Single-photon emission from two-dimensional perovskites channeled through low-energy edge states

Domain: physics
Published: 2026-03-23
Editorial outcome: minor_revision

Abstract
Two-dimensional perovskites, an emerging family of van der Waals materials with an innate
quantum-well structure, exhibit remarkable optical characteristics and compositional tunability.
However, their potential as a source for quantum emitters remains unexplored. Here, we
investigate low-energy edge states unique to two-dimensional perovskites using comprehensive
photoluminescence imaging and spectroscopy, to further leverage them towards uncovering
quantum emitters. In contrast to above-bandgap excitation, spatially localized single-photon
emitters are realized through sub-bandgap energy excitation at the edges in exfoliated single
crystals. Specifically, the sub-bandgap excitation facilitates efficient access to deeply confined
states within the bandgap, as charge carriers can c hannel through low-energy edge states while
inhibiting competing processes. Furthermore, we demonstrate single-photon emitters from
artificially created edges via strain as well as from two-dimensional perovskites with different
quantum-well thicknesses. Our findings provide promising opportunities in the development of
two-dimensional quantum emitters, where two-dimensional perovskites may provide added
functionalities with versatility in material design, fabrication, and scalability.

Introduction
Efficient, stable, and scalable single-photon emission from solid-state quantum emitters is
important for the advancement of next-generation quantum cryptography and information
technology1,2. For example, vacancy-dopant defects in diamond and self-assembled semiconductor
quantum dots have been extensively investigated as single-photon emitters3-8. However, the
limitations on defect growth in targeted locations and the integration with high-quality cavities
make their practical implementation challenging. Van der Waals two-dimensional (2D) materials,
including transition-metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), are also
considered viable platforms for single-photon generation due to their advantageous photon
manipulation properties9-17. Nonetheless, achieving precise and reproducible control of quantum
emitters in 2D van der Waals materials, along with the scalable production of high-quality
monolayers for effective integration into photonic circuits, remains a significant challenge18,19. It
is thus essential to explore a diverse set of prospective 2D material candidates and to have a deeper
understanding of their emission characteristics to develop optimal single-photon applications.
2D hybrid halide perovskites have recently garnered considerable interest as an emerging
family of 2D materials due to their inherent quantum-well structure and exceptional optoelectronic
properties20,21. In contrast to conventional 2D materials, 2D perovskites do not require reduction
to a monolayer to exhibit strong quantum confinement effect, due to their alternating organic-
inorganic hybrid superlattice-like architecture22. These materials possess a general chemical
formula of (L) (A) M X , where L represents a long-chain organic cation, A is a small organic
2 n-1 n 3n+1
cation (e.g., methylammonium [MA+]), M is a metal cation (such as lead or tin), and X is a halide
anion; the integer n defines the quantum-well thickness per unit cell. Variations in each component
present a viable approach to tailor specific properties23-25. Due to their high tunability in both
organic and inorganic layers, multi-faceted design principles including molecular engineering,
lattice dynamics control, and quantum confinement modulation can offer material optimization
necessary for reliable single-photon generation. Moreover, 2D perovskites are of low-cost and
easily solution processable at scale while maintaining the advantages of van der Waals materials
such as efficient device integration as well as quantum-state manipulation through defect control
and strain engineering. Thus, achieving single-photon emission from 2D perovskites will present
a potential foundation for scalable and reliable quantum emitters while inspiring new design
strategies that extend beyond our current understanding of these materials.
In this work, we explore low-energy edge states that are unique to 2D perovskites with n ≥ 2
by using detailed photoluminescence (PL) imaging and spectroscopy, aiming to uncover their
potential as alternative quantum emitters. Our investigation indicates that spatially confined single-
photon emitters can be realized by stimulating the edges in exfoliated single crystals using sub-
bandgap energy optical excitation. Through extensive photoluminescence excitation (PLE)
analysis, we uncover that sub-bandgap optical excitation effectively allows charge carriers to
access deeply confined states within the bandgap through low-energy edge states while
suppressing other recombination channels including band-edge emission or shallow defects. In
addition, emitter-site control is achieved using nanostructures that induce edge-like effects even
within the interior region of exfoliated sheets. We further develop single-photon emitters from 2D
perovskites with varying quantum-well thicknesses, offering insight into their tunability.

Results
Distinct optoelectronic properties have been observed in 2D perovskites, with differences
noted between the interior and the edges of exfoliated single crystals or grains26-30. In particular,
edge states exhibit significant differences in PL spectra and lifetimes, effective charge-carrier
transport, and exciton dissociation in 2D perovskites with two or more inorganic layers (n ≥ 2),
which are attributed to edge-termination-induced reconstruction or termination-dependent
electrostatics29,31. Therefore, we investigate the edge regions of exfoliated single-crystal sheets of
2D perovskites with varying inorganic layer thicknesses using diverse pump wavelengths (Fig. 1a).
We examine three types of 2D perovskites: (BA) PbI (BA n=1), (BA) (MA)Pb I (BA n=2), and
2 4 2 2 7
(BA) (MA) Pb I (BA n=3), where BA stands for butylammonium, to identify the unique optical
2 2 3 10
characteristics of each compound and uncover potential quantum emitters. Figure 1b illustrates the
representative alternating organic-inorganic hybrid crystal structure of BA n=2 together with its
corresponding quantum-well band structure. Phase -pure bulk single crystals were obtained via a
slow cooling crystallization method and subsequently vacuum-dried for mechanical exfoliation
into thin sheets (see Methods). Supplementary Fig. 1 shows the PXRD results for each single
crystal batch, confirming their high compositional purity.
Figure 1c shows a top-view scanning electron microscope (SEM) image of an exfoliated
single-crystal sheet of BA n=2, distinctly illustrating the terrace-like morphology of the edge
region, with the inset providing a closer view of the edge from an alternative viewing angle. In
addition, a notable difference in lattice characteristics is observed between the interior and edge
regions of a thin exfoliated sheet, as shown in the high-angle annular dark-field scanning
transmission electron microscopy (HAADF STEM) image (Fig. 1d, left). The interior region
exhibits clear lattice planes with an interplanar spacing of 0.315 nm corresponding to lattice
spacing along the [202] direction, as confirmed by the fast Fourier transform (FFT) pattern (Fig.
1d, right). In contrast, the edge region has a less defined structure, with the FFT pattern
highlighting the structural inhomogeneity between the interior and edge regions. Although the
extent of the edge is on the order of a few nanometers as shown in the STEM image, the single-
crystal sheets investigated in this work hereon were carefully controlled during exfoliation to
exhibit a continuous terrace-like edge on the order of micrometers as shown in the inset of Fig. 1c,
enabling selective examination of the edges to ascertain their distinct properties.
Comprehensive PL imaging and characterization of 2D perovskites were performed to
examine the relationship between emission and pump wavelengths, as well as to locate spatially
confined emitters. Figure 2a shows the optical microscope image of an exfoliated single-crystal
sheet of BA n=2, which was used for PL measurements in its interior and edge regions at 4 K. The
PL spectra in Fig. 2b indicate that the interior region is characterized by the narrow emission at a
wavelength of around 580 nm, corresponding to the emission from band-edge exciton
recombination of BA n=2. In contrast, the edge region displays a pronounced dominance of lower-
energy broad emission. Spatial PL mapping was performed to further clarify that these distinct
features are from the edge. First, the narrow emission component was acquired using a 600 nm
short-pass filter in conjunction with above-bandgap excitation from a 532 nm continuous-wave
(CW) laser (Fig. 2c). No substantial difference in PL intensity was observed between the interior
and edge regions. Next, we performed spatial PL mapping of the broad emission component using
a 650 nm long-pass filter at various pump wavelengths (Figs. 2d-g). Figure 2d shows the PL
mapping of broad emission under above-bandgap excitation at 532 nm, which is the same as that
of Fig. 2c; however, the PL intensity shows increased intensity in the edge region than in Fig. 2c.
To investigate this feature more clearly, we further increased the pump wavelength. Near-bandgap
excitation at 580 nm increased the relative intensity of the edge region in the PL map of broad
emission (Fig. 2e), whereas sub-bandgap excitation at 600 nm led to almost completely localized
emission in the edge region (Fig. 2f). Finally, spatial PL mapping under even lower-energy sub-
bandgap excitation at 633 nm yielded strongly localized emitters at the edges (Fig. 2g).
