Optical heterostructure in a two-dimensional organic crystal

Domain: physics
Published: 2025-12-29
Editorial outcome: minor_revision

Abstract
Optical heterostructures, which feature spatially heterogeneous optical properties at the subwavelength scale, represent a key frontier for next-generation integrated photonics and optoelectronics. While typically realized by joining dissimilar materials, achieving such heterogeneity in single-component systems has remained a fundamental challenge. Here, we report an intrinsic optical heterostructure in a uniform organic nanosheet, manifesting as strongly enhanced fluorescence in the inner zone compared to the outer zone. We demonstrate that this emission heterogeneity stems from a spatially localized solid-state transition in the central top layer, which transforms the initial single crystal into an out-of-plane twin structure and significantly enhances the radiative recombination efficiency. This transition is driven by the competitive interplay between molecule-substrate and intermolecular interactions, as corroborated by multiscale structural, optical, and theoretical analyses. Our findings not only establish a platform for realizing optical heterostructures in organic materials but also open avenues for exploring structural-dynamics-governed photonic phenomena, offering broad implications for future materials design and micro-optical applications.

Introduction
A heterostructure is composed of heterogeneous materials with different properties, which exhibit superior functionalities compared to homogeneous materials1. Similarly, an optical heterostructure is defined as a system consisting of two or more units with different optical properties, which spatially exhibit rich and unique optical properties with respect to individual material2,3. Owing to their considerable application potential, these structures have garnered significant interest in materials science. They offer a promising platform for high-resolution optical coding, data encryption, anti-counterfeiting, optical multiplexing, and multi-wavelength lasing2–4. Furthermore, the unique combination of spatially confined luminescence and tunable efficiency positions them as compelling candidates for developing next-generation optoelectronic components, including on-chip light sources, microscale lasers, and integrated photonic circuits3,5,6. Typically, optical heterostructures are achieved by lateral connection of materials with distinct chemical components or lattice structures. However, the mismatch between these materials often necessitates intricate design and artificial control of assembling processes2,3,7.

Comparatively, optical heterostructures in single-component system offer distinct advantages, including simple chemical fabrication strategies, minimal lattice mismatches, and avoidable defects. Recent advancements in inorganic single-component systems have demonstrated the realization of optical heterostructure through the induction of crystalline phase transitions, grain boundary formations, defect engineering, and other structural differences5,8–10. Within these layers, atoms are interconnected by robust covalent bonds, as depicted at the top of Fig. 1a, forming solid periodic lattices. These microscopic structural differences can easily disrupt the strict lattice periodicity and influence the energy band details, ultimately affecting the macroscopic optical performance along in-plane direction2,6,7,11.

In contrast, the observation of optical heterostructure in organic single-component systems is much more challenging due to the fundamental differences between organic and inorganic materials. Organic crystals hold weak van der Waals forces along both the in-plane and interlayer directions, as illustrated at the bottom of Fig. 1a, compared to the strong covalent bonds that unite atoms within inorganic layers. Consequently, organic materials exhibit significant in-plane structural flexibility and fault tolerance, making it difficult to effectively regulate macroscopic properties via microscopic structural differences12,13. Additionally, the lack of experimental strategies to control molecular packing orientation further complicates the precise creation of structural difference in organic materials. Therefore, the realization of optical heterostructures within organic materials has primarily relied on approaches such as cavity effects or multi-component systems3,14–18.

In this work, we demonstrate the intrinsic optical heterostructure achieved in a single organic-component nanosheet. The inner and outer zones of such a nanosheet exhibit distinct photoluminescence (PL) behaviors, despite having no visual difference in composition, lattice structure, and thickness. The optical heterostructure is attributed to a twin structure phase transition occurring in the top inner part of the nanosheet, driven by the synergistic and competitive cooperation between intermolecular forces and molecule-substrate interactions. Our findings highlight the crucial role of interfacial interaction, beyond intermolecular interactions, in designing promising structures and functionalities for organic materials.

Results
The optical heterostructure was serendipitously discovered in nanosheets of single organic component 9,9’-(2,5-dibromo-1,4-phenylene)bis[9H-carbazole] (C30H18Br2N2, hereafter referred as PDBCz). These nanosheets were fabricated using the physical vapor deposition (PVD) method, which prevents any unintended chemical reactions and enable the nanosheets with single component. X-ray diffraction (XRD) analysis of nanosheets is consist with that of single-crystals (Supplementary Fig. 1), which confirm the crystal phase and demonstrates the space group of P2/c19. Figure 1b and Supplementary Fig. 2 depict the unit cell structure. Optical microscopy images presented in the upper panel of Fig. 1c demonstrate that regardless of their varying thicknesses, the nanosheets exhibit a uniformly smooth surface and a distinctive parallelogram morphology.

A prominent feature of these nanosheets is their intrinsic optical heterostructure, characterized by a consistently brighter inner zone as revealed by fluorescence imaging (Fig. 1c bottom) and PL mapping (Fig. 1d inset). Spectral analysis of a representative PDBCz nanosheet shows the PL intensity in the inner zone (Iinner) is ~454% stronger than the outer zone (Iouter), despite sharing an identical emission maximum at ~552 nm (Fig. 1d). Critically, the near-overlapping transmittance and reflectance spectra of both zones (Fig. 1e) indicate comparable optical absorption and photon excitation rates20, collectively pointing to a superior radiative recombination efficiency in the inner zone as the sole origin of the enhanced emission (Supplementary Note I). The regular geometric shape of this high-efficiency region starkly contrasts with the disorderly zones in inorganic analogs due to randomly formed grain boundaries, defects, or phases5,8–10. It is also noteworthy that the outer edge can exhibit brighter emission due to optical confinement effects like waveguide modes17,21, which is distinct from the optical heterostructure reported here.

We systematically investigated the origin of the optical heterostructure. Our initial hypothesis of optical microcavity effects was definitively ruled out, as the enhanced emission in the inner zone persists even when the nanosheet’s regular shape is disrupted (Supplementary Fig. 3) Optical image and atomic force microscopy (AFM) image (Fig. 1f) revealed no visible differences in the surface morphologies between the inner and outer zones. Notably, transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (TEM-EDS) (Fig. 1g) shows that the distribution of elements (such as carbon and bromine) is uniform throughout the entire nanosheet. Furthermore, the crystal structures of the inner and outer zones are highly consistent, as evidenced by the matching selected area electron diffraction (SAED) patterns obtained from different zones (Fig. 2a and Supplementary Figs. 4, 5). Therefore, the optical heterostructure phenomenon cannot be attributed to differences in surface morphology, elemental distribution, or crystal structure between the inner and outer zones.

Strikingly, crystallographic analysis revealed a twin structure along the b-axis in PDBCz, which is seldom observed in organic crystals. This is definitively demonstrated by the selected area electron diffraction patterns (Fig. 2a and Supplementary Figs. 4, 5), as the electron diffraction patterns from both the inner and outer zones consistently displayed two sets of spots. These patterns match perfectly with the simulated twinned m1 + m2 structure with mirror symmetry (Fig. 2b), in clear contrast to the single set of spots expected for a single crystal (Fig. 2c). Typically, the formation of twin structures is the result of cooperative effects of multiple interactions, which is challenging to achieve in organic materials, especially in bulk form. When organic crystals become molecularly thin, however, the molecule-substrate interaction becomes comparable and competitive with intermolecular interactions22,23. Therefore, the most likely reason for the formation of the twin structure in PDBCz nanosheets is the cooperative effects of intermolecular and molecule-substrate interactions.

We designed a series of control experiments to investigate the interactions between the PDBCz nanosheets and substrates. Solution-processed methods are known to yield free-standing materials with ignorable molecule-substrate interaction. As shown in Fig. 2d, the PDBCz nanosheets prepared by solution method exhibit only one set of diffraction pattern, with no twin structure or optical heterostructure phenomenon. Similarly, PDBCz nanosheets grown on other types of non-siliceous substrates (such as BN, mica, and sapphire substrates) using PVD methods also lack twin structure and do not exhibit optical heterostructure phenomenon as shown in Fig. 2e and Supplementary Fig. 6. In contrast, PDBCz nanosheets grown on various siliceous substrates (such as SiO2/Si, Si, and quartz substrates) all exhibit twin structure and the optical heterostructure phenomenon (Supplementary Fig. 7). To further probe the role of bromine, we examined structurally similar molecules without Br atoms, such as 1,4-di(-H-carbazol-9-yl)benzene (referred as PDCz)24 and 1.3-bis(N-carbazoly)benzene (referred as mCP)25. When grown on silicon substrates via the same PVD method, these Br-free nanosheets show no optical heterostructure (Supplementary Fig. 8), highlighting the critical role of Br. Given the potential for covalent Si-Br bonding26, we calculated the adsorption and intermolecular interaction energies for these systems (Supplementary Note II and Supplementary Table 1). The results reveal a distinct covalent Br-Si interaction between PDBCz and SiO2 (Fig. 2f and Supplementary Fig. 9), which is absent for PDBCz on BN, as well as for Br-free PDCz on either substrate (Fig. 2f, g). Energetically, the PDBCz-SiO2 system exhibits a markedly lower adsorption energy (–0.764 eV) than its intermolecular interaction energy (–0.723 eV), whereas the PDCz-SiO2 system shows the opposite trend (Fig. 2h). This contrast confirms that the PDBCz-SiO2 interface features strong binding—primarily mediated by Si-Br bonding—which surpasses both the material’s own intermolecular cohesion and the adsorption of Br-free analogs.

To gain insights into the formation and evolution of the twin structure, we conducted in-situ and real-time investigations on the dynamic growth process of PDBCz nanosheets, as shown in the bottom of Fig. 3a. From the optical images, we observed that the crystal nuclei initially appear as separated islands with circular or elliptical shapes. As the growth time increases, these nuclear islands gradually grow to fractal and compact islands, and then form into nanosheets. This growth process follows an island growth mode (top of Fig. 3a)27,28, which differs from the layer-by-layer growth mode commonly observed for organic crystals22,29,30. Guided by island growth, we calculated the surface energies and relative growth rates of the major crystal facets to unravel the growth kinetics (Supplementary Note III). The (020) surface exhibits a distinctly lower free energy than other facets (Fig. 3c), leading to its slowest growth rate and thereby defining the crystal morphology as the largest exposed face—a finding consistent with our experimental results (Fig. 3a, bottom). Overall, the observed island growth mode with a substrate-parallel orientation represent the most energetically favored configuration.

The temporal evolution of the structure and optical properties provides direct insight into the underlying mechanism. Initially, neither the optical heterostructure nor the twin structure is observed (Fig. 3b). They emerges simultaneously once the nanosheet develops a regular parallelogram shape and are conserved during subsequent growth, establishing a direct correlation between the structural transition and the emergent optical phenomenon.

Based on this, we propose a coherent growth model (Fig. 3e). In the initial stage (I), strong molecule-substrate interactions (Fig. 2f, g, h) enforce an identical m1 lattice in both layers, which is dominant for small-sized nanomaterials22,23. As the nanosheet grows, intermolecular interactions become increasingly influential. Upon reaching a critical size (Stage II), this interplay triggers a localized solid-state transition exclusively in the top layer, which rotates into the m2 orientation, forming a mirror-twin structure (m1 + m2) with the substrate-pinned bottom layer. This process is fundamentally enabled by strong specific interactions with the siliceous substrate. During subsequent lateral growth (Stage III), the system propagates this heterostructure, with the bottom and top layers perpetuating the m1 and m2 lattices, respectively, thereby preserving the twin configuration and the associated optical heterostructure.

