Atomically confined excitons in 2D perovskites for bright and sub-nanosecond scintillation

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

Abstract
Ultrafast radiation detection is crucial for advancing medical imaging, high-energy physics, astronomy, and industrial applications, offering high spatio-temporal resolution and reduced radiation exposure. However, the bottleneck lies in the formidable challenge of achieving scintillators with both ultrafast response and high efficiency. Contrasting with previous approaches that focused on molecular-level confinement, here we propose the strategy of pushing exciton confinement to the limit of atomic scale. Using 2D perovskites as a model, we design organic A-site cations to selectively enhance in-plane distortion to localize excitons, while suppressing out-of-plane and intra-octahedral distortions to minimize the formation of inefficient, long-lived self-trapped excitons. Specifically, (1,4-CMA)PbBr4 exhibits a rare combination of fast response (0.62 ns) and high light yield (19,700 photons MeV−1), surpassing leading commercial and research scintillators. These properties enable breakthroughs in advanced imaging, including fast positron emission tomography with timing precision of 43.3 ps and high-resolution X-ray imaging (32 lp mm−1).

Introduction
Scintillators, as a fundamental technology, are widely applied in medicine, high-energy physics, astronomy, and industry, where fast response and high efficiency are universally pursued goals1–3. Ideal scintillators with these properties enable high spatiotemporal resolution while minimizing radiation doses for medical X-ray computed tomography (CT) and positron emission tomography (PET) scans, as well as aid in registering intense events for vertex localization in high-energy physics, astronomy, and nuclear reaction4,5. Nevertheless, achieving scintillators with both ultrafast response and high efficiency remains a formidable challenge6,7. According to Fermi’s Golden Rule, response speed is proportional to the transition moment between initial and final states, whereas efficiency corresponds to the density of emission states8,9. Conventional inorganic scintillators (e.g., LaBr3:Ce, LYSO:Ce) exhibit long response times on the order of tens of nanoseconds due to the limited dipole moment of rare earth d-f transitions7,10,11. While core-valence and intra-band luminescence (e.g., BaF2) can enhance transition matrix and achieve sub-nanosecond response, charge migration within bands seriously reduces the density of emission states, leading to poor efficiencies (<3000 photons MeV−1)12.

Exciton-based scintillators have emerged as promising alternatives13. Stronger exciton confinement enhances both the transition dipole moment and the emission state density, offering the potential for both ultrafast response and high efficiency. As a result, materials like nanocrystals14–16, organic materials17–21, and 2D perovskites22,23 have achieved response speeds below 10 ns. However, the sub-nanosecond barrier has rarely been broken. The fundamental limitation lies in the insufficient degree of exciton confinement in current schemes, which only allow for molecular-level confinement, typically involving hundreds to thousands of atoms (Fig. 1a). For example, nanocrystals comprise >1000 atoms, while organic molecules contain >100 atoms. In 2D perovskites, although excitons are confined in the out-of-plane direction within a single lattice (three atoms), they still extend across tens or more atoms in the in-plane direction, resulting in either unsatisfactory light yield or slow response speed.

To overcome this dilemma, we propose the strategy of pushing exciton confinement to the limit of atomic scale. Using 2D perovskites as a model system, we apply in-plane lattice distortion along with out-of-plane dielectric screening to achieve atomic-level exciton confinement (Fig. 1b). The key lies in the synergistic regulation of steric hindrance and polarity of organic A-site cations to selectively enhance in-plane distortion while suppressing out-of-plane and intra-octahedral distortions. The latter two distortions, if not properly controlled, can lead to inefficient, long-lived self-trapped exciton (STE) states (Supplementary Fig. 1)24–31.

Following these rules, we design a series of 2D perovskites with tailored A-site cations, including cyclohexanemethylamine (CMA), 4-aminomethyl-1-cyclohexanecarboxylate (CMA-COOH), and cyclohexane-1,4-diyldimethanamine (1,4-CMA) (Supplementary Fig. 2). All three compounds fall into the rare category of high light yield and fast response (Fig. 1c, Supplementary Table 1). Among them, (1,4-CMA)PbBr4 achieves a remarkable level of exciton confinement by combining strong in-plane tilting with intense out-of-plane dielectric confinement. This effective confinement in all dimensions endows (1,4-CMA)PbBr4 with a rare combination of sub-nanosecond radioluminescence lifetime (0.62 ns) and high light yield (19,700 photons MeV−1). These properties enable breakthroughs in advanced imaging applications. We demonstrate PET detection with a coincidence timing resolution (CTR) of 43.3 ps, allowing for reconstruction-free PET imaging with significantly reduced radiation dose and acquisition time. Additionally, its fast response ensures minimal afterglow, enabling dynamic X-ray imaging with high resolution (32 lp mm−1).

Results
Structural distortions influenced by A-site cations
First, we investigate the exciton confinement behavior in 2D perovskites. While their layered structure inherently isolates adjacent layers and the low dielectric constant of A-site cations ensures well-established out-of-plane quantum and dielectric confinement, the in-plane exciton behavior and the influence of lattice distortions remains less understood.

Structural distortions in 2D perovskites primarily fall into three categories: intra-octahedral distortions, where Pb2+ deviates from the octahedral center, altering the Br-Pb-Br bond angles; out-of-plane octahedral tilting, which changes the Pb-Br-Pb bond angles along the stacking direction; and in-plane octahedral tilting, which affects Pb-Br-Pb bond angles within the inorganic plane (Fig. 2a). By analyzing our synthesized 2D perovskite materials alongside previously reported results, we identify a common trend—both intra-octahedral distortions and out-of-plane tilting tend to induce STE states, leading to slow and inefficient emission (Supplementary Fig. 3). Consequently, in-plane tilting emerges as a rare but effective strategy to achieve exciton confinement without triggering STE formation.

We further examine the structural origins of these three types of distortions and their correlation with A-site molecular properties (Fig. 2b–d). The intra-octahedral distortion is influenced by the symmetric environment created by the nearest eight A-site cations, which exert electrostatic attraction and repulsion on the central Pb2+32. In a highly symmetric arrangement, Pb2+ remains at the octahedral center, whereas asymmetric polarization induces its displacement (Fig. 2b). Thus, intra-octahedral distortion value (θ2) is primarily affected by the C-N symmetry of the nearest A-site molecules, as well as the spatial arrangement of their outer carbon skeletons. For example, in [S-N]2PbBr4 (S-N represents S-(−)−1-(1-naphthyl)ethylammonium), the highly asymmetric C-N configuration induces significant polarity, resulting in pronounced intra-octahedral distortion (Supplementary Fig. 4)33. Meanwhile, out-of-plane tilting (Dout) is more significantly affected by the centrosymmetry of the four organic molecules in the adjacent upper and lower layers (Fig. 2c). In (1,3-PDA)PbBr4 (1,3-PDA represents 1,3-phenylenediammonium), the asymmetric arrangement of both the C-N ends and phenylamine across the upper and lower octahedral layers leads to substantial out-of-plane distortion (Supplementary Fig. 5)34. On the other hand, the steric hindrance of the functional ammonium group, which fits in the gap between Pb-Br octahedra, determines the in-plane tilting (Fig. 2d). Smaller functional groups induce more contraction and in-plane tilting (Supplementary Fig. 6)35,36.

Based on these findings, we identify key design principles for ideal A-site cations: 1) Strong symmetry in both the C-N configuration and the outer molecular skeleton to minimize intra-octahedral distortions. 2) Centrosymmetric arrangement across layers, eliminating polarity-driven out-of-plane tilting. 3) Small functional ammonium groups to facilitate in-plane tilting.

Structure-property relationship
To implement these principles, we design non-conjugated molecular additives (CMA) and their derivatives as A-site cations to create a highly symmetric environment (Supplementary Fig. 2 and Supplementary Fig. 7). Unlike previously reported materials, three compounds—(CMA)2PbBr4, (CMA-COOH)2PbBr4 and (1,4-CMA)PbBr4—exhibit pronounced in-plane tilting while maintaining minimal intra-octahedral and out-of-plane distortions (<2°) (Fig. 3a, b, Supplementary Fig. 3 and Supplementary Tables 2–6). Their photoluminescence spectra feature remarkably narrow emission peaks (FWHM of ~30 nm), and temperature-dependent measurements reveal minimal linewidth broadening, providing strong evidence against STE formation (Fig. 3c).

To further validate our design rationale, we introduce an asymmetric A-site cation, 1,3-CMA, as a control to induce an asymmetric polarization environment around Pb-Br octahedron. As expected, (1,3-CMA)PbBr4 exhibits significant out-of-plane tilting and intra-octahedral distortion, accompanied by a broad STE emission peak (Inset of Fig. 3c).

We then quantify exciton diffusion dynamics by performing transient photoluminescence microscopy (TPLM) on single crystals (Fig. 3d and Supplementary Fig. 8-9 and Supplementary Note I). The diffusion constant D is determined by D=(σt2−σ02)/4t, where σt2 represents the time-dependent variance of the exciton population, and σ02 corresponds to the initial variance. The diffusion coefficient of (1,4-CMA)PbBr4 is 0.0238 cm2 s−1, significantly smaller than BM2PbBr4 (0.0925 cm2 s−1), indicating much stronger in-plane exciton confinement than BM2PbBr4. The serious in-plane exciton diffusion of BM2PbBr4 also explains its relatively low light yield (~3,000 photons MeV−1)23.

We also calculate the electrostatic potentials of the organic amine molecules (Fig. 3e and Supplementary Fig. 10). 1,4-CMA exhibits the lowest electrostatic potential, which enables stronger out-of-plane dielectric confinement37,38. Consequently, the exciton binding energy (Eb) of (1,4-CMA)PbBr4 reaches 363.4 meV, slightly higher than (CMA-COOH)2PbBr4 (305.9 meV) and (CMA)2PbBr4 (265.4 meV) (Fig. 3f), and much higher than previously reported 2D perovskites, including (PEA)2PbBr4 (190 meV) and (BA)2PbBr4 (260 meV)23,39.

We further evaluate the photoluminescence performance through fluence-dependent photoluminescence quantum yield (PLQY) measurements, which reveal that all samples exhibit a characteristic rise-to-roll-off trend (Supplementary Fig. 11). The initial rise suggests efficient filling of non-radiative trap states at lower excitation densities. The subsequent universal roll-off is attributed to intensified exciton-exciton interactions at high exciton densities, where the decreased average distance between excitons significantly raises the probability of synergistic energy loss through multiple recombination pathways31.