Conventional above-bandgap excitations are incapable of producing spatially confined
emitters in 2D perovskites. Lower-energy excitation leads to increasingly localized PL emissions
in the edge region, ultimately yielding highly confined emissive spots. These features observed at
4 K are also present at room temperature (Supplementary Fig. 2), indicating that lowering the
temperature does not substantially alter the characteristics of the edge states. In addition, BA n=3
showed similar results to BA n=2, revealing pronounced broad PL emission characteristics in the
edges and localized emitters with sub-bandgap excitations (Supplementary Fig. 3). However,
exfoliated sheets of BA n=1 exhibit both narrow and broad emission without clear spatial
confinement features, regardless of changes in the pump wavelength (Supplementary Fig. 4);
spatially localized emitters were not identified even at lower-energy excitations. Broad emissions
from n=1 2D perovskites are typically attributed to defects or self-trapped excitons, which cannot
be directly excited and must transition from band-edge excitons32,33. Our results are consistent with
prior observations that n=1 2D perovskites do not exhibit edge-state PL, as opposed to n=2 and
334. Thus, we focus on BA n=2 and BA n=3 for further development into single-photon emitters
hereon in this work.
We then performed systematic optical experiments to analyze the highly confined emissive
spots observed at the edges in BA n=2 under sub-bandgap excitation to identify evidence of photon
anti-bunching. Figure 3a shows an optical microscope image of the exfoliated BA n=2 single-
crystal sheet used in the measurements. PL emission was measured at 4 K using a 650 nm long-
pass filter in conjunction with sub-bandgap excitation from a 633 nm CW laser (see Methods). 
The exfoliated sheets were initially scanned to search for bright localized spots, and following the
screening process, a small area containing several bright emitters was selected as a zone of interest
for further investigation (Fig. 3b).
Each local bright spot in Fig. 3b, indicated by the dashed circles labeled 1, 2, and 3, were
examined for single-photon emissions. PL measurements of the emitters at spots 1 and 2 (white
dashed circles) exhibited multiple sharp peaks emitted simultaneously, potentially representing
different confined emitters adjacent to each other. However, the spectral proximity and overlap of
the peaks result in a lack of unambiguous evidence for clear photon anti-bunching behavior (Fig.
3c). On the other hand, the emitter at spot 3 (red dashed circle) showed a fully isolated single peak
at a wavelength of 683.0 nm with a full width at half maximum (FWHM) of 0.65 nm (Fig. 3d).
The second-order correlation function g(2)() was measured using an Hanbury Brown and Twiss
(HBT) setup (see Methods). The g(2)(0) value for the peak of spot 3 was 0.397 ± 0.109 (Fig. 3e),
which clearly indicates photon anti-bunching behavior. The measurement of integrated PL
intensity as a function of the pump power showed a saturation emission intensity of 44,171
counts/s at a power of 2.8 W (Fig. 3e, inset). In addition, the time-resolved PL (TRPL)
measurement revealed the decay time of the emitter in Fig. 3d to be 870 ± 3.73 ps (Fig. 3f).
Taken together, the results in Fig. 3 indicate that single-photon emission is distinctly
observable within localized bright spots located at the edges of exfoliated sheets of BA n=2. The
rapid decay time of 2D perovskites, in contrast to TMD monolayers (about a few ns)35,36 and both
boron- and carbon-related vacancies in hBN (also about a few ns)37,38, can directly contribute to
the enhanced rate of quantum emission. A single-photon emitter exhibiting similar properties was
also observed in BA n=3 exfoliated sheets under sub-bandgap excitation, yielding a g(2)(0) value
of 0.278 ± 0.103 (Supplementary Fig. 5). BA n=3 showed a red-shifted single-photon emission
peak at 743.4 nm compared to BA n=2 (683.0 nm) due to the smaller bandgap in BA n=3 with a
thicker inorganic layer. In addition, the PL lifetime of BA n=3 was measured to be longer ( =
1.729 ± 0.032 ns) than that of BA n=2 (Supplementary Fig. 6). Overall, these findings establish a
unique approach that leverages sub-bandgap, low-energy excitations to generate single-photon
emitters in 2D perovskites.
Next, we performed sub-bandgap pulsed excitation measurements on BA n=2 at 4 K.
Compared with CW excitation, pulsed excitation suppresses residual background emission,
allowing the intrinsic single-photon statistics of the emitter to be measured more reliably and
yielding improved g(2)(0) values. Figure 4a shows an optical microscope image of the exfoliated
BA n=2 single-crystal sheet used for these measurements. Similar to Figs. 2 and 3, spatial PL
mapping with a 650 nm long-pass filter revealed increased intensity in the edge region under near-
bandgap excitation ( = 600 nm), whereas sub-bandgap excitation ( = 633 nm) led to localized
p p
bright spots at the edges (Figs. 4b and c). We selected three representative spots (1–3) and observed
isolated narrow emission lines with FWHM of ~0.65–1.31 nm (Fig. 4d). HBT measurements under
pulsed excitation yielded clear photon anti-bunching for all three spots, with g(2)(0) = 0.229 ± 0.041
(spot 1), 0.326 ± 0.054 (spot 2), and 0.345 ± 0.059 (spot 3) (Fig. 4e). The measurement of the
integrated PL intensity as a function of the pump power exhibited saturation behavior, with a
saturation emission intensity of 67,213 counts/s at a pump power of 22.2 W (Fig. 4f).
We also measured power-dependent g(2)() on the same emitter (spot 1 in Fig. 4c) and found
that g(2)(0) improves at a lower pump power (g(2)(0) ~0.20 at 20 W and ~0.23 at 40 W)
(Supplementary Fig. 7). This result supports the fact that the observed anti-bunching is dominated
by a single emitter, rather than being an artifact arising from background noise. While sub-bandgap
excitation yielded single-photon emitters at the edges, we rarely observed localized bright spots in
the interior that exhibited anti-bunching behavior, with g(2)(0) = 0.471 ± 0.047 (Supplementary Fig.
8). However, the emission-wavelength stability of these interior point-defect single-photon
emitters is distinct from that of edge-state emitters. For quantitative evaluation, we performed
time-resolved spectral tracking for both interior point-defect and natural edge-state single-photon
emitters (Supplementary Fig. 9). The measurements indicate that edge-state single-photon emitters
are less prone to bleaching and exhibit better spectral stability than point-defect single-photon
emitters.
To gain deeper insight into three distinct emission characteristics of 2D perovskites, including
narrow, broad, and single-photon emissions, we performed photoluminescence excitation (PLE)
spectroscopy and TRPL experiments on the edge region of the sample. The PLE map of the BA
n=2 exfoliated sheet was obtained by first locating a spot with a confined emitter in the edge region
and acquiring PL data from that spot by continuously decreasing the excitation energy from above
(528 nm) to below (630 nm) the bandgap (Fig. 5a). The measurement results reveal the following
key features. First, the broad emission in the edge region is clearly observed for pump wavelengths
ranging from above-bandgap to below-bandgap. This indicates the presence of an energy state at
the edge that lies below the conduction band, with the emission from this transition contributing
to the broad emission. Second, the broad emission substantially decreases at the pump wavelengths
near 590 nm, which corresponds to the bandgap energy of ~2.1 eV at the edge. The transition from
the edge state is weakened by excitation at an energy corresponding to the bandgap. This
phenomenon occurs as a result of resonant pumping, which enables efficient excitation to the
conduction band edge26,39. Third, a sharp peak is observed at 708.5 nm, with a significant increase
in intensity when the pump wavelength changes from above-bandgap to near-bandgap and
subsequently to sub-bandgap excitation. This feature is particularly evident in Fig. 5b. To verify
the single-photon characteristic of the peak, we assessed the second-order correlation function,
g(2)(). The fitted value of g(2)(0) was determined to be 0.376 ± 0.167, indicating photon anti-
bunching (Fig. 5c). Furthermore, we conducted temperature-dependent PL measurements on the
sharp peak, which decreases with increasing temperature and disappears at temperatures over 60
K (Supplementary Fig. 10).
In addition, TRPL experiments were conducted to examine the changes in decay
characteristics at different pump wavelengths, using a band-pass filter to isolate the single-photon
emission peak (see Methods). The TRPL data were analyzed using a biexponential function:
𝐼(𝑡) = 𝐴1𝑒−𝑡⁄𝜏1 +𝐴2𝑒−𝑡⁄𝜏2 , where 1 represents the radiative decay time and 2 represents the
non-radiative decay time. We measured  = 3.18 ± 0.14 ns and 2.59 ± 0.08 ns, and  = 0.40 ±
0.004 ns and 0.33 ± 0.002 ns for the pump wavelengths of 550 nm and 633 nm, respectively (Fig.
5d). We note that τ and τ showed shorter decay times with sub-bandgap excitation at 633 nm.
The decrease in both decay times signifies that sub-bandgap excitation activates a channel that
allows excitons to be rapidly directed into a mid-gap state and undergo an accelerated
recombination process, thereby enhancing the probability and rate of single-photon emission.
Furthermore, we performed temperature-dependent TRPL measurements on an edge-state emitter
in BA n=2 at 4, 10, 20, 40, and 60 K, and analyzed the spectral characteristics, including the
emission linewidth (FWHM) and the integrated PL intensity (Supplementary Fig. 11). We also
extracted the temperature dependence of the PL lifetime. A key feature of these measurements is
that the practical temperature limit is primarily set by phonon-driven loss of spectral selectivity
together with activated competition from nonradiative/escape channels, rather than a single
mechanism alone.