To elucidate why the lateral growth of the top layer consistently maintains the m2 orientation in the outer zone rather than reverting to m1 after the structural transition, we conducted structural relaxation simulations (Fig. 3d and Supplementary Fig. 10). These calculations demonstrate that when m1 and m2 molecules are positioned adjacently within the same a-c plane, the system undergoes a spontaneous reorientation toward a uniform molecular arrangement. This confirms that mixed m1-m2 packing is energetically unfavorable in the lateral plane and is intrinsically eliminated during structural optimization. The result provides a theoretical basis for the observed structural stability, explaining how the coherent m2 domain extends into the outer zone during continued growth while preserving the well-defined twin boundary.

Building on this structural model, we present direct experimental evidence linking the localized twin transition to the optical heterostructure. First, we employed Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM) to probe the local electronic and structural properties. These techniques sensitively map variations in surface potential and electrostatic force gradients, which serve as indicators of crystallinity and structural heterogeneity31,32. On a thick, as-grown nanosheet, we observed a pronounced contrast in both surface potential and EFM phase between the inner and outer zones (Fig. 4a), consistent with the crystallographic difference induced by the structural transition. Second, we leveraged the inherent fragility of materials at structural transition regions33–35. Thermal annealing of PVD-grown nanosheets on siliceous substrates caused degradative damage exclusively within the inner zone (Fig. 4a), directly confirming that the structural transition is spatially confined to this region. In contrast, solution-grown nanosheets, which lack the transition, showed no such differential damage (Supplementary Fig. 11). Together, these results unequivocally correlate the confined structural transition with the emergence of the optical heterostructure.

To further validate that the structural transition is confined to the top layer of the inner zone, we investigated its dependence on nanosheet thickness. According to our model, the transition requires a sufficient vertical distance from the substrate and should therefore be absent in sufficiently thin nanosheets. This was confirmed by two complementary approaches. First, we progressively thinned thick nanosheets via plasma etching (Supplementary Note IV and Supplementary Fig. 12). As the thickness decreased, the luminescence contrast between the inner and outer zones diminished accordingly, and vanished completely at a critical thickness of ~20 nm (Supplementary Figs. 13–15). Concurrently, the contrast in KPFM and EFM images also disappeared (Supplementary Fig. 15). Second, as-grown thin nanosheets (≤20 nm) inherently exhibited no optical heterostructure nor any KPFM/EFM contrast (Fig. 4b). Critically, neither the artificially thinned nor the as-grown thin nanosheets (including solution-processed ones) showed any damage after thermal annealing (Fig. 4b and Supplementary Fig. 16), in stark contrast to the localized damage observed in the thick nanosheets. Furthermore, selected area electron diffraction confirmed that these thin nanosheets exhibit only a single-crystal pattern without any twin feature (Fig. 4c, inset). Collectively, these results demonstrate that a minimum thickness is required for the structural transition to occur, confirming its localization to the top layer and directly linking the transition to the emergence of the optical heterostructure.

The thickness-dependent behavior of the structural transition was quantitatively verified by analyzing the PL intensity ratio (Iinner/Iouter) across nanosheets of different thicknesses (Supplementary Fig. 17). The experimental data are well described by the model: Iinner/Iouter = [It × (t–20) + It × 20] / (I0 × t), where I0 is the PL intensity per unit thickness from the bottom layer and the outer zone of the top layer, and It is the enhanced intensity from the inner zone of the top layer due to the structural transition (Fig. 4c and Supplementary Note V). The excellent agreement between the model and experimental data strongly validates our proposed mechanism, confirming that the optical heterostructure originates from a transition confined to the top layer beyond a ~20 nm thickness.

These results collectively confirm that the structural transition is directly responsible for the optical heterostructure. To further solidify this causal link, we performed a statistical analysis of the transition zone and the PL-enhanced inner zone across 120 nanosheets (Supplementary Figs. 18 and 19). The transition zone area (defined by the initial emergence of both the twin structure and optical heterostructure during growth) and the inner zone area (the final bright central region) were both found to be approximately 5 µm2 (Fig. 4d). Furthermore, during nanosheet growth, the outer zone expands while the inner zone area remains constant (Supplementary Fig. 20). The statistical equivalence and constant nature of the inner zone area provide strong support for the conclusion that the structural transition defines the region of enhanced emission, thereby establishing it as the origin of the observed optical heterostructure.

We then sought to elucidate how the structural transition gives rise to the optical heterostructure phenomenon. It is known that structural transitions or local disorder can induce exciton localization, which suppresses exciton diffusion to nonradiative trap sites and thereby enhances radiative recombination efficiency36–38. This mechanism is consistent with previous reports showing that crystal packing and order-disorder polytypes in PDBCz modulate luminescence properties24,39, and nanoscale disorder in organic molecular solids can induce exciton localization36,40,41. To directly probe the recombination dynamics in our system, we performed time-resolved photoluminescence (TRPL) measurements. The results show that the inner zone exhibits a markedly shorter PL lifetime together with a significantly stronger emission intensity compared to the outer zone (Fig. 4e). This coexistence of a shorter lifetime and enhanced emission suggests the introduction of an additional, fast radiative pathway.

Further analysis of the PL decay kinetics reinforces this interpretation. The decay curve in the outer zone is well fitted by a bi-exponential function with lifetimes τ1 = 0.463 ns and τ2 = 2.417 ns. In contrast, the inner zone requires a tri-exponential function, with lifetimes τ1 = 0.467 ns, τ2 = 2.412, and an additional, ultrafast component τ3 = 0.151 ns. The two longer components (~0.46 ns and ~2.4 ns), present in both zones, are attributed to the intrinsic recombination channels of PDBCz. The exclusive presence of the ultrafast τ3 component in the inner zone signifies an extra radiative recombination channel opened by the structural transition. This additional fast pathway accelerates the overall exciton recombination, resulting in the observed shorter lifetime and higher PL efficiency in the inner zone, and conclusively linking the structural transition to the formation of the optical heterostructure.

In conclusion, we report the observation of an intrinsic optical heterostructure and its associated twin structure in a single-component organic nanosheet. This photonic system, characterized by strongly enhanced fluorescence in the inner zone, originates from a spatially localized solid-state transition confined to the upper region—specifically, the top layer beyond ~20 nm from the substrate. This localized restructuring creates a distinct transition region that significantly enhances radiative recombination efficiency, thereby directly generating the observed emission heterogeneity. The entire process is governed by the competitive interplay between molecule-substrate and intermolecular interactions, which evolves as the nanosheet grows.

This work not only establishes a platform for realizing optical heterogeneity in organic materials but also highlights structural dynamics as a fundamental design principle for organic micro-nano photonics. The discovery of a tunable twin structure coupled to optical functionality opens a compelling research direction, calling for further exploration through advanced in-situ characterization and multi-scale theoretical modeling. A deeper understanding of such structure-property relationships will be essential for harnessing structural transitions as a versatile tool for tailoring light-matter interactions. We anticipate that this emerging avenue will not only expand the frontiers of organic material science but also accelerate the development of innovative integrated photonic and optoelectronic devices.

Methods
Sample preparation
9H-carbazole (1.0 g, 6.0 mmol) and KOH (0.54 g, 9.6 mmol) were added to a glass flask and dissolved in dimethylformamide (40 mL) at 40 °C, then stirred for 2 hours. 1,4-Dibromo-2,5-difluorobenzene (0.65 g, 2.4 mmol) was introduced into the mixture and heated at 110 °C for 4 hours, followed by cooling to room temperature and filtration. The raw product was purified using silica column chromatography with dichloromethane and petroleum ether as eluents. White powders were obtained with a yield of 60%. A schematic diagram of the synthesis process is shown in Supplementary Fig. 21.

PDBCz nanosheets were synthesized by the physical vapor deposition (PVD) method. In a typical process, PDBCz powder was loaded into an alumina boat and placed at the center of a tube furnace, while a substrate pretreated by O2 plasma was positioned downstream at a fixed distance from the source. After evacuation to a base pressure of ~0.1 Pa, high-purity Ar gas (99.995%) was introduced as the carrier at a constant flow. The furnace temperature was ramped to 310 °C at a rate of 10 °C min-1 and maintained for 1 hour, followed by natural cooling to room temperature. This setup enabled the reproducible growth of uniform PDBCz nanosheets. Detailed synthetic procedure for PDBCz nanosheets, together with schematic illustrations of the experimental setup, are provided in Supplementary Note VI and Supplementary Fig. 22.

In addition, nanosheets were also obtained by a solution-based self-assembly method, in which controlled solvent evaporation induced molecular ordering on the substrate surface. First, 5.66 mg of PDBCz powder was dispersed in 10 mL of trichloromethane (TCM) and sonicated for 5 minutes to form a homogeneous solution. Subsequently, 1 mL of N,N-dimethylformamide (DMF) and 1 mL of dichloromethane (DCM) were added, and the resulting transparent solution was allowed to stand at room temperature for 3 days. A 20 µL aliquot of the mixed solution was then drop-cast onto a pre-cleaned substrate and placed in a sealed Petri dish saturated with ethanol vapor. Controlled evaporation of the mixed solvent promoted the self-assembly of PDBCz molecules, resulting in the formation of nanosheets on the substrate surface within 2 hours. A schematic illustration of the experimental setup is provided in Supplementary Fig. 23.

Characterization
Optical and fluorescence images were recorded using a Nikon DS-Ri2 microscope under UV excitation from a mercury lamp (330-380 nm). Photoluminescence (PL) spectra of the nanosheets were measured with a micro-Raman system (Zolix Finder Smart FST2-MPL501-405C1/WITEC Alpha300R) equipped with a 405 nm continuous-wave laser. PDBCz powders and single crystals were characterized by X-ray diffraction (Nexsa G2). AFM, KPFM, and EFM measurements were performed on an Oxford MFP-3D system, and TEM images were obtained with a JEOL ARM200F operated at 200 kV. Detailed PL measurement conditions, including optical setup, excitation parameters, and data acquisition procedures, are provided in Supplementary Note VII and Supplementary Fig. 24.

