretation that both the\nAgGaS inner shell and the Zn Ga S outer shell retain a chalcopyrite-type tetragonal structure. If the\n2 1-3/2x x\ncore, inner shell, and outer shell share similar crystallographic symmetry, the absence of clear lattice-\nfringe contrast between these regions is therefore consistent with this interpretation.\nTo further support this interpretation, we performed X-ray diffraction (XRD) analysis on the core-only\nAgIn Ga S samples, the AgIn Ga S /AgGaS samples, and the AgIn Ga S /AgGaS /ZnS samples.\nx 1-x 2 x 1-x 2 2 x 1-x 2 2\nAll samples exhibit diffraction patterns consistent with a tetragonal chalcopyrite-based structure.\nNotably, even after the ZnS shelling step, the outer layer is identified as Zn Ga S by APT analysis,\n1-3/2x x\nwhile the diffraction patterns remain essentially unchanged, aside from minor peak shifts attributable\nto slight variations in lattice parameters. While the diffraction peaks are inevitably broadened due to the\nnanocrystalline nature of the QDs, the comparative analysis remains meaningful and indicates that the\noverall crystal structure is preserved throughout the shell formation process. Therefore, the absence of\na clear lattice-fringe contrast between the core and shell in high-resolution TEM images can be\nattributed to their strong crystallographic similarity. From a local structural perspective, even if partial\nAg depletion occurs near the surface, the underlying S sublattice and the tetragonal framework are\nexpected to remain intact. Within this context, the Zn Ga S outer layer can be described as adopting\n1-3/2x x\na chalcopyrite-type tetragonal structure, which provides a plausible explanation for the absence of\ndistinct lattice-fringe contrast between the core and shell regions in the HAADF-STEM images. We\nhave added the corresponding XRD results to the Supplementary Information to support the structural\nsimilarity of the QDs before and after shell formation.\nThe manuscript has been revised as follows:\nPlease see Page # 6,\n“Moreover, the high-resolution STEM results reveal that the QDs are structurally similar before and\nafter the ZnS precursor step, with all crystal planes showing a lattice spacing of ~0.29 nm, which almost\ncorresponds to the interplanar spacing of the (200) planes of chalcopyrite-structured AgGaS . The lattice\n2\nfringes are clearly evident not only inside the AgIn Ga S -based QDs but also at the edges, indicating\nx 1-x 2\nthe high crystallinity across the entire QD structure. (Figs. 1b and 1e). X-ray diffraction (XRD)\nmeasurements were further performed on the core-only AgIn Ga S , AgIn Ga S /AgGaS , and\nx 1-x 2 x 1-x 2 2\nAgIn Ga S /AgGaS /ZnS QDs to corroborate the structural similarity before and after shell formation.\nx 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, and the overall\ndiffraction features remain essentially unchanged, even after the Zn precursor step (Supplementary Fig.\n4). This comparative analysis indicates that the crystal structure is preserved throughout the shell\nformation process.”\nPlease see Page #15,\n“This process forms a compositionally mixed Zn Ga S outer shell with a chalcopyrite structure,\n1-3/2x x\nwhich is the overall crystallographic framework of the AgGaS inner shell.18,53,60-65 Fortuitously, the\n2\nformation of Zn Ga S with a chalcopyrite-like structure, which is the same as that of AgGaS , is\n1-3/2x x 2\ncrystallographically preferable over zincblende-structured ZnS.”\nPlease see Supplementary Fig. 4,\nSupplementary Fig. 4 XRD patterns of AgIn Ga S -based QDs. XRD patterns of AgIn Ga S\nx 1-x 2 x 1-x 2\n(AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs.\nx 1-x 2 2 x 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, with only minor peak\nshifts attributable to slight lattice parameter variations.\nReferences have been added as follows:\n18. Lee, H. J. et al. Coherent heteroepitaxial growth of I-III-VI Ag(In,Ga)S colloidal\n2 2\nnanocrystals with near-unity quantum yield for use in luminescent solar concentrators. Nat. Commun.\n14, 3779 (2023). 10.1038/s41467-023-39509-y.\n53. Uematsu, T., Tepakidareekul, M., Hirano, T., Torimoto, T. & Kuwabata, S. Facile high-yield\nsynthesis of Ag–In–Ga–S quaternary quantum dots and coating with gallium sulfide shells for narrow\nband-edge emission. Chem. Mater. 35, 1094-1106 (2023). 10.1021/acs.chemmater.2c03023.\n60. Yan, D. et al. High photoluminescence Ag-In-Ga-S quantum dots based on ZnX -treated\n2\nsurface passivation. Nano Res. 17, 7533-7541 (2024). 10.1007/s12274-024-6724-0.\n61. Jang, J. S., Borse, P. H., Lee, J. S., Choi, S. H. & Kim, H. G. Indium induced band gap tailoring\nin AgGa In S chalcopyrite structure for visible light photocatalysis. J. Chem. Phys. 128 (2008).\n1− x x 2\ndoi.org/10.1063/1.2900984.\n62. Asadullayeva, S. G., Ismayilova, N. A., Musayev, M. A. & Abbasov, I. I. Optical and electronic\nproperties of defect chalcopyrite ZnGa S . Int. J. Mod. Phys. B 38, 2450007 (2024).\n2 4\n10.1142/S0217979224500073.\n63. Bai, L., Lin, Z., Wang, Z., Chen, C. & Lee, M. H. Mechanism of linear and nonlinear optical\neffects of chalcopyrite AgGaX (X=S, Se, and Te) crystals. J. Chem. Phys. 120, 8772-8778 (2004).\n2\n10.1063/1.1687338.\n64. Kaga, H. & Kudo, A. Cosubstituting effects of copper(I) and gallium(III) for ZnGa S with\n2 4\ndefect chalcopyrite structure on photocatalytic activity for hydrogen evolution. J. Catal. 310, 31-36\n(2014). 10.1016/j.jcat.2013.08.025.\n65. Sahariya, J., Kumar, P. & Soni, A. Structural and optical investigations of ZnGa X (X = S, Se)\n2 4\ncompounds for solar photovoltaic applications. Mater. Chem. Phys. 199, 257-264 (2017).\n10.1016/j.matchemphys.2017.07.003.\nThus, the present manuscript does not contain novel results. The manuscript may not be suitable for the\npublication in the preset journal.\nReviewer #3 (Remarks to the Author):\nGeneral Comment: The authors present an important study on the atomistic level for Quantum Dots\nusing APT. The interface design and the intermixing of the components is still a great uncertainty for\nquantum dots, therefore, a knowledge of the distribution of the components is very important. APT is\nvery promising for obtain more important and exciting results about the intermixing which can use for\na better tuning of the synthesis of QDs. All in all, the manuscript is very important contribution in this\nfield. But there are some points which should be clarified before publication:\nAnswer to General Comment: We sincerely thank the reviewer for the positive assessment of our\nwork and for recognizing the importance of APT to achieve an atomistic-level understanding of\ncompositional distributions in AgIn Ga S -based quantum dots. We fully agree with the reviewer’s\nx 1-x 2\nperspective on the potential of APT to guide improved synthesis strategies. In response to the reviewer’s\ncomments, we have made every effort to further enhance the reliability of our analysis and to clarify\ndata interpretation and visualization throughout the manuscript. All comments raised by the reviewer\nhave been carefully addressed, and the manuscript has been revised accordingly. Detailed responses to\neach comment are provided below.\nComment 1. In Figs. 2, 4, and 6 different particles are shown in (a), in Figs. 2 and 4 there are five. It\nis unclear if all particles of them were further evaluated or only one of them was evaluated.\nAnswer to Comment 1: We thank the reviewer for seeking clarification. In all APT datasets shown\nin Figs. 2, 4, and 6, five quantum dots (QDs) were analyzed within a single reconstructed volume.\nFigure 2 presents the APT results for the AgIn Ga S /AgGaS QDs, while Figures 4 and 6 correspond\nx 1-x 2 2\nto the AgIn Ga S /AgGaS /ZnS QDs.\nx 1-x 2 2\nFigures 4 and 6 are based on the same APT dataset and include the same five QDs; however, Figure 6\nuses a different visualization to highlight the spatial distribution of Cl. No Cl signal was detected in the\nAgIn Ga S /AgGaS QDs shown in Fig. 2. Therefore, the analysis was not limited to a single particle,\nx 1-x 2 2\nand all five QDs in each reconstructed dataset were evaluated consistently.\nComment 2. In Figs. 4 and 6 the five partices are not homogeneous, in QD 4 and QD 5 (Fig.4) Ag\nseems be not so present, in Fig. 6 In was inhomegenously distributed.\nAnswer to Comment 2: We thank the reviewer for the careful observation. The apparent\ncompositional inhomogeneities observed in Figs. 4 and 6, such as the reduced apparent Ag signal in\nQD#4 and QD#5 in Fig. 4 or the seemingly inhomogeneous In distribution in Fig. 6, are largely\ninfluenced by geometric and visualization-related effects rather than from intrinsic compositional\nvariations among the quantum dots (QDs).\nIn the APT specimen, the QDs are not arranged in a single line or perfectly aligned along the analysis\n(z) direction, but instead exhibit different lateral positions, with each QD occupying a distinct position\nin the x–y plane. As a result, sliced-view images such as Fig. 4b cannot intersect all QDs at an identical\ncentral plane. Similarly, the 2D compositional profiles in Fig. 4d were extracted uniformly along the z\ndirection from a fixed cylindrical sampling volume (3 × 3 × 21 nm3). Consequently, in some QDs the\nprofiles partially sample the shell region, whereas in others they primarily intersect the core, which can\ngive rise to apparent dot-to-dot variations in the compositional profiles.