Based on the results of the PLE and TRPL experiments, the model depicted in Fig. 5e
elucidates the series of transitions that charge carriers can possibly experience in different regions
of 2D perovskite sheets under different energy excitations. First, under above-bandgap excitation,
the interior region exhibits a narrow emission characteristic aligned with the bandgap energy (Fig.
5e, left). In the edge region, three distinct channels exist due to the edge state located below the
conduction band (Fig. 5e, middle): narrow emission from band-edge recombination, broad
emission caused by the transition from the edge state to the valence band, and nonradiative decay
through shallow traps21,40,41 situated just below the conduction band. In contrast, sub-bandgap
excitation creates a unique charge carrier transition scenario at specific locations of the edges
where mid-gap defect states exist (Fig. 5e, right). Excitons directly excited to the edge state are
effectively channeled towards a single mid-gap defect state, enhancing radiative recombination
and promoting single-photon emission while other recombination is suppressed, as shown in the
PLE measurement in Fig. 5a. We note that the edge state can provide a lower hurdle for excitons
to efficiently transition to deeper mid-gap states, likely due to their closer energy levels.
Next, to artificially induce edge-like effects within the interior regions of 2D perovskites, an
exfoliated BA n=2 single-crystal sheet was transferred onto a SiO /Si substrate including patterned
polymethyl methacrylate (PMMA) nanorods. Figures 6a and 6b show SEM and optical microscope
images, respectively, of the exfoliated sheet on 500-nm-high nanorods fabricated using PMMA
overdose, resulting in a tensile-strained morphology in BA n=2. Spatial PL mapping was
performed at 4 K under pulsed excitation using a 650 nm long-pass filter, comparing near-bandgap
excitation ( = 600 nm) and sub-bandgap excitation ( = 633 nm) around the artificially
p p
engineered edges (Figs. 6c-d). Five representative spots (1–5) were selected, where the PL spectra
exhibited distinct narrow emission peaks ranging from 655.7 to 726.5 nm (Fig. 6e). In addition,
the second-order correlation function, g(2)(), measured from spots 2 and 3 showed anti-bunching
behavior, with g(2)(0) values of 0.451 ± 0.045 and 0.491 ± 0.067, respectively (Fig. 6f).
Furthermore, TRPL measurements revealed a decay time of 0.696 ± 0.028 ns for the emitter at
spot 3 (Fig. 6g). PLE spectroscopy of spot 2 further confirms that the localized emission can be
efficiently accessed under sub-bandgap excitation (Fig. 6h).
Spatial PL mapping under lower-energy sub-bandgap excitation reveals strongly localized
emitters at the strain-engineered regions, with the localization becoming more pronounced as the
pump wavelength is increased from 532 nm to 633 nm (Figs. 6c-d and Supplementary Fig. 12).
Similar behavior was also observed for BA n=3 samples with artificially engineered edges
(Supplementary Fig. 13). Because these strain-induced emitters exhibit the same characteristics as
natural edge-state emitters, our results support the idea that tensile strain can generate edge-like
defects. Taken together, introducing artificial edges via strain engineering provides a promising
route for improved control and optimization of single-photon emitters in 2D perovskites.
In conclusion, we successfully demonstrated 2D perovskite-based single-photon emitters for
the first time. These emitters form from deep mid-gap states that are channeled via low-energy
edge states through unconventional sub-bandgap excitation. With conventional above-bandgap
excitation, the interior region of exfoliated sheets exhibited narrow PL emission, whereas the edge
regions displayed broad emission characteristics. Adjusting the pump wavelength revealed that
sub-bandgap excitation induces localized bright spots at the edges of exfoliated sheets of 2D
perovskites, particularly in BA n ≥ 2. These localized bright spots had distinct sharp peaks, which
were verified as single-photon emitters by second-order correlation measurements. Exciton
dynamics were further investigated through PLE and TRPL measurements, revealing the presence
of low-energy edge states within the band that can provide efficient access to deeper mid-gap states
responsible for single-photon emission. In particular, 2D perovskite single-photon emitters showed
high single-photon purity and brightness accompanied by their rapid decay time. Single-photon
emitters were also achieved in artificially engineered edges in exfoliated sheets, which pave an
additional avenue for potential emitter site control.
We summarized the emission energies of single-photon emitters for two compositions (BA
n=2 and BA n=3) and categorized them into three types (Supplementary Fig. 14). Statistical
analysis reveals distinct emission ranges depending on the composition and emitter type. For both
BA n=2 and BA n=3, for example, single-photon emission predominantly appears in an energy
range of ~300–400 meV below the bandgap. These results indicate that, although individual
emitter characteristics vary from site to site, selecting the n value and controlling the emission type
enable practical control over the achievable single-photon emission range. We also note that
isolated single peaks are not uniformly observed when rastering along the material edges. For
future optimization of emission sites and to enable practical implementation, we envision that
integrating artificial-edge single-photon emitters with a high-quality optical cavity supporting a
single resonant mode could provide reliable single-peak quantum emission.
Overall, 2D perovskites offer distinct advantages, combining solution processability with the
favorable properties typically associated with epitaxially grown van der Waals quantum emitters.
Furthermore, owing to their intrinsic quantum-well structure, 2D perovskite quantum emitters are
free from the monolayer constraint of TMDs, while retaining precise emitter-site control
capabilities that are inaccessible to quantum dot-based emitters. Therefore, the unique low-energy
edge-state mediated single-photon emission mechanism in 2D perovskites highlights their
fundamentally distinct photophysical nature, opening new opportunities for controllable quantum
light generation.

Methods
Synthesis of 2D perovskites. A slow cooling crystallization method was used to obtain bulk 2D
perovskite single crystals. PbI (0.4 mmol) and BAI (0.4 mmol) precursors were mixed into an
2
acid mixture containing 0.9 mL of hydroiodic acid (HI, 57 wt % in H O) and 0.1 mL of
2
hypophosphorous acid (H PO , 50 wt % in H O) in a glass vial for (BA) PbI (BA n=1). PbI (0.4
3 2 2 2 4 2
mmol), BAI (0.2 mmol), and MAI (0.35 mmol) precursors were mixed into the same acid mixture
for (BA) (MA)Pb I (BA n=2). PbI (0.6 mmol), BAI (0.2 mmol), and MAI (0.55 mmol)
2 2 7 2
precursors were mixed into the same acid mixture for (BA) (MA) Pb I (BA n=3). After mixing
2 2 3 10
the precursors and solution, the contents of the sample vial were magnetically stirred and heated
to 100°C in a water bath until materials were completely dissolved and the solution looked clear.
The vials were then moved to a Dewar flask water bath at 100°C to cool down for 72 hours until
it reached room temperature. The crystals were then retrieved from the solution through vacuum
filtration for further use.
Sample fabrication. 2D perovskites single-crystal sheets were isolated from bulk single crystals
through mechanical exfoliation using polyvinyl chloride low-adhesion cleanroom tape (blue tape,
DN-280). The exfoliated single-crystal sheets were initially transferred onto PDMS using the same
tape and subsequently onto a SiO /Si substrate via the PDMS stamping method at room
2
temperature.