First-principles calculations
Structure optimization calculations based on Density Functional Theory (DFT) were carried out using the Vienna Ab initio Simulation Package (VASP)42–44. The exchange-correlation potentials were described through the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) formalism45. A plane wave basis set with an energy cutoff of 450 eV was utilized to expand the electronic wavefunctions, and the criterion for the total energy tolerance was set below 10-4 eV. Detailed calculations of adsorption energy and growth rate are provided in Supplementary Notes II and III.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Kan Liao et al reported their work on a novel 2D organic crystal that shows built-in optical heterostructure (OH) formed by out of plan twinning formed in the core of the crystal from protected substrate interactions. This is demonstrated in the enhanced photoluminescence (PL) in the inner zone of the crystal. The uniqueness of the work is the creation of OH in a single component system which subsequently eliminates the complication from common practice with lateral arrangement of multiple moieties. This discovery potentially could advance the field of organic optoelectronic devices.\nPros:\n1. A novel synthesis that created a single-component organic OH, which overcome the challenge that low dimensional organic crystals are facing: controllability of structural and optical properties due to the flexibility both in-plane and out-of-plane from the weak van der Waals interaction among molecules, as well as the stacking order among layers.\n2. Very beautiful images with high resolution, and impressive illustrations.\nCons:\n1. Missing a comprehensive review of relevant work. The authors claimed ‘Therefore, the realization of OH within organic materials has, to date, been confined to multi-component systems3,14,15,16’. However, this is overstated. For instance, in 2022 single component organic OH was created in a cubic microdisk via self-assembly which showed high contrast of PL between the edge and the body, and with high optical gain [ref. Adv. Optical Mater. 2022, 10, 2101931].\n2. In general, this MS lacks in-depth discussion about the mechanisms behind the observations. Also, the submission is incomplete and misses content about the material synthesis and characterization methods. The SI only contains figures with no explanations. It would greatly enhance the scientific merits of the MS if the SI were more self-sufficient.\n3. I have multiple specific questions/concerns about the current MS:\n(i) There is no description of PL measurements which are critical for the claim.\nQuestion about the illumination method: if the beam size is smaller than the whole crystal (~8um), then it is expected that the center would be brighter than the edge. The authors should show PLQE (not just the PL intensity as in Fig. 1d) normalized by the incident light intensity.\n(ii) The authors cited a few references {ref. 31-33} to explain their observation of nonuniformity in PL. However, the material systems in the citations are different from their crystal, the reviewer feels that it is important that they conduct further in-depth analysis to gain more insights about the OH phenomenon. For instance, measuring PLQE of the inner and outer zones, varying the excitation energy and intensity, and mapping the PL using microscopic setup.\n(iii) I have a problem with the label in SI fig. 14. From the top drawing, t should be more than 20nm (probably the substrate SiO2 thickness). Yet, a few of the bottom graphs have t<20nm which is physically impossible, unless t is mislabeled: it should be the thickness of the crystal nanosheet, not the combined thickness. The same labeling appears in Fig. 4b as well. Suggest the authors to double check and make appropriate correction.\n(iv) The explanation illustrated in Fig. 3c has unanswered question: if the formation of top layer crystal structure to m2 orientation is due to the increasing intermolecular interaction, one would expect this to continue as the size grows bigger passing the critical size, yet, the m2 orientation mysteriously stopped and the rest of the crystal structure is back to m1 orientation similar to the bottom layer where the strong interaction helps to hold the structure. The authors need to explain more about the ‘thermodynamically-driven island growth mode’.\n(v) I do not understand the merit of the fitting formula used in Fig. 4b, and how that is a proof that ‘This confirms that the enhancement of PL is arising from the structural transition.’ , In addition, it looks to me that there is a wide variation of the inner zone area shown in SI Fig. 16, unlike what the authors claimed ‘Remarkably, both areas are approximately 5μm2 (Fig. 4c).’.\n(vi) Also, I fail to see the connection between simulation in Fig. 2f and the experimental observations. How does the simulation predict a critical size of the nanosheet when OH occurs? How is the critical size related to the simulated adsorption energy in Fig. 2g? what factors would alter these parameters? Without answers to these questions, this MS remains at the laboratory report level and cannot provide guidance to help advancement of the science in the field of OH. Furthermore, it will be helpful for the community if the authors could elaborate on potential applications of their cool crystal nanosheet.", "reviewer_2": "The manuscript submitted by Kan Liao et al. reports an intrinsic OH uniquely observed in a 2D nanosheet composed of a single organic component. This discovery is highly intriguing, as it represents the first demonstration of such a phenomenon in purely organic 2D systems. The authors provide a convincing mechanistic explanation, attributing the OH to an island growth process and the presence of a twin structure, both of which are rarely reported in organic nanosheets. These findings not only shed light on the fundamental physics of organic 2D materials but also open new directions for the design of multifunctional optical materials. I therefore strongly recommend publication in Nature Communications after minor revisions. A few clarifications and minor issues should be addressed prior to acceptance.\n1. The authors identified a twin structure in the PDBCz organic nanosheets. To facilitate a deeper understanding of its crystallographic features, please provide the molecular packing structures based on the unit cell along the a-, b-, and c-crystallographic axes, respectively.\n2. The authors propose that the nanosheet follows an island growth mode. It is recommended to further clarify the relative growth rates and corresponding surface energies of the major crystal facets, in order to gain deeper insight into the growth kinetics.\n3. Figure S12 shows that plasma treatment can effectively eliminate the optical heterojunction phenomenon in the upper layer. To further verify the reliability of this method, it is recommended to provide experimental results on thinner samples after plasma treatment.\n4. The authors propose that the twin structure transition in the nanosheets originates from a synergistic competition between intermolecular interactions and the interaction between Br atoms and the silicon substrate. To verify the generality of this mechanism, it is recommended to include observations of the OH phenomenon on SiO₂/Si substrates using structurally similar molecules that do not contain Br atoms.\n5. On page 6 of the manuscript, the authors provide adsorption energy values for both SiO2 and BN substrates, along with intermolecular interaction energies. How these values were computationally derived?", "reviewer_3": "This work reports an optical heterostructure that exhibits different emission intensities. I agree that optical heterostructures composed of single-component organic molecules are rare. However, I cannot recommend acceptance of this work due to the following concerns:\nWhy do the inner and outer zones exhibit different emission intensities? The authors attribute this to structural transition but fail to provide a clear explanation.\nThe authors suggest that the formation of the heterostructure is strongly influenced by interactions between the PDBCz nanosheets and substrates, yet no experiments were conducted to investigate these interfacial interactions.\nMany key conclusions in the manuscript are speculative and lack direct evidence, which is not scientifically rigorous. The authors should perform more detailed mechanistic studies.\nDid the authors examine the emission properties from different sides of the bulk crystals? Is there any variation in emission intensity?"}, "author_response": "Response Letter (Manuscript ID: NCOMMS-25-44606-T)\nRevisions and list of changes:\nIn Main Text:\n1. In the author list, we have added Wenheng Xu, Zhongjing Xia, Zilong Mao, and\nYan Lv, in recognition of their contributions to the experiments and the mechanistic\ninterpretation in the revised manuscript.\n2. The abstract has been revised and optimized, as highlighted in yellow on page 2.\n3. The Introduction has been revised to include an updated perspective on the\npotential future applications of the optical heterostructure phenomenon on page 3,\nwith changes highlighted in yellow.\n4. The discussion regarding the formation of optical heterostructures through optical\nmicrocavities or multi-component systems has been refined, and additional\nreferences have been included (as highlighted in yellow on page 4).\n5. We have added subtitles to each section of the Results and Discussion part.\n6. The description of the PL mapping has been added to the Results and Discussion\n(page 5), with changes highlighted in yellow.\n7. We have added a discussion on the absorption in the inner and outer zones, as\nhighlighted in yellow on page 5.\n8. The explanation for the brighter emission at the edges has been optimized and\nupdated on page 5, with changes highlighted in yellow.\n9. The paragraph discussing the exclusion of certain possible mechanisms for the\noptical heterostructure phenomenon has been optimized and updated on page 5,\nwith changes highlighted in yellow.\n10. The discussion regarding the twin structure being oriented along the b-axis has\nbeen optimized and updated on page 6, with changes highlighted in yellow.\n11. The discussion regarding the theoretical calculations for the nanosheets with\nsimilar molecular structures but without Br has been updated on page 7 of the main\ntext, with changes highlighted in yellow.\n12. In the discussion of the island growth mechanism, we have incorporated theoretical\ncalculations of growth rate and surface energy, as indicated by the yellow-\n1\nhighlighted text on page 8.\n13. In the discussion of our proposed scenario, we have incorporated theoretical\ncalculations to verify that the outer zone of the top layer remains in the m structure\n2\nwhile that of the bottom layer retains the m structure, as indicated by the yellow-\n1\nhighlighted text on page 8.\n14. The discussion regarding the evidence for the existence of the structural transition\nconfined to the inner zone has been updated and optimized, as shown in the yellow-\nhighlighted text on pages 9-10.\n15. In the section discussing the correlation between structural transition and the\noptical heterostructure phenomenon, we reorganized the logic by first describing\nthat the structural transition region coincides in area with the inner zone of\nenhanced PL emission, and then explaining why the structural transition leads to\nthe emergence of the optical heterostructure phenomenon, as indicated by the\nyellow-highlighted text on pages 9-11.\n16. The discussion on formula fitting based on our proposed scenario has been revised\nand updated, as indicated by the yellow-highlighted section on page 10.\n17. The discussion on why the structural transition induces the optical heterostructure\nphenomenon has been updated and optimized, with the addition of a discussion on\nTRPL data, as indicated by the yellow-highlighted text on pages 10-11.\n18. The conclusions section has been revised and optimized, as highlighted in yellow\non pages 11-12.\n19. The experimental methods have been optimized and updated as indicated by the\nyellow-highlighted section on pages 12-13.\n20. We have added a Data Availability section on page 13.\n21. The references have been updated and adjusted accordingly based on the revisions\nin the main text.\n22. We have added an Additional information section on page 17.\n23. A PL mapping image has been added as an inset in Fig. 1d, and the corresponding\nfigure caption has been revised accordingly.\n24. The transmission and reflection spectra have been added as Fig. 1e, and the\ncorresponding caption has been revised accordingly.\n2\n25. The TEM-EDS data have been added as Fig. 1g, and the corresponding caption has\nbeen revised accordingly.