\nTo evaluate the intrinsic compositional uniformity more accurately, we therefore extracted and analyzed\nthe full 3D volumes of individual QDs, as shown in Fig. 5. These analyses confirm that, while minor\nvariations exist, there are no significant compositional anomalies, such as a pronounced depletion of\nAg in specific QDs. Moreover, the sliced 3D atom maps in Fig. 4b provide the most direct visualization\nof the internal spatial distributions, clearly showing that Ag and In are appropriately distributed within\neach QD at their respective positions.\nFigure 6 is based on the same APT dataset as Fig. 4 and was visualized differently to emphasize the\nspatial distribution of Cl. In this figure, the entire 3D reconstruction volume is displayed rather than a\nsliced view. Consequently, depending on the projection direction, QDs located along the line of sight\ncan partially overlap along the projection direction and obscure one another, making some elemental\ndistributions—such as In—appear more inhomogeneous. Nevertheless, these different visualization\nmodes were intentionally employed to illustrate both the internal spatial distributions and the overall\ncompositional distribution tendencies of the QDs. To avoid potential misinterpretation of the\ncompositional maps, we have clarified these visualization and geometry related effects in the revised\nmanuscript.\nThe manuscript has been revised as follows:\nPlease see Page #8,\n“Because the QDs are closely packed in a non-collinear configuration within the APT specimen and\nmay therefore partially overlap in projection views, 3D reconstructions with C- and H-related ions\nremoved are also provided in Supplementary Fig. 7 to better visualize the individual QDs.”\nPlease see Page #12,\n“The 2D concentration profile along the analysis direction offers additional insights into the internal\nstructure of each QD (Fig. 4d). Although the elemental concentrations vary slightly depending on the\nprofiling path owing to the spatial offset of individual QDs in the APT specimen, the 2D profile confirms\nthe significant presence of Ga atoms in the outer shell, albeit at lower concentrations than Zn atoms.”\nPlease see Fig. 2,\nFig. 2 3D reconstruction and compositional analysis of AgIn Ga S /AgGaS QDs using APT. a\nx 1-x 2 2\nReconstructed 3D atom map of the entire analyzed volume, showing five slightly ellipsoidal QDs. b\nSlice-view atom maps of Ag, In, Ga, and S, illustrating their internal spatial distributions. Ga and S\natoms are more broadly distributed than Ag and In. c Ag iso-surface map and proxigram showing 3D\ncompositional trends; the proxigram was obtained from five QDs. The Ag and In contents increase\ntoward the QD core, while the Ga content increases toward its outer surface, confirming the core/shell\nheterostructure. (Scale bars: 2 nm in a–c.) Error bars in c indicate one-sigma counting statistics.\nPlease see Fig. 4,\nFig. 4 3D reconstruction and compositional analysis of AgIn Ga S /AgGaS /ZnS QDs using APT.\nx 1-x 2 2\na Reconstructed 3D atom map of the entire analyzed volume, showing five slightly ellipsoidal QDs. b\nSlice-view atom maps of Ag, In, Ga, Zn, and S, illustrating their internal spatial distributions. Although\nmost Ag and In atoms are in the core regions, a significant amount of Ga is distributed over the shell\nregion. Zn atoms predominantly exist in the outer shell. c Ag and Zn iso-surface and proxigram obtained\nfrom Ag iso-surfaces; the proxigram was obtained from five QDs. The Ag and In contents increase\ntoward the QD core, while the content of Zn increases toward the outer shell of the QD. A significant\namount of Ga also appears in the outer shell. d 2D compositional profile extracted from a cylindrical\nvolume (3 × 3 × 21 nm3) through the 3D reconstruction, confirming the core/shell/shell structure. (Scale\nbars: 2 nm in a–d). The slight variations in the compositional distributions from QD to QD arise from\nthe spatial offsets of individual QDs within the APT specimen. Error bars in c, d indicate one-sigma\ncounting statistics.\nComment 3. Was the Cl correlated with some of the cations? I am missing a statement about this\nquestion?\nAnswer to Comment 3: We thank the reviewer for this question regarding the correlation of Cl with\nspecific cations. As discussed in the manuscript, Cl originates from the ZnCl precursor used during the\nThe APT results show that Cl is predominantly localized in the outer shell region and is not detected in\nthe AgIn Ga S /AgGaS quantum dots (QDs) prior to the ZnS precursor step. Importantly, Cl exhibits\nx 1-x 2 2\na strong spatial correlation with Zn, as clearly shown in Fig. 6, whereas no comparable correlation is\nobserved with other cations. This is consistent with the shell composition revealed by APT, where the\nouter shell is identified as Zn Ga S. Because no additional Ga precursor is introduced during the\n1-3/2x x\nshell formation step, the observed Cl distribution is attributed to Zn-related species, indicating that Cl\nis correlated with Zn rather than with Ag or In.\nTo further support the reliability of the APT results, we performed complementary X-ray photoelectron\nspectroscopy (XPS) measurements. While XPS does not provide 3D spatial information, it clearly\nshows that Cl is nearly absent in the AgIn Ga S /AgGaS QDs and becomes distinctly detectable only\nx 1-x 2 2\nafter the ZnS precursor step, that is, in the AgIn Ga S /AgGaS /ZnS QDs. The Cl 2p core-level spectra\nx 1-x 2 2\nreveal the presence of metal–Cl chemical states exclusively in the AgIn Ga S /AgGaS /ZnS samples,\nx 1-x 2 2\nfurther supporting the strong Zn-Cl association observed by APT. Given that no significant metal–Cl\ncomponent is observed in the AgIn Ga S /AgGaS QDs, this Cl signal is most reasonably attributed\nx 1-x 2 2\nto Zn–Cl–related chemical states introduced during the ZnS precursor step. This observation is fully\nconsistent with the APT data and supports the conclusion that Cl is introduced during the Zn-based shell\nformation process. As discussed in the manuscript, although Cl is clearly associated with the Zn-\ncontaining outer shell, its precise role, whether acting as a trap related defect or as an inorganic ligand,\ncannot be conclusively determined within the scope of the present study. The revised manuscript has\nbeen updated accordingly, and the corresponding XPS results have been added to the Supplementary\nInformation.\nThe manuscript has been revised as follows:\nPlease see Page #17,\n“As shown in Fig. 6 and Supplementary Fig. 19, Cl atoms are detected only in the AgIn Ga\nx 1-\nS /AgGaS /ZnS QDs and are localized in the outer shell region in only trace amounts. The Cl atoms,\nx 2 2\nderived from the ZnCl precursor used in the ZnS precursor step, show a strong spatial correlation with\nXPS measurement further confirms that Cl is absent in AgIn Ga S cores and nearly absent in\nx 1-x 2\nAgIn Ga S /AgGaS QDs, whereas the Cl signal is pronounced in the AgIn Ga S /AgGaS /ZnS QDs,\nx 1-x 2 2 x 1-x 2 2\nexhibiting metal–Cl chemical states attributable to Zn–Cl-related species (Supplementary Fig. 20 and\nSupplementary Table 2).”\nPlease see Supplementary Fig. 15,\nSupplementary Fig. 20 XPS Cl 2p core-level spectra of AgIn Ga S -based QDs. XPS spectra of Cl\nx 1-x 2\n2p core-level acquired from AgIn Ga S (AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga\nx 1-x 2 x 1-x 2 2 x 1-\nS /AgGaS /ZnS (AIGS/AGS/ZnS) QDs. Cl-related signals are observed only in the AIGS/AGS/ZnS\nx 2 2\nQDs, indicating the presence of metal–Cl chemical states associated with the Zn-based shell introduced\nduring the ZnS precursor step.\nPlease see Supplementary Table 2,\nAtomic composition (%) (XPS)\nSpecimen\nCl Zn Ga Ag In\nC O F S\n(in Cl (in Zn (in Ga (in Ag (in In\n(in C 1s) (in O 1s) (in F 1s) (in S 2s)\n2p) 2p ) 2p ) 3d) 3d )\n3/2 3/2 5/2\nAIGS 76.29 1.91 0 11.54 0 0 1.1 4.46 4.7\nAIGS/AGS 69.37 2.83 0 14.69 0.23 0 5.16 5.9 1.83\nAIGS/AGS/ZnS 62.92 6.58 0 13.93 2.07 6.07 4.54 2.85 1.06\nAtomic composition (%) (XPS)\nSpecimen\nS Ga Ag In\n(in S 2s) (in Ga 2p ) (in Ag 3d) (in In 3d )\n3/2 5/2\nAIGS 52.92 5.07 20.44 21.57\nAIGS/AGS 53.25 18.71 21.41 6.63\nAIGS/AGS/ZnS 62.28 20.28 12.73 4.72\nAtomic composition (%) (XPS)\nSpecimen\nAg Zn In Ga\n(in Ag 4d) (in Zn 3d) (in In 4d) (in Ga 3d)\nAIGS 36.1 0 37.7 26.2\nAIGS/AGS 37.1 0 10.0 52.9\nAIGS/AGS/ZnS 21.3 30.5 5.1 43.1\nSupplementary Table 2. Atomic compositions of AgIn Ga S /AgGaS (AIGS/AGS) and\nx 1-x 2 2\nAgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs measured by XPS. The Ga-to-Ag ratio is\nx 1-x 2 2\nconsistently higher in the AIGS/AGS/ZnS QDs than in the AIGS/AGS QDs, suggesting the formation\nof a Ga-containing outer shell. This trend is observed both when comparing Ag MNN and Ga 2p and\nwhen comparing Ag 4d and Ga 3d, all of which lie in similar binding energy ranges.\nComment 4. The general problem of all microscopic studies are the statistical relevance. Some\ncomments to this issue would be very helfpul for the readership of this manuscript.\nAnswer to Comment 4: We thank the reviewer for raising the important issue of statistical relevance,\nwhich is a general limitation of all microscopy-based techniques. As correctly pointed out, microscopic\nanalyses inherently probe a limited number of particles compared to bulk characterization techniques\nthat are more representative of the overall sample, and this limitation has now been explicitly\nacknowledged in the revised manuscript.