Optical measurements. Optical measurements were conducted on the samples at 4 K using a
home-built confocal microscope mounted on a helium-flow cryostat (Montana Instruments, s50)
(Supplementary Fig. 15). A high-resolution raster scan was performed using two scanning
galvanometric mirrors and a 4f confocal alignment system. The PL emission from the sample was
collected using a 100× objective lens with a numerical aperture of 0.85 and directed to either a
monochromator coupled with a CCD (Princeton Instruments, PIXIS 400 BRX) or to avalanche
photodiodes (Excelitas, SPCM AQRH 16). Photon statistics were measured using the HBT setup,
wherein photons were detected by two identical avalanche photodiodes coupled with time-tagging
electronics (PicoQuant, Picoharp 300). The g(2)(0) values were subsequently extracted from well-
isolated peaks observed in the samples. For TRPL measurements, the emitters were optically
excited using a supercontinuum pulsed laser (NKT Photonics) operating at a repetition rate of 20
MHz. PLE measurements were conducted with a supercontinuum pulsed laser (NKT Photonics)
coupled with a SuperK VARIA filter module (NKT Photonics) to selectively tune the pump
wavelength.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Edge states of layered halide perovskites have recently been recognized as key factors in enhancing the performance of thin film solar cells, photodetectors, and light emitting diodes, owing to their ability to dissociate excitons into free carriers with lower energy and longer lifetime. In this work, spatially confined single photon emitters were realized at the edges of exfoliated perovskite single crystals through comprehensive spectroscopic measurements. The study is interesting and brings fresh idea on the edge states of 2D halide perovskites. This work is therefore suitable for publication in Nature Communications, if the following concerns can be adequately addressed.\n1. The manuscript states that the main features of edge states are their lower energy and longer lifetime compared with bulk states. The authors present PL spectra of n = 2 and n = 3 samples at 4 K, which differ significantly from previous reports at room temperature. Although PL mapping shows bright edge emission under sub bandgap pumping, are the low temperature edge states physically the same as those at room temperature? Could the extremely low temperature substantially alter the behavior of the edge states?\n2. In Figure 3, the authors demonstrate single photon emission at selected edge sites with different emission energies. However, similar emission spots are also observed in the interior region. Hence, the emission appears more likely to be related to point defects. How can the authors clearly separate the contributions from point defects and edge states? Is further experimental or analytical evidence needed to support this distinction?\n3. Multiple emission peaks are detected in the perovskite single photon emitters. For a given material, can the energy/wavelength of its single photon emission be predicted? Is it possible to establish a correlation between structure/composition and emission characteristics to guide material design and device applications?\n4. The authors artificially created “edge like defects” by covering exfoliated crystals on a SiO₂/Si substrate patterned with PMMA nanorods. The nanorods undoubtedly introduce tensile strain on the crystals. Because the lattice is discontinuous at the edges, the physical environment at the crystal edge is more complex than in the bulk. Why do the authors believe that tensile strain can produce “edge like” defects? Is there any structural or other experimental evidence to support this assumption?\n5. The occurrence of edge states in a family of 2D A2PbBr4 (n=1) should be mentioned in the background. (Adv. Mater. 2025, 37, 2419606)", "reviewer_2": "In this work, Dou and Park et al introduce a conceptually appealing framework in which ‘edge-state’ mediated excitation pathways in layered RP perovskites leads to localized single-photon emitters, representing an extension of perovskite photophysics into the quantum-emitter domain. The work’s primary strength lies in establishing a connection between material dimensionality, edge electronic structure, and the activation of deep, localized radiative centers that exhibit sub-Poissonian photon statistics. However, the central weaknesses are that the claimed single-photon emission requires more conclusive evidence, as the modest antibunching depth and limited emitter statistics leave open alternative explanations such as background mixing, multiple emitters within the diffraction-limited spot, or blinking-related artifacts, and that the mechanistic interpretation remains largely qualitative, with no explicit identification of the underlying defect structure or atomistic origin. Addressing these issues would substantially elevate the manuscript’s rigor and strengthen its suitability for publication at the Nature Communications level. Below are further technical issues for the authors to consider:\n1. The reported uncorrected g(2)(0) values (~0.28–0.47) indicate antibunching but remain well above those typical for high-purity SPEs (e.g., ~0.05–0.15 for defect centers in hBN, <0.10 for epitaxial III–V materials). To accurately assess single-photon purity, the manuscript is encouraged to provide background-corrected g(2)(0) values, explicit signal-to-background ratios at each emitter, and ideally power-dependent antibunching measurements to rule out multi-emitter contributions or background-induced artefacts. Residual background emission, whether originating from edge states, trap-related PL, or substrate fluorescence, can artificially suppress or distort the measured dip in g(2)(0), leading to an apparent antibunching signature that does not necessarily reflect the intrinsic single-photon statistics of the emitter itself.\n2. The analysis is currently based on a limited number of representative emitters. To establish reproducibility, the authors are encouraged to report distributions of linewidths, brightness, emission wavelength stability, and emitter density along edges, etc. Even a modest statistical set would significantly strengthen the generality of the conclusions.\n3. While the edge-state funneling picture is plausible, the manuscript lacks concrete identification of the emitting defect. The authors are encouraged to discuss realistic defect candidates (halide vacancies, reduced-edge octahedra, termination configurations, etc.), ideally supported by DFT levels or relevant literature, and clarify how alternative explanations (e.g., inclusions, local strain pockets, impurities) were excluded.\n4. Since narrow-line emission disappears above ~60 K (Fig. S6), the authors could include a more realistic assessment of the temperature-limiting mechanisms (e.g., phonon broadening, defect ionization, edge-state delocalization?).\n5. Key metrics such as spectral wandering, blinking dynamics, and emission intermittency are not characterized. These are critical for evaluating SPE usability. Time-resolved spectral tracking and intensity autocorrelation analysis over minutes to hours would meaningfully strengthen the case for stable quantum emission.", "reviewer_3": "The manuscript reports single-photon emission from mid-gap states in (BA) (MA)Pb I and (BA) (MA) Pb I . The reported 2 27 2 2 310 synthesis and characterization of these materials are methodologically sound, and claims regarding the lower energy emission are generally compelling based on the evidence provided. My assessment is that this manuscript represents a contribution to the field, particularly toward understanding of the electronic landscape in two-dimensional perovskites, and I would recommend its publication. However, I believe the following concerns should be addressed:\nSingle-photon emission with regard to practical use in quantum emitters depends on a reliable source of single photons on demand. The g(2) values reported here do meet the criterion for anti-bunching behavior, however, they indicate a relatively high probability of multi-photon emission. The results here represent a characterization of the mid-gap states in the materials studied with interesting results, but these results do not support a claim for practical utility. The manuscript is well-written and pleasant to read, but embellished, and I suggest generally reframing the abstract, introduction, and other sections of the paper to avoid the inference that these materials are of practical use as single-photon emitters.\nThe second-order correlation measurements in Fig. 3. e., 4. c., and Supplementary Fig. 5. e. exhibit relatively low signal-to-noise ratios, which is unfortunate considering the measurement’s importance in establishing the claimed single-photon emission and the quantification of photon purity. Can the reliability of the data be improved, for instance, to the standard of Fig. 5 e.? Were additional pulsed measurements attempted?\nCan the authors comment on the general probability and ease of finding a single peak, as opposed to a multitude, when rastering the edges of the materials measured here? Did the artificially engineered edges facilitate isolation of single peaks? Commenting on this could guide future optimization of emission sites and realistically define practical use, if any.\nIn addition:\nWhat is the photoluminescence lifetime of the mid-gap state emission measured for BA n=3 crystals (Fig. 5, Supplementary Fig. 2. and 5.)? How does the lifetime depend on the energetic position of the specific mid-gap state probed, and how does this vary between n=2, n=3, … n=N materials?\nWhat is the identity of the emission peak at approximately 694 nm, predominantly visible under above-bandgap excitation (Fig. 4. a, b), and is it significant to the properties of the mid-gap state measured?\nCan the authors comment on the thermally activated depopulation of the mid-gap state emission (Supplementary Fig. 6.)? In a recent publication, dark excitons have been reported to localize at the edges of two-dimensional perovskites (Bailey, C. G. et al. Advanced Energy Materials 15, 2501593, 2025). In addition to the edge state, could such fine structure states mediate the population and depopulation of the mid-gap states studied here? Additional temperature-dependent photoluminescence data could address the origins of the thermal quenching and elucidate relaxation pathways relevant to the single-photon emission state."