\n26. In Figs. 2f-h, we incorporated simulations of the bonding configurations of Br-free\nPDCz molecules on SiO and BN substrates, together with recalculated adsorption\n2\nenergies. The corresponding figure captions have been revised accordingly.\n27. Theoretical calculations of surface energies and growth rates for different facets of\nthe nanosheets have been added as Fig. 3c. The related figure captions have been\nrevised accordingly.\n28. Molecular arrangements before and after structural relaxation has been added as\nFig. 3d. The related figure captions have been revised accordingly.\n29. The schematic illustration in Fig. 3e has been optimized and modified to more\nclearly present the structural transition in the inner zone of top layer. The\ncorresponding figure captions have been revised accordingly.\n30. Figure 4 has been reorganized. Optical images before and after thermal annealing,\nas well as KPFM, PFM and AFM images, have been added for as-grown thick\nsamples and thin samples. The plot in Fig. 4d showing the PL intensity ratio\nbetween the inner and outer zones as a function of thickness has been modified by\nadding schematic illustrations of nanosheets with different thicknesses and\noptimizing the inset. TRPL data have also been included. The corresponding figure\ncaptions have been revised accordingly.\n31. The figure numbering in the main text has been updated accordingly.\nIn Supplementary Information Materials:\n1. Molecular structure diagrams of m and m oriented along the a, b, and c\n1 2\ncrystallographic axes have been added as Supplementary Figure 2.\n2. A detailed description of the qualitative comparison of PLQE has been added as\nSupplementary Note I.\n3. Supplementary illustrations of the molecular structures of PDCz and mCP\nmolecules (Br-free, structurally similar to PDBCz), along with their optical and\nfluorescence images, have been added as Supplementary Figure 8.\n4. A description of the adsorption energy calculation method has been added as\n5. A description of the surface energy and growth rate calculation method has been\nadded as Supplementary Note III.\n6. Molecular arrangements before and after structural relaxation for the alternative\nconfiguration have been added as Supplementary Figure 10.\n7. A detailed description of the plasma-treated samples, along with a corresponding\nschematic illustration, has been added as Supplementary Note IV and\nSupplementary Figure 12.\n8. Optical, AFM, and fluorescence images of a nanosheet thinned to 8.7 nm via\nplasma treatment have been added as Supplementary Figure 14.\n9. Optical and fluorescence images of an artificially thinned PDBCz nanosheet (~18\nnm) before and after thermal annealing have been added as Supplementary Figure\n16.\n10. AFM data have been added to Supplementary Figure 17, and the schematic\nillustration has been optimized and revised.\n11. A detailed description of the fitting procedure for the experimental I /I ratio\ninner outer\nas a function of thickness has been added as Supplementary Note V.\n12. The image used to extract the area of the inner enhanced emission region of the\nnanosheet has been optimized, primarily by adjusting the scale bar so that the\ncentral emission area appears consistent in size across the image, as shown in\nSupplementary Figure 19.\n13. A detailed description and schematic illustration of the PDBCz powder synthesis\nhave been added as Supplementary Note VI and Supplementary Figure 21.\n14. A detailed description of the PDBCz nanosheet synthesis via the PVD method has\nbeen added, and the corresponding schematic illustration has been revised and\noptimized. These have been included as Supplementary Note VII and\nSupplementary Figure 22.\n15. A detailed description of the PDBCz nanosheet synthesis via the solution method,\nalong with a corresponding schematic illustration, has been added as\nSupplementary Note VIII and Supplementary Figure 23.\n16. A detailed description of the PL measurements, along with a corresponding\n4\nschematic illustration, has been added as Supplementary Note IX and\nSupplementary Figure 24.\n17. All figure captions in the Supplementary Information have been revised to provide\ndetailed descriptions.\n18. The figure numbering in the Supplementary Information has been updated to align\nwith the revised main text.\nKan Liao et al. reported their work on a novel 2D organic crystal that shows built-in\noptical heterostructure (OH) formed by out of plane twinning formed in the core of the\ncrystal from protected substrate interactions. This is demonstrated in the enhanced\nphotoluminescence (PL) in the inner zone of the crystal. The uniqueness of the work is\nthe creation of OH in a single component system which subsequently eliminates the\ncomplication from common practice with lateral arrangement of multiple moieties. This\ndiscovery potentially could advance the field of organic optoelectronic devices.\nPros:\n1. A novel synthesis that created a single-component organic OH, which overcome\nthe challenge that low dimensional organic crystals are facing: controllability of\nstructural and optical properties due to the flexibility both in-plane and out-of-plane\nfrom the weak van der Waals interaction among molecules, as well as the stacking order\namong layers.\n2. Very beautiful images with high resolution, and impressive illustrations.\nReplies to Reviewer #1’s Comments:\nWe greatly appreciate Reviewer #1’s positive and encouraging assessment of our\nwork. We are delighted that the novelty of constructing a single-component organic\noptical heterostructure, as well as the quality of our experimental data and visualizations,\nhave been recognized. Guided by the reviewer’s insightful comments, we have\ncarefully revised and refined the manuscript to further enhance its clarity, coherence,\nand overall presentation. Below, we provide detailed, point-by-point replies to each\ncomment.\nComment 1:\nMissing a comprehensive review of relevant work. The authors claimed ‘Therefore,\nthe realization of OH within organic materials has, to date, been confined to multi-\ncomponent systems [3,14,15,16]. However, this is overstated. For instance, in 2022\nsingle component organic OH was created in a cubic microdisk via self-assembly which\nshowed high contrast of PL between the edge and the body, and with high optical gain\n[ref. Adv. Optical Mater. 2022, 10, 2101931].\nWe sincerely appreciate the reviewer’s careful reading and for pointing out the\nrelevant report in ref. [Adv. Optical Mater. 2022, 10, 2101931], for which we have made\nthe description more rigorous in our revised manuscript. The optical heterostructure\nphenomenon described in that work is fundamentally different from the optical\nheterostructure observed in our study. To facilitate a clearer understanding of the\ndistinct luminescent regions and mechanisms in the optical heterostructure, we have\nincluded simplified schematic diagrams in Figure R1.1. In ref. [Adv. Optical Mater.\n2022, 10, 2101931], the emission contrast appears at the very edges of cubic microdisks\nand is primarily attributed to the optical waveguide and whispering-gallery-mode\n(WGM) effects. By contrast, our nanosheets exhibit an intrinsic optical heterostructure\nwithin the central region, where the inner zone shows significantly stronger\nphotoluminescence than the surrounding outer zone. This phenomenon cannot be\nexplained by cavity or waveguide effects but instead originates from a unique out-of-\nplane twin structural transition driven by the interplay between molecule-substrate and\nintermolecular interactions.\nFigure R1.1. The top panels present schematic illustrations of the luminescent regions\nand mechanisms in the o-MSB microdisk [Adv. Optical Mater. 2022, 10, 2101931] (a)\nand the PDBCz nanosheet in our work (b), while the bottom panels show their\ncorresponding fluorescence images.\nNotably, in addition to this optical heterostructure phenomenon with significantly\n7\nstronger photoluminescence than the surrounding outer zone, our nanosheets also\ndisplay brighter outer edges due to the optical waveguide effect. Accordingly, ref. [Adv.\nOptical Mater. 2022, 10, 2101931] has been cited again in the main text within the\nsentence discussing that “It is also noteworthy that the outer edge can exhibit brighter\nemission due to optical confinement effects like waveguide modes, which is distinct\nfrom the optical heterostructure reported here.”\nComment 2:\nIn general, this MS lacks in-depth discussion about the mechanisms behind the\nobservations. Also, the submission is incomplete and misses content about the material\nsynthesis and characterization methods. The SI only contains figures with no\nexplanations. It would greatly enhance the scientific merits of the MS if the SI were\nmore self-sufficient.\nReply to Comment 2:\nWe sincerely thank the reviewer for this valuable and constructive comment. In the\nrevised manuscript, we have added detailed descriptions of the material synthesis and\ncharacterization methods in the main text to ensure completeness. A more\ncomprehensive version of these methods, together with additional datasets, has been\nprovided in the Supplementary Information (Supplementary Notes IV, VI-IX;\nSupplementary Figures 4 and 21-24). Furthermore, all figures in the Supplementary\nInformation are now accompanied by detailed captions to improve clarity, consistency,\nand scientific rigor.\nThe observed optical heterostructure phenomenon is originating from a structural\ntransition, as accompanied with the emergence of twin structures. To address the\nreviewer’s concern and provide a clearer mechanistic framework, we have reorganized\nthe manuscript to present a concise logical progression of the underlying mechanism,\nwith key experimental evidence summarized in Tables R1.1-1.5.\nThe logic of our work can be outlined as follows:\n(1) Interfacial Interaction Dominance: At the early growth stage, strong molecule-\nsubstrate interactions, particularly those involving the Si-Br bond, dominate molecular\npacking and lead to the formation of uniform m lattice.\n1\n(2) Structural Transition and Twin Formation: As the nanosheet laterally expands,\n8\nthe molecule-substrate interaction becomes less dominant compared to the\nintermolecular interaction, triggering a structural transition in the top layer from m to\n1\nm , forming an out-of-plane twin structure (m + m ).\n2 1 2\n(3) Optical Consequence: The structural transition area exhibits a new\nrecombination pathway (~0.15 ns), resulting in enhanced photoluminescence efficiency\nin the inner region.\nIn summary, the structural transition (Tables R1.1-1.2) → twin formation (Table R1.1)\nand PL enhancement (Table R1.3) → optical heterostructure (Table R1.4) sequence\nprovides a coherent explanation for the observed phenomena. We also carried out a\nseries of material characterizations aimed at excluding simple and conventional\nexplanations (Table R1.5). To further substantiate this mechanism, we have\nincorporated additional PL mapping, KPFM/EFM measurements,\ntransmittance/reflectance spectra, TRPL data, and theoretical calculations in the revised\nmanuscript (highlighted in yellow in the main text). Moreover, we have expanded the\nSupplementary Information with detailed synthesis procedures, characterization\nmethods, and explanatory notes to make it fully self-contained and scientifically\nrigorous.\nTable R1.1 Structural transition and twin formation induced by molecule-substrate\ninterfacial interactions originating from Si-Br bonding.\nObservations\nDescriptions Figures\n/Experiments\nPDBCz on siliceous Figure 1c and\nTwin structure and OH\nsubstrates Supplementary Figure 7\nPDBCz on non-siliceous One set diffraction and Figure 2e and\nsubstrates non-OH Supplementary Figure 6\nSimilar molecular without\nNon-OH Supplementary Figure 8\nBr on siliceous substrates\nPDBCz on siliceous\nOne set diffraction and Figure 2d and\nsubstrates by solution\nnon-OH Supplementary Figure 11\nmethod\nObservations\nDescriptions Figures\n/Experiments\nThermal annealing Transition zone is more sensitive to heat Figure 4a\nCrystallinity difference between inner and\nKPFM and EFM outer zones induced by the structural Figure 4a\ntransition\nThick and thin (< 20 nm) samples show\nPlasma-thinned and totally different behaviors in most Figure 4b and\nas-grown thin characterizations, suggesting the Supplementary\nnanosheet structural transition only appears at the top Figure 13-15\nlayer of inner zone\nTable R1.3 Structural-transition-induced PL enhancement in the inner zone.