\nThe primary objective of this work is to directly visualize and elucidate key physical and chemical\nphenomena in AgIn Ga S -based quantum dots (QDs), namely compositional gradients, precursor\nx 1-x 2\nreactivity, and cation exchange-driven shell formation, using APT. A central contribution of this study\nis to demonstrate that such nanoscale processes can be directly accessed and analyzed using APT,\nthereby providing methodological guidance for future studies of complex QD systems.\nTo strengthen the statistical relevance beyond the limited number of QDs analyzed by APT and TEM,\nwe performed complementary characterizations using X-ray photoelectron spectroscopy (XPS). XPS is\na surface-sensitive technique that probes elemental composition and chemical bonding states within the\ntop few nanometers of a sample, and therefore provides averaged information from a large number of\nQDs rather than 3D distributions within individual particles. XPS spectra acquired from different core-\nlevel regions confirm that the QDs consist of Ag–S, In–S, Ga–S, and Zn–S chemical bonds as designed,\nwithout detectable oxidation. Although XPS cannot resolve 3D spatial distributions, its surface\nsensitivity makes it suitable for inferring compositional changes near the outer region of the QDs. To\nassess the compositional trends, we compared the relative Ag and Ga signals before and after the ZnS\nshelling process using multiple core-level regions, including Ag MNN vs Ga 2p and Ag 4d vs Ga 3d,\neach of which occupies an equivalent binding energy range. In all cases, the Ga-to-Ag ratios were\nconsistently higher in the AgIn Ga S /AgGaS /ZnS QDs than in the AgIn Ga S /AgGaS QDs.\nx 1-x 2 2 x 1-x 2 2\nThis systematic increase allows us to infer that substantial Ga-related components are present in the\noutermost shell region after the ZnS precursor step. This XPS result is fully consistent with the APT\nobservations that reveal Ga-containing outer shells, and suggests that the compositional features\nidentified by APT are not limited to a small number of individual QDs but are representative of the\noverall sample.\nIn addition, the overall elemental compositions obtained from APT are consistent with inductively\ncoupled plasma optical emission spectrometry measurements, supporting the representativeness of the\nanalyzed volumes. Finally, X-ray diffraction (XRD) measurements confirm that the crystal structure\nremains similar before and after shell formation, with core-only AgIn Ga S , AgIn Ga S /AgGaS ,\nx 1-x 2 x 1-x 2 2\nand AgIn Ga S /AgGaS /ZnS QDs all exhibiting a comparable chalcopyrite-based tetragonal structure.\nx 1-x 2 2\nThis structural consistency measured by XRD, also observed by transmission electron microscopy,\nfurther supports the representativeness of the microscopic observations. We now explicitly discuss the\ngeneral issue of statistical representativeness inherent to microscopy-based analyses in the revised\nmanuscript. In addition, we strengthen the interpretation by corroborating the APT results with multiple\ncomplementary characterization techniques.\nThe manuscript has been revised as follows:\nPlease see Page # 13,\n“Because APT inherently probes only a limited number of individual QDs and may therefore suffer\nfrom limited statistical representativeness, the compositional features identified by APT were further\nexamined using complementary techniques. XPS analyses performed on the same samples reveal\nconsistently higher Ga-to-Ag ratios after the Zn precursor step, evaluated from multiple core-level\nregions, including Ag MNN vs Ga 2p and Ag 4d vs Ga 3d. This systematic increase in the Ga-to-Ag\nratios supports the formation of Ga-containing outer shells and is consistent with the Zn Ga S shell\n1-3/2x x\nformation revealed by APT (Supplementary Fig. 15 and Supplementary Table 2). Inductively coupled\nplasma analyses performed on the same samples show overall elemental compositions consistent with\nthe APT results (Supplementary Tables 3 and 4). Taken together, these results indicate that the structural\nand compositional features identified by APT are not limited to a small number of individual QDs but\nare representative of overall sample.”\nPlease see Supplementary Fig. 15,\nSupplementary Fig. 15 XPS analysis results of AgIn Ga S -based QDs. XPS spectra of a Ga 2p, b\nx 1-x 2\nZn 2p , c In 3d, d Ag 3d, and e S 2s core levels acquired from AgIn Ga S (AIGS), AgIn Ga\n2/3 x 1-x 2 x 1-\nS /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs using an Al K-alpha\nx 2 2 x 1-x 2 2\nX-ray source. All QDs contain Ag–S, In–S, Ga–S, and Zn–S chemical bonds, as designed without\ndetectable oxidation. f Valence band spectra of AIGS, AIGS/AGS, and AIGS/AGS/ZnS QDs. The Ga-\nto-Ag ratio is consistently higher in the AIGS/AGS/ZnS QDs than in the AIGS/AGS QDs. The detailed\ncompositions are provided in Supplementary Table 2. These results further support the presence of a\nGa-containing outer shell, consistent with the Zn Ga S shell identified by APT after the ZnS\n1-3/2x x\nprecursor step.\nPlease see Supplementary Table 3,\nAtomic ratio (ICP–OES)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.40 0.12 0.44 0\nAIGS/AGS/ZnS 0.13 0.08 0.35 0.46\nSupplementary Table 3. Atomic ratios measured of AgIn Ga S /AgGaS (AIGS/AGS) and\nx 1-x 2 2\nAgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) determined by ICP–OES.\nx 1-x 2 2\nPlease see Supplementary Table 4,\nAtomic ratio (APT)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.43 0.13 0.39 0\nAIGS/AGS/ZnS 0.15 0.08 0.30 0.48\nSupplementary Table 4. Atomic ratios of AgIn Ga S /AgGaS (AIGS/AGS) and AgIn Ga\nx 1-x 2 2 x 1-\nS /AgGaS /ZnS (AIGS/AGS/ZnS) determined by APT.\nx 2 2\nComment 5. It would be helpful, to mark the single particles in Fig. 2, 4 and 6, then the reader can see\nthe borders of the particles.\nAnswer to Comment 5: We thank the reviewer for this helpful suggestion. We agree that marking\nindividual particles can assist readers in identifying the boundaries of the quantum dots (QDs) in the\n3D reconstructions.\nHowever, in the present APT datasets, the QDs are not aligned along a single common axis but are\narranged in a non-collinear, close-packed configuration within the specimen. As a result, depending on\nthe viewing direction, some particles partially overlap in projection views, making it nontrivial to\nunambiguously define particle boundaries within the main reconstruction figures.\nTo address the reviewer’s suggestion while taking this limitation into account, we have prepared\nadditional images in which the approximate outlines of individual QDs are indicated with dashed guides\nbased on the 3D ion maps. Although the particles are not perfectly spherical, simplified spherical guides\nfor visual reference were used to provide a clear and intuitive visualization of particle separation. To\navoid introducing visual bias, such as a misleading visual impression or encouraging over-interpretation\nof the reconstructions in the main figures, these annotated images have been added to the Supplementary\nInformation. The revised manuscript now explicitly refers to them.\nThe manuscript has been revised as follows:\nPlease see Page #8,\n“Figure 2a shows the 3D reconstructed APT atom maps of the AgIn Ga S /AgGaS QDs, representing\nx 1-x 2 2\na hemispherical APT specimen containing five QDs (see Supplementary Fig. 6 for dashed guides\nindicating the approximate outlines of individual QDs). The QDs appear as slightly ellipsoidal features\nin the APT reconstruction, and a slice-view image along the x–z plane is presented in Fig. 2b to visualize\ntheir internal elemental distribution.”\nPlease see Page #11,\n“Following the ZnS precursor step to form the outermost shell, APT was conducted to investigate the\ncompositional evolution of AgIn Ga S /AgGaS /ZnS QDs. Figure 4a shows a reconstructed APT 3D\nx 1-x 2 2\natom map containing five nearly spherical AgIn Ga S /AgGaS /ZnS QDs (see Supplementary Fig. 9\nx 1-x 2 2\nfor dashed guides indicating the approximate outlines of individual QDs). To further clarify the presence\nof these five QDs within the reconstructed volume and their internal structures, slice-view images along\nthe x–z planes are provided in Fig. 4b.”\nPlease see Supplementary Fig. 6,\nSupplementary Fig. 6 APT 3D reconstructions of AgIn Ga S /AgGaS QDs with delineated\nx 1-x 2 2\nparticle boundaries. APT 3D reconstructions showing multiple AgIn Ga S /AgGaS QDs within the\nx 1-x 2 2\nanalyzed volume. Dashed ovals mark the boundaries between individual QDs. In the APT specimen,\nthe QDs are arranged in a close-packed, non-collinear configuration, which can lead to partial overlap\nin the projection; the similar elemental compositions of neighboring QDs further reduce the visual\ncontrast between adjacent particles. (Scale bars: 2 nm.)\nPlease see Supplementary Fig. 9,\nSupplementary Fig. 9 APT 3D reconstructions of AgIn Ga S /AgGaS /ZnS QDs with delineated\nx 1-x 2 2\nparticle boundaries. APT 3D reconstructions showing multiple AgIn Ga S /AgGaS /ZnS QDs within\nx 1-x 2 2\nthe analyzed volume. Dashed guides mark the boundaries between individual QDs. In the APT\nspecimen, the QDs are arranged in a close-packed, non-collinear configuration, leading to partial\noverlap in the projection. (Scale bars: 2 nm.)\nReviewer #4 (Remarks to the Author):\nGeneral Comment: The paper highlights an additional pathway to uncover the complex internal\nstructure of quaternary AgInGaS core-shelled quantum dots using atom probe tomography (APT). It is\na well-written paper, providing elemental compositional profiles across nanocrystals to obtain detailed\ninformation about he compositional variations of core-shell and core-shell-shell nanocrystals.