}, "author_response": "Response to Reviewer #1.\nComment. Edge states of layered halide perovskites have recently been recognized as key\nfactors in enhancing the performance of thin film solar cells, photodetectors, and light emitting\ndiodes, owing to their ability to dissociate excitons into free carriers with lower energy and\nlonger lifetime. In this work, spatially confined single photon emitters were realized at the\nedges of exfoliated perovskite single crystals through comprehensive spectroscopic\nmeasurements. The study is interesting and brings fresh idea on the edge states of 2D halide\nperovskites. This work is therefore suitable for publication in Nature Communications, if the\nfollowing concerns can be adequately addressed.\nOur response. We thank Reviewer #1 for his/her positive evaluation of the novelty and\nimportance of our work. We are happy to have the opportunity to address the reviewer’s\nimportant questions and specific suggestions.\nComment 1. The manuscript states that the main features of edge states are their lower energy\nand longer lifetime compared with bulk states. The authors present PL spectra of n = 2 and n = 3\nsamples at 4 K, which differ significantly from previous reports at room temperature. Although\nPL mapping shows bright edge emission under sub bandgap pumping, are the low temperature\nedge states physically the same as those at room temperature? Could the extremely low\ntemperature substantially alter the behavior of the edge states?\nOur response. We thank the reviewer for raising this insightful question. However, we would\nlike to clarify that we did not state in the manuscript that the lifetime of the edge states is longer\nthan that of the bulk states. In the revised manuscript, we further investigated the temperature\ndependence of the edge states by performing PL mapping on the same BA n=2 flake at 4 K and\nroom temperature (new Supplementary Fig. 2).\nOur results suggest that low temperature does not fundamentally change the main features of\nthe edge states. Under various pump wavelengths (532, 580, 600, and 633 nm), broad emission\nfrom the flake edges is also enhanced at room temperature, similar to the low-temperature case.\nLocalized emission spots are observed at the edges even at room temperature, although their\nbrightness under 600 nm and 633 nm excitation is weaker than at 4 K. These observations\nindicate that the edge-state-related emission mechanism is already present at room temperature.\nIn addition, the fact that edge emission occurs over a broad temperature range but becomes\nmore spectrally resolved at low temperature is consistent with a previous report [ACS Nano\n13, 1635 (2019)].\nTo respond to the reviewer’s comment, we added new Supplementary Fig. 2 to the revised\nSupplementary Information. In addition, we added a sentence to the revised manuscript (page\n9): “These features observed at 4 K are also present at room temperature (Supplementary Fig.\n2), indicating that lowering the temperature does not substantially alter the characteristics of\nthe edge states.”\n[New Supplementary Fig. 2]\nComment 2. In Figure 3, the authors demonstrate single photon emission at selected edge sites\nwith different emission energies. However, similar emission spots are also observed in the\ninterior region. Hence, the emission appears more likely to be related to point defects. How can\nthe authors clearly separate the contributions from point defects and edge states? Is further\nexperimental or analytical evidence needed to support this distinction?\nOur response. We thank the reviewer’s insightful comment. As the reviewer pointed out,\nsimilar emission spots can also be observed in the interior region. Although such interior point-\ndefect single-photon emitters are relatively rare, they can be distinguished from edge-state\nsingle-photon emitters based on their spectral and dynamical signatures. A comparison of these\nemitters, together with the artificial-edge single-photon emitters, is summarized in new\nSupplementary Fig. 14.\nWe note that, in contrast to point-defect single-photon emitters, edge-state single-photon\nemitters occupy a systematically lower-energy band; for BA n=2, they are emphasized at ~300–\n400 meV below bandgap energy (E = 2.17 eV), consistent with our proposed mechanism. In\ng\naddition, to evaluate emission-wavelength stability, we included time-resolved spectral\ntracking for both interior point-defect and natural edge-state single-photon emitters (new\nSupplementary Fig. 9). These results show that the edge-state single-photon emitters are less\nprone to bleaching and exhibits better spectral stability than the point-defect single-photon\nemitters.\nTo respond to the reviewer’s comment, we added new Supplementary Figs. 9 and 14 to the\nrevised Supplementary Information. In addition, we added the following sentences to the\nrevised manuscript (page 14): “While sub-bandgap excitation yielded single-photon emitters\nat the edges, we rarely observed localized bright spots in the interior that exhibited anti-\nbunching behavior, with g(2)(0) = 0.471 ± 0.047 (Supplementary Fig. 8). However, the\nemission-wavelength stability of these interior point-defect single-photon emitters is distinct\nfrom that of edge-state emitters. For quantitative evaluation, we performed time-resolved\nspectral tracking for both interior point-defect and natural edge-state single-photon emitters\n(Supplementary Fig. 9). The measurements indicate that edge-state single-photon emitters are\nless prone to bleaching and exhibit better spectral stability than point-defect single-photon\nemitters.”.\n[New Supplementary Fig. 14]\n[New Supplementary Fig. 9]\nComment 3. Multiple emission peaks are detected in the perovskite single photon emitters.\nFor a given material, can the energy/wavelength of its single photon emission be predicted? Is\nit possible to establish a correlation between structure/composition and emission characteristics\nto guide material design and device applications?\nOur response. As the reviewer noted, multiple emission peaks are observed in the perovskite\nsingle-photon emitters. As discussed in the original manuscript, the single-photon emitters\noriginate from deep mid-gap states, and thus their exact emission wavelengths cannot be\ndeterministically predicted. However, the emission-wavelength range can be experimentally\nidentified, as shown in new Supplementary Fig. 14a (Comment 2).\nSpecifically, we summarized the emission energies of single-photon emitters for two\ncompositions (BA n=2, E ~2.17 eV; BA n=3, E ~2.03 eV), and categorized the emitters into\ng g\nthree types such as interior point-defect, natural edge-state, and artificial edge-state single-\nphoton emitters. The statistical analysis reveals distinct emission ranges depending on the\ncomposition and type. For example, for both BA n=2 and BA n=3, single-photon emission\npredominantly appears in an energy range ~300–400 meV below E . These results indicate that,\ng\nalthough individual emitter characteristics vary from site to site, selecting the n value and\ncontrolling the emission type enables practical control over the achievable single-photon\nemission range.\nTo respond to the reviewer’s comment, we added a paragraph to the revised manuscript (page\n21): “We summarized the emission energies of single-photon emitters for two compositions\n(BA n=2 and BA n=3) and categorized them into three types (Supplementary Fig. 14).\nStatistical analysis reveals distinct emission ranges depending on the composition and emitter\ntype. For both BA n=2 and BA n=3, for example, single-photon emission predominantly\nappears in an energy range of ~300–400 meV below the bandgap. These results indicate that,\nalthough individual emitter characteristics vary from site to site, selecting the n value and\ncontrolling the emission type enable practical control over the achievable single-photon\nemission range.”\nComment 4. The authors artificially created “edge like defects” by covering exfoliated crystals\non a SiO /Si substrate patterned with PMMA nanorods. The nanorods undoubtedly introduce\n2\ntensile strain on the crystals. Because the lattice is discontinuous at the edges, the physical\nenvironment at the crystal edge is more complex than in the bulk. Why do the authors believe\nthat tensile strain can produce “edge like” defects? Is there any structural or other experimental\nevidence to support this assumption?\nOur response. We thank the reviewer for raising this important question. The artificially\nengineered single-photon emitters formed by tensile strain exhibit the same distinctive\ncharacteristics as the natural edge-state single-photon emitters. Since the experimentally\nobserved physical behavior and excitation-dependent signatures are essentially identical, our\nresults support that tensile strain can produce “edge-like” defects.\nTo clarify this point, we performed additional experiments using an exfoliated BA n=2 single-\ncrystal sheet transferred onto a SiO /Si substrate patterned with polymethyl methacrylate\n2\n(PMMA) nanorods (new Fig. 6 and new Supplementary Fig. 12). The measurements reveal\nseveral key features. First, spatial PL mapping under lower-energy sub-bandgap excitation\nproduces strongly localized emitters at the strain-engineered regions. This localization becomes\nincreasingly pronounced as the pump wavelength is increased from 532 nm to 633 nm (new\nFig. 6c-d and new Supplementary Fig. 12c-f). Second, under deeper sub-bandgap excitation\n( = 633 nm), multiple localized bright spots around the engineered edge-like morphology\np\nexhibit distinct narrow emission peaks spanning 655.7–726.5 nm (new Fig. 6e). Corresponding\nHBT measurements yield g(2)(0) = 0.451 ± 0.045 and 0.491 ± 0.067 (new Fig. 6f). Third, TRPL\nmeasurements indicate an emitter decay time of 0.696 ± 0.028 ns (new Fig. 6g). PLE\nspectroscopy further confirms that the localized emission can be efficiently accessed under sub-\nbandgap excitation (new Fig. 6h).\nTaken together, these optical characteristics closely match those observed for natural edge-state\nsingle-photon emitters, supporting our interpretation. The introduction of artificial edges\nthrough strain engineering represents a promising approach for additional control and\noptimization of single-photon emitters in 2D perovskites.