\nObservations\nDescriptions Figures\n/Experiments\nThe trend is in good agreement with\nI /I vs t Figure 4c\ninner outer\nour proposed scenario\nSizes of transition Transition zone is consisted with the\nFigure 4d\nand inner zones PL-enhanced inner zone\nSimultaneous disappearance of\nPlasma-thinned and Figure 4b and\ntransition and PL-enhancement in the\nas-grown thin Supplementary 13-\ninner zone for both types of thin\nnanosheet 15\nsamples\nThe additional fast radiative process\nTR-PL in the transition zone leads to Figure 4e\nenhanced PL\nObservations\nDescriptions Figures\n/Experiments\nFluorescence images Brighter inner zone than outer zone Figure 1c\nPL spectra Stronger PL in the inner zone Figure 1d\nPLQE Higher PLQE in the inner zone Supplementary Note I\nPL Mapping Stronger PL in the inner zone Figure 1d\nTable R1.5 Exclusion of possible mechanisms for the OH phenomenon.\nObservations\nDescriptions Figures\n/Experiments\nNo visible difference between inner and outer\nOptical images Figure 1c\nzones\nNo thickness difference between inner and\nAFM images Figure 1e\nouter zones\nUniform elemental distribution in the inner\nTEM-EDS Figure 1g\nand outer zones\nHighly consistent crystal structures of the\nTEM-SAED Figure 2a\ninner and outer zones\nOptical Supplementary\nOH persists despite disrupted nanosheet shape\nmicrocavity effect Figure 3\nComment 3:\nI have multiple specific questions/concerns about the current MS:\n(i) There is no description of PL measurements which are critical for the claim.\nQuestion about the illumination method: if the beam size is smaller than the whole\ncrystal (~8 um), then it is expected that the center would be brighter than the edge. The\nauthors should show PLQE (not just the PL intensity as in Fig. 1d) normalized by the\nincident light intensity.\nWe sincerely thank the reviewer for this valuable comment. Photoluminescence (PL)\nspectra and mapping of the nanosheets were performed using a micro-Raman system\n(Zolix Finder Smart FST2-MPL501-405C1 or WITec Alpha 300R). The substrate was\nmounted under a confocal optical microscope, where a 405 nm continuous-wave laser\nwas focused onto the sample surface through a dry objective lens (NA = 0.55), yielding\na ~1 μm spot size. All measurements were conducted at room temperature. The PL was\ncollected by the same objective, filtered through a 405 nm long-pass filter, and directed\nto a spectrometer equipped with a CCD detector, with a typical integration time of 1 s.\nThe corresponding optical path is illustrated in Figure R1.2 (also see Supplementary\nNote IX and Supplementary Figure 24). Fluorescence images were recorded on a Nikon\nDS-Ri2 microscope camera under UV excitation from a mercury lamp (Nikon\nINTENSILIGHT C-HGFI) with a 330-380 nm bandpass filter.\nFigure R1.2 Schematic diagram of PL measurement setup.\nWe also sincerely thank the reviewer for raising this important point regarding the\ndirect measurement of the PLQE for the inner and outer zones. We fully agree that\nquantifying the absolute PLQE would provide a more direct measure of the emission\nefficiency difference.\nPLQE measurement presents significant challenges. The established and most reliable\nmethod for absolute PLQE determination is the integrating sphere approach1, 2. This\ntechnique, however, is inherently a macro-scale measurement that collects and averages\nthe total photon flux from the entire sample area illuminated by the excitation beam. It\nlacks the spatial resolution to isolate and compare signals from two distinct sub-micron\nregions within a single nanosheet, particularly given that the inner zone’s dimensions\nare confined to under 2 μm.\nUnfortunately, after an extensive search, we have not been able to identify a\ncommercially available or custom-built system that can perform direct and absolute\nPLQE measurements with the required sub-2μm spatial resolution on our samples.\nGiven this technical limitation, we employed a series of indirect but compelling\nqualitative and semi-quantitative experiments to robustly compare the relative emission\nefficiency between the zones:\n(1) Spatially resolved PL spectroscopy and mapping:\nUsing a confocal micro-PL system, we separately acquired PL spectra and performed\nPL mapping for the inner and outer zones (Figure 1d). The PL intensity in the inner\nzone is consistently and substantially stronger than that in the outer zone, confirming\npronounced emission heterogeneity at the microscale.\n(2) Comparison of optical absorption:\nTo further determine whether the difference in PL intensity originates from variations\nin absorption, we measured the reflectance and transmittance spectra of both regions.\nAs shown in Figure 1e of the main text, the two spectra are nearly identical, indicating\nnegligible differences in optical absorption and thus comparable photon excitation\nconditions between the inner and outer zones3.\nSince the absorption is comparable while the PL emission is markedly stronger in the\ninner zone, we can qualitatively conclude that its PLQE is significantly higher than that\nof the outer zone.\nComment 4:\n(ii) The authors cited a few references (ref. 31-33) to explain their observation of\n13\nnonuniformity in PL. However, the material systems in the citations are different from\ntheir crystal, the reviewer feels that it is important that they conduct further in-depth\nanalysis to gain more insights about the OH phenomenon. For instance, measuring\nPLQE of the inner and outer zones, varying the excitation energy and intensity, and\nmapping the PL using microscopic setup.\nReply to Comment 4:\nWe thank the reviewer for this valuable comment. We agree that it is important to\nprovide literature evidence more closely related to our material system. In the revised\nmanuscript, we have added relevant studies on PDBCz itself and structurally analogous\ncarbazole derivatives. For example, Shi et al. reported that PDBCz exhibits highly\nefficient ultralong phosphorescence whose efficiency is strongly affected by crystal\npacking and local structural motifs4. Moreover, crystallographic studies revealed that\nPDBCz crystals adopt order-disorder polytypes, which highlights the intrinsic presence\nof structural heterogeneity in this system5. These works directly support our conclusion\nthat local structural transitions and disorder can influence exciton dynamics in PDBCz.\nFurthermore, we have cited additional reports on exciton transport in organic\nmolecular solids, which demonstrate that structural disorder leads to exciton\nlocalization, and thereby enhances radiative recombination6-8. These studies provide a\nbroader mechanistic foundation for understanding the suppression of exciton\nannihilation in disordered molecular crystals.\nTo address the comment regarding PL quantification, we have supplemented our\nour\nstudy with micro-PL mapping data. This provides direct spatial visualization of the\nemission heterogeneity, which we have now included as an inset in Figure 1d of the\nrevised manuscript. Furthermore, as detailed in our response above, we have thoroughly\nexplained the challenges associated with direct micro-scale PLQE measurements and\nthe compelling indirect evidence supporting the higher radiative efficiency in the inner\nzone.\nComment 5:\n(iii) I have a problem with the label in SI Fig. 14. From the top drawing, t should be\nmore than 20 nm (probably the substrate SiO thickness). Yet, a few of the bottom\n2\ngraphs have t < 20 nm which is physically impossible, unless t is mislabeled: it should\n14\nbe the thickness of the crystal nanosheet, not the combined thickness. The same labeling\nappears in Fig. 4b as well. Suggest the authors to double check and make appropriate\ncorrection.\nReply to Comment 5:\nWe sincerely thank the reviewer for this careful observation. As illustrated in the\nschematic diagram of Figure R1.3, a nanosheet exhibiting the optical heterostructure\nphenomenon comprises a top layer with m orientation (blue) and a bottom layer with\n2\nm orientation (pink). The total thickness of the nanosheet is denoted as t. For\n1\nnanosheets that display the optical heterostructure phenomenon, t exceeds 20 nm, with\nthe bottom layer maintained at ~20 nm and the top layer corresponding to (t – 20) nm.\nIn contrast, nanosheets with a total thickness below 20 nm lack the top structural\ncomponent, and their entire thickness consists solely of the m orientation.\nnm do not exhibit the optical heterostructure phenomenon.\nFigure R1.3 The schematic diagram of a PDBCz nanosheet with thickness t, exhibiting\nthe optical heterostructure phenomenon, consisting a top layer with m orientation (blue,\n2\nt – 20 nm) and a bottom layer with m orientation (pink, ~20 nm).\ntext and Supplementary Figure 17 have been updated to explicitly indicate the total\nthickness t on the right side. In addition, the corresponding AFM images for each\nnanosheet shown in Supplementary Figure 17 have been included in the revised SI.\n(iv) The explanation illustrated in Fig. 3c has unanswered question: if the formation\nof top layer crystal structure to m orientation is due to the increasing intermolecular\n2\ninteraction, one would expect this to continue as the size grows bigger passing the\ncritical size, yet, the m orientation mysteriously stopped and the rest of the crystal\n2\nstructure is back to m orientation similar to the bottom layer where the strong\n1\ninteraction helps to hold the structure. The authors need to explain more about the\n‘thermodynamically-driven island growth mode’.\nReply to Comment 6:\nWe appreciate the reviewer’s insightful question. Through in-situ and real-time\ninvestigations of the optical morphology of PDBCz nanosheets at different growth\nstages, we found that the process follows an island growth mode (top of Figure 3a).\nUnder this mode, the nanosheets undergo predominant lateral expansion with only\nminimal variation in thickness throughout growth.\nTo obtain deeper insights into the growth kinetics, we performed theoretical\ncalculations to elucidate the relative growth rates and surface energies of the major\ncrystal facets. The theoretical calculations were performed using Materials Studio 2017,\nwhere the growth and equilibrium morphologies of PDBCz were evaluated with the\nForcite energy method. The simulations employed the universal force field with\nmedium accuracy. Experimentally, monoclinic PDBCz was identified to crystallize in\nthe space group P2 /c, with lattice parameters a = 8.710 Å, b = 17.149 Å, c = 8.657 Å,\n1\nα = γ = 90°, and β = 116.339°, in good agreement with those used in our calculations9.\nAs shown in Figure R1.4a, PDBCz nanosheet can adopt two growth orientations on\nsubstrates: perpendicular (standing-up) or parallel (lying-down). To elucidate the\ngrowth mechanism and packing mode, both growth and equilibrium morphology\nmethods were employed. The growth morphology was analyzed using the attachment\nenergy method, which relates the growth rate of a given surface to the potential energy\nper unit cell released upon the addition of a new layer in vacuum. This method provides\ninsights into crystal habits under non-equilibrium growth conditions. The attachment\nenergy (E ) is defined as the energy released when a growth slice attaches to the crystal\natt\nsurface10. It can be calculated as11:\nE = E – E\natt latt slice\n16\nwhere E is lattice energy of the crystal and E is the energy of a growth slice with\nlatt slice\nthickness equal to the interplanar distance. The growth rate of a crystal facet is assumed\nto be proportional to its attachment energy: facets with the lowest E grow most slowly\natt\nand thus dominate the morphology. In contrast, the equilibrium morphology method\nconsiders that, under equilibrium conditions, crystal habits minimize the total surface\nfree energy12. The surface free energy not only governs the crystal growth process but\nalso determines the molecular orientation in the lattice. Indeed, organic molecular\npacking generally follows the principle of minimizing total surface free energy13.\nAs shown in Figure R1.4b, our calculations of the (100), (011), and (020) facets\nreveal that the (020) surface exhibits the lowest surface free energy, corresponding to\nthe slowest growth rate and hence the largest exposed facet. This result is in excellent\nagreement with experimental observations in our work. The small difference between\nthe surface free energies of the (100) and (011) planes indicates comparable growth\nrates, reflecting the slight anisotropy in the lattice parameters along these directions.\nOverall, the horizontal growth model, with the crystal oriented parallel to the substrate,\nminimizes the system’s surface energy and thus represents the most favorable growth\nconfiguration.\nFigure R1.4 (a) The crystal growth model, where the arrows denote the preferred\ngrowth orientations. (b) The calculated surface free energies and corresponding growth\nrates of the (100), (011), and (020) facets.