\nFurthermore, insights are provided about relative precursor reactivity and potential substitution\nreactions occurring, as to why and how some of these layers were formed. I recommend publication in\nnature communications, with minor revisions.\nAnswer to General Comment: We sincerely thank the reviewer for the positive assessment of our\nmanuscript and for recommending its publication in Nature Communications. We particularly\nappreciate the reviewer’s recognition of the value of atom probe tomography in revealing detailed\ncompositional profiles and internal structural variations in AgIn Ga S -based quaternary core–shell\nx 1-x 2\nand core–shell–shell quantum dots, as well as the insights into precursor reactivity and possible\nsubstitution reactions during synthesis. We agree with the reviewer’s comments aimed at further\nstrengthening the manuscript through additional clarification and literature support. In response, we\nhave revised the manuscript accordingly, as detailed in the point-by-point responses below.\nComment 1. The authors make broad claims about how the compositional gradient would improve\nbasically every QD property (quantum yield, stability, longevity, etc). The authors should provide some\nliterature references that have shown this or indicate that this should be the case.\nAnswer to Comment 1: We thank the reviewer for pointing out that our original statements regarding\nthe impact of compositional gradients may appear overly broad.\nWe acknowledge that our original wording may not have sufficiently conveyed the intended level of\ncaution regarding the impact of compositional gradients on quantum dot (QD) properties. Accordingly,\nthe manuscript has been revised to adopt a more cautious and clearly articulated tone, emphasizing that\na compositional gradient is expected to be beneficial based on prior studies, rather than asserting\nguaranteed improvements. More specifically, numerous previous studies have reported that introducing\ncompositionally graded shells or intermediate (mid-)shell layers can lead to enhanced\nphotoluminescence quantum yield (QY) and improved photostability in colloidal QDs. These\nimprovements are commonly attributed to reduced lattice mismatch at core/shell interfaces and\nsuppressed defect formation. For example, compositionally graded or alloyed shell structures have been\ndemonstrated to improve optical performance and stability in InP-based and related QD systems.\nBased on established literature precedents, it is reasonable to expect that the compositionally graded\nshells observed in our QDs could contribute to improved QY and stability. In response to the reviewer’s\ncomment, the revised manuscript has been updated to reflect this more conservative interpretation and\nto explicitly cite relevant literature supporting the potential benefits of compositional gradients, without\noverstating their general applicability or certainty.\nThe manuscript has been revised as follows:\nPlease see Page #13,\n“Interestingly, the compositional distributions of Ag, In, and Ga are gradual rather than discrete across\nthe layers. From a structural perspective, this compositional gradient is expected to offer advantages\nsuch as minimizing lattice mismatch between the core and inner shell and effectively reducing the\nlikelihood of internal defect formation. Such structural features—often referred to as compositional-\ngradient shells or gradient alloy shells—have been widely reported in InP- and CdSe-based QDs to\ncontribute to enhancing both the QY and stability.54-58”\nReferences have been added as follows:\n54. Lee, S. H. et al. The effects of discrete and gradient mid-shell structures on the\nphotoluminescence of single InP quantum dots. Nanoscale 11, 23251-23258 (2019).\n10.1039/C9NR06847C.\n55. Lim, J. et al. InP@ ZnSeS, core@ composition gradient shell quantum dots with enhanced\nstability. Chem. Mater. 23, 4459-4463 (2011). 10.1021/cm201550w.\n56. Bae, W. K., Char, K., Hur, H. & Lee, S. Single-step synthesis of quantum dots with chemical\ncomposition gradients. Chem. Mater. 20, 531-539 (2008). 10.1021/cm070754d.\n57. Duan, X. et al. InP quantum dots with a strain-engineered gradient shell for enhanced optical\nperformance and stability. Nano Lett. 25, 13539-13548 (2025). 10.1021/acs.nanolett.5c03042.\n58. Yu, P. et al. Highly efficient green InP-based quantum dot light-emitting diodes regulated by\ninner alloyed shell component. Light Sci. Appl. 11, 162 (2022). 10.1038/s41377-022-00855-z.\nComment 2. A prominent tail is seen in the PL spectra after the addition of a ZnS shell (or alloyed\nZnGaS layer as the authors later conclude). Could the others discuss the origin of this tail? Is it related\nto the compositional gradients observed in the QDs.\nAnswer to Comment 2: We thank the reviewer for pointing out the pronounced long-wavelength\n(tail) emission observed after the addition of the ZnS shell (or alloyed Zn Ga S layer).\n1-3/2x x\nLong-wavelength emission following ZnS shell coating has been widely reported in AgInS - and related\n2\npathways rather than band-edge excitonic transitions. Multiple mechanisms may contribute to such\nemission, including unpassivated intrinsic defects, newly formed interfacial trap states, and defects\nintroduced during cation exchange. To date, however, the precise origin of this tail emission has not\nbeen conclusively established in the literature.\nIn the present system, we attribute the emergence of the long-wavelength emission primarily to defect\nformation during the ZnS precursor step and the associated cation exchange process. Our transmission\nelectron microscopy and X-ray diffraction analyses indicate that the shell retains a chalcopyrite-related\ntetragonal structure rather than forming an incoherent zinc-blende-type ZnS phase, suggesting that Zn\n1-\nGa S is formed via cation exchange through substitution of Ag+ and Ga3+ by Zn2+.\n3/2x x\nThis intrinsically non-charge-neutral process can generate a substantial density of cation vacancies,\nwhich act as intragap trap states in I–III–VI semiconductors. Such defect-related intragap states are\n2\ncommonly associated with donor–acceptor-pair–like recombination responsible for long-wavelength\nemission in I–III–VI\nConsistent with our APT observations, the inward migration of Zn2+ species into the particle interior\ncan further promote the formation of cation vacancies and associated defect complexes. When these\ndefects are introduced in the vicinity of In-rich domains, where charge carriers are preferentially\nlocalized, they can give rise to intragap trap states that locally perturb carrier confinement and facilitate\ntrap-assisted recombination.\nWith respect to the compositional gradients observed in the QDs, we note that these gradients are\nunlikely to be the direct origin of the tail emission. Rather, the compositional gradients arise from the\ncation exchange-driven shell formation process, which can locally modulate the chemical environment\nand defect distribution within the QDs. In this sense, the compositional gradients may indirectly\ninfluence the spatial distribution and density of defect states, particularly near the core/shell and\ninner/outer shell regions, thereby contributing to the prominence of defect-related tail emission,\nwithout being its primary cause. We also attempted to further elucidate the nature of this emission using time-\nresolved photoluminescence (TR-PL) measurements. However, the long-wavelength feature could not\nbe reliably decomposed into distinct lifetime components, indicating that it originates from a broad\ndistribution of energetically and spatially heterogeneous trap states. Accordingly, the revised\nmanuscript now explicitly discusses that the observed tail emission is consistent with defect-related\nrecombination processes associated with the ZnS precursor step.\nThe manuscript has been revised as follows:\nPlease see Page #5,\n“In addition to the dominant green emission, the AgIn Ga S /AgGaS /ZnS QDs exhibit a long-\nx 1-x 2 2\nwavelength emission tail in the 600–700 nm range (Fig. 1d). The area ratio of this long-wavelength\nemission, defined as the fraction of the total PL intensity in the long-wavelength region (λ > λ +\nPL max\n50 nm), increases from ~3.7% in AgIn Ga S /AgGaS QDs to ~14.9% in AgIn Ga S /AgGaS /ZnS\nx 1-x 2 2 x 1-x 2 2\nQDs. Such long-wavelength emission is generally attributed to defect- or trap-assisted recombination\nrather than band-edge excitonic transitions in AgInS -based QDs and related I–III–VI -based QDs.36-40”\n2 2\nPlease see Page #6,\n“X-ray diffraction (XRD) measurements were further performed on the core-only AgIn Ga S ,\nx 1-x 2\nAgIn Ga S /AgGaS , and AgIn Ga S /AgGaS /ZnS QDs to corroborate the structural similarity\nx 1-x 2 2 x 1-x 2 2\nbefore and after shell formation. All XRD patterns are consistent with a chalcopyrite-based tetragonal\nstructure, and the overall diffraction features remain essentially unchanged, even after the Zn precursor\nstep (Supplementary Fig. 4). This comparative analysis indicates that the crystal structure is preserved\nthroughout the shell formation process.”