\nTo respond to the reviewer’s question, we have added new Fig. 6 and new Supplementary Fig.\n12. We also added the corresponding paragraph to the revised manuscript: “Next, to artificially\ninduce edge-like effects within the interior regions of 2D perovskites, an exfoliated BA n=2\nsingle-crystal sheet was transferred onto a SiO /Si substrate including patterned polymethyl\n2\nmethacrylate (PMMA) nanorods. Figs. 6a and 6b show SEM and optical microscope images,\nrespectively, of the exfoliated sheet on 500-nm-high nanorods fabricated using PMMA\noverdose, resulting in a tensile-strained morphology in BA n=2. Spatial PL mapping was\nperformed at 4 K under pulsed excitation using a 650 nm long-pass filter, comparing near-\nbandgap excitation ( = 600 nm) and sub-bandgap excitation ( = 633 nm) around the\np p\nartificially engineered edges (Figs. 6c-d). Five representative spots (1–5) were selected, where\nthe PL spectra exhibited distinct narrow emission peaks ranging from 655.7 to 726.5 nm (Fig.\n6e). In addition, the second-order correlation function, g(2)(), measured from spots 2 and 3\nshowed anti-bunching behavior, with g(2)(0) values of 0.451 ± 0.045 and 0.491 ± 0.067,\nrespectively (Fig. 6f). Furthermore, TRPL measurements revealed a decay time of 0.696 ±\n0.028 ns for the emitter at spot 3 (Fig. 6g). PLE spectroscopy of spot 2 further confirms that\nthe localized emission can be efficiently accessed under sub-bandgap excitation (Fig. 6h).”\n(page 18) and,\n“Spatial PL mapping under lower-energy sub-bandgap excitation reveals strongly localized\nemitters at the strain-engineered regions, with the localization becoming more pronounced as\nthe pump wavelength is increased from 532 nm to 633 nm (Figs. 6c-d and Supplementary Fig.\n12). Similar behavior was also observed for BA n=3 samples with artificially engineered edges\n(Supplementary Fig. 13). Because these strain-induced emitters exhibit the same characteristics\nas natural edge-state emitters, our results support the idea that tensile strain can generate ‘edge-\nlike’ defects. Taken together, introducing artificial edges via strain engineering provides a\npromising route for improved control and optimization of single-photon emitters in 2D\nperovskites.” (page 20).\n[New Supplementary Fig. 12]\n[New Fig. 6]\nComment 5. The occurrence of edge states in a family of 2D A2PbBr4 (n=1) should be\nmentioned in the background. (Adv. Mater. 2025, 37, 2419606)\nOur response. We thank the reviewer for the suggestion. We have cited the recommended\nwork [Adv. Mater. 37, 2419606 (2025)] as Ref. 30 in the revised reference list.\nResponse to Reviewer #2.\nComment. In this work, Dou and Park et al introduce a conceptually appealing framework in\nwhich ‘edge-state’ mediated excitation pathways in layered RP perovskites leads to localized\nsingle-photon emitters, representing an extension of perovskite photophysics into the quantum-\nemitter domain. The work’s primary strength lies in establishing a connection between material\ndimensionality, edge electronic structure, and the activation of deep, localized radiative centers\nthat exhibit sub-Poissonian photon statistics. However, the central weaknesses are that the\nclaimed single-photon emission requires more conclusive evidence, as the modest\nantibunching depth and limited emitter statistics leave open alternative explanations such as\nbackground mixing, multiple emitters within the diffraction-limited spot, or blinking-related\nartifacts, and that the mechanistic interpretation remains largely qualitative, with no explicit\nidentification of the underlying defect structure or atomistic origin. Addressing these issues\nwould substantially elevate the manuscript’s rigor and strengthen its suitability for publication\nat the Nature Communications level. Below are further technical issues for the authors to\nconsider:\nOur response. We thank Reviewer #2 for his/her constructive comments and suggestions. We\nare happy to have the opportunity to address the reviewer’s important questions and specific\nsuggestions.\nComment 1. The reported uncorrected g(2)(0) values (~0.28–0.47) indicate antibunching but\nremain well above those typical for high-purity SPEs (e.g., ~0.05–0.15 for defect centers in\nhBN, <0.10 for epitaxial III–V materials). To accurately assess single-photon purity, the\nmanuscript is encouraged to provide background-corrected g(2)(0) values, explicit signal-to-\nbackground ratios at each emitter, and ideally power-dependent antibunching measurements to\nrule out multi-emitter contributions or background-induced artefacts. Residual background\nemission, whether originating from edge states, trap-related PL, or substrate fluorescence, can\nartificially suppress or distort the measured dip in g(2)(0), leading to an apparent antibunching\nsignature that does not necessarily reflect the intrinsic single-photon statistics of the emitter\nitself.\nOur response. We thank the reviewer for raising this important point. To improve the reliability\nof the data, we performed additional pulsed measurements, which increased the signal-to-noise\nratio of the second-order correlation function g(2)() (new Fig. 4). Compared with CW\nexcitation, pulsed excitation suppresses residual background emission, allowing the intrinsic\nsingle-photon statistics of the emitter to be measured more reliably and yielding improved g(2)(0)\nvalues. Representative values are g(2)(0) = 0.229 ± 0.041, 0.326 ± 0.054, and 0.345 ± 0.059 for\nspots 1, 2, and 3, respectively, on the same BA n=2 flake (new Fig. 4e).\nWe also performed power-dependent g(2)() measurements on a single-photon emitter under\npulsed excitation (new Supplementary Fig. 7). The observation that g(2)(0) improves at lower\npump power (g(2)(0) ~0.20 at 20 W and ~0.23 at 40 W) supports that the anti-bunching is\ndominated by a single emitter rather than being an artifact caused by background noise.\nAlthough the single-photon purity in 2D perovskites is currently lower than that of established\nplatforms such as h-BN or epitaxial III-V materials, we emphasize that developing single-\nphoton emitters in non-epitaxially grown materials is pivotal for realizing practical, low-cost,\nand scalable quantum light sources.\nTo respond to the reviewer’s comment, we added new Fig. 4 and a corresponding paragraph in\nthe revised manuscript (page 12): “Next, we performed sub-bandgap pulsed excitation\nmeasurements on BA n=2 at 4 K. Compared with CW excitation, pulsed excitation suppresses\nresidual background emission, allowing the intrinsic single-photon statistics of the emitter to\nbe measured more reliably and yielding improved g(2)(0) values. Fig. 4a shows an optical\nmicroscope image of the exfoliated BA n=2 single-crystal sheet used for these measurements.\nSimilar to Figs. 2 and 3, spatial PL mapping with a 650 nm long-pass filter revealed increased\nintensity in the edge region under near-bandgap excitation ( = 600 nm), whereas sub-bandgap\np\nexcitation ( = 633 nm) led to localized bright spots at the edges (Figs. 4b and c). We selected\np\nthree representative spots (1–3) and observed isolated narrow emission lines with FWHM of\n~0.65–1.31 nm (Fig. 4d). HBT measurements under pulsed excitation yielded clear photon\nanti-bunching for all three spots, with g(2)(0) = 0.229 ± 0.041 (spot 1), 0.326 ± 0.054 (spot 2),\nand 0.345 ± 0.059 (spot 3) (Fig. 4e). The measurement of the integrated PL intensity as a\nfunction of the pump power exhibited saturation behavior, with a saturation emission intensity\nof 67,213 counts/s at a pump power of 22.2 W (Fig. 4f).”\nIn addition, we added new Supplementary Fig. 7 and two sentences to the revised manuscript\n(page 14): “We also measured power-dependent g(2)() on the same emitter (spot 1 in Fig. 4c)\nand found that g(2)(0) improves at a lower pump power (g(2)(0) ~0.20 at 20 W and ~0.23 at 40\nW) (Supplementary Fig. 7). This result supports the fact that the observed anti-bunching is\ndominated by a single emitter, rather than being an artifact arising from background noise.”\nWhile the g(2)(0) values in Fig. 3e and old Fig. 4c exhibit relatively low signal-to-noise ratios,\nas the reviewer noted, these panels are primarily included to highlight the main observations—\nthe localized bright spots at the edges (Fig. 3e) and the PLE measurements of single-photon\nemitters (old Fig. 4c).\n[New Fig. 4]\n[New Supplementary Fig. 7]\nComment 2. The analysis is currently based on a limited number of representative emitters. To\nestablish reproducibility, the authors are encouraged to report distributions of linewidths,\nbrightness, emission wavelength stability, and emitter density along edges, etc. Even a modest\nstatistical set would significantly strengthen the generality of the conclusions.\nOur response. We thank the reviewer for raising this important point. As suggested by the\nreviewer, we measured a larger number of emitters (new Fig. 4 and new Fig. 6) and added\nreproducibility and statistical analyses (new Supplementary Fig. 14). As shown in new\nSupplementary Fig. 14, we summarized the emission wavelength/energy distributions,\nlinewidth distributions, and the corresponding g(2)(0) values for three types of single-photon\nemitters (interior point-defect, natural edge-state, and artificial edge-state single-photon\nemitters) across two 2D perovskite compositions (BA n=2 and BA n=3). In addition, to evaluate\nemission wavelength stability, we included time-resolved spectral tracking for both interior\npoint-defect single-photon emitters and natural edge-state single-photon emitters (new\nSupplementary Fig. 9).\nRegarding emitter density along edges, we agree with the reviewer that this is also an important\nparameter. However, it strongly depends on the sample and the measurement conditions.\nTherefore, rather than reporting a value, we present PL maps acquired under various pump\nwavelengths in the main and supplementary figures.\nTo respond to the reviewer’s comment, we added new Supplementary Figs. 9 and 14 to the\nrevised Supplementary Information. Also, we added a paragraph to the revised manuscript\n(page 21): “We summarized the emission energies of single-photon emitters for two\ncompositions (BA n=2 and BA n=3) and categorized them into three types (Supplementary Fig.