\nExperimentally, through real-time optical images captured at different time intervals,\nit is evident that the lateral dimensions of the nanosheets increase significantly over\ntime (Figure 3a in the main text). Although atomic force microscopy (AFM)\nmeasurements could not be performed during the dynamic growth process, the\n17\nconsistent coloration observed in the optical images suggests that the thickness of the\nnanosheets remains largely unchanged. This observation further corroborates the\ntheoretical calculations.\nTo elucidate why the outer zone of the top layer remains in the m orientation during\n2\nlateral growth after the structural transition, rather than reverting to m , we carried out\n1\ntheoretical structural relaxation simulations (Figure R1.5). When m (or m ) molecules\n1 2\nare placed positioned adjacent m (or m ) molecules within the lateral a-c plane,\n2 1\nrelaxation drives the system toward a uniform molecular orientation and ordered\npacking, confirming that heterogeneous m -m arrangements within the lateral a-c plane\n1 2\nare energetically unfavorable and spontaneously eliminated.\nFigure R1.5 Molecular arrangements before and after structural relaxation. When m\n1\n(or m ) molecules are placed positioned adjacent m (or m ) molecules within the lateral\n2 2 1\na-c plane, relaxation drives the system toward a uniform molecular orientation and\nordered packing, confirming that heterogeneous m -m arrangements within the lateral\n1 2\na-c plane are energetically unfavorable and spontaneously eliminated.\nComment 7:\n(v) I do not understand the merit of the fitting formula used in Fig. 4b, and how that\nis a proof that ‘This confirms that the enhancement of PL is arising from the structural\n18\ntransition.’ In addition, it looks to me that there is a wide variation of the inner zone\narea shown in SI Fig. 16, unlike what the authors claimed ‘Remarkably, both areas are\napproximately 5 μm2 (Fig. 4c).’\nReply to Comment 7:\nWe appreciate the reviewer’s insightful comment on the fitting formula used in\nFigure 4b (Figure 4c in the revised manuscript). In our experiments, the PL spectra of\nthe inner and outer zones were recorded under 405 nm excitation, which can penetrate\nand excite the bottom layer14-16. The excitation spot size was ~1 μm2, smaller than the\ninner zone area (~5 μm2). We define the PL intensity per unit thickness of the bottom\nlayer and the outer zone of the top layer as I , and that of the inner zone of the top layer\n0\nas I, which differs from I due to structural transition.\nt 0\nAccordingly, the PL intensity of the inner zone (I ) can be expressed as:\ninner\nI = [I × (t – 20) + I × 20] × (excitation spot size)\ninner t 0\nwhile that of the outer zone (I ) is:\nouter\nI = (I × t) × (excitation spot size).\nouter 0\nThus, the ratio can be written as :\nI /I = [I × (t – 20) + I × 20]/(I × t)\ninner outer t 0 0\nUsing this relation, the experimental data of I /I as a function of nanosheet\ninner outer\nthickness (t) were fitted, as shown in Figure 4c. The excellent agreement between\nexperiment and fitting confirms the validity of the model.\nWe apologize for the misunderstanding caused by Supplementary Figure 19 in\nrevised version, where the non-uniform scale bars created the impression of a large\nvariation in the inner zone. In the revised version, we have standardized the scale bars\nto ensure consistency.\nComment 8:\n(vi) Also, I fail to see the connection between simulation in Fig. 2f and the\nexperimental observations. How does the simulation predict a critical size of the\nnanosheet when OH occurs? How is the critical size related to the simulated adsorption\nenergy in Fig. 2g? what factors would alter these parameters? Without answers to these\nquestions, this MS remains at the laboratory report level and cannot provide guidance\n19\nto help advancement of the science in the field of OH. Furthermore, it will be helpful\nfor the community if the authors could elaborate on potential applications of their cool\ncrystal nanosheet.\nComment 8:\nWe appreciate the reviewer’s insightful question. Figure 2f presents the theoretical\nsimulations of a PDBCz molecule on SiO and BN substrates to evaluate their\n2\nadsorption energies. The adsorption energy of PDBCz-SiO system (−0.764 eV) is\n2\nlower than both its intermolecular interaction energy (−0.723 eV) and that of the\nPDBCz-BN system (close to 0 eV). This reveals that the PDBCz-SiO₂ interface\npossesses markedly stronger binding strength, primarily induced by Si-Br bonding. The\ncritical size is defined as the characteristic nanosheet dimension at which a single- to\ntwin-crystalline structural transition occurs. This transition arises when the influence of\ninterfacial interactions becomes less dominant than intermolecular interactions as the\nnanosheet grows larger. We regret that the theoretical calculations cannot be performed\nat this “critical size” scale of several micrometers, as the computational cost would be\nprohibitively high.\nHowever, we analyzed a large number of PDBCz nanosheets to identify the onset of\nthe optical heterostructure phenomenon and twin structure during the growth process\nto obtain the critical size (Supplementary Figure 18). The critical size is approximately\nthe same across samples, indicating that it is not altered by other factors.\nAlso, we find that, while varying the initial orientation or position of PDBCz\nmolecules relative to either the substrate or another PDBCz molecule may lead to\nquantitative differences in the calculated adsorption or interaction energies, the\nqualitative conclusions remain unchanged. The fundamental physical behaviors we\nobserved are robust against such variations in initial configurations.\nThe demonstrated nanosheets with intrinsic optical heterostructure open up several\npotential application opportunities. First, the distinct PL contrast between the inner and\nouter zones can be exploited for high‐resolution optical coding, data encryption, and\nanticounterfeiting, where spatially confined optical responses are highly desirable.\nSecond, the enhanced emission efficiency in the inner zone offers promising prospects\nfor light‐emitting devices and integrated photonic components, in particular for\nmicroscale lasers and optical waveguides, where locally regulated luminescence is\n20\ncritical. Third, the well‐defined structural transition underlying the optical\nheterostructure provides a new platform for constructing logic elements and\noptoelectronic circuits that utilize spatially separated emissive regions as functional\nunits. Finally, the simple single‐component nature and vapor‐phase growth\ncompatibility of these nanosheets make them attractive for scalable integration into\nflexible and multifunctional organic optoelectronic devices. We therefore believe that\nthe unique structural-optical correlation uncovered here will inspire new design\nstrategies for advanced organic materials and their device applications.\nThe manuscript submitted by Kan Liao et al. reports an intrinsic OH uniquely\nobserved in a 2D nanosheet composed of a single organic component. This discovery\nis highly intriguing, as it represents the first demonstration of such a phenomenon in\npurely organic 2D systems. The authors provide a convincing mechanistic explanation,\nattributing the OH to an island growth process and the presence of a twin structure, both\nof which are rarely reported in organic nanosheets. These findings not only shed light\non the fundamental physics of organic 2D materials but also open new directions for\nthe design of multifunctional optical materials. I therefore strongly recommend\npublication in Nature Communications after minor revisions. A few clarifications and\nminor issues should be addressed prior to acceptance.\nReplies to Reviewer #2’s Comments:\nWe sincerely appreciate Reviewer #2’s positive evaluation, recognizing that our\nfindings not only provide new insights into the fundamental physics of organic 2D\nmaterials but also open promising avenues for the design of multifunctional optical\nmaterials. In response to the reviewer’s constructive comments, we have carefully\nrevised and improved the manuscript. Below, we provide detailed, point-by-point\nreplies to each comment.\nComment 1:\nThe authors identified a twin structure in the PDBCz organic nanosheets. To facilitate\na deeper understanding of its crystallographic features, please provide the molecular\npacking structures based on the unit cell along the a-, b-, and c-crystallographic axes,\nrespectively.\nReply to Comment 1:\nWe appreciate the reviewer’s insightful suggestion. Figure R2.1 presents the\nmolecular packing structures of the unit cell viewed along the a-, b-, and c-\ncrystallographic axes. The white, gray, blue, and brown spheres correspond to hydrogen,\ncarbon, nitrogen, and bromine atoms, respectively. The three colored arrows denote the\ncrystallographic axes a, b, and c, while the black solid lines mark the boundary of a\nsingle unit cell. The PDBCz crystal belongs to the P2₁/c space group, with lattice\nparameters of a = 8.710 Å, b = 17.149 Å, c = 8.657 Å, α = γ = 90°, and β = 116.339°.\nand c-crystallographic axes for m (a-c) and m (d-f) structure. The white, gray, blue,\n1 2\nand brown spheres correspond to hydrogen, carbon, nitrogen, and bromine atoms,\nrespectively. The three colored arrows denote the crystallographic axes a, b, and c,\nwhile the black solid lines mark the boundary of a single unit cell. The PDBCz crystal\nbelongs to the P2₁/c space group, with lattice parameters of a = 8.710 Å, b = 17.149 Å,\nc = 8.657 Å, α = γ = 90°, and β = 116.339°.\nComment 2:\nThe authors propose that the nanosheet follows an island growth mode. It is\nrecommended to further clarify the relative growth rates and corresponding surface\nenergies of the major crystal facets, in order to gain deeper insight into the growth\nkinetics.\nReply to Comment 2:\nWe appreciate the reviewer’s valuable suggestion. To obtain deeper insights into the\ngrowth kinetics, we performed theoretical calculations to elucidate the relative growth\nrates and surface energies of the major crystal facets.\nThe theoretical calculations were performed using Materials Studio 2017, where the\n23\ngrowth and equilibrium morphologies of PDBCz were evaluated with the Forcite\nenergy method. The simulations employed the universal force field with medium\naccuracy. Experimentally, monoclinic PDBCz was identified to crystallize in the space\ngroup P2₁/c, with lattice parameters a = 8.710 Å, b = 17.149 Å, c = 8.657 Å, α = γ =\n90°, and β = 116.339°, in good agreement with those used in our calculations9.\nAs shown in Figure R2.2 a, PDBCz nanosheet can adopt two growth orientations on\nsubstrates: perpendicular (standing-up) or parallel (lying-down). To elucidate the\ngrowth mechanism and packing mode, both growth and equilibrium morphology\nmethods were employed. The growth morphology was analyzed using the attachment\nenergy method, which relates the growth rate of a given surface to the potential energy\nper unit cell released upon the addition of a new layer in vacuum. This method provides\ninsights into crystal habits under non-equilibrium growth conditions. The attachment\nenergy (E ) is defined as the energy released when a growth slice attaches to the crystal\natt\nsurface10. It can be calculated as11:\nE = E – E\natt latt slice\nwhere E is lattice energy of the crystal and E is the energy of a growth slice with\nlatt slice\nthickness equal to the interplanar distance. The growth rate of a crystal facet is assumed\nto be proportional to its attachment energy: facets with the lowest E grow most slowly\natt\nand thus dominate the morphology. In contrast, the equilibrium morphology method\nconsiders that, under equilibrium conditions, crystal habits minimize the total surface\nfree energy12. The surface free energy not only governs the crystal growth process but\nalso determines the molecular orientation in the lattice. Indeed, organic molecular\npacking generally follows the principle of minimizing total surface free energy13.\nAs shown in Figure R2.2 b, our calculations of the (100), (011), and (020) facets\nreveal that the (020) surface exhibits the lowest surface free energy, corresponding to\nthe slowest growth rate and hence the largest exposed facet. This result is in excellent\nagreement with experimental observations in our work. The small difference between\nthe surface free energies of the (100) and (011) planes indicates comparable growth\nrates, reflecting the slight anisotropy in the lattice parameters along these directions.\nOverall, the horizontal growth model, with the crystal oriented parallel to the substrate,\nminimizes the system’s surface energy and thus represents the most favorable growth\nconfiguration.