Scintillation performance
We then characterize the radioluminescence (RL) properties. During the scintillation process under X/γ-ray excitation, primary high-energy electrons are initially generated and rapidly thermalize (on the picosecond to femtosecond timescale) to form band-edge excitons. The response speed of scintillators is determined by the slow recombination process of these excitons near band edges (Fig. 4a). The atomic-level confined exciton in these 2D perovskite enable efficient and fast recombination. To verify their radioluminescence properties, we grow high-quality, large single crystals of (CMA)2PbBr4, (CMA-COOH)2PbBr4, (1,4-CMA)PbBr4, and (1,3-CMA)PbBr4 using the slow cooling solution method according to their respective solubility curves (Supplementary Fig. 12). The basic properties, including optical images, absorption spectra and band structures, are shown in Supplementary Figs. 13–15.

As presented in Fig. 4b, the RL spectra of these single crystals are measured using an integrating sphere, with the well-studied (PEA)2PbBr4 single crystal of the same size and thickness serving as a reference. The RL spectra closely resemble their corresponding PL spectra excited by ultraviolet light (inset of Fig. 3c), indicating the same radiative recombination pathway. (CMA)2PbBr4, (CMA-COOH)2PbBr4, and (1,4-CMA)PbBr4 all exhibit narrow band RL emission while (1,3-CMA)PbBr4 displays broadband STE emission. The scintillation light yields of (CMA)2PbBr4, (CMA-COOH)2PbBr4, and (1,4-CMA)PbBr4 are evaluated relative to a LYSO:Ce reference (32,000 photons MeV−1) using standard pulse-height analysis. In this setup, the crystals are optically coupled to a photomultiplier tube (PMT), and the signals excited by 241Am (59.5 keV) are recorded on an oscilloscope. The acquired spectra are presented in Fig. 4c. We calculate the yield based on the channel number of the full-energy peak centroid, incorporating a correction for the PMT’s wavelength-dependent efficiency (Supplementary Fig. 16). The full energy peak centroids for the three crystals are located at the channel number 68.52, 28.99, and 27.99, which are 1.47, 0.62, and 0.60 times that of the LYSO:Ce standard, respectively. Thus, the scintillation yields of (CMA)2PbBr4, (CMA-COOH)2PbBr4, and (1,4-CMA)PbBr4 are estimated to be 50,460, 20,460, and 19,730 photons MeV−1, respectively.

Furthermore, we record the PL and RL decay dynamics of these scintillators using a pulsed 367 nm laser and a pulsed X-ray source, respectively. The average lifetime of (CMA)2PbBr4, (CMA-COOH)2PbBr4 and (1,4-CMA)PbBr4 under laser excitation was 5.23 ns, 0.98 ns and 0.46 ns, respectively (Fig. 4d). However, the RL decay was slightly slower than PL decay (Fig. 4e), primarily because X-rays excites high-energy electrons, which are more likely to escape from their exciton-bound states, leading to longer lifetimes. This difference is further accentuated by enhanced non-radiative recombination at the surface under PL excitation—a contrast to the bulk-dominated carrier generation and comparatively lower defect-mediated recombination in RL. This also implies that, due to the energy being 3-5 orders of magnitude higher, achieving fast RL is much more challenging than for materials like phosphors or OLEDs, making exciton confinement even more crucial. For a more rigorous comparison, we adopt RL decay time. We find that (1,4-CMA)PbBr4 achieves an average scintillation lifetime of 0.62 ns, while (CMA-COOH)2PbBr4 and (CMA)2PbBr4 exhibit average lifetimes of 1.09 ns and 8.4 ns, respectively. The fast and slow component of RL decay for (CMA)2PbBr4 is 4.37 ns and 9.61 ns, while for (CMA-COOH)2PbBr4, they are 0.61 and 2.13 ns, respectively. Notably, the fast and slow component of RL decay for (1,4-CMA)PbBr4 reach as short as 0.25 and 1.16 ns, respectively (Supplementary Table 7). In comparison, BaF240—one of the fastest commercial scintillators—exhibit the fast component of 0.6 ns, lagging behind (1,4-CMA)PbBr4, underscoring the advantage of our materials.

We also assess the stability of the fastest scintillator (1,4-CMA)PbBr4 toward moisture and continuous radiation (Fig. 4f and Supplementary Fig. 17). No detectable weight loss is recorded for the sample following a 35-day period at 70% relative humidity. In terms of radiation stability, the light output is measured while the bare single crystal (without encapsulation) is irradiated with X-rays in an ambient environment (25 °C and 70% relative humidity), as shown in Fig. 4f. After continuous X-ray irradiation (916.09 mGy s−1) and repeated on-off X-ray cycles, accumulating a total dose of 1,099 Gyair, no deterioration in light output was observed. This dose is equivalent to 20,000 times CT scans, demonstrating exceptional radiation hardness. The structural and optical integrity after testing are confirmed by XRD and PL measurements, which show negligible degradation (Supplementary Figs. 18–19). The scintillation kinetics are largely retained post-irradiation (Supplementary Fig. 20), and the variations are consistent with the creation and evolution of point defects41,42, as evidenced by the stable crystal structure (Supplementary Fig. 18).

Applications in fast radiation detection and imaging
Next, we investigate the advantages of the scintillator for time-of-flight PET (TOF-PET) and dynamic X-ray imaging. In TOF-PET, CTR serves as a critical performance parameter, where a lower CTR results in better spatial resolution43. LYSO:Ce,Ca crystals were selected as a benchmark reference due to their rapid timing characteristics and high stopping power (Supplementary Fig. 21). An alternating stack consisting of three naturally grown (1,4-CMA)PbBr4 single crystal layers and four manually cut LYSO:Ce,Ca layers was constructed, with all surfaces polished. The overall dimensions were 3 × 2.8 × 3 mm3 and covered with enhanced specular reflector films, wrapped in Teflon thereafter, except for the light extraction face. This dual-layer coating can effectively reduce light escape from the crystal corners44. The reference and sample crystals are directly coupled to the optical windows of the SiPMs. A pair of detectors detects coincidence signals from two back-to-back 511 keV γ-photons produced by positron annihilation from a 22Na radioactive source. A coincidence event was recorded when two photons trigger both detectors within energy and time difference windows. The signals were selected in a coincidence window of ±1 ns filtered through an energy/voltage filter to isolate those in the photopeak region of the energy spectrum, using the valley between Compton edge and photopeak as the reference point (Fig. 5a). The acquisitions were post-processed using custom Python code to detect features like different CTR voltage thresholds for each voltage bias, analyzing and sanitizing acquired events, as detailed in previous work45.

The integrated charge spectrum shows clear full energy peaks with an energy resolution of 10.4% at 511 keV (Fig. 5a). To enhance the effective count rate of the system, a laminated configuration consisting of LYSO and perovskite was implemented. By employing a filter (0/1, 130 nV s) to process the coincidence pairs of output pulses and performing overlapping integration, a CTR of 79.6 ± 0.8 ps was achieved for the LYSO/(1,4-CMA)PbBr4 laminated detector (Fig. 5b, c), smaller than the intrinsic CTR of the LYSO detector itself of 83 ± 3 ps. To further obtain the intrinsic CTR of (1,4-CMA)PbBr4 independently, a single-layer perovskite was directly coupled to SiPM surfaces through optical grease. Remarkably, a record CTR of 43.3 ps of (1,4-CMA)PbBr4 detector was obtained (Fig. 5d). This performance significantly surpasses that of commercial scanners (~200 ps), previously reported BM2PbBr4 (207 ps), and (PEA)2PbBr4 (119 ps), and Zr-DPA MOF (85 ps), highlighting the excellent temporal resolution of the (1,4-CMA)PbBr4 (Fig. 5e)22,23,46. The improved timing resolution for TOF-PET implies the ability to diagnose smaller lesions or tumors. When the timing resolution is sufficiently precise to directly localize the source, a new regime is entered, where images can be obtained directly without the need for reconstruction steps. As shown in Fig. 5f, traditional TOF-PET requires image reconstruction to precisely locate the source position. Our detector, leveraging its ultra-short resolution, eliminates the need for reconstruction and directly accomplishes the task with just a pair of detectors. Beyond medical applications, the fast response and high efficiency of these scintillators make them highly promising for astrophysical cosmic ray detection, nuclear monitoring, border control, and high-energy physics experiments, all of which require fast response to mitigate pile-up background and accurately localize event vertexes.

Benefiting from the short decay time, (1,4-CMA)PbBr4 demonstrates a suppressed afterglow. Residual images of a round nut with a camera after 10 sec of X-ray exposure were captured (Supplementary Fig. 22). (1,4-CMA)PbBr4 presents negligible residual images in all frames, whereas the commercial CsI:Tl film shows distinct residual images even after 2 s. As shown in Fig. 5g, a screw was mounted on a slide-traction plate, and the steel plate applied a pulling force to the screw while continuous X-ray irradiation occurred (Supplementary Fig. 23). The image was captured in an interval of 30 ms. As the screw moved, the serrated structure on the outer edge of the screw was clearly visible with no residual shadow.

Furthermore, featuring a high light yield, (1,4-CMA)PbBr4 exhibits a spatial resolution of 32.3 lp mm−1 at a modulation transfer function (MTF) of 0.2, measured by the slanted-edge method (Fig. 5h). This performance outperforms commercial CsI:Tl (3–5 lp mm−1) and GOS:Tb (4–8 lp mm−1), and also exceeds recently reported perovskite (22 lp mm−1) and organic scintillators (18 lp mm−1)13,20,47–49. X-ray images of a standard resolution test chart indicate that the line pairs at 30 lp mm−1 remain clearly resolvable (Fig. 5i), validating this high spatial resolution. This exceptional imaging resolution enables the precise identification of ultra-fine internal structures, as demonstrated in a campus card (showing embedded components), an integrated circuit chip (revealing trace distributions), and a bird leg (visualizing bone structure) (Fig. 5j).

Finally, we evaluate the practical performance of (1,4-CMA)PbBr4 by comparing its radiation hardness and X-ray stopping power with commercial BaF2 (Supplementary Fig. 24). While BaF2 exhibit superior radiation resistance and higher stopping power, (1,4-CMA)PbBr4 demonstrates reasonable radiation hardness (~2% degradation after 5 kGyair) despite its lower attenuation coefficient.

Methods
Materials
Cyclohexylmethylammonium (97%), 4-(aminomethyl)cyclohexanecarboxylic acid (99%), 1,3-bis(aminomethyl)cyclohexane (98%) were purchased from Adamas Reagent Co. Ltd. 1,4-cyclohexane-diyl-dimethanamine (98%), hydrobromic acid (HBr) (40%), were purchased from Aladdin Chemical Co. Ltd. Lead bromide (99.999%) was purchased from Advanced Election technology Co. Ltd. All the chemicals were used as received without any further purification.