\n14). Statistical analysis reveals distinct emission ranges depending on the composition and\nemitter type. For both BA n=2 and BA n=3, for example, single-photon emission predominantly\nappears in an energy range of ~300–400 meV below the bandgap. These results indicate that,\nalthough individual emitter characteristics vary from site to site, selecting the n value and\ncontrolling the emission type enable practical control over the achievable single-photon\nemission range.”\n[New Supplementary Fig. 14]\n[New Supplementary Fig. 9]\nComment 3. While the edge-state funneling picture is plausible, the manuscript lacks concrete\nidentification of the emitting defect. The authors are encouraged to discuss realistic defect\ncandidates (halide vacancies, reduced-edge octahedra, termination configurations, etc.), ideally\nsupported by DFT levels or relevant literature, and clarify how alternative explanations (e.g.,\ninclusions, local strain pockets, impurities) were excluded.\nOur response. We thank the reviewer for raising this point. Identifying the exact origin of\nsingle-photon emitters in halide perovskite materials remains experimentally challenging\nbecause these materials are intrinsically soft and fragile. In our previous attempts, we used\nSTEM to probe the emitters, but the samples degraded rapidly under the electron beam,\npreventing reliable defect identification. Very recently, a new TEM technique was developed\nto image defects in halide perovskite materials [Nature 647, 364 (2025)]; however, this\napproach requires sophisticated sample preparation and advanced data analysis. We plan to\nestablish new collaborations with TEM experts in the near future to investigate the structural\norigin of the single-photon emitters in more detail. Even if sufficient structural information can\nbe obtained, unambiguously assigning an emission peak to a specific defect site is still difficult\nat the present stage. We thus believe that disclosing the exciting observation of single-photon\nemission in 2D perovskite materials in a timely fashion will benefit the community and help\nattract additional imaging and theory experts to study this type of materials.\nInstead, as suggested by the reviewer, we have added a discussion of realistic defect candidates\nby outlining possible explanations for edge states based on relevant literature. First, edge-\ntermination-induced reconstruction (i.e., “dangling/relaxed” terminal PbX octahedra) can\n6\ngenerate reduced-energy edge-localized states, as predicted by first-principles calculations\n[Adv. Mater. 34, 2201666 (2022)]. Alternatively, the edges in exfoliated single crystals can be\ndescribed as layer-edge states stabilized by internal electric fields arising from dipole alignment\nand termination-dependent electrostatics [Nano Lett. 21, 182 (2021)]. Together, these\nmechanisms support interpreting our emitters as originating from low-energy “edge states” that\nare unique to 2D perovskites, rather than from generic bulk point defects.\nTo address the reviewer’s comment, we added one sentence to the revised manuscript (page 5):\n“which are attributed to edge-termination-induced reconstruction or termination-dependent\nelectrostatics29,31”.\nComment 4. Since narrow-line emission disappears above ~60 K (Fig. S6), the authors could\ninclude a more realistic assessment of the temperature-limiting mechanisms (e.g., phonon\nbroadening, defect ionization, edge-state delocalization?).\nOur response. We thank the reviewer for raising this point regarding the temperature-limit\nmechanisms. To provide a more realistic assessment, we performed temperature-dependent\nTRPL measurements on an edge-state emitter in BA n=2 at 4, 10, 20, 40, and 60 K, and\nanalyzed the spectral characteristics, including the emission linewidth (FWHM) and the\nintegrated PL intensity (new Supplementary Fig. 11). From these measurements, we extracted\nthe temperature dependence of the PL lifetime.\nThe results reveal several key features. First, the linewidth (FWHM) increases from 0.8 nm at\nspectral fluctuations at elevated temperatures. Second, the integrated PL intensity decreases\nrapidly above 20 K, and the lifetime decreases from ~6 ns near 10 K to ~2 ns at 60 K, indicating\nthat nonradiative pathways become increasingly competitive as temperature rises. Third, the\nnon-monotonic lifetime trend, an increase from 4 K to 10 K followed by a decrease, suggests\na crossover in the dominant decay pathways. Overall, these results indicate that the practical\ntemperature limit is primarily set by phonon-driven loss of spectral selectivity together with\nactivated competition from nonradiative/escape channels, rather than a single mechanism alone.\nTo respond to the reviewer’s comment, we added new Supplementary Fig. 11 and the following\nsentences to the revised manuscript (page 17): “Furthermore, we performed temperature-\ndependent TRPL measurements on an edge-state emitter in BA n=2 at 4, 10, 20, 40, and 60 K,\nand analyzed the spectral characteristics, including the emission linewidth (FWHM) and the\nintegrated PL intensity (Supplementary Fig. 11). We also extracted the temperature dependence\nof the PL lifetime. A key feature of these measurements is that the practical temperature limit\nis primarily set by phonon-driven loss of spectral selectivity together with activated\ncompetition from nonradiative/escape channels, rather than a single mechanism alone.”\n[New Supplementary Fig. 11]\nComment 5. Key metrics such as spectral wandering, blinking dynamics, and emission\nintermittency are not characterized. These are critical for evaluating SPE usability. Time-\nresolved spectral tracking and intensity autocorrelation analysis over minutes to hours would\nmeaningfully strengthen the case for stable quantum emission.\nOur response. We thank the reviewer for raising this important point regarding emission\nstability. As suggested by the reviewer, we performed time-resolved spectral tracking (new\nSupplementary Fig. 9), in which PL spectra are acquired every 10 s for up to 5 min for both a\npoint-defect single-photon emitter (~663 nm) and an edge-state single-photon emitter (~695\nnm). These results show that the edge-state single-photon emitter is less prone to bleaching and\nexhibits better spectral stability than the point-defect single-photon emitter.\n[New Supplementary Fig. 9]\nTo address the reviewer’s comment, we added new Supplementary Fig. 9 and the following\nsentences to the revised manuscript (page 14): “While sub-bandgap excitation yielded single-\nphoton emitters at the edges, we rarely observed localized bright spots in the interior that\nexhibited anti-bunching behavior, with g(2)(0) = 0.471 ± 0.047 (Supplementary Fig. 8).\nHowever, the emission-wavelength stability of these interior point-defect single-photon\nemitters is distinct from that of edge-state emitters. For quantitative evaluation, we performed\ntime-resolved spectral tracking for both interior point-defect and natural edge-state single-\nphoton emitters (Supplementary Fig. 9). The measurements indicate that edge-state single-\nphoton emitters are less prone to bleaching and exhibit better spectral stability than point-defect\nsingle-photon emitters.”\nResponse to Reviewer #3.\nComment. The manuscript reports single-photon emission from mid-gap states in\n(BA) (MA)Pb I and (BA) (MA) Pb I . The reported synthesis and characterization of these\n2 2 7 2 2 3 10\nmaterials are methodologically sound, and claims regarding the lower energy emission are\ngenerally compelling based on the evidence provided. My assessment is that this manuscript\nrepresents a contribution to the field, particularly toward understanding of the electronic\nlandscape in two-dimensional perovskites, and I would recommend its publication. However,\nI believe the following concerns should be addressed:\nOur response. We thank Reviewer #3 for his/her positive evaluation of the novelty and\nimportance of our work. We are happy to have the opportunity to address the reviewer’s\nimportant questions and specific suggestions.\nComment 1. Single-photon emission with regard to practical use in quantum emitters depends\non a reliable source of single photons on demand. The g(2) values reported here do meet the\ncriterion for anti-bunching behavior, however, they indicate a relatively high probability of\nmulti-photon emission. The results here represent a characterization of the mid-gap states in\nthe materials studied with interesting results, but these results do not support a claim for\npractical utility. The manuscript is well-written and pleasant to read, but embellished, and I\nsuggest generally reframing the abstract, introduction, and other sections of the paper to avoid\nthe inference that these materials are of practical use as single-photon emitters.\nOur response. We thank the reviewer for the positive comments on our results. We also\nappreciate the reviewer’s point regarding the practical use of single-photon emitters. While our\nmanuscript was not intended to claim immediate practical applicability, we recognize that some\nstatements could be interpreted in that way. As suggested by the reviewer, we have therefore\nrevised and toned down several expressions in the Abstract, Introduction, and other sections to\navoid potential overstatement and unnecessary confusion. For example, we removed the\nfollowing sentences in the revised manuscript (page 21): “Beyond these advantages which\ndirectly address the limitations of existing quantum emitter materials,” and “Our demonstration\nof 2D perovskite single-photon emitters, combined with their exceptional chemical tunability\nand structural versatility, establishes them as a promising and scalable candidate for developing\nefficient quantum light sources for emerging quantum technologies.”\nComment 2. The second-order correlation measurements in Fig. 3. e., 4. c., and Supplementary\nFig. 5. e. exhibit relatively low signal-to-noise ratios, which is unfortunate considering the\nmeasurement’s importance in establishing the claimed single-photon emission and the\nquantification of photon purity. Can the reliability of the data be improved, for instance, to the\nstandard of Fig. 5 e.? Were additional pulsed measurements attempted?