\ngrowth orientations. (b) The calculated surface free energies and corresponding growth\nrates of the (100), (011), and (020) facets. The (020) surface exhibits the lowest surface\nfree energy, corresponding to the slowest growth rate and hence the largest exposed\nfacet. Overall, the island growth model, with the crystal oriented parallel to the substrate,\nminimizes the system’s surface energy and thus represents the most favorable growth\nconfiguration.\nComment 3:\nFigure S12 shows that plasma treatment can effectively eliminate the optical\nheterojunction phenomenon in the upper layer. To further verify the reliability of this\nmethod, it is recommended to provide experimental results on thinner samples after\nplasma treatment.\nReply to Comment 3:\nWe appreciate the reviewer’s valuable suggestion. As shown in the top row of Figure\nR2.3, a PDBCz nanosheet with thickness of 204.5 nm exhibits optical heterostructure\nphenomenon. After plasma etching, the nanosheet was thinned to 8.7 nm, which is even\nthinner than the 17.8 nm sample discussed in the main text. As shown in the bottom\nrow of Figure R2.3, the optical heterostructure phenomenon disappears in the etched\nnanosheet, confirming that plasma treatment can effectively eliminate the optical\nheterostructure phenomenon in the upper layer.\n(the top row) and after (the bottom row) plasma etching treatment.\nComment 4:\nThe authors propose that the twin structure transition in the nanosheets originates\nfrom a synergistic competition between intermolecular interactions and the interaction\nb interaction\nbetween Br atoms and the silicon substrate. To verify the generality of this mechanism,\nit is recommended to include observations of the OH phenomenon on SiO₂/Si substrates\nusing structurally similar molecules that do not contain Br atoms.\nReply to Comment 4:\nWe appreciate the reviewer’s valuable suggestion. We have synthesized 1,4-di(-H-\ncarbazol-9-yl)benzene (PDCz)4 and 1.3-bis(N-carbazoly)benzene (mCP)17 nanosheets\nas shown in Figure R2.4, whose molecular structures are similar to that of PDBCz but\nwithout bromine atoms. Specifically, PDCz adopts a para-carbazole configuration,\nwhereas mCP adopts an ortho-carbazole configuration. As shown in their fluorescence\nimages, neither material exhibits the optical heterostructure phenomenon. Theoretical\ncalculations also indicate that the PDCz-SiO system (−0.443 eV) exhibits a weaker\n2\nmolecule-substrate interaction compared with its intermolecular interaction energy\n(−1.071 eV), suggesting that the interfacial interaction is weaker than the\n26\nintermolecular coupling (Figure 2h in the main text).\nFigure R2.4 The molecular structure, optical and fluorescence images of PDCz (the\ntop row) and mCP (the bottom row) nanosheet on SiO₂/Si substrates.\nComment 5:\nOn page 6 of the manuscript, the authors provide adsorption energy values for both\nSiO and BN substrates, along with intermolecular interaction energies. How these\n2\nvalues were computationally derived?\nReply to Comment 5:\nWe thank the reviewer for the question. Regarding the first-principles calculations,\nwe employed the VASP software, with detailed computational methodologies already\ndescribed in the Method of main text. For the adsorption energy calculations, we strictly\nfollowed the formula:\nE =E – E – E\nads total PDBCz sub\nwhere E gives the adsorption energy, E represents the energy of a single PDBCz\nads total\nmolecule adsorbed on the substrate, E denotes the energy of an isolated PDBCz\nPDBCz\nmolecule, and E is the energy of the bare substrate. Both pre-adsorption and post-\nsub\nadsorption structures were fully relaxed, with the detailed numerical results presented\nin Table R2.1.\nRegarding the intermolecular interactions between PDBCz molecules, we calculated:\n1) The energy of two isolated PDBCz molecules (-359.936 × 2 = -79.872 eV), 2) The\n27\nenergy of two PDBCz molecules interacting with each other (-720.595 eV). The\nintermolecular interaction energy was then obtained by taking the difference between\nthese values (-0.723 eV).\nWhile regarding the intermolecular interactions between PDCz molecules, we\ncalculated: 1) The energy of two isolated PDCz molecules (-363.863 × 2 = -727.726\neV), 2) The energy of two PDCz molecules interacting with each other (-728.797 eV).\nThe intermolecular interaction energy was then obtained by taking the difference\nbetween these values (-1.071 eV).\nTable R2.1 The calculated adsorption energy for both SiO and BN substrates.\n2 2\nE /E (eV) -356.231 -356.143 -360.711 -360.711\nPDBCz PDBCz\nE (eV) -115.293 -1425.022 -115.293 -1425.022\nsub\nE (eV) -472.288 -1781.166 -476.447 -1785.733\ntotal\nE (eV) -0.764 -0.001 -0.443 0\nads\nThis work reports an optical heterostructure that exhibits different emission\nintensities. I agree that optical heterostructures composed of single-component organic\nmolecules are rare. However, I cannot recommend acceptance of this work due to the\nfollowing concerns.\nReplies to Reviewer #3’ comments:\nWe sincerely thank Reviewer #3 for acknowledging the rarity of realizing optical\nheterostructures in single-component organic systems. We fully understand the\nreviewer’s concerns regarding the mechanistic explanation and experimental evidence.\nIn the revised manuscript, we have substantially strengthened the work to address these\npoints.\nComment 1:\nWhy do the inner and outer zones exhibit different emission intensities? The authors\nattribute this to structural transition but fail to provide a clear explanation.\nReply to Comment 1:\nWe sincerely thank the reviewer for raising this important question. The distinct\nemission intensities between the inner and outer zones arise from a structural transition\nconfined to the inner zone in the top layer of the nanosheet.\n1). Evidence of structural-transition-induced optical heterostructure.\n1) From direct experimental observations, the region undergoing the structural\ntransition precisely coincides with the area exhibiting enhanced photoluminescence at\nthe nanosheet center. Quantitatively, the area of the transition zone—defined as the\nnanosheet region where both the optical heterostructure phenomenon and twin\nstructures first emerge during growth—closely matches that of the PL-enhanced inner\nzone in fully developed nanosheets (both ≈ 5 μm2; Figure R3.1, also see Figure 4d in\nthe main text). Moreover, as the nanosheets grow laterally, the outer zone continuously\nexpands while the inner zone remains unchanged, reflecting the fixed size of the\ntransition zone (Supplementary Figure 20). This one-to-one spatial correspondence\nbetween the structural transition and the PL enhancement region provides compelling\nevidence that the OH phenomenon originates directly from the structural transition.\nzones.\nFigure R3.2 Time-resolved PL (TRPL) spectra of the inner and outer zones, revealing\na shorter lifetime and an additional fast radiative recombination pathway in the inner\nzone.\n(2) Mechanistically, the enhanced emission in the inner zone stems from the\nstructural transition. Time-resolved photoluminescence (TRPL) measurements reveal\nthat the inner zone—corresponding to the structural transition region—exhibits a\n30\nmarkedly shorter PL lifetime but a significantly higher emission intensity than the outer\nzone (Figure R3.2, also see Figure 4e in the main text). Detailed fitting of the PL decay\ncurves reveals an additional ultrafast radiative component (~0.15 ns) unique to the inner\nzone, which is absent in the outer region. This newly introduced fast radiative\nrecombination pathway, triggered by the structural transition, accelerates carrier\nrecombination and enhances the overall PL efficiency, thereby accounting for the\npronounced emission enhancement localized within the inner zone. To determine\nwhether the difference in PL intensity originates from variations in absorption, we have\nalso measured the reflectance and transmittance spectra of both regions (as shown in\nFigure 1e of the revised main text), indicating negligible differences in optical\nabsorption between the inner and outer zones.\nTogether, these results unambiguously demonstrate that the structural transition\nserves as the physical origin of the optical heterostructure, both in spatial\ncorrespondence and in carrier recombination dynamics.\n2). Evidences for the existence of the structural transition.\n(1) Clear contrasts in surface potential and phase are observed between the inner and\nouter zones of the nanosheet, as revealed by Kelvin probe force microscopy (KPFM)\nand electrostatic force microscopy (EFM) (Figure R3.3a, also see Figure 4a in the main\ntext). These contrasts arise from variations in the local work function and electrostatic\nforce gradient, respectively—both of which are highly sensitive to crystallinity\ndifferences induced by the structural transition18, 19.\nFigure R3.3 Optical microscope (OM), fluorescence (FL), Kelvin probe force\nmicroscopy (KPFM) and electrostatic force microscopy (EFM) images of a pristine\nthick PDBCz nanosheet (a, ~ 128 nm) and a pristine thin nanosheet (b, ~ 19 nm),\nrespectively. After annealing, the OM and atomic force microscope (AFM) images\n31\nreveal significant structural degradation localized in the central region of the thick\nnanosheet, whereas the thin nanosheet remains intact.\n(2) Since structural transitions typically render materials fragile and thermally\nunstable20-22, we performed thermal annealing tests on PDBCz nanosheets. Remarkably,\ndegradative damage occurred exclusively in nanosheets exhibiting the optical\nheterostructure phenomenon, and this damage was consistently confined to the inner\nzone, as revealed by optical and AFM images (Figure R3.3a, also see Figure 4a in the\nmain text). In contrast, nanosheets without the OH phenomenon—whether artificially\nthinned, as-grown thin, or solution-processed—showed no discernible difference\nbetween the inner and outer zones after annealing (Figure R3.3b, also see Figure 4b in\nthe main text and Supplementary Figures 11 and 16). These results demonstrate that the\nstructural transition is present only in nanosheets displaying the OH phenomenon and\nis spatially confined to the PL-enhanced inner region.\n(3) We conducted a series of thinning experiments to confirm the existence of the\nstructural transition in the top layer. Thick nanosheets were gradually thinned by plasma\netching. As the etching time increased, the nanosheet thickness decreased, and the\nluminescence contrast between the inner and outer zones progressively diminished\n(Supplementary Figure 13). Notably, when the thickness was reduced to a critical value\nof ~20 nm, the optical heterostructure phenomenon disappeared completely\n(Supplementary Figures 13-15). Similarly, as-grown nanosheets thinner than 20 nm\nexhibited no optical heterostructure behavior (Figure R3.3b, also see Figure 4b in the\nmain text). Remarkably, both the artificially thinned and as-grown thin nanosheets\nremained intact after thermal annealing (Figure R3.3b, also see Fig. 4b and\nSupplementary Figure 16). Furthermore, the diffraction patterns of these thin\nnanosheets displayed only a single lattice orientation without any twin feature (inset of\nFigure 4c in the main text). These results confirm that both the structural transition and\nthe OH phenomenon occur only in relatively thick nanosheets.\n3). Origin of the structural transition.\nDuring the island growth of the nanosheets, the top and bottom layers initially share\nthe same lattice structure (m , Figures 2c and 3b in the main text) at the early growth\n1\nstage (stage I in Figure R3.4, also see Figure 3e in the main text). This uniformity arises\n32\nfrom strong molecule-substrate interactions (Figures 2g-h in the main text), which\ndominate in small-sized nanosheets23, 24. As the nanosheet expands laterally,\nintermolecular interactions become increasingly dominant. When the nanosheet\nreaches a critical size (stage II in Figure R3.4, also see Figure 3e in the main text), the\nlattice of the top layer undergoes a transition to the m orientation—maintaining the\n2\nsame crystal framework but rotating relative to m —while the bottom layer remains\n1\nlocked in the m orientation due to its intimate contact with the substrate. This leads to\n1\nthe emergence of a twin structure comprising m + m domains (Figures 2b and 3b).\n1 2\nThe structural transition is enabled by the siliceous substrate, which forms strong\ninterfacial interactions with the molecules, possibly through Si-Br covalent bonding25\n(Figures 2g-h in the main text). Following the structural transition, the nanosheets\nundergo further lateral growth, during which the bottom layer retains the m lattice\n1\nwhile the newly formed top layer preserves the m lattice (stage III in Figure R3.4, also\n2\nsee Figure 3e in the main text).\nFigure R3.4 Schematic diagram of the optical heterostructure mechanism of a PDBCz\nnanosheet with three important stages of growth process.