Preparation of single crystals and film
The (1,4-CMA)PbBr4 crystal was synthesized using the solution cooling method. PbBr2 (0.125 mmol) and 1,4-cyclohexane-diyl-dimethanamine (0.15 mmol) were dissolved in 1 mL and 4 mL of HBr, respectively. The solutions were mixed, heated to 120 °C until dissolved, then cooled at 0.5 °C h−1 to promote crystal growth. The (CMA)2PbBr4 crystal was synthesized similarly. PbBr2 (0.5 mmol) and cyclohexylmethylammonium (CMA) (1 mmol) were separately dissolved in 1 mL and 6 mL of HBr. Then the solutions were mixed, heated to 125 °C, then cooled at 2 °C h−1 to allow crystal growth. The (CMA-COOH)2PbBr4 crystal was synthesized by dissolving PbBr2 (0.5 mmol) and 4-(aminomethyl)cyclohexanecarboxylic acid (CMA-COOH) (1 mmol) in HBr. The mixed solution was heated to 100 °C and cooled at 2 °C h−1 to promote crystal growth. The (1,3-CMA)PbBr4 crystal was synthesized by dissolving PbBr2 (0.5 mmol) and 1,3-Bis(aminomethyl)cyclohexane (1,3-CMA) (0.5 mmol) in 1 mL and 3 mL of HBr, respectively. The mixed solution was heated to 125 °C and cooled at 2 °C h−1 for growth. The (1,4-CMA)PbBr4 crystal was mechanically ground for 30 min to obtain micron-sized powders. Polymethyl methacrylate (PMMA) was dissolved in ortho-xylene by magnetic stirring and heating at 100 °C until fully dissolved. The prepared powders were then mixed with PMMA at a mass ratio of 2:1, followed by mechanical stirring for 24 h. Finally, the mixture was dropped onto a mold. After drying at room temperature for 5–6 h, the film was formed and could be peeled off.

Materials characterizations
Single-crystal X-ray diffraction measurements were conducted on an XtaLAB PRO MM007HF X-ray diffractometer equipped with a liquid nitrogen cooling system and a Cu Kα radiation source. Powder XRD measurements were performed using a Rigaku MiniFlex600 X-ray diffractometer, utilizing a Mo Kα X-ray tube (λ = 0.71073 Å), operating at 40 kV and 15 mA, with a step size of approximately 0.08° s−1. Photoluminescence and photoluminescence excitation spectra were recorded using an FL3-22 time-resolved fluorescence spectrometer. Ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra were obtained using a Lambda 750s spectrophotometer in transmission mode. Time-resolved fluorescence spectroscopy measurements were carried out with a Spirit 1040-8-SHG06900 fR Rack LCR1 spectrometer, using a 340 nm pulsed laser for excitation. PLQY was measured using an XPQY-EQE photoluminescence efficiency measurement system. For RL and X-ray imaging, we used X-ray tube (Moxtek MAGPRO 70kV 12W) as the source. X-ray photons were generated under an accelerating voltage of 50 kV and operating currents from 20 to 140 μA. The distance between the X-ray tube and the detector varied from 10 to 80 cm. The X-ray dose rate was calibrated using an ion chamber dosimeter (IBA Dosimetry MagicMax). The stability was recorded using an oscilloscope (RIGOL DS1202Z-E). The RL spectra were recorded with a PG2000-PRO spectrometer. X-ray images were captured using a digital camera (FL 20BW). The RL decay dynamics of the scintillators were characterized using a picoX5084 X-ray excited scintillation lifetime/decay (TRRL) testing system developed by Shanghai UPU Optoelectronic Technology Co., Ltd. The system employs a N16432 photo-excited X-ray tube (Hamamatsu), which generates pulsed X-rays controlled by the illumination of a photocathode. By utilizing a picosecond laser to produce high-repetition-rate, sub-100-picosecond X-ray pulses, the system enables rapid measurements based on time-correlated single-photon counting (TCSPC).

Structural distortion calculation and statistics
Structural distortion parameters have been calculated (using VESTA software) from the crystallographic data, including both published data and data from crystals synthesized in this work. The intra-octahedral distortion was evaluated by mean of the following parameters50–53:
θ2 = 1/111 Σi=112 |αi − 90| (1)
where αi are the Br-Pb-Br angles.
The extent of structural distortion within the inorganic framework was quantified through a detailed analysis of the bridging Pb-Br-Pb angles. All calculations were performed based on the Cartesian atomic coordinates derived from the single-crystal X-ray diffraction structure refinement.
The total octahedral tilting (Dtilt) for a given Pb-Br-Pb angle is defined as the angular deviation from the ideal linear geometry: Dtilt = 180° - θPb-Br-Pb, where θPb-Br-Pb was measured from the refined crystallographic information file (CIF) using the angle measurement tool in the VESTA software.
To decouple the distinct components of the total tilt, the three-dimensional Pb-Br-Pb angle was vectorially decomposed into its out-of-plane and in-plane projections. This decomposition was carried out by defining a best-fit plane through the three Pb atoms (Pbi, Pbj, and an adjacent Pbk) constituting the local inorganic layer environment. This approach of using atomic positions to define a local plane, rather than relying on the crystallographic (001) plane, was adopted because in certain cases the Pb atoms do not lie perfectly in the (001) plane.
The unit normal vector (n̂) of this locally defined plane was computed. The vectors corresponding to the Pb-Br-Pb bonds were then projected onto the direction of n̂ (out-of-plane component) and onto the plane itself (in-plane component). The angles between these projected vectors, θout and θin, were calculated using the standard arccosine dot product formula (For vectors a and b, the angle between them is given by θ = cos−1[(a·b)/(|a||b|)]).
From these projections, the respective distortion parameters were defined as: Out-of-plane tilting (Dout): Dout = 180° - θout, In-plane tilting (Din): Din = 180° - θin. Here, θout and θin represent the angles between the out-of-plane and in-plane projections of the Pb-Br bond vectors, respectively. The uncertainty δf in an arbitrary function f(x) is δf = |df/dx|·δx, hence δ(cos−1x) = |(1-x2)−1/2|·δx. This results in higher error in D values when the Pb-Br-Pb angles are closer to 180°. This does not represent lower precision in a particular X-ray structure, but is a necessary result of correct error propagation.
Following the calculations, the validity of the vector decomposition was corroborated by verifying that the following relation holds approximately: cos2(θtilt) ≈ cos2(θin) + cos2(θout) – 1, which derives from the spherical law of cosines.

TOF-PET measurement
TOF-PET measurements were conducted using a custom-made standard apparatus. This setup incorporated electronic boards providing dual outputs (energy and timing) and NUV-HD-MT SiPMs manufactured by Fondazione Bruno Kessler (FBK), tested with various coupling materials54,55. The sample crystal pixel was coupled to the SiPM surface using optical grease, and a LYSO:Ce layer was positioned above it. The device has an overall volume of 3 × 3 × 0.4 mm3 and an extraction face of about 3 × 3 mm2. For the referenced detector, a bulk 3 × 3 × 5 mm3 LYSO:Ce,Ca crystal sourced from SIPAT (China) was employed. A 22Na source is placed between a pair of facing gamma-ray detectors to form the coincidence system. The detector unit comprises a scintillator for gamma-to-light conversion, followed by a SiPM that generates electrical signals from these photons. These signals are then conditioned by front-end electronics while preserving a high signal-to-noise ratio. To capture the full pulse dynamics and extract all relevant features, we utilized a high-bandwidth (8 GHz) oscilloscope to digitize the signals from the SiPM channels45. The device was configured to trigger only on specific voltage thresholds, thereby performing coincidence filtering directly within the hardware.

Calculation of scintillation light yield
We performed a cross-verification using multiple standard scintillators with different light yields and emission wavelengths to ensure the accuracy of our test setup and calibration procedure. We employed two certified reference scintillators (LYSO:Ce and BGO). LYSO:Ce (purchased from Meishan Bora New Materials Co., Ltd., China): Certified light yield of ~32,000 photons MeV−1, emission peak at ~428 nm; BGO (purchased from Meishan Bora New Materials Co., Ltd., China): Certified light yield of 10,000 photons MeV−1, emission peak at ~480 nm.
We measured the pulse-height spectra of both LYSO:Ce and BGO under a 241Am (59.5 keV) γ-ray excitation using a photomultiplier tube (PMT, model R2059, Hamamatsu). The quantum efficiency (QE) curve of this PMT was provided by the manufacturer and is included in Supplementary Fig. 16. A high-voltage power supply (556, Ortec) was used to provide the necessary voltage to the PMT. The output signal from the PMT was read using an oscilloscope (MSO54B 5-BW-2000, Tektronix). The radioactive 241Am, with an activity of 2.85 × 108 Bq, irradiated the crystal through a Be window from a distance of 1 cm.
Both the PMT and the 241Am source were placed inside a completely lightproof shielding box. A total of 50,000 pulses were collected using the oscilloscope, processed into a pulse height spectrum, and fitted using a Gaussian function. The light yield of one standard was used to predict the other, following the full spectral correction methodology. The corrected light yield is calculated as:
LYsample = LYcertified · (Cmeasured / Ccertified) · (φcertified / φmeasured) (2)
where C denotes the channel number, and φ is the wavelength dependent detection efficiency correction constant, calculated by convolving each scintillator’s emission spectrum with the PMT’s QE curve:
φj = ∫ I(λ)j · S(λ) dλ (3)
where I(λ)j represent the RL intensity of the scintillator, and S(λ) is the wavelength-dependent PMT detection efficiency.
Using LYSO:Ce (LY ~32,000 photons MeV−1) to predict BGO’s LY: φBGO = 0.1920, φLYSO = 0.2691. The measured photopeak channel ratio was CBGO / CLYSO = 0.24 (Fig. 4c). Predicted LYBGO = 32,000 × 0.24 × (0.2691/0.192) = 10,764 photons MeV−1. This value is in agreement with the certified LY of BGO (~10,000 photons MeV−1), well within the acceptable uncertainty range for such measurements, considering factors like surface finish and coupling.
We then measured our 2D perovskite scintillator under identical conditions. The calculated φ values were 0.2691 for the LYSO:Ce reference, and 0.2503, 0.2611, and 0.2614 for (CMA)2PbBr4, (CMA-COOH)2PbBr4, and (1,4-CMA)PbBr4, respectively. For each sample, the light yield was calculated using the established comparative formula:
LYsample = LYLYSO · (Csample / CLYSO) · (φLYSO / φsample) (4)
This rigorous calibration procedure, applying all necessary corrections, obtained the final light yields of 50,460, 20,460, and 19,730 photons MeV−1 for (CMA)2PbBr4, (CMA-COOH)2PbBr4, and (1,4-CMA)PbBr4, respectively.