\nPlease see Page #15,\n“The emergence of the Zn Ga S outer shell via cation exchange offers a plausible origin for the long-\n1-3/2x x\nwavelength emission tail observed after the ZnS precursor step. In our system, the substitution of Ag+\nand Ga3+ by Zn2+ during shell formation inherently introduces cation vacancies and local charge-\ncompensating defects. Consistent with our APT observations, the inward diffusion of Zn2+ into the\ninterior of the QDs further induces cation exchange and promotes the formation of such defects. When\nthese defects form in the vicinity of In-rich regions, where carriers are preferentially confined, they can\nintroduce intragap trap states that locally perturb the confinement potential and promote trap-assisted\nrecombination. These defect-related states are commonly associated with donor–acceptor-pair-like\nrecombination pathways that give rise to long-wavelength emission in I–III–VI semiconductors.23,38-\n2\n40,66-68”\nPlease see Fig. 1,\n“Fig. 1 Comparison of AgIn Ga S /AgGaS QDs and AgIn Ga S /AgGaS /ZnS QDs. a, d\nx 1-x 2 2 x 1-x 2 2\nSchematic representation of the internal structure, band diagrams, and UV–vis absorption, and PL\ncharacteristics. Both QDs exhibit bright green luminescence at ~530 nm, with the QY increasing from\n85% to 92% after the ZnS precursor step to form the outer shell. In addition, AgIn Ga S /AgGaS /ZnS\nx 1-x 2 2\nQDs exhibit a long-wavelength emission tail in the ~600–700 nm range.”\nPlease see Supplementary Fig. 4,\nSupplementary Fig. 4 XRD patterns of AgIn Ga S -based QDs. XRD patterns of AgIn Ga S\nx 1-x 2 x 1-x 2\n(AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs.\nx 1-x 2 2 x 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, with only minor peak\nshifts attributable to slight lattice parameter variations.\nReferences have been added as follows:\n23. Kim, J. H. et al. Synthesis of widely emission-tunable Ag–Ga–S and its quaternary derivative\nquantum dots. Chem. Eng. J. 347, 791-797 (2018). 10.1016/j.cej.2018.04.167.\n36. Dai, M. et al. Tunable photoluminescence from the visible to near-infrared wavelength region\nof non-stoichiometric AgInS nanoparticles. J. Mater. Chem. 22, 12851-12858 (2012).\n2\n10.1039/C2JM31463K.\n37. Chen, Y. et al. Green and facile synthesis of high-quality water-soluble Ag-In-S/ZnS core/shell\nquantum dots with obvious bandgap and sub-bandgap excitations. J. Alloys Compd. 753, 364-370\n(2018). 10.1016/j.jallcom.2018.04.242.\n38. Park, S. M. et al. Red Ag-based I–III–VI quantum dots as competitive alternatives to InP\nemitters. ACS Energy Lett. 10, 3005-3013 (2025). 10.1021/acsenergylett.5c00962.\n39. Farid, A. et al. One-pot synthesis of luminescent Ag−Cu−Ga−S/ZnS quantum dots bridging\nthe cyan gap for ultrahigh-color-rendering white-light-emitting diodes. ACS Appl. Nano Mater. 8,\n14703-14712 (2025). 10.1021/acsanm.5c02386.\n40. Park, S. et al. Suppressing tail emission from AgIn Ga S /AgGaS quantum dots by GaI –\n1−x x 2 2 3\nassisted interface reinforcement. ACS Nano 19, 26831-26842 (2025). 10.1021/acsnano.5c07418.\n66. Shen, F. et al. Photophysics and photovoltaic properties of Zn-alloyed Ag-In-S quantum dots\nsensitized solar cells. J. Alloys Compd. 922, 166296 (2022). 10.1016/j.jallcom.2022.166296.\n67. Rivaux, C. et al. Continuous flow aqueous synthesis of highly luminescent AgInS and\n2\n68. Kameyama, T. et al. Controlling the electronic energy structure of ZnS–AgInS solid solution\n2\nnanocrystals for photoluminescence and photocatalytic hydrogen evolution. J. Phys. Chem. C 119,\n24740-24749 (2015). 10.1021/acs.jpcc.5b07994.\nComment 3. In Figures 2c and 4c: is the iso-surface proxigram shown just for one quantum dot? How\ndoes it different between the 5 QDs identified?\nAnswer to Comment 3: The iso-surface proxigrams shown in Figures 2c and 4c were extracted by\naveraging the data from all five quantum dots (QDs) identified in each APT dataset, rather than from a\nsingle QD. As shown in the individual 2D concentration profiles in Figures 3c and 3f, as well as Figures\n5c and 5f, the absolute compositions vary slightly from one QD to another; however, the overall\ncompositional trends and distribution profiles are highly consistent across the five QDs. Proxigrams\nextracted from a single QD can be relatively noisy due to limited ion counts, whereas averaging over\nmultiple QDs improves statistical robustness and more clearly reveals the general reproducible\ncompositional trends.\nBecause proxigram analysis is particularly useful for capturing and visualizing the compositional\ntendencies in nanoscale heterostructures, we included the proxigrams in Figures 2c and 4c to represent\nthe common behavior among the analyzed QDs. This approach allows the key compositional features\nto be conveyed clearly while minimizing dot-to-dot variability. We have now explicitly clarified in the\nfigure captions that the proxigrams represent averaged profiles obtained from five QDs.\nThe manuscript has been revised as follows:\nPlease see Fig. 2,\nc Ag iso-surface map and proxigram showing 3D compositional trends; the proxigram was obtained\nfrom five QDs. The Ag and In contents increase toward the QD core, while the Ga content increases\ntoward its outer surface, confirming the core/shell heterostructure.\nPlease see Fig. 4,\nc Ag and Zn iso-surface and proxigram obtained from Ag iso-surfaces; the proxigram was obtained\nfrom five QDs. The Ag and In contents increase toward the QD core, while the content of Zn increases\ntoward the outer shell of the QD. A significant amount of Ga also appears in the outer shell.\nComment 4. What happens if quantum dots aggregate? Is it possible to differentiate using the APT\ntechniqe aggregated regions of QDs from individual qds that have clearly defined domains?\nAnswer to Comment 4: We thank the reviewer for raising this important question regarding quantum\ndot (QD) aggregation and the capability of APT to distinguish aggregated regions from individual QDs\nwith well-defined domains.\nIn general, APT analysis of colloidal QDs is technically challenging, primarily because QDs are not\nembedded in a rigid matrix, which complicates specimen preparation for APT. To address this issue,\nwe prepared APT specimens from QDs in a dried powder form. In this state, the organic ligands\nsurrounding the QDs contract during drying, resulting in QDs being positioned in close proximity to\none another. However, this proximity does not indicate irreversible aggregation, as the dried QDs\nreadily disperse uniformly when re-dissolved in solvent.\nAs shown by the APT reconstructions, the QDs do not appear as a single aggregated mass within the\nreconstructed volume but remain distinguishable as individual particles with separable volumes. This\nallows us to perform volume extraction and analyze each QD individually. To further support this\ninterpretation, we provide complementary transmission electron microscopy images of the QDs\nprepared in the same dried powder form, which show that individual QDs remain spatially\ndistinguishable despite their close packing.\nIf the QDs were present as a single, fully merged structure, the APT data would not be expected to\nreveal distinct core/shell/shell architectures for individual particles, nor would ligand-derived C and H\nsignals be observed separating adjacent QDs, as seen in our datasets. In that case, a single continuous\ncompositional profile would be expected, rather than the discrete, particle-resolved structures observed\nhere. Therefore, while QDs are closely packed in the dried powder state, APT enables us to differentiate\nindividual QDs from aggregated regions within the present datasets by resolving distinct compositional\ndomains and interparticle boundaries.\nThe manuscript has been revised as follows:\nPlease see Page #7,\n“For the APT analysis in this work, dried QDs were prepared in powder form and analyzed following\nour previously described method.41 In that study, we demonstrated that APT could faithfully resolve\ncore/shell architectures and 3D compositional distributions in commercially available CdSe/ZnS\ncore/shell QDs, in good agreement with TEM observations.”\nPlease see Page #21,\n“APT specimen preparation\nNeedle-shaped specimens for APT analysis were prepared from dried QDs in powder form using the\nfocused ion beam (FIB) lift-out method (Helios5 HX, Thermo Fisher Scientific). Initially, a 100 nm-\nthick Pt layer was deposited using an electron beam, followed by depositing a 1 μm-thick Pt layer with\na Ga-ion beam to passivate a region of interest measuring 12 μm × 1.7 μm on the sample surface. The\ntarget region was then extracted and mounted onto a sharpened W tip. The specimens on the W tip were\nfurther shaped into a needle-like geometry using an annular milling pattern (30 kV, 80 pA). To refine\nthe samples and reduce damage induced by the Ga-ion beam, the region of interest was thinned again\nwith an annular pattern at 5 kV and 8 pA. The morphology and dispersion of the QDs prepared by this\nprocedure were confirmed by HAADF–STEM (Supplementary Fig. 21).”\nPlease see Supplementary Fig. 21,\nSupplementary Fig. 21 Morphology and dispersion of AgIn Ga S /AgGaS /ZnS QDs in dried\nx 1-x 2 2\npowder form. HAADF–STEM images of AgIn Ga S /AgGaS /ZnS QDs in dried powder form,\nx 1-x 2 2\nprepared by FIB, showing the representative particle morphology and overall dispersion. The QDs\nexhibit sizes and shapes comparable to those observed in the solution-dispersed QDs. (Scale bars: 10\nnm.)\nReferences have been added as follows:\n41. Chae, B. G. et al. Direct three-dimensional observation of core/shell-structured quantum dots\nwith a composition-competitive gradient. ACS Nano 12, 12109-12117 (2018).\n10.1021/acsnano.8b05379.\nComment 5. For the core-shell-shell structure (with ZnS), is the first shell still Ag-deficient like with\nthe core-shell structure (as mentioned in line 76)?\nAnswer to Comment 5: We thank the reviewer for the question regarding whether the first shell\nremains Ag-deficient after the ZnS precursor step.