\nComment 2:\nThe authors suggest that the formation of the heterostructure is strongly influenced\nby interactions between the PDBCz nanosheets and substrates, yet no experiments were\nconducted to investigate these interfacial interactions.\nReply to Comment 2:\nWe sincerely thank the reviewer for this valuable comment. We fully agree that direct\ncharacterization of molecule-substrate interfacial interactions is extremely challenging,\nespecially for molecularly thin organic nanosheets, where the interface is buried and\nconventional surface‐sensitive techniques (such as XPS or STM) are difficult to apply\nwithout disturbing the crystal structure. To address this issue, we performed a\n33\nsystematic series of controlled experiments to indirectly yet convincingly probe the\ninterfacial effect, as summarized in Table R3.1 as follows.\nFirst, PDBCz nanosheets grown by physical vapor deposition on siliceous substrates\n(SiO₂/Si, Si, and quartz) consistently exhibited the optical heterostructure phenomenon\nand the twin structure (Figure 1c in the main text and Supplementary Figure 7). In\ncontrast, PDBCz nanosheets grown on non‐siliceous substrates (BN, mica, and sapphire)\nand solution‐processed nanosheets (where molecule-substrate interactions are\nnegligible) displayed uniform photoluminescence without any optical heterostructure\nfeature (Figures 2d-e in the main text and Supplementary Figure 6).\nSecond, to further isolate the chemical contribution of substrate elements, we\nsynthesized and deposited structurally similar carbazole derivatives lacking bromine\natoms, namely 1,4-di(9H-carbazol-9-yl)benzene (PDCz) and 1,3-bis(N-\ncarbazolyl)benzene (mCP), on the SiO /Si substrates. Neither of these bromine-free\n2\nmolecules exhibited the optical heterostructure phenomenon. These results indicate that\nthe interfacial interaction between Si and Br plays a crucial role in triggering the\nstructural transition responsible for the optical heterostructure (Supplementary Figure\n8).\nThird, first-principles calculations further support this interpretation. For the\nPDBCz-SiO system, the adsorption energy (−0.764 eV) is lower than both its\n2\nintermolecular interaction energy (−0.723 eV) and that of the PDBCz-BN system (close\nto 0 eV), while the PDCz-SiO system (−0.443 eV) shows a higher adsorption energy\n2\nthan its intermolecular interaction energy (−1.071 eV) (Figure 2h in the main text). This\ncontrast reveals that the PDBCz-SiO₂ interface possesses markedly stronger binding\nstrength, primarily induced by Si-Br bonding. Consistently, charge‐density analysis\n(Supplementary Figure 9) clearly shows electron density accumulation between Si and\nBr atoms, confirming the formation of covalent Si-Br bonds at the interface.\nTogether, these results provide compelling evidence that the optical heterostructure\nphenomenon originates from strong interfacial coupling between PDBCz and siliceous\nsubstrates via Si-Br bonding, which modulates the balance between interfacial and\nintermolecular interactions during crystal growth. Although direct measurement of this\nburied interface is experimentally challenging, the combined comparative experiments\nand theoretical calculations unambiguously reveal the decisive role of interfacial\ninteractions in the formation of the heterostructure.\nPresence Presence Observation of Adsorption\nGrowth Substrate Molecular\nof Si of Br optical energy Interpretation\nMethod Type Structure\nElement Element heterostructure (eV)\nLower than its intermolecular interaction\nenergy (−0.723 eV). Strong Si-Br\nPVD SiO\n2\n/Si PDBCz ✓ ✓ Yes −0.764\ninterfacial interaction induces twin\nstructure and optical heterostructure.\nStrong Si-Br interfacial interaction\nPVD Quartz PDBCz ✓ ✓ Yes −− induces twin structure and optical\nheterostructure.\nWeak van der Waals interaction, absence\nPVD BN PDBCz ✗ ✓ No 0\nof Si-Br bonding.\nNon-siliceous substrates, weak\nPVD Mica/Sapphire PDBCz ✗ ✓ No −−\ninterfacial coupling.\nHigher than its intermolecular interaction\nPVD SiO\n2\n/Si PDCz/mCP ✓ ✗ No −0.443 energy (−1.071 eV). Lack of Br prevents\nSi-Br bond formation.\nSolution Weak molecule-substrate interaction\nSiO\n2\n/Si PDBCz ✓ ✓ No −−\ngrowth under solution growth.\nDid the authors examine the emission properties from different sides of the bulk\ncrystals? Is there any variation in emission intensity?\nReply to Comment 3:\nWe thank the reviewer for raising this important point about facet-dependent\nemission, a key property for bulk crystals. In our work, we intentionally employ a PVD\nmethod based on strong molecule-substrate interactions and island growth to form\noriented nanosheets, as these are directly relevant for the optical heterostructures we\ntarget. While this approach does not yield the bulk crystals necessary for the suggested\nexperiment, it allows us to probe a different but equally critical aspect: in-plane\nuniformity. Critically, we would like to emphasize that the observed contrast in\nemission between the inner and outer zones is not a consequence of differing\ncrystallographic orientations. We confirm that all PDBCz nanosheets exhibit identical\ncrystallographic orientation along the b-axis, as demonstrated by both selected-area\nelectron diffraction (SAED) and X-ray diffraction (XRD) analyses.\nSpecifically, the SAED patterns collected from multiple zones of the same nanosheet\n(Figure 2a in the main text and Supplementary Figures 4-5) show consistent diffraction\nspots corresponding to the (020) family planes, indicating that the entire nanosheet—\nincluding both the inner and outer regions—shares the same in-plane orientation. The\nXRD data (Supplementary Figure 1) further show sharp and single (020) diffraction\npeaks, confirming the preferred orientation of all nanosheets along the b-axis. Therefore,\nthe observed PL contrast between the inner and outer regions cannot originate from\ndifferent exposed crystal facets, as the crystal orientation remains identical across the\nnanosheet.\nFurthermore, theoretical calculations of the relative growth rates and surface free\nenergies of the major crystal facets reveal that the (020) surface—which corresponds to\nthe b-axis orientation—possesses the lowest surface energy and slowest growth rate\n(Figure 3c in the main text). This theoretical result is fully consistent with the\nexperimental observation that PDBCz nanosheets preferentially grow with the (020)\nfacet exposed parallel to the substrate.\nTogether, these results unequivocally demonstrate that all nanosheets are oriented\nalong the b-axis, and the PL enhancement in the inner zone does not stem from different\ncrystal faces. Instead, it arises from the out-of-plane twin structural transition induced\n36\nby the interplay between molecule-substrate and intermolecular interactions, as\ndescribed in the main text.\nComment 4:\nMany key conclusions in the manuscript are speculative and lack direct evidence,\nwhich is not scientifically rigorous. The authors should perform more detailed\nmechanistic studies.\nReply to Comment 4:\nWe sincerely thank the reviewer for this critical comment, which has helped us\nsignificantly strengthen the mechanistic discussion in our manuscript. We agree that\nscientific rigor is paramount and have undertaken a comprehensive effort to move from\nthe initial observation of a novel phenomenon toward a more substantiated mechanistic\nunderstanding.\nThe core of our revised argument is that the observed optical heterostructure—\nthe brighter emission from the inner zone of the nanosheet—exists precisely\nbecause a solid-state structural transition to a twin crystal occurs locally in the top\nlayer of the inner zone, and this transition locally enhances the photoluminescence\nquantum yield.\nThis unified model is supported by the following key lines of evidence, which we\nhave elaborated in the revised text:\nA. Evidence for a Localized Twin-Structure Transition:\n1. Direct Structural Evolution: Structural characterization of nanosheets at\ndifferent growth stages reveals a clear evolution: the selected area electron diffraction\n(SAED) pattern transitions from a single-crystal (m ) pattern to a twin-crystal (m + m )\n1 1 2\npattern (Figure 3b).\n2. Spatial Localization to the Center: The inner zone exhibits distinct behavior\nunder thermal stress (showing damage) and in electrical potential (KPFM/EFM),\nconfirming its unique structural identity compared to the outer zone without transition\nzone (new data in Figures 4a-b).\n3. Confinement to the Top Layer: Thin nanosheets (<20 nm) exhibit only the\nsingle-crystal pattern and show no differences between inner and outer zones in all\n37\ncharacterizations, proving that the structural transition is confined to the top layers of\nthe nanosheet (new data in Figure 4b and Supplementary Figures 13-15).\n4. Driving Force Identified: The strong molecule-substrate interaction, primarily\nvia Si-Br bonding, is the key driver. This is confirmed by control experiments:\nnanosheets grown from solution method, on BN substrates, or using Br-free analogous\nmolecules all fail to form the twin structure (Figures 2d-h including some new data).\n5. Energetic Plausibility of the Growth Mode: Theoretical calculations confirm\nthat the observed island-growth mode and the resulting coherent lattice between inner\nand outer zones are energetically favorable (new data in Figure 3c-d).\nB. Evidence Correlating the Twin Transition with the Optical Heterostructure:\n1. Temporal Co-occurrence: The twin structure and the optical heterostructure\nemerge simultaneously during the dynamic growth process (Figure 3b).\n2. Universal Co-existence: The two phenomena consistently appear together\nacross a wide range of conditions, including different molecular designs, growth\nmethods, substrates, and thicknesses, as shown in Figures 2d-e and Supplementary\nFigures 7-8 with some new data.\n3. Universal Absence in their Absence: Thin nanosheets (< 20 nm) that do not\nundergo the structural transition also show no sign of the OH, demonstrating that one\nis a prerequisite for the other (new data in Figure 4b).\n4. Spatial Correlation: The spatial extent of the structurally transformed region\naligns almost perfectly with the bright inner zone region of the optical heterostructure\n(Figure 4d).\n5. Mechanism for Enhanced Emission: Time-resolved PL (new data in Figure\n4e) reveals a faster and more complex decay in the inner zone, while absorbance\nremains unchanged (new data in Figure 1e), directly pointing to an enhanced radiative\nefficiency in the transition region.\n6. Quantitative Model Validation: The proposed model—where only the top\nlayer of the inner zone has enhanced intensity (I) and all other regions have a baseline\nt\nintensity (I )—perfectly fits the PL statistical data from a large number of nanosheets\n0\nof different thicknesses (Figure 4c with new description in Supplementary Note V).\nWe acknowledge the profound challenge of obtaining direct, molecular-level\n38\nevidence of dynamic structural transitions in fragile organic micro-nano systems, where\nreal-time tracking of transient phases remains exceptionally difficult. Over the past six\nyears, we have exhaustively employed all accessible characterization techniques to\nconstruct the multi-faceted, corroborating evidence presented here.\nWe are convinced that the discovery of an optically distinct heterostructure arising\nfrom a solid-state transition in an organic system opens a new and fertile platform for\nexploration. To actively encourage this, we have explicitly outlined the open questions\nand future possibilities in the revised manuscript. By framing our work in this way, we\nhope to invite the broader community to join us in uncovering deeper mechanistic\ninsights and more compelling evidence. We believe this phenomenon will stimulate\nfurther thinking and highlight that organic micro-nano crystals host a wealth of\nunexplored science related to structural dynamics and property control. We hope the\nreviewer finds our strengthened evidence and clarified discussion persuasive and agrees\nthat our work provides a valuable and intriguing foundation for future studies.\n1. Leyre, S., et al. Absolute determination of photoluminescence quantum efficiency\nusing an integrating sphere setup. Rev. Sci. Instrum. 85, 12 (2014)\n2. Würth, C., et al. 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The authors should add a sentence clarifying this to avoid misunderstanding.\n2. regarding the fitting formula for Fig. 4c: suggest adding more clear description to It and I0, to reflect that It only occurs for the top layer (with m2 structure).", "reviewer_2": "I am satisfied with the revision and it could be accepted.", "reviewer_3": "I appreciate the authors for their efforts on revising the manuscript, and the paper can be accepted as its current form."}, "author_response": ""}]

Decision letter