Calculation of X-ray imaging spatial resolution
The spatial resolution of the X-ray imaging system was quantitatively evaluated using the MTF based on the slanted-edge method. Sharp-edge images were acquired by imaging a copper sheet approximately 0.15 mm in thickness. The edge spread function (ESF) was obtained from the edge profile, followed by differentiation to generate the line spread function (LSF). Finally, the MTF was computed by applying a Fourier transform to the LSF, as expressed in the equation below:
MTF(ν) = F(LSF(x)) = F(dESF(x)/dx) (5)
where ν is the spatial frequency.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a highly innovative and potentially transformative strategy for achieving atomic-scale exciton confinement in 2D perovskites to break the long-standing efficiency-speed trade-off in scintillators through ingenious lattice engineering. The design of tailored A-site cations (e.g., 1,4-CMA) to selectively control lattice distortions demonstrates exceptional insight. The reported performance metrics (0.62 ns lifetime, 19,500 photons/MeV) are outstanding and, if rigorously validated, represent a significant leap over state-of-the-art scintillators. The demonstrated applications (43.3 ps PET timing, 32 lp/mm X-ray imaging) further highlight the practical impact. While the scientific premise is strong and the results are exciting, several issues require clarification and additional evidence to fully support the claims.\n1. Validation of the Reported High Light Yield: The reported light yield of 19,500 photons/MeV is remarkably high for a scintillator exhibiting such fast decay kinetics. To enable critical assessment and future replication of this exceptional result, providing more comprehensive details regarding the calibration procedure is essential. Specifically, please explicitly state the reference scintillators employed along with their certified light yield values, detail the photomultiplier tube (PMT) specifications including the quantum efficiency curve relevant to the emission wavelengths, and fully describe the calibration methodology. This should encompass aspects such as the comparison of photopeak positions and any correction factors applied.\n2. Clarification of Decay Kinetics Reporting: The reported fast decay time of 0.62 ns is a key feature of this scintillator. To ensure unambiguous interpretation and reproducibility, please explicitly clarify whether this value represents the lifetime of the prompt component (τ1) or the weighted average lifetime (τavg). Additionally, specifying the decay model used for fitting (e.g., bi-exponential, tri-exponential) and reporting the relative amplitudes (or intensity fractions) of all resolved decay components are essential for a complete understanding of the scintillation kinetics.\n3. Radiation Stability Metrics: Although light output stability is demonstrated (Fig. 3f), the scintillation decay kinetics under prolonged irradiation were not evaluated. I suggest adding radioluminescence (RL) decay measurements after a dose of 1,000 mGyair to confirm the absence of speed degradation.\n4. Radioluminescence Decay Discrepancy: The authors should clarify why the radioluminescence (RL) decay time (0.62 ns) is slower than the photoluminescence (PL) decay time (0.46 ns) observed in Fig. 3d-e, given the description of identical recombination pathways. Additionally, discussing whether potential trap-state involvement could account for this difference would strengthen the analysis.\n5. The potential trade-offs associated with in-plane distortion, such as induced brittleness or phase instability, should be discussed.\n6. Stability Data Presentation: To strengthen the evidence for material stability under moisture and radiation exposure, I recommend including post-stability-test characterization data, such as X-ray diffraction (XRD) and photoluminescence (PL) spectra. This would help confirm the retention of structural integrity after testing.\n7. While the 24-hour stability at 60% RH provides initial data, this duration may be insufficient to fully assess material robustness for potential demanding applications (e.g., medical imaging or industrial monitoring). To better evaluate long-term stability, I suggest including extended aging tests (e.g., over 30 days) under similar or relevant conditions, potentially in the Supplementary Information.\n8. The authors are recommended to cite some latest research results, such as\nD. Zhang et al., Ultrafast (600 ps) Vacuum-UV Reflective Scintillation Enabled by Surface Exciton Recombination in\nLayered Perovskite PEA2PbBr4. Angew. Chem., Int. Ed. n/a, e202505665 (2025).\nThis will help to update the literature review and show the connection between the current work and the latest research in the field.", "reviewer_2": "In this manuscript, the authors designed a series of 2D perovskites with fast exciton recombination as X-ray scintillator. In brief, they selectively enhanced in-plane lattice distortion through the design of organic A-site cations to achieve exciton localization, while effectively suppressing out-of-plane and intra-octahedral distortions, successfully achieving emission at nanosecond-scale. This work demonstrates the potential of low-dimensional perovskites for ultrafast emission tomography detection. Overall, this result is interesting and impressive; however, there are still some concerns that need to be addressed. In my view, fast radiative recombination is very common in 2D perovskites, but a 2D perovskite that shows both fast recombination and excellent PL/RL properties is challenging. The authors should discuss more about the correlations between radiative recombination and crystal structure, as well as provide more evidence showing the advantages of these materials.\n1. The authors propose that enhanced in-plane lattice distortion facilitates exciton localization, thereby achieving ultrafast recombination processes. While exciton localization is indeed a prerequisite for rapid recombination, this phenomenon is quite common in 2D lead-based perovskite systems. I believe that the key is to reduce the electron-phonon interaction to reduce the non-radiative losses. The authors should experimentally investigate the phonon interactions.\n2. Although the reported material demonstrates a good light yield, both photoluminescence and radioluminescence spectra exhibit significant noise. Why?\n3. The current light yield data were obtained through relative comparative methods rather than absolute measurements. It would be more reliable to directly measure the absolute light yield of single-crystal samples using a vertical geometry configuration.\n4. The authors should measure fluence-dependent emission properties, especially for quantum yields.\n5. Considering the similarities of RL and electroluminescence, is it possible for these materials to generate efficient EL?", "reviewer_3": "This paper “Atomically Confined Excitons in 2D Perovskites for Bright and Sub-Nanosecond Scintillation” by Liu and co-authors presents an interesting and in depth study on atomic-level exciton confinement in 2D perovskites, focusing on (1,4-CMA)PbBr₄ and derivatives. The work demonstrates the interplay between A-site cation symmetry, lattice distortions, and scintillation performance, culminating in record-breaking sub-nanosecond radioluminescence lifetimes with high light yields. The paper also shows strong result for X-ray imaging, both very fast CRT and high spatial resolution, close to current CsI detectors.\nI would recommend the manuscript for publication with minor revisions:\nHow does radiation hardness and X-ray stopping power compare? There should be a more detailed discussion about it, e,g, BaF₂, while slightly slower that the 2D-PVK here investigated, has vastly higher radiation hardness and stopping power.\nWhile the link between in-plane tilting and exciton confinement as shown in the manuscript is convincing, the physical picture of why this confinement is so effective at suppressing self-trapped excitons could be elaborated a bit more.\nFigures are a bit dense and could be worth splitting them especially Fig. 2.\nIn the experimental section I would recommend to include more specifics on how distortion angles were extracted."}, "author_response": "Response: We are deeply grateful to the reviewer for this crucial comment, which\nallows us to more thoroughly elaborate on our rigorous calibration process. In response\nto this comment and to provide the strongest possible support for our findings, we have\nexpanded the Methods Section (specifically, the \"calculation of scintillation light yield\"\nsubsection) to include the following comprehensive details.\nOur calibration approach was twofold:\n1. Firstly, to validate the reliability of the test system and methodology: We performed\na cross-verification using multiple standard scintillators with different light yields and\nemission wavelengths to ensure the accuracy of our test setup and calibration procedure.\n2. Secondly, to accurately measure our sample: After confirming the system's reliability,\nwe proceeded to measure our sample, applying all necessary corrections.\nValidation of the Measurement System Using Multiple Reference Scintillators:\nTo critically validate the accuracy of our entire measurement and calibration\nsystem prior to measuring our sample, we employed two certified reference scintillators:\nLYSO:Ce and BGO.\n LYSO:Ce (purchased from Meishan Bora New Materials Co., Ltd., China):\nCertified light yield of ~32,000 photons/MeV, emission peak at ~428 nm.\n BGO (purchased from Meishan Bora New Materials Co., Ltd., China): Certified\nlight yield of 10,000 photons/MeV, emission peak at ~480 nm.\nCalibration Cross-Check Procedure:\nWe measured the pulse-height spectra of both LYSO:Ce and BGO under a 241Am\n(59.5 keV) γ-ray excitation using a photomultiplier tube (PMT, model R2059,\nHamamatsu). The quantum efficiency (QE) curve of this PMT was provided by the\nmanufacturer and is included in the Supporting Information. A high-voltage power\nsupply (556, Ortec) was used to provide the necessary voltage to the PMT. The output\nsignal from the PMT was read using an oscilloscope (MSO54B 5-BW-2000, Tektronix).\nThe radioactive 241Am, with an activity of 2.85 × 108 Bq, irradiated the crystal through\na Be window from a distance of 1 cm. Both the PMT and the 241Am source were placed\ninside a completely lightproof shielding box. A total of 50000 pulses were collected\nusing the oscilloscope, processed into a pulse height spectrum, and fitted using a\nGaussian function. The light yield of one standard was used to predict the other,\nfollowing the full spectral correction methodology:\nThe corrected light yield is calculated as:\n𝐶𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝜑𝑐𝑒𝑟𝑡𝑖𝑓𝑖𝑒𝑑\nwhere C deno𝐿te𝑌s 𝑚 t 𝑒 h 𝑎 e 𝑠 c 𝑢 h 𝑟𝑒 a 𝑑 nn=el𝐿 n𝑌u 𝑐 m 𝑒𝑟 b 𝑡𝑖 e 𝑓 r 𝑖 , 𝑒 a 𝑑 n⋅d is the w⋅avelength dependent detection\n𝐶𝑐𝑒𝑟𝑡𝑖𝑓𝑖𝑒𝑑 𝜑𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑\nefficiency correction constant, calculated by convolving each scintillator's emission\n𝜑\nspectrum with the PMT's QE curve:\n∫𝐼𝑗(𝜆)𝑆(𝜆)𝑑𝜆\n𝜑𝑗 =\nwhere I\nj\n(λ) represents the RL spe∫c𝐼t𝑗ru(𝜆m) 𝑑o𝜆f the scintillator, and S(λ) is the\nwavelength-dependent PMT detection efficiency.