\nBased on our APT results, the first shell remains Ag-deficient even after the ZnS precursor step. As\nshown in the 2D concentration profiles in Figures 5c and 5f, the region immediately inside the Zn-based\nouter shell, identified as the Zn Ga S shell, exhibits a relatively higher Ga concentration and a\n1-3/2x x\ncorrespondingly lower Ag concentration. This compositional trend is consistent with that observed for\nthe Ag-deficient shell in the core/shell structure prior to the ZnS precursor step, as shown in Figures 3c\nand 3f. These observations indicate that the Ag-deficient nature of the first shell is preserved during the\nsubsequent Zn-based shelling process, rather than being eliminated or homogenized. Accordingly, the\nfinal core/shell/shell architecture consists of an AgIn Ga S core, an AgGaS inner shell, an Ag-\nx 1-x 2 2\ndeficient AgGaS layer, and a Zn Ga S outer shell.\n2 1-3/2x x\nComment 6. Since the nanocrystal size does not change after ZnS shelling, does this mean the core-\nsize decreases after addition of the ZnS shell? Or does the thickness of the first shell (AgGaS2 shell)\ndecrease instead? Is it possible to estimate the diameter of the core and the thickness of two shells using\nthis technique?\nAnswer to Comment 6: We thank the reviewer for the question regarding whether the unchanged\nnanocrystal size after the ZnS precursor step implies a reduction in the core size or a change in the\nthickness of the first shell, and whether the core and shell thicknesses can be estimated using APT.\nBased on our transmission electron microscopy (TEM) and APT results, the AgIn Ga S core does not\nx 1-x 2\ndecrease in size after the ZnS precursor step. Instead, a portion of the AgGaS inner shell is converted\n2\ninto the Zn Ga S outer shell through a cation exchange process, rather than through epitaxial growth\n1-3/2x x\nof a new ZnS layer. Consequently, the overall nanocrystal diameter remains nearly unchanged, while a\nZn Ga S outer shell forms at the expense of the part of the AgGaS inner shell. To further corroborate\n1-3/2x x 2\nthis interpretation, we additionally performed XRD measurements in this revision. The core-only\nAgIn Ga S , AgIn Ga S /AgGaS , and AgIn Ga S /AgGaS /ZnS quantum dots (QDs) all exhibit\nx 1-x 2 x 1-x 2 2 x 1-x 2 2\ndiffraction patterns consistent with a chalcopyrite-based tetragonal structure, and the overall diffraction\nfeatures remain essentially unchanged after the Zn precursor step, aside from minor peak shifts.\nThese XRD results further support that the overall crystallographic framework is preserved during the\nZnS precursor step.\nRegarding dimensional analysis, APT is well suited for resolving 3D compositional distributions and\nidentifying compositional gradients and interfaces within individual QDs. However, determining the\nabsolute thickness of each layer with high precision using APT alone remains challenging. This\nlimitation arises because different materials exhibit different evaporation fields, which can introduce\nlocal magnification effects and distort reconstructed dimensions. Despite these effects, numerous\nstudies have demonstrated that APT provides reliable information on compositional distributions in\nnanostructures, particularly along the analysis (z) direction. Therefore, while APT reliably captures the\nrelative compositional trends and the core/shell/shell architecture, precise and absolute measurements\nof the core diameter and individual shell thicknesses remain subject to uncertainty. In principle, TEM\ncan provide complementary size information; however, because TEM images represent projected views\nthrough the nanocrystal, accurately determining the thickness of individual shell layers remains\ninherently challenging.\nThe manuscript has been revised as follows:\nPlease see Page #8,\n“The QDs appear as slightly ellipsoidal features in the APT reconstruction, and a slice-view image along\nthe x–z plane is presented in Fig. 2b to visualize their internal elemental distribution. We note that APT\nanalyses of nanostructures may exhibit reconstruction artifacts, such as apparent elongation along the\nanalysis (z) direction and interfacial broadening, primarily arising from local magnification and\ntrajectory aberrations.42-52 Accordingly, the slightly ellipsoidal morphology observed here is attributed\nto such effects rather than the intrinsic particle shape. However, compositional distributions along the\nz-direction have been shown to provide more reliable quantitative information than lateral directions\nbecause the reduced trajectory overlap suppresses artificial intermixing.42-44,51,52 Under these conditions,\ncompositional distributions and interfacial characteristics closer to the intrinsic distributions can be\nmore faithfully evaluated.”\nReferences have been added as follows:\n42. Jang, K. et al. Three-dimensional atomic mapping of ligands on palladium nanoparticles by\natom probe tomography. Nat. Commun. 12, 4301 (2021). 10.1038/s41467-021-24620-9.\n43. Kim, S. H. et al. Characterization of Pd and Pd@ Au core-shell nanoparticles using atom probe\ntomography and field evaporation simulation. J. Alloys Compd. 831, 154721 (2020).\n10.1016/j.jallcom.2020.154721.\n44. Tedsree, K. et al. Hydrogen production from formic acid decomposition at room temperature\nusing a Ag–Pd core–shell nanocatalyst. Nat. Nanotechnol. 6, 302-307 (2011). 10.1038/nnano.2011.42.\n45. Grenier, A. et al. 3D analysis of advanced nano-devices using electron and atom probe\ntomography. Ultramicroscopy 136, 185-192 (2014). 10.1016/j.ultramic.2013.10.001.\n46. Khan, M. A., Ringer, S. P. & Zheng, R. Atom probe tomography on semiconductor\ndevices. Adv. Mater. Interfaces 3, 1500713 (2016). 10.1002/admi.201500713.\n47. Hatzoglou, C., Radiguet, B. & Pareige, P. Experimental artefacts occurring during atom probe\ntomography analysis of oxide nanoparticles in metallic matrix: quantification and correction. J. Nucl.\nMater. 492, 279-291 (2017). 10.1016/j.jnucmat.2017.05.008.\n48. Philippe, T., Gruber, M., Vurpillot, F. & Blavette, D. Clustering and local magnification effects\nin atom probe tomography: a statistical approach. Microsc. Microanal. 16, 643-648 (2010).\n10.1017/S1431927610000449.\n49. Lawitzki, R., Stender, P. & Schmitz, G. Compensating local magnifications in atom probe\ntomography for accurate analysis of nano-sized precipitates. Microsc. Microanal. 27, 1–12 (2021).\n10.1017/S1431927621000180.\n50. Beinke, D., Oberdorfer, C. & Schmitz, G. Towards an accurate volume reconstruction in atom\nprobe tomography. Ultramicroscopy 165, 34-41 (2016). 10.1016/j.ultramic.2016.03.008.\n51. Takahashi, J. & Kawakami, K. Position artifacts in 3D reconstruction of plate‐shaped\nprecipitates in steels depending on the analysis direction of atom probe tomography. Surf. Interface\nAnal. 53, 982-995 (2021). 10.1002/sia.7001.\n52. Maruyama, N., Smith, G. D. W. & Cerezo, A. Interaction of the solute niobium or molybdenum\nwith grain boundaries in α-iron. Mater. Sci. Eng. A 353, 126-132 (2003). 10.1016/S0921-\n5093(02)00678-0.\nComment 7. As mentioned in line 235 to 245, why was the compositional inhomogeneity more\npronounced in the core versus the shell for the samples presented? Also, why does this inhomogeneity\nget more noticeable/severe after shelling with ZnS?\nAnswer to Comment 7: We thank the reviewer for the insightful question regarding the origin of the\nmore pronounced compositional inhomogeneity observed in the core compared to the shell, and its\napparent enhancement after the ZnS precursor step.\nThe relatively stronger compositional inhomogeneity observed in the core is primarily attributed to the\nintrinsic complexity of quaternary Ag–In–Ga–S core formation. During core synthesis, multiple\nprecursors with different chemical reactivities are simultaneously present, and the formation of the core\ndoes not follow a simple, single-step nucleation-and-growth process. In particular, the AgIn Ga S\nx 1-x 2\ncore is synthesized by heating a mixed precursor solution from low temperature, a process during which\nlocal clustering and compositionally heterogeneous regions are likely to form due to differences in\nprecursor reactivity and diffusion kinetics. Previous studies have shown that AgIn Ga S core\nx 1-x 2\nformation does not proceed via a simple nucleation-and-growth mechanism, but rather involves\nintermediate phases and cation exchange processes. For example, several reports indicate that AgGaS\n2\nor AgInS seeds form initially, followed by the incorporation of In and Ga through cation exchange to\n2\ngenerate the chalcopyrite AgIn Ga S lattice. This multistep and dynamically perturbed growth\nx 1-x 2\npathway is therefore likely to increase the likelihood of local compositional fluctuations within the core.\nSuch effects are intrinsically more pronounced in quaternary systems than in binary or ternary quantum\ndots (QDs), explaining why compositional inhomogeneity is stronger in the core than in the\nsubsequently formed shells. In addition, the ZnS precursor step involves an additional thermal treatment\nof the pre-formed cores, which can further promote cation diffusion and redistribution within the QDs,\nthereby amplifying pre-existing compositional fluctuations to some extent.