\nUsing LYSO:Ce (LY~32,000 ph/MeV) to predictBGO's LY:\n= 0.1920, = 0.2691, The measured photopeak channel ratio was C /\nBGO\n𝜑 C L𝐵Y𝐺S𝑂O = 0.24 (F 𝜑 ig𝐿.𝑌 4𝑆𝑂c). Predicted LY BGO = 32000 × 0.24 × (0.2691/0.192) = 10,764\nph/MeV. This value is in agreement with the certified LY of BGO (~10,000 ph/MeV),\nwell within the acceptable uncertainty range for such measurements, considering\nfactors like surface finish and coupling.\nMeasurement and Calibration of Our Perovskite Sample:\nHaving established high confidence in our system's accuracy, we then measured\nour 2D perovskite scintillator under identical conditions. The calculated φ values were\n0.2691 for the LYSO:Ce reference, and 0.2503, 0.2611, and 0.2614 for (CMA) PbBr ,\n2 4\n(CMA-COOH) PbBr ,\n2 4\nand (1,4-CMA)PbBr ,\n4\nrespectively.\nFor each sample, the light yield was calculated using the established comparative\nformula:\n𝐶𝑠𝑎𝑚𝑝𝑙𝑒 𝜑𝐿𝑌𝑆𝑂\n𝐿𝑌𝑠𝑎𝑚𝑝𝑙𝑒 = 𝐿𝑌𝐿𝑌𝑆𝑂 ⋅ ⋅\nThis rigorous calibration procedure, appl𝐶y 𝐿 i 𝑌 n 𝑆 g 𝑂 all 𝜑n 𝑠 e 𝑎 c 𝑚 es 𝑝 s 𝑙 a 𝑒 ry corrections, obtained\nthe final light yields of 50,460, 20,460, and 19,730 photons MeV-1 for (CMA) PbBr ,\n2 4\n(CMA-COOH) PbBr ,\n2 4\nand (1,4-CMA)PbBr ,\n4\nrespectively.\n1400\n1200\n1000\n800\n600\n400\n200\n0\n0 10 20 30 40 50 60 70 80 90\nFig.4 c Pulse height spectra of LYSO:Ce, BGO, (CMA) PbBr , (CMA-COOH) PbBr and\n2 4 2 4\n(1,4-CMA)PbBr under 241Am (59.5 keV) excitation.\n4\n2. Clarification of Decay Kinetics Reporting: The reported fast decay time of 0.62 ns\nis a key feature of this scintillator. To ensure unambiguous interpretation and\nreproducibility, please explicitly clarify whether this value represents the lifetime of\nthe prompt component (τ1) or the weighted average lifetime (τavg). Additionally,\nspecifying the decay model used for fitting (e.g., bi-exponential, tri-exponential) and\nreporting the relative amplitudes (or intensity fractions) of all resolved decay\ncomponents are essential for a complete understanding of the scintillation kinetics.\nResponse: We sincerely thank the reviewer for this insightful comment and for\nhighlighting the need for clarity regarding the decay kinetics reporting. We have\nclarified these points in the revised manuscript and provide a point-by-point response\nbelow:\n1. Lifetime Value Clarification: The reported fast decay time of 0.62 ns for (1,4-\nCMA)PbBr under X-ray excitation indeed represents the weighted average lifetime\n4\nstnuoC\nLYSO:Ce\nBGO\n(CMA) PbBr\n2 4\n(CMA-COOH) PbBr 2 4\n(1,4-CMA)PbBr\n(τ ), not the lifetime of the prompt component (τ ). This has now been explicitly\navg 1\nstated in the caption of the updated Supplementary Table 7.\n2. Decay Model and Amplitudes: The decay curves were fitted using a bi-exponential\ndecay function. The complete fitting parameters, including the amplitudes (A , A )\n1 2\nand lifetimes (τ , τ ) for both PL and RL of all samples, are now comprehensively\n1 2\npresented in the updated Supplementary Table 7.\nSupplementary Table 7:\n(CMA) PbBr (CMA-COOH) PbBr (1,4-CMA)PbBr\n2 4 2 4 4\nA 1.20 1.05 1.06\n1\nτ (ns) 3.12 0.44 0.26\n2\nτ (ns) 9.03 1.16 0.85\n2\nτ (ns) 5.23 0.98 0.46\naverage\nA 0.37 3.21 1.18\n1\nτ (ns) 4.37 0.61 0.25\n2\nτ (ns) 9.61 2.13 1.16\n2\nτ (ns) 8.44 1.09 0.62\naverage\n3. Radiation Stability Metrics: Although light output stability is demonstrated (Fig.\n3f), the scintillation decay kinetics under prolonged irradiation were not evaluated. I\nsuggest adding radioluminescence (RL) decay measurements after a dose of 1,000\nG to confirm the absence of speed degradation.\nyair\nResponse: We thank the reviewer for this insightful suggestion. As recommended, we\nhave performed RL decay measurements on our scintillators after exposure to a\nradiation dose of 1,000 Gy to evaluate the potential speed degradation under\nair\nprolonged irradiation.\nThe measured RL decay lifetimes before and after irradiation are summarized\nbelow:\nLifetime Lifetime\nSample\nbefore irradiation (ns) after irradiation (ns)\n(CMA) PbBr 8.4 7.71\n2 4\n(CMA-COOH) PbBr 1.09 1.57\n2 4\n(1,4-CMA)PbBr 0.62 0.89\nCOOH) PbBr and (1,4-CMA)PbBr show a slight increase in lifetime, indicating a\n2 4 4\nspeed degradation as pointed out by the reviewer.\nCrucially, XRD characterization on (1,4-CMA)PbBr after irradiation confirms no\n4\nphase transformation or structural degradation (Supplementary Figure 18).\nFurthermore, the maintained light output stability (Fig. 4f) rules out the massive\ncreation of non-radiative recombination centers, which typically shorten carrier\nlifetimes.\nTherefore, we attribute the prolonged RL decay lifetime in (CMA-COOH) PbBr\n2 4\nand (1,4-CMA)PbBr to the formation of radiation-induced shallow traps. In the\n4\nscintillation process, these traps temporarily capture charge carriers during their\nmigration to the luminescent centers. The subsequent delayed release of these carriers\nextends the overall radiative recombination pathway, manifesting as a longer decay\ntime in the RL kinetics (Journal of the American Chemical Society 2019, 141, 15557-\n15566; Angew. Chem. Int. Ed. 2024, 63, e202411499).\nWe have added the following concise statement to the Discussion section (Page\n11, Paragraph 2): “The scintillation kinetics were largely retained post-irradiation\n(Supplementary Figure 20), and the variations are consistent with the creation and\nevolution of point defects,as evidenced by the stable crystal structure (Supplementary\nFigure 18).”This addition has been highlighted in the revised manuscript for clarity.\nThe corresponding RL decay curves are presented below for the reviewer's\ninspection.\n100 (CMA)\n4\n(CMA-COOH) PbBr\n2 4\n(1,4-CMA)PbBr 4\n10−1\n10−2\n0 10 20 30 40\nSupplementary Figure 20: RL decay of 2D perovskites after 1,000 Gy irradiation.\nair\n4. Radioluminescence Decay Discrepancy: The authors should clarify why the\nradioluminescence (RL) decay time (0.62 ns) is slower than the photoluminescence\n(PL) decay time (0.46 ns) observed in Fig. 3d-e, given the description of identical\nrecombination pathways. Additionally, discussing whether potential trap-state\ninvolvement could account for this difference would strengthen the analysis.\nResponse: We sincerely thank the reviewer for this insightful comment on the\ndiscrepancy between RL and PL decay times. As initially suggested, one contributing\nfactor is that under X-ray excitation, high-energy electrons are generated which are\nmore likely to escape from exciton-bound states, leading to a slightly prolonged\nradioluminescence decay. Furthermore, the difference in decay kinetics can be\nexplained by the distinct excitation depths and the resulting interactions with defects\nassociated with PL and RL excitation modes. The key difference lies in the penetration\ndepth of the excitation source:\n1. Photoluminescence (PL): is excited by a laser source (e.g., 365 nm) with very\nlimited penetration, generating electron–hole pairs predominantly near the\nsurface. Surfaces generally exhibit a higher density of defects (e.g., vacancies,\ndangling bonds), which act as non-radiative recombination centers. The faster PL\ndecay (0.46 ns) is likely due to this enhanced non-radiative recombination near\n).u.a(\nytisnetni\nLR\nτ = 7.71 ns\nave\nτ = 1.57 ns\nave\nτ = 0.89 ns\nave\nTime (ns)\nthe surface, which shortens the observed lifetime.\n2. Radioluminescence (RL): is excited by high-energy X-rays, which penetrate\ndeeply and generate carriers throughout the bulk. The interior of the bulk crystal\nis expected to have a lower defect density. Thus, non-radiative recombination is\nless dominant, allowing the radiative recombination to exhibit a longer decay\ntime (0.62 ns).\nWe have revised the manuscript to include this discussion. “However, the RL decay\nwas slightly slower than PL decay (Fig. 4e), primarily because X-rays excites high-\nenergy electrons, which are more likely to escape from their exciton-bound states,\nleading to longer lifetimes. This difference is further accentuated by enhanced non-\nradiative recombination at the surface under PL excitation—a contrast to the bulk-\ndominated carrier generation and comparatively lower defect-mediated recombination\nin RL.”This addition has been highlighted in the revised manuscript for clarity.\n5. The potential trade-offs associated with in-plane distortion, such as induced\nbrittleness or phase instability, should be discussed.\nResponse: We thank the reviewer for this insightful comment. We fully agree that such\nstructural features must be carefully evaluated for their impact on mechanical and phase\nstability. In our system, our data indicate that the in-plane tilting does not lead to\nsignificant brittleness or phase instability, based on the following considerations:\n1. Tolerance Factor and Cation Design: The organic cations were explicitly designed\nto maintain a Goldschmidt tolerance factor within the stable range for 2D\nperovskites. The observed in-plane tilting represents a predictable and benign\nstructural adaptation to A-site steric effects, rather than an indication of intrinsic\ninstability.\n2. Interlayer Interactions: The symmetric configuration of the organic cations\npromotes strong van der Waals interactions between adjacent layers, enhancing\nmechanical cohesion and mitigating brittleness.\n3. Experimental Validation: The crystals showed robust handling during polishing\nand device integration, indicating satisfactory mechanical properties. Furthermore,\nthe material exhibited exceptional phase stability under both continuous X-ray\nirradiation (up to 1099 Gy dose) and elevated humidity (70% RH for 35 days),\nas shown in Fig. 4f and Supplementary Figure 17.\n0 200 400 600 800 1000 1200\nFig.4 f Radiation stability of (1,4-CMA)PbBr\nIn summary, the in-plane tilting is a controlled feature within a stable lattice\nframework, and no adverse trade-offs were observed. Instead, this structural feature is\nintegral to achieving the desired optoelectronic properties.\n).u.a(\nytisnetni\nLR\nOn/Off cycles\nContinuous irradiation\nTotal dose: 1099 Gy\nair\nTime (s)\nWe thank the reviewer for raising this important point. We have added the\nfollowing statement in the Conclusion section: “Importantly, the in-plane tilting in our\nsystem is a controlled structural feature within a stable lattice framework, which does\nnot compromise the mechanical integrity or phase stability, as evidenced by our\nstability tests.” This addition has been highlighted in the revised manuscript for clarity.\n6. Stability Data Presentation: To strengthen the evidence for material stability under\nmoisture and radiation exposure, I recommend including post-stability-test\ncharacterization data, such as X-ray diffraction (XRD) and photoluminescence (PL)\nspectra. This would help confirm the retention of structural integrity after testing.\nResponse: We sincerely thank the reviewer for this insightful suggestion. As\nrecommended, we have conducted additional post-stability-test characterizations to\nfurther demonstrate the material’s stability under both moisture and radiation exposure.\nSpecifically, we performed X-ray diffraction (XRD) and photoluminescence (PL)\nmeasurements on (1,4-CMA)PbBr after subjecting it to accelerated aging conditions\n4\n(35 days at 70% RH) and a total radiation dose of 1099 Gy. The resulting XRD patterns\nand PL spectra are presented below in comparison with those of the pristine material.\n10 20 30 40 50 60 70 80\n).u.a(\nytisnetnI\nCalculated\nTwo theta(°)\n).u.a(\nytisnetnI\nAfter\n).u.a(\nytisnetnI\nBefore\nSupplementary Figure 18: XRD patterns of (1,4-CMA)PbBr before (blue) and after (black)\n4\nstability tests. The negligible change in diffraction patterns confirms the structural integrity after 35\ndays at 70% humidity and a 1099 Gy radiation dose.\nSupplementary Figure 19: PL spectra of (1,4-CMA)PbBr before (black) and after (red) stability\n4\ntests. The negligible change in peak shape and position confirms the robustness of its optical\nproperties.\nThe XRD data confirm that the crystal structure remains intact after stability\ntesting, with no detectable phase degradation or structural alteration. Similarly, the PL\nspectra show negligible change in the emission profile and peak position, indicating\nwell-preserved optoelectronic properties. These results strongly support the exceptional\nstability of (1,4-CMA)PbBr .\n[11] “The structural and optical integrity after testing were confirmed by XRD and PL\nmeasurements, which show negligible degradation (Supplementary Figure 18-19).”\nand have included the corresponding figures (Supplementary Figure 18-19) in the\nSupplementary Information. This addition has been highlighted in the revised\nmanuscript for clarity.\n7. While the 24-hour stability at 60% RH provides initial data, this duration may be\ninsufficient to fully assess material robustness for potential demanding applications\n(e.g., medical imaging or industrial monitoring). To better evaluate long-term\nstability, I suggest including extended aging tests (e.g., over 30 days) under similar\nor relevant conditions, potentially in the Supplementary Information.\nResponse: We sincerely thank the reviewer for this insightful suggestion. We fully\nagree that long-term stability is a critical metric for evaluating the practical potential of\nscintillation materials. As recommended, we have conducted an extended aging test for\n35 days under a even harsher condition of 70% relative humidity (RH) at 25 °C to more\nrigorously assess our material's robustness.\nThe results are highly encouraging and strongly support the exceptional stability\nof (1,4-CMA)PbBr . There is no weight change after 35 days of exposure under 70%\nby 2.4% under the same condition. The corresponding stability test data and figures are\nprovided below and in the Supplementary Information (Supplementary Figure 17).\nSupplementary Figure 17: Moisture stability. The weight change of (1,4-CMA)PbBr crystal and\n8. The authors are recommended to cite some latest research results, such as：\nD. Zhang et al., Ultrafast (600 ps) Vacuum-UV Reflective Scintillation Enabled by\nSurface Exciton Recombination in Layered Perovskite PEA PbBr . Angew. Chem.,\n2 4\nInt. Ed. n/a, e202505665 (2025).\nThis will help to update the literature review and show the connection between the\ncurrent work and the latest research in the field.\nResponse: We thank the reviewer for pointing us to this highly relevant and recent\nstudy. We have now cited Zhang et al. (Angew. Chem., Int. Ed., 2025, e202505665) in\nthe revised manuscript. This work provides strong independent support for the core\npremise of our study by demonstrating that exciton localization—achieved in their case\nvia lattice contraction—is a highly effective strategy for achieving ultrafast decay time.\nTo incorporate this suggestion, we have added the following concise statement to\nthe Discussion section (Page 4, Paragraph 1):\"This approach is further supported by a\nrecent demonstration of ultrafast decay (600 ps) via exciton localization induced by\nlattice contraction in a related perovskite system.\" This addition has been highlighted\nin the revised manuscript for clarity.\nReviewer #2: In this manuscript, the authors designed a series of 2D perovskites with\nfast exciton recombination as X-ray scintillator. In brief, they selectively enhanced in-\nplane lattice distortion through the design of organic A-site cations to achieve exciton\nlocalization, while effectively suppressing out-of-plane and intra-octahedral distortions,\nsuccessfully achieving emission at nanosecond-scale. This work demonstrates the\npotential of low-dimensional perovskites for ultrafast emission tomography detection.\nOverall, this result is interesting and impressive; however, there are still some concerns\nthat need to be addressed. In my view, fast radiative recombination is very common in\n2D perovskites, but a 2D perovskite that shows both fast recombination and excellent\nPL/RL properties is challenging. The authors should discuss more about the\ncorrelations between radiative recombination and crystal structure, as well as provide\nmore evidence showing the advantages of these materials.\n1. The authors propose that enhanced in-plane lattice distortion facilitates exciton\nlocalization, thereby achieving ultrafast recombination processes. While exciton\nlocalization is indeed a prerequisite for rapid recombination, this phenomenon is\nquite common in 2D lead-based perovskite systems. I believe that the key is to reduce\nthe electron-phonon interaction to reduce the non-radiative losses. The authors\nshould experimentally investigate the phonon interactions.\nResponse: We thank the reviewer for highlighting the crucial roles of exciton\nlocalization and suppressed electron-phonon interaction in achieving high-performance\nscintillators. We fully agree with this perspective. The novelty of our work lies in\nprecisely addressing these two aspects simultaneously through a unique structural\ndesign.\nWhile exciton localization is common in 2D perovskites, it primarily arises from\ninter-layer confinement due to quantum-well effects. In contrast, in-plane exciton\nlocalization is far less common. As recently demonstrated (Nat. Commun. 2024, 15,\n1893), many 2D perovskites exhibit high in-plane carrier mobility (tens to hundreds of\ncm2/V·s), indicating delocalized in-plane exciton behavior. In such systems, the lack of\nin-plane localization leads to significant exciton diffusion.\nThe key innovation of our work is the achievement of strong in-plane exciton\nlocalization without introducing the detrimental electron-phonon interaction that\ntypically characterizes such strongly distorted systems (Nat. Mater.2018, 17, 550; Nat.\nCommun. 2020, 11, 4699). It is well-established in the literature that pronounced lattice\ndistortions, while capable of localizing excitons, often lead to the formation of self-\ntrapped excitons (STEs). This results in inefficient and slow radiative recombination\nprocesses, which is highly detrimental for fast scintillation.\nOur work strategically circumvents this long-standing trade-off. This is\naccomplished through a targeted structural design that incorporates pronounced in-\nplane tilting (D ) to localize excitons within the plane, while maintaining minimal\nin\nintra-octahedral distortion (θ2) and out-of-plane tilting (D ) (<2°) to suppress the\nout\nelectron-phonon interaction typically associated with these distortions\n(Supplementary Table 2).\nThis design distinctly differentiates our material from:\n Conventional 2D perovskites (e.g., BA PbBr ), which exhibit pronounced D\n2 4 out\n(Supplementary Figure 3) that lead to electron-phonon interaction and limit\nradiative efficiency (Nat. Mater. 2018, 17, 550).\n Systems with minimal D (e.g., BM PbBr ), which lack the necessary D (Fig.\nout 2 4 in\n3b) to localize excitons, resulting in rapid carrier diffusion (Fig. 3d) and\nconsequently poor luminescence (Adv. Mater. 2023, 35, 2211769).\nThus, our materials occupy a unique structural sweet spot (as illustrated in Fig. 3b\nand Supplementary Figure 3), enabling efficient and fast radiative recombination by\nspecifically solving the problem of in-plane exciton delocalization while\nsimultaneously avoiding the strong electron-phonon interaction. Moreover,\ntemperature-dependent measurements reveal minimal linewidth broadening, providing\nstrong evidence against electron-phonon interaction (Fig. 3c). We are confident that our\ncomprehensive structural and optoelectronic data provide compelling evidence for this\nconclusion. We thank the reviewer again for raising this point, which has allowed us to\nbetter highlight the novelty of our material design strategy.\nFig.3c Temperature-dependent FWHM of PL spectra for the synthesized 2D perovskites. The inset\nis the PL spectra at room temperature.\nTo better highlight this key point as suggested, we have now added a concise\nstatement in the Conclusion section: “Our strategy overcomes a fundamental limitation\nin luminescent materials: the typical correlation between strong exciton localization\nand inefficient STE emission. By precisely engineering the lattice distortions, we\nachieve in-plane exciton confinement without inducing the strong electron-phonon\ninteraction that dictates STE formation.” This addition has been highlighted in the\nrevised manuscript for clarity.\n2. Although the reported material demonstrates a good light yield, both\nphotoluminescence and radioluminescence spectra exhibit significant noise. Why?\nResponse: We sincerely thank you for this insightful comment regarding the noise\npresent in photoluminescence (PL) and radioluminescence (RL) spectra. We believe\nthis is primarily due to the limited sensitivity and higher inherent noise floor of the\nPG2000-PRO fiber optic spectrometer used in our initial measurements, an effect that\nbecame particularly pronounced under the low-light conditions typical of RL detection.\nTo directly address this concern, we have re-measured the key spectra under\noptimized conditions, including a significant increase in integration time and signal\naveraging over multiple scans. This approach effectively suppressed random noise,\nyielding the clean spectral profiles now presented in Fig. 3c and Fig. 4b. These high-\nfidelity spectra unequivocally confirm our initial findings regarding the emission\nprofiles and peak positions.\nWe apologize for any confusion caused by the initial data and are grateful for the\nopportunity to strengthen our manuscript with these more robust and reliable results.