\nAnother contributing factor to the observed compositional inhomogeneity, particularly when comparing\nmultiple QDs, is inherent particle-to-particle variability. During quaternary QDs formation, individual\nQDs tend to exhibit a broader distribution of compositions compared to simpler systems. As supporting\nevidence, we present complementary STEM–EDS compositional profiles. Although STEM–EDS does\nnot provide 3D information in this experiment, it enables a rough assessment of compositional\ndistributions across a larger number of QDs. We analyzed the elemental distributions of six individual\nQDs from both the AgIn Ga S /AgGaS and AgIn Ga S /AgGaS /ZnS samples. While the overall\nx 1-x 2 2 x 1-x 2 2\ncompositional tendencies, including the core/shell and core/inner-shell/outer-shell architectures, are\nconsistent, the relative compositions of the constituent elements vary from particle to particle. These\nresults confirm clear particle-to-particle compositional variations, which are consistent with the trends\nobserved by APT. These points have been addressed and discussed in the revised manuscript and the\nSupplementary Information.\nThe manuscript has been revised as follows:\nPlease see Page #14,\n“Interestingly, directly comparing two individual QDs reveals compositional differences, even among\nco-synthesized QDs. These differences are particularly pronounced in the core region, with noticeable\nvariations in the Ag, In, and Ga contents; some QDs exhibit cores with a very low Ga content (Figs. 4d\nand 5c). Similar results were observed in the AgIn Ga S /AgGaS QDs, albeit to a lesser extent. The\nx 1-x 2 2\npronounced inhomogeneity in the cores likely originates from the intrinsic complexity of the quaternary\nAg–In–Ga–S system. The synthesis of the core simultaneously involves multiple cation precursors with\ndifferent reactivities, and the cores are formed under a continuous heating process. Previous studies\nhave also shown that AgIn Ga S core formation proceeds through intermediate phases and cation-\nx 1-x 2\nexchange processes rather than a simple single-step growth process.18,53,59 More specifically, AgGaS\n2\nor AgInS seeds form initially, followed by the incorporation of In and Ga to generate the chalcopyrite\nx 1-x 2\nFurthermore, the subsequent ZnS precursor step introduces an additional thermal treatment, which can\npromote cation diffusion and redistribution, thereby amplifying pre-existing compositional fluctuations\nwithin the cores.”\nPlease see Page #14,\n“Consistent with the APT observations, STEM–EDS line profiles collected from multiple QDs reveal\nnoticeable particle-to-particle variations in the relative Ag, In, and Ga distributions (Supplementary Figs.\n16 and 17). For each QD, several independent line profiles were extracted and compared, all of which\nshow dot-to-dot compositional fluctuations. These results indicate that the observed compositional\ninhomogeneity is an intrinsic feature of the synthesized quaternary QDs, thereby supporting the features\nrevealed by APT. A deviation from the ideal stoichiometry in the core can change the emission\nwavelengths. Thus, such compositional inhomogeneity in the quaternary semiconductor cores may\ncontribute to the inhomogeneous spectral broadening of the UV–Vis absorption and PL bands for both\nQDs, along with other factors such as the size distribution. Our APT analysis directly shows\ncompositional inhomogeneity among the cores and highlights the importance of controlling such\nvariations, along with defects, to improve the color purity of the green emission.”\nPlease see Supplementary Fig. 16,\nSupplementary Fig. 16 Detailed elemental distributions of AgIn Ga S /AgGaS QDs measured\nx 1-x 2 2\nby STEM–EDS. STEM image and corresponding EDS line profiles of individual AgIn Ga S /AgGaS\nx 1-x 2 2\nQDs. Although the QDs show similar compositional trends, the elemental compositions vary from\nparticle to particle. Owing to the projected nature and limited spatial resolution of STEM–EDS, these\ndata provide qualitative compositional trends rather than a definitive 3D core/shell/shell assignment.\n(Scale bars: 7 nm.)\nPlease see Supplementary Fig. 17,\nSupplementary Fig. 17 Detailed elemental distributions of AgIn Ga S /AgGaS /ZnS QDs\nx 1-x 2 2\nmeasured by STEM–EDS. STEM image and corresponding EDS line profiles of individual AgIn Ga\nx 1-\nS /AgGaS /ZnS QDs. Zn is distributed over a broader region than Ag and In, indicating the formation\nx 2 2\nof a Zn-containing outer region. Although the QDs show similar overall compositional tendencies, the\nelemental compositions vary among individual QDs, indicating particle-to-particle variability. Owing\nto the projected nature and limited spatial resolution of STEM–EDS, these data provide qualitative\ncompositional trends rather than a definitive 3D core/shell/shell assignment. (Scale bars: 7 nm.)\nReferences have been added as follows:\n18. Lee, H. J. et al. Coherent heteroepitaxial growth of I-III-VI Ag(In,Ga)S colloidal\n2 2\nnanocrystals with near-unity quantum yield for use in luminescent solar concentrators. Nat. Commun.\n14, 3779 (2023). 10.1038/s41467-023-39509-y.\n53. Uematsu, T., Tepakidareekul, M., Hirano, T., Torimoto, T. & Kuwabata, S. Facile high-yield\nsynthesis of Ag–In–Ga–S quaternary quantum dots and coating with gallium sulfide shells for narrow\nband-edge emission. Chem. Mater. 35, 1094-1106 (2023). 10.1021/acs.chemmater.2c03023.\n59. Xie, X. et al. Narrow-bandwidth I–III–VI semiconductor nanocrystals: synthesis,\nluminescence and applications in quantum-dot light-emitting diodes. Adv. Phys. Res. 3, 2400071 (2024).\n10.1002/apxr.202400071.\nComment 8. X-ray diffraction pattern not included. Can the XRD of the QDs before and after ZnS\nshelling be included as supporting information? This would further corroborate the high-resolution\nSTEM imaging showing that the particles are structurally similar before and after coating with ZnS\n(lines 114 to 119).\nAnswer to Comment 8: We thank the reviewer for this valuable suggestion. To further corroborate\nthe structural similarity of the quantum dots (QDs) before and after ZnS shelling, we have performed\nX-ray diffraction (XRD) analysis on the core-only AgIn Ga S samples, the AgIn Ga S /AgGaS\nx 1-x 2 x 1-x 2 2\nsamples, and the AgIn Ga S /AgGaS /ZnS samples.\nx 1-x 2 2\nAll samples exhibit diffraction patterns consistent with a chalcopyrite-based tetragonal structure.\nImportantly, the overall diffraction features remain largely unchanged after the ZnS precursor step,\nexcept for minor peak shifts attributable to slight lattice parameter variations associated with shell\nformation. Although the diffraction peaks are broadened due to the nanocrystalline nature of the QDs,\nthe comparative analysis indicates that the overall crystal structure is preserved throughout the shell\nformation process. The corresponding XRD patterns have now been included in the Supplementary\nInformation, and the revised manuscript has been updated to reflect this additional structural\ncharacterization in the Supplementary Information.\nThe manuscript has been revised as follows:\nPlease see Page #6,\n“X-ray diffraction (XRD) measurements were further performed on the core-only AgIn Ga S ,\nx 1-x 2\nAgIn Ga S /AgGaS , and AgIn Ga S /AgGaS /ZnS QDs to corroborate the structural similarity\nx 1-x 2 2 x 1-x 2 2\nbefore and after shell formation. All XRD patterns are consistent with a chalcopyrite-based tetragonal\nstructure, and the overall diffraction features remain essentially unchanged, even after the Zn precursor\nstep (Supplementary Fig. 4). This comparative analysis indicates that the crystal structure is preserved\nthroughout the shell formation process.”\nPlease see Supplementary Fig. 4,\nSupplementary Fig. 4 XRD patterns of AgIn Ga S -based QDs. XRD patterns of AgIn Ga S\nx 1-x 2 x 1-x 2\n(AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs.\nx 1-x 2 2 x 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, with only minor peak\nshifts attributable to slight lattice parameter variations.\nComment 9. Do the authors have elemental analysis (such as ICP-OES or ICP-MS) of the sample to\ncompare/confirm the elemental composition obtained from atomic probe tomography (ATP)?\nAnswer to Comment 9: We thank the reviewer for the suggestion to include elemental analysis to\nvalidate the compositional results obtained from APT.\nWe have performed inductively coupled plasma optical emission spectroscopy (ICP-OES)\nmeasurements on the samples. It should be noted that the ICP-OES results represent bulk-averaged\nelemental compositions rather than information from individual quantum dots. In contrast, APT enables\nthe extraction of compositional information from the reconstructed analysis volume, as well as from\nselected local regions. Despite this fundamental difference in sampling volume, we find that the overall\ncompositional trends obtained from ICP-OES are consistent with those derived from APT. To facilitate\ndirect comparison and to improve transparency, the ICP-OES results have been added to the\nSupplementary Information.\nThe manuscript has been revised as follows:\nPlease see Page #13,\n“Because APT inherently probes only a limited number of individual QDs and may therefore suffer\nfrom limited statistical representativeness, the compositional features identified by APT were further\nexamined using complementary techniques. XPS analyses performed on the same samples reveal\nconsistently higher Ga-to-Ag ratios after the Zn precursor step, evaluated from multiple core-level\nregions, including Ag MNN vs Ga 2p and Ag 4d vs Ga 3d. This systematic increase in the Ga-to-Ag\nratios supports the formation of Ga-containing outer shells and is consistent with the Zn Ga S shell\n1-3/2x x\nformation revealed by APT (Supplementary Fig. 15 and Supplementary Table 2). Inductively coupled\nplasma analyses performed on the same samples show overall elemental compositions consistent with\nthe APT results (Supplementary Tables 3 and 4). Taken together, these results indicate that the structural\nand compositional features identified by APT are not limited to a small number of individual QDs but\nare representative of overall sample.”