\nFig.3c Temperature-dependent FWHM of PL spectra for the synthesized 2D perovskites. The inset\nis the PL spectra at room temperature.\n(CMA) PbBr\n2 4\n(PEA) PbBr\n2 4\n(CMA-COOH) PbBr\n2 4\n(1,4-CMA)PbBr\n4\n400 450 500 550 600\nFig.4 b Normalized radioluminescence spectra of 2D perovskites.\n3. The current light yield data were obtained through relative comparative methods\nrather than absolute measurements. It would be more reliable to directly measure the\nabsolute light yield of single-crystal samples using a vertical geometry configuration.\n).u.a(\nytisnetnI\nWavelength (nm)\nResponse: We sincerely thank you for raising this important point regarding the\nmeasurement of absolute light yield. We fully agree that obtaining accurate absolute\nlight yield values is highly desirable. However, as the reviewer is likely aware, direct\nabsolute light yield measurement remains inherently challenging due to several\nfundamental experimental limitations: 1) The uncertainty in absolute light yield\nmeasurements originates primarily in the difficulty of accurate gain calibration of PMT.\nThis process is challenging due to the limited single photoelectron resolution of PMTs,\nwhich is compromised by noise, dark current, and afterpulses. Consequently, the light\nyield values obtained through this absolute method are generally considered to have an\nuncertainty on the order of 10% (Eur. Phys. J. C, 2022, 82, 344). 2) The nonlinear\nresponse of PMT/SiPM, especially in high-gain regimes, further complicates absolute\nquantification and can introduce additional systematic errors.\nGiven these constraints, we employed a rigorous cross-calibration methodology,\nas detailed in the \"Calculation of Scintillation Light Yield\" subsection of our revised\nMethods section. Our calibration approach was twofold:\n1. Firstly, to validate the reliability of the test system and methodology: We performed\na cross-verification using multiple standard scintillators with different light yields and\nemission wavelengths to ensure the accuracy of our test setup and calibration procedure.\n2. Secondly, to accurately measure our sample: After confirming the system's reliability,\nwe proceeded to measure our sample, applying all necessary corrections.\nValidation of the Measurement System Using Multiple Reference Scintillators:\nTo critically validate the accuracy of our entire measurement and calibration\nsystem prior to measuring our sample, we employed two certified reference scintillators:\nLYSO:Ce and BGO.\n LYSO:Ce (purchased from Meishan Bora New Materials Co., Ltd., China):\nCertified light yield of ~32,000 photons/MeV, emission peak at ~428 nm.\n BGO (purchased from Meishan Bora New Materials Co., Ltd., China): Certified\nlight yield of 10,000 photons/MeV, emission peak at ~480 nm.\nCalibration Cross-Check Procedure:\nWe measured the pulse-height spectra of both LYSO:Ce and BGO under a 241Am\n(59.5 keV) γ-ray excitation using a photomultiplier tube (PMT, model R2059,\nHamamatsu). The quantum efficiency (QE) curve of this PMT was provided by the\nmanufacturer and is included in the Supporting Information. A high-voltage power\nsupply (556, Ortec) was used to provide the necessary voltage to the PMT. The output\nsignal from the PMT was read using an oscilloscope (MSO54B 5-BW-2000, Tektronix).\nThe radioactive 241Am, with an activity of 2.85 × 108 Bq, irradiated the crystal through\na Be window from a distance of 1 cm. Both the PMT and the 241Am source were placed\ninside a completely lightproof shielding box. A total of 50000 pulses were collected\nusing the oscilloscope, processed into a pulse height spectrum, and fitted using a\nGaussian function. The light yield of one standard was used to predict the other,\nfollowing the full spectral correction methodology:\nThe corrected light yield is calculated as:\n𝐶𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝜑𝑐𝑒𝑟𝑡𝑖𝑓𝑖𝑒𝑑\nwhere C deno𝐿te𝑌s 𝑚 t 𝑒 h 𝑎 e 𝑠 c 𝑢 h 𝑟𝑒 a 𝑑 nn=el𝐿 n𝑌u 𝑐 m 𝑒𝑟 b 𝑡𝑖 e 𝑓 r 𝑖 , 𝑒 a 𝑑 n⋅d is the w⋅avelength dependent detection\n𝐶𝑐𝑒𝑟𝑡𝑖𝑓𝑖𝑒𝑑 𝜑𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑\nefficiency correction constant, calculated by convolving each scintillator's emission\n𝜑\nspectrum with the PMT's QE curve:\n∫𝐼𝑗(𝜆)𝑆(𝜆)𝑑𝜆\n𝜑𝑗 =\nwhere I\nj\n(λ) represents the RL int∫en𝐼𝑗s(it𝜆y) 𝑑o𝜆f the scintillator, and S(λ) is the\nwavelength-dependent PMT detection efficiency.\nUsing LYSO:Ce (LY~32,000 ph/MeV) to predictBGO's LY:\n= 0.1920, = 0.2691, The measured photopeak channel ratio was C /\nBGO\n𝜑 C L 𝐵 Y 𝐺 S 𝑂 O = 0.24 (F 𝜑 ig𝐿.𝑌 4𝑆𝑂c). Predicted LY BGO = 32000 × 0.24 × (0.2691/0.192) = 10,764\nph/MeV. This value is in agreement with the certified LY of BGO (~10,000 ph/MeV),\nwell within the acceptable uncertainty range for such measurements, considering\nfactors like surface finish and coupling.\nMeasurement and Calibration of Our Perovskite Sample:\nHaving established high confidence in our system's accuracy, we then measured\nour 2D perovskite scintillator under identical conditions. The calculated φ values were\n0.2691 for the LYSO:Ce reference, and 0.2503, 0.2611, and 0.2614 for (CMA) PbBr ,\n2 4\n(CMA-COOH) PbBr ,\n2 4\nand (1,4-CMA)PbBr ,\n4\nrespectively.\nFor each sample, the light yield was calculated using the established comparative\nformula:\n𝐶𝑠𝑎𝑚𝑝𝑙𝑒 𝜑𝐿𝑌𝑆𝑂\n𝐿𝑌𝑠𝑎𝑚𝑝𝑙𝑒 = 𝐿𝑌𝐿𝑌𝑆𝑂 ⋅ ⋅\nThis rigorous calibration procedure, appl𝐶y 𝐿 i 𝑌 n 𝑆 g 𝑂 all 𝜑n 𝑠 e 𝑎 c 𝑚 es 𝑝 s 𝑙 a 𝑒 ry corrections, obtained\nthe final light yields of 50,460, 20,460, and 19,730 photons MeV-1 for (CMA) PbBr ,\n2 4\n(CMA-COOH) PbBr ,\n2 4\nand (1,4-CMA)PbBr ,\n4\nrespectively.\n1400\n1200\n1000\n800\n600\n400\n200\n0\n0 10 20 30 40 50 60 70 80 90\nFig.4 c Pulse height spectra of LYSO:Ce, BGO, (CMA) PbBr , (CMA-COOH) PbBr and\n2 4 2 4\n(1,4-CMA)PbBr under 241Am (59.5 keV) excitation.\ncounting, we believe our calibrated relative approach provides reliable and reproducible\nstnuoC\nLYSO:Ce\nBGO\n(CMA) PbBr\n2 4\n(CMA-COOH) PbBr 2 4\n(1,4-CMA)PbBr\nresults that are scientifically robust for comparative performance evaluation. Thank you\nonce again for allowing us to clarify this aspect of our work.\n4. The authors should measure fluence-dependent emission properties, especially for\nquantum yields.\nResponse: We are grateful to the reviewer for raising this important point. In direct\nresponse to this comment, we have systematically measured the fluence-dependent\nPLQY for our 2D perovskites. The new dataset and a detailed analysis have been\nintegrated into the revised manuscript to address this point thoroughly. The key figure\n(Supplementary Figure 11) is provided in the supplementary materials, as discussed\non page [9] of the manuscript.\nThe core discussion we added is as follows:\nWe further evaluated the fluorescence performance through fluence-dependent\nPLQY measurements, which revealed that all samples exhibited a characteristic rise-\nto-roll-off trend, achieving peak PLQY of 7.8%, 21.7%, and 39.5% for (1,4-\nCMA) PbBr , (CMA-COOH) PbBr , and (CMA) PbBr , respectively (Supplementary\n2 4 2 4 2 4\nFigure 11). The initial rise suggests efficient filling of non-radiative trap states at lower\nexcitation densities. The subsequent universal roll-off is attributed to intensified\nexciton-exciton interactions at high exciton densities, where the decreased average\ndistance between excitons significantly raises the probability of synergistic energy loss\nthrough multiple recombination pathways (Angew. Chem., Int. Ed., 2025, e202505665).\nSupplementary Figure 11: Fluence-dependent PLQY. PLQY as a function of excitation power\ndensity for (1,4-CMA)PbBr , (CMA-COOH) PbBr , and (CMA) PbBr . All samples exhibit an\n4 2 4 2 4\ninitial increase followed by a decrease in PLQY with increasing power density.\nWe thank the reviewer once again for this insightful suggestion, which has\nundoubtedly improved our work.\n5. Considering the similarities of RL and electroluminescence, is it possible for these\nmaterials to generate efficient EL?\nResponse: We sincerely thank the reviewer for raising this insightful question. We fully\nagree with the reviewer that radioluminescence (RL) and electroluminescence (EL)\nshare similarities as high-energy excitation processes, and indeed, materials exhibiting\nefficient RL may in principle demonstrate EL under appropriate conditions.\nIn fact, the material requirements for efficient scintillation and EL devices differ\nsignificantly. Scintillators typically require substantial thickness (often hundreds of\nmicrometers or more) to ensure sufficient X-ray absorption. In contrast, EL devices—\nsuch as OLEDs, QLEDs, and perovskite LEDs—demand ultra-thin active layers\n(typically below 1 µm, often around 50 nm) to facilitate effective charge injection,\ntransport, and recombination, as well to minimize Joule heating and electrical\nquenching.\nGiven that our study focuses on bulk single-crystal scintillators optimized for high\nX-ray absorption and RL efficiency, their direct application in EL devices is challenging\ndue to their considerable thickness. That said, we appreciate the reviewer’s implicit\nsuggestion regarding the potential EL applicability of this material system. We believe\nthat through further structural engineering—such as exfoliating thin crystals or growing\nlarge-area single-crystal thin films via self-assembly or confined crystallization—these\nmaterials may indeed exhibit promising EL performance. Such directions, while beyond\nthe scope of the current work, open exciting avenues for future research.\nIn response to the reviewer's valuable suggestion regarding electroluminescence,\nwe have now incorporated a brief outlook in the Conclusion section: “While the bulk\ncrystals in this study are optimized for scintillation, the underlying photophysics\nsuggests that nanostructured or thin-film versions of these materials could open\navenues for efficient electroluminescence.” This addition has been highlighted in the\nrevised manuscript for clarity.\nReviewer #3: This paper “Atomically Confined Excitons in 2D Perovskites for Bright\nand Sub-Nanosecond Scintillation” by Liu and co-authors presents an interesting and\nin depth study on atomic-level exciton confinement in 2D perovskites, focusing on (1,4-\nCMA)PbBr and derivatives. The work demonstrates the interplay between A-site\n4\ncation symmetry, lattice distortions, and scintillation performance, culminating in\nrecord-breaking sub-nanosecond radioluminescence lifetimes with high light yields.\nThe paper also shows strong result for X-ray imaging, both very fast CRT and high\nspatial resolution, close to current CsI detectors.\nI would recommend the manuscript for publication with minor revisions:\n1. How does radiation hardness and X-ray stopping power compare? There should\nbe a more detailed discussion about it, e,g, BaF₂, while slightly slower that the\n2D-PVK here investigated, has vastly higher radiation hardness and stopping power.\nResponse: We sincerely appreciate the reviewer’s insightful comment regarding the\ncomparison of radiation hardness and X-ray stopping power between our 2D-PVK\nscintillator and conventional scintillators such as BaF . We fully agree that these are\n2\ncritical parameters for practical applications of scintillation materials.\nIn response to the comment, we have conducted systematic experiments and\nsimulations to quantitatively compare these properties. To evaluate the radiation\nhardness, we exposed BaF and 2D-PVK (with identical volume and thickness) to\n2\nuniform X-ray irradiation using a calibrated X-ray tube (W target, 70 kV) at a constant\ndose rate of 89.28 Gy s-1. The steady-state RL intensity was monitored at increasing\nair\ncumulative doses up to 5 kGy. The normalized RL integral intensities are as follows:\n1.0\n0.8\n0.6\n0.4\n0.2 BaF\n2\n(1,4-CMA)PbBr\n4\n0.0\n1 2 3 4 5\nSupplementary Figure 24: Comparison of radiation hardness and X-ray stopping power\nbetween a standard BaF scintillator and (1,4-CMA)PbBr . (a) Normalized steady-state RL\n2 4\nintensity of BaF and (1,4-CMA)PbBr as a function of cumulative radiation dose (X-rays, W target,\n2 4\n70 kV, dose rate 89.28 Gy s-1).\nair\nThese results confirm that BaF exhibits excellent radiation hardness, while our\n2\n).u.a(\nytisnetni\nlargetnI\nLR\nDose (kGy)\n2D-PVK shows a larger but still relatively moderate decrease (~2% loss under the same\nconditions).\nRegarding X-ray stopping power, we calculated the total linear attenuation\ncoefficients (with scattering) for both materials using the XCOM Photon Cross Sections\nDatabase (NIST). As shown in the attached Supplementary Figure 24, the attenuation\ncurve of 2D-PVK lies below that of BaF across the energy range of interest, indicating\n2\nthat BaF indeed possesses a higher stopping power due to its higher density and\n2\neffective atomic number.\n105\n104\n103\n102\n101\n100\n10−1\n10−3 10−2 10−1 100 101 102 103 104 105\nSupplementary Figure 24: Comparison of radiation hardness and X-ray stopping power\nbetween a standard BaF scintillator and (1,4-CMA)PbBr . (b) Total linear attenuation\n2 4\ncoefficients (with scattering) for BaF and (1,4-CMA)PbBr as a function of photon energy,\n2 4\ncalculated using the NIST XCOM database.\nWe fully acknowledge that established scintillators such as BaF exhibit superior\n2\nperformance in terms of radiation hardness and stopping power—properties essential\nfor many high-dose and high-energy detection applications. However, our study focuses\non addressing the long-standing trade-off between scintillation decay time and light\nyield. 2D-PVK scintillator achieves an ultrafast decay time while maintaining high light\nyield, a combination that may open up new possibilities for applications demanding\nhigh temporal resolution, even though its radiation hardness and stopping power remain\nlower than those of mature scintillators like BaF .\n2\n)1-mc(\ntneiciffeoc\nnoitaunettA\nBaF\n2\n(1,4-CMA)PbBr\nWe have added a more detailed discussion on these points in the revised\nmanuscript (Page 15, Paragraph 2:“Finally, we evaluated the practical performance of\n(1,4-CMA)PbBr by comparing its radiation hardness and X-ray stopping power with\n4\ncommercial BaF (Supplementary Figure 24). While BaF exhibits superior radiation\n2 2\nresistance and higher stopping power, (1,4-CMA)PbBr demonstrates reasonable\n4\nradiation hardness (~2% degradation) despite its lower attenuation coefficient”). This\naddition has been highlighted in the revised manuscript for clarity.\nThank you once again for this valuable suggestion, which has helped us improve\nthe depth and balance of our discussion.\n2. While the link between in-plane tilting and exciton confinement as shown in the\nmanuscript is convincing, the physical picture of why this confinement is so\neffective at suppressing self-trapped excitons could be elaborated a bit more.\nResponse: We thank the reviewer for this insightful comment. Elaborating the physical\npicture of why self-trapped excitons (STEs) are so effectively suppressed is indeed the\ncore of our material design strategy, which depends on the distinct roles of different\nlattice distortions.\nIn this work, structural design achieves strong in-plane exciton localization while\nsimultaneously minimizing the electron-phonon interaction that drives the formation of\nSTEs. This is accomplished through two concurrent effects:\n Enhanced in-plane confinement via in-plane tilting: The pronounced in-plane\ntilting creates a disordered potential landscape that localizes excitons within the\n2D plane. This drastically reduces their in-plane diffusion length, funneling them\ndirectly to radiative sites.\n Suppressed electron-phonon interaction via minimal θ2/ D : As our systematic\nout\nanalysis of numerous 2D perovskites reveals (Supplementary Figure 3 and\nSupplementary Table 2), a minimal intra-octahedral distortion (θ2) and out-of-\nplane tilting (D ) is critically important for suppressing electron-phonon\nout\ninteraction. Our material is specifically designed to exhibit exceptionally small\nvalues of these distortions, thereby preventing the lattice softening and\nconfigurational rearrangement that are prerequisites for forming stable STEs in 2D\nperovskite system.\nWe thank the reviewer for raising this important point, which has allowed us to\nbetter articulate the unique physical mechanism behind the effective suppression of\nself-trapped excitons in our material system.\nFollowing the reviewer's insightful suggestion to elaborate the physical picture,\nwe have incorporated a clear statement in the Conclusion section: “Our strategy\novercomes a fundamental limitation in luminescent materials: the typical correlation\nbetween strong exciton localization and inefficient STE emission. By precisely\nengineering the lattice distortions, we achieve in-plane exciton confinement without\ninducing the strong electron-phonon interaction that dictates STE formation.” This\naddition has been highlighted in the revised manuscript for clarity.\n3. Figures are a bit dense and could be worth splitting them especially Fig. 2.\nResponse: Thank you for your thoughtful comment regarding the organization of the\nfigures. We agree that the original Fig. 2 was somewhat dense and could benefit from\nreorganization to improve clarity.\nIn response to your suggestion, we have split the original Fig. 2 into two separate\nfigures: Fig. 2 now focuses specifically on the structural distortions influenced by A-\nsite cations; Fig. 3 now presents the subsequent structure-property relationships derived\nfrom these structural insights.\nWe believe this reorganization significantly enhances the clarity and logical flow\nof the data presentation. We are grateful for the suggestion, as it has undoubtedly\nimproved the quality of our paper.\n4. In the experimental section I would recommend to include more specifics on how\ndistortion angles were extracted.\nResponse: We sincerely thank the reviewer for this insightful comment. We agree that\na more detailed explanation of the methodology for extracting the distortion angles is\ncrucial for reproducibility and clarity. In our revised manuscript, we have substantially\nexpanded the “Structural distortion calculation and statistics” section in the Methods\nto provide a comprehensive description of our analytical process.\nOur revised approach no longer relies solely on a provided MATLAB script but\nincorporates a rigorous vector decomposition method, aided by structural visualization\nsoftware (VESTA) and manual mathematical validation. The key specifics of the\nmethodology are now detailed as follows:\n“Structural distortion calculation and statistics\nThe extent of structural distortion within the inorganic framework was quantified\nthrough a detailed analysis of the bridging Pb-Br-Pb angles. All calculations were\nperformed based on the Cartesian atomic coordinates derived from the single-crystal\nX-ray diffraction structure refinement.\nThe total octahedral tilting (D ) for a given Pb-Br-Pb angle is defined as the\ntilt\nangular deviation from the ideal linear geometry: D = 180° - θ , where θ\ntilt Pb-Br-Pb Pb-Br-Pb\nwas measured from the refined crystallographic information file (CIF) using the angle\nmeasurement tool in the VESTA software.\nTo decouple the distinct components of the total tilt, the three-dimensional Pb-Br-\nPb angle was vectorially decomposed into its out-of-plane and in-plane projections.\nThis decomposition was carried out by defining a best-fit plane through the three Pb\natoms (Pb, Pb, and an adjacent Pb ) constituting the local inorganic layer environment.\ni j k\nThis approach of using atomic positions to define a local plane, rather than relying on\nthe crystallographic (001) plane, was adopted because in certain cases the Pb atoms do\nnot lie perfectly in the (001) plane.\nThe unit normal vector (n̂) of this locally defined plane was computed. The vectors\ncorresponding to the Pb-Br-Pb bonds were then projected onto the direction of n̂ (out-\nof-plane component) and onto the plane itself (in-plane component). The angles\nbetween these projected vectors, θ and θ , were calculated using the standard\nout in\narccosine dot product formula (For vectors a and b, the angle between them is given by\nθ = cos-1[ (a • b) / (|a| |b|) ]).\nFrom these projections, the respective distortion parameters were defined as:\n Out-of-plane tilting (D out ): D out = 180° - θ out\n In-plane tilting (D in ): D in = 180° - θ in\nHere, θ and θ represent the angles between the out-of-plane and in-plane\nout in\nprojections of the Pb-Br bond vectors, respectively. The uncertainty δf in an arbitrary\nfunction f(x) is δf = |df/dx|·δx, hence δ(cos-1x) = |(1-x2)-1/2|·δx. This results in higher\nerror in D values when the Pb-Br-Pb angles are closer to 180°. This does not represent\nlower precision in a particular X-ray structure, but is a necessary result of correct error\npropagation.\nFollowing the calculations, the validity of the vector decomposition was\ncorroborated by verifying that the following relation holds approximately: cos2(θ ) ≈\ntilt\ncos2(θ ) + cos2(θ ) – 1, which derives from the spherical law of cosines.”\nin out\nWe believe this detailed description, now present in the revised manuscript,\nprovides the necessary specifics on the extraction of distortion angles and underscores\nthe robustness of our quantitative analysis. Once again, we thank the reviewer for this\nvaluable suggestion, which has significantly improved the clarity and reproducibility\nof the methodological presentation in our manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I would like to congratulate the authors for the thorough job they have done in responding to my comments. Well done and\nwell deserved for your paper to be published!", "reviewer_2": "All my concerns have been thoroughly addressed by the authors. I can now recommend the acceptance of this manuscript."}, "author_response": ""}]

Decision letter