\nPlease see Supplementary Table 3,\nAtomic ratio (ICP–OES)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.40 0.12 0.44 0\nAIGS/AGS/ZnS 0.13 0.08 0.35 0.46\nSupplementary Table 3. Atomic ratios measured of AgIn Ga S /AgGaS (AIGS/AGS) and\nx 1-x 2 2\nAgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) determined by ICP–OES.\nx 1-x 2 2\nPlease see Supplementary Table 4,\nAtomic ratio (APT)\nSpecimen\nAg/S In/S Ga/S Zn/S\nAIGS/AGS 0.43 0.13 0.39 0\nAIGS/AGS/ZnS 0.15 0.08 0.30 0.48\nSupplementary Table 4. Atomic ratios of AgIn Ga S /AgGaS (AIGS/AGS) and AgIn Ga\nx 1-x 2 2 x 1-\nS /AgGaS /ZnS (AIGS/AGS/ZnS) determined by APT.\nx 2 2\nComment 10. Method section (line 330) indicates that core-only AgInGaS were synthesized as well.\nHow does the size, morphology, crystal structure and elemental composition between the core-only and\ncore-shell (and core-shell-shell) nanocrystals differ? Was APT conducted on the core-only QDs? If so,\nare the elemental composition trends seen for the core-only, similar to the cores of the shelled QDs?\nIs inhomogeneity between different nanocrystals seen also for the core-only samples? Or is it a\nby-product of shelling?\nAnswer to Comment 10: We thank the reviewer for the detailed questions regarding the size,\nmorphology, crystal structure, and elemental composition of the core-only AgIn Ga S quantum dots\nx 1-x 2\n(QDs), and whether APT analysis was performed on these samples.\nAPT analysis of colloidal QDs is technically very challenging, and this limitation becomes particularly\nsignificant in the case of core-only QDs. Because of their small size and the absence of a rigid\nembedding matrix, specimen preparation for APT is highly nontrivial. More importantly, reliable APT\nanalysis of individual QDs requires sufficient compositional contrast to distinguish neighboring\nparticles within the reconstructed volume. In systems such as AgIn Ga S /AgGaS or AgIn Ga\nx 1-x 2 2 x 1-\nS /AgGaS /ZnS, the presence of chemically distinct shell layers provides such contrast, enabling the\nx 2 2\nidentification and extraction of individual QDs for analysis. In contrast, for core-only AgIn Ga S QDs,\nx 1-x 2\nwhich consist of a single composition without a surrounding shell or matrix, differentiating adjacent\nparticles in APT reconstructions becomes extremely difficult. This situation is similar to analyzing\nnanoparticles without a surrounding matrix in APT, where particle segmentation becomes unreliable\nand subsequent compositional interpretation can be significantly reduced. This limitation represents one\nof the major challenges in applying APT to colloidal QDs and is one of the key reasons why APT\nstudies on colloidal QDs remain scarce in the literature to date.\nNevertheless, the size, size distribution, crystal structure, and overall elemental composition of the core-\nonly AgIn Ga S QDs were thoroughly characterized using complementary techniques. X-ray\nx 1-x 2\ndiffraction (XRD) analysis clearly shows that the core-only AgIn Ga S QDs crystallize in a\nx 1-x 2\nchalcopyrite-based tetragonal structure, which is also preserved after AgGaS and ZnS shelling\n2\nprocesses. Transmission electron microscopy further confirms that the core-only QDs exhibit a round-\ncornered polyhedral or spherical shape with an average diameter of 3.58 ± 0.59 nm and clear lattice\nfringes, indicating high crystallinity consistent with the XRD results.\nWe suggest that the compositional inhomogeneity discussed in this work is not solely a by-product of\nshelling, but rather largely reflects intrinsic characteristics of quaternary core formation. This\ninterpretation is consistent with the intrinsically complex formation mechanism of quaternary AgIn Ga\nx 1-\nS cores, in which multiple precursors with different reactivities participate in a non-simple, multistep\nx 2\ngrowth process involving intermediate phases and cation exchange reactions. Such dynamically\nperturbed core formation pathways are well known to promote local compositional fluctuations in\nquaternary I–III–VI QDs. The shelling process itself may modify local compositional distributions\n2\nduring shell formation, and the observed compositional inhomogeneity may also reflect inherent\nparticle-to-particle variability, as discussed in our response to Comment #7. Accordingly, the\ncompositional features revealed by APT are interpreted as arising from a combination of intrinsic\nheterogeneity established during quaternary core formation and additional structural and compositional\nmodifications during shell synthesis. These features are therefore not interpreted as being solely induced\nby shelling, nor as artifacts of APT visualization. The additional results have been incorporated into the\nrevised manuscript and the Supporting Information.\nThe manuscript has been revised as follows:\nPlease see Page #6,\n“Interestingly, after the ZnS precursor step for the outer shell formation, the QDs grow only negligibly,\ndespite the addition of more precursors, and remain similar in both size and shape, with an average\ndiameter of 6.11 ± 1.03 nm (Supplementary Fig. 2; see also Supplementary Fig. 3 for HAADF–STEM\nimages of the core-only AgIn Ga S QDs). Moreover, the high-resolution STEM results reveal that the\nx 1-x 2\nQDs are structurally similar before and after the ZnS precursor step, with all crystal planes showing a\nlattice spacing of ~0.29 nm, which almost corresponds to the interplanar spacing of the (200) planes of\nchalcopyrite-structured AgGaS .”\n“X-ray diffraction (XRD) measurements were further performed on the core-only AgIn Ga S ,\nx 1-x 2\nAgIn Ga S /AgGaS , and AgIn Ga S /AgGaS /ZnS QDs to corroborate the structural similarity\nx 1-x 2 2 x 1-x 2 2\nbefore and after shell formation. All XRD patterns are consistent with a chalcopyrite-based tetragonal\nstructure, and the overall diffraction features remain essentially unchanged, even after the Zn precursor\nstep (Supplementary Fig. 4). This comparative analysis indicates that the crystal structure is preserved\nthroughout the shell formation process.”\nPlease see Page #14,\n“Previous studies have also shown that AgIn Ga S core formation proceeds through intermediate\nx 1-x 2\nphases and cation-exchange processes rather than a simple single-step growth process.18,53,59 More\nspecifically, AgGaS or AgInS seeds form initially, followed by the incorporation of In and Ga to\n2 2\ngenerate the chalcopyrite AgIn Ga S lattice, which is likely to promote local compositional\nx 1-x 2\nfluctuations within the core. Furthermore, the subsequent ZnS precursor step introduces an additional\nthermal treatment, which can promote cation diffusion and redistribution, thereby amplifying pre-\nexisting compositional fluctuations within the cores.”\nPlease see Supplementary Fig. 3,\nSupplementary Fig. 3 HAADF–STEM image of core-only AgIn Ga S QDs. a Low-magnification\nx 1-x 2\nand b high-magnification HAADF–STEM images of core-only AgIn Ga S QDs. The AgIn Ga S\nx 1-x 2 x 1-x 2\nQDs are crystalline and exhibit an average diameter of 3.58 ± 0.59 nm. (Scale bars: 20 nm in a and 5nm\nin b.)\nPlease see Supplementary Fig. 4,\nSupplementary Fig. 4 XRD patterns of AgIn Ga S -based QDs. XRD patterns of AgIn Ga S\nx 1-x 2 x 1-x 2\n(AIGS), AgIn Ga S /AgGaS (AIGS/AGS), and AgIn Ga S /AgGaS /ZnS (AIGS/AGS/ZnS) QDs.\nx 1-x 2 2 x 1-x 2 2\nAll XRD patterns are consistent with a chalcopyrite-based tetragonal structure, with only minor peak\nshifts attributable to slight lattice parameter variations.\nReferences have been added as follows:\n18. Lee, H. J. et al. Coherent heteroepitaxial growth of I-III-VI Ag(In,Ga)S colloidal\n2 2\nnanocrystals with near-unity quantum yield for use in luminescent solar concentrators. Nat. Commun.\n14, 3779 (2023). 10.1038/s41467-023-39509-y.\n53. Uematsu, T., Tepakidareekul, M., Hirano, T., Torimoto, T. & Kuwabata, S. Facile high-yield\nsynthesis of Ag–In–Ga–S quaternary quantum dots and coating with gallium sulfide shells for narrow\nband-edge emission. Chem. Mater. 35, 1094-1106 (2023). 10.1021/acs.chemmater.2c03023.\n59. Xie, X. et al. Narrow-bandwidth I–III–VI semiconductor nanocrystals: synthesis,\nluminescence and applications in quantum-dot light-emitting diodes. Adv. Phys. Res. 3, 2400071 (2024).\n10.1002/apxr.202400071."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The quality of the revised ms has been improved. All the concerns have been addressed. I now recommend acceptance of this work in Nature Communications.", "reviewer_2": "The present manuscript has been appropriately revised and may be suitable for publication.", "reviewer_3": "The authors revise their manuscript appropriately and adress all my concerns.", "reviewer_4": "This is a very interesting manuscript, addressing the challenge of compositional analysis and mapping in individual nanocrystals with chemical complexity--alloys, doped nanocrystals, core/shell, etc. The technique provides information that is not readily accessed using other techniques. I think that the authors have adequately addressed the reviewer comments in their revision. This manuscript should continue to inspire researchers in the field to utilize this technique to analyze their materials."}, "author_response": ""}]

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