\nOur response. We thank the reviewer for raising this important point. To improve the reliability\nof the data, we performed additional pulsed measurements, which increased the signal-to-noise\nratio of the second-order correlation function g(2)() (new Fig. 4). Compared with CW\nexcitation, pulsed excitation suppresses residual background emission, allowing the intrinsic\nsingle-photon statistics of the emitter to be measured more reliably and yielding improved g(2)(0)\nvalues. Representative values are g(2)(0) = 0.229 ± 0.041, 0.326 ± 0.054, and 0.345 ± 0.059 for\nspots 1, 2, and 3, respectively, on the same BA n=2 flake (new Fig. 4e).\nWhile the g(2)(0) values in Fig. 3e and old Fig. 4c exhibit relatively low signal-to-noise ratios,\nas the reviewer noted, these panels are primarily included to highlight the main observations—\nthe localized bright spots at the edges (Fig. 3e) and the PLE measurements of single-photon\nemitters (old Fig. 4c).\nTo respond to the reviewer’s comment, we added new Fig. 4 and a corresponding paragraph to\nthe revised manuscript (page 12): “Next, we performed sub-bandgap pulsed excitation\nmeasurements on BA n=2 at 4 K. Compared with CW excitation, pulsed excitation suppresses\nresidual background emission, allowing the intrinsic single-photon statistics of the emitter to\nbe measured more reliably and yielding improved g(2)(0) values. Fig. 4a shows an optical\nmicroscope image of the exfoliated BA n=2 single-crystal sheet used for these measurements.\nSimilar to Figs. 2 and 3, spatial PL mapping with a 650 nm long-pass filter revealed increased\nintensity in the edge region under near-bandgap excitation ( = 600 nm), whereas sub-bandgap\np\nexcitation ( = 633 nm) led to localized bright spots at the edges (Figs. 4b and c). We selected\np\nthree representative spots (1–3) and observed isolated narrow emission lines with FWHM of\n~0.65–1.31 nm (Fig. 4d). HBT measurements under pulsed excitation yielded clear photon\nanti-bunching for all three spots, with g(2)(0) = 0.229 ± 0.041 (spot 1), 0.326 ± 0.054 (spot 2),\nand 0.345 ± 0.059 (spot 3) (Fig. 4e). The measurement of the integrated PL intensity as a\nfunction of the pump power exhibited saturation behavior, with a saturation emission intensity\nof 67,213 counts/s at a pump power of 22.2 W (Fig. 4f).”.\n[New Fig. 4]\nComment 3. Can the authors comment on the general probability and ease of finding a single\npeak, as opposed to a multitude, when rastering the edges of the materials measured here? Did\nthe artificially engineered edges facilitate isolation of single peaks? Commenting on this could\nguide future optimization of emission sites and realistically define practical use, if any.\nOur response. We thank the reviewer for raising this question. In our measurements, isolated\nsingle peaks are not uniformly observed when rastering along the edges of the materials. For\nthe artificially engineered edges, because they exhibit emission characteristics consistent with\nthose of natural edges (as discussed in Comment 4 of Reviewer 1), isolated single peaks can\nsimilarly be observed. However, artificial-edge emitters are found only at selected localized\nsites, rather than uniformly along the entire edge (new Fig. 6). For future optimization of\nemission sites and to realistically define practical use, we envision that combining artificial-\nedge single-photon emitters with a high-Q optical cavity that supports a single resonant mode\ncould enable reliable single-peak quantum emission.\nTo respond to the reviewer’s comment, we added two sentences to the revised manuscript (page\n21): “We also note that isolated single peaks are not uniformly observed when rastering along\nthe material edges. For future optimization of emission sites and to enable practical\nimplementation, we envision that integrating artificial-edge single-photon emitters with a high-\nquality optical cavity supporting a single resonant mode could provide reliable single-peak\nquantum emission.”\n[New Fig. 6]\nComment 4. What is the photoluminescence lifetime of the mid-gap state emission measured\nfor BA n=3 crystals (Fig. 5, Supplementary Fig. 2. and 5.)? How does the lifetime depend on\nthe energetic position of the specific mid-gap state probed, and how does this vary between\nn=2, n=3, … n=N materials?\nOur response. We thank the reviewer for raising this point. We performed TRPL measurements\non a BA n=3 emitter at 4 K to obtain an accurate PL lifetime for the mid-gap state emission.\nThe measured PL lifetime is  = 1.729 ± 0.032 ns (new Supplementary Fig. 6). For comparison,\nthe representative lifetime for BA n=2 is ~0.87 ns (Fig. 3).\nAt present, we only have TRPL datasets for BA n=2 and n=3. Thus, we cannot establish how\nthe lifetime varies among n = 2, 3, …, N materials. Additional material-dependent studies may\nbe addressed in future work.\nTo address the reviewer’s comment, we added new Supplementary Fig. 6 and the following\nsentence to the revised manuscript (page 12): “In addition, the PL lifetime of BA n=3 was\nmeasured to be longer ( = 1.729 ± 0.032 ns) than that of BA n=2 (Supplementary Fig. 6).”\n[New Supplementary Fig. 6]\nComment 5. What is the identity of the emission peak at approximately 694 nm, predominantly\nvisible under above-bandgap excitation (Fig. 4. a, b), and is it significant to the properties of\nthe mid-gap state measured?\nOur response. We thank the reviewer for this careful observation. In our excitation-energy-\ndependent interpretation, multiple recombination pathways exist at the edge (e.g., band-edge\nemission, edge-state broad emission, and shallow-trap emission). The emission peak at ~694\nnm can appear from one of these channels when carriers are generated under above-bandgap\nexcitation. On the other hand, the g(2)() and TRPL properties of the mid-gap state discussed in\nFig. 4 were assessed by spectrally isolating the targeted sharp peak using appropriate filtering.\nThus, the presence of the ~694 nm peak does not affect the measured properties of the mid-gap\nemitter.\nComment 6. Can the authors comment on the thermally activated depopulation of the mid-gap\nstate emission (Supplementary Fig. 6.)? In a recent publication, dark excitons have been\nreported to localize at the edges of two-dimensional perovskites (Bailey, C. G. et al. Advanced\nEnergy Materials 15, 2501593, 2025). In addition to the edge state, could such fine structure\nstates mediate the population and depopulation of the mid-gap states studied here? Additional\ntemperature-dependent photoluminescence data could address the origins of the thermal\nquenching and elucidate relaxation pathways relevant to the single-photon emission state.\nOur response. We thank the reviewer for raising this point regarding the temperature-limit\nmechanisms. To provide a more realistic assessment, we performed temperature-dependent\nTRPL measurements on an edge-state emitter in BA n=2 at 4, 10, 20, 40, and 60 K, and\nanalyzed the spectral characteristics, including the emission linewidth (FWHM) and the\nintegrated PL intensity (new Supplementary Fig. 11). From these measurements, we extracted\nthe temperature dependence of the PL lifetime.\nThe results reveal several key features. First, the linewidth (FWHM) increases from 0.8 nm at\nspectral fluctuations at elevated temperatures. Second, the integrated PL intensity decreases\nrapidly above 20 K, and the lifetime decreases from ~6 ns near 10 K to ~2 ns at 60 K, indicating\nthat nonradiative pathways become increasingly competitive as temperature rises. Third, the\nnon-monotonic lifetime trend, an increase from 4 K to 10 K followed by a decrease, suggests\na crossover in the dominant decay pathways. Overall, these results indicate that the practical\ntemperature limit is primarily set by phonon-driven loss of spectral selectivity together with\nactivated competition from nonradiative/escape channels, rather than a single mechanism alone.\nThe reviewer’s suggestion that dark-exciton fine-structure states may mediate the population\nand depopulation of the mid-gap states is intriguing and represents a possible pathway.\nHowever, since our observations can be consistently interpreted within the edge-state model,\nwe did not further investigate this mechanism in the present study.\nTo respond to the reviewer’s comment, we added new Supplementary Fig. 11 and the following\nsentences to the revised manuscript (page 17): “Furthermore, we performed temperature-\ndependent TRPL measurements on an edge-state emitter in BA n=2 at 4, 10, 20, 40, and 60 K,\nand analyzed the spectral characteristics, including the emission linewidth (FWHM) and the\nintegrated PL intensity (Supplementary Fig. 11). We also extracted the temperature dependence\nof the PL lifetime. A key feature of these measurements is that the practical temperature limit\nis primarily set by phonon-driven loss of spectral selectivity together with activated\ncompetition from nonradiative/escape channels, rather than a single mechanism alone.”\n[New Supplementary Fig. 11]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns.I would like to recommend its publication in Nature Communications now.", "reviewer_2": "All my concerns have been well addressed. I am supportive of the publication of this work.", "reviewer_3": "Considering the scope of the work, I believe the concerns regarding the inference of practical use, the SNR of the g(2) measurements, and acquisition of TRPL data have been adequately addressed. Furthermore, the clarifications regarding the isolated single peaks and spectral features are satisfactory. I have no further concerns."}, "author_response": "Response to Reviewer #1.\nComment. The authors have addressed all my concerns. I would like to recommend its\npublication in Nature Communications now.\nOur response. We thank Reviewer #1 for his/her positive evaluation of our work and the\nexplicit recommendation for publication.\nResponse to Reviewer #2.\nComment. All my concerns have been well addressed. I am supportive of the publication\nof this work.\nOur response. We thank Reviewer #2 for his/her positive evaluation of our work and the\nexplicit recommendation for publication.\nResponse to Reviewer #3.\nComment. Considering the scope of the work, I believe the concerns regarding the\ninference of practical use, the SNR of the g(2) measurements, and acquisition of TRPL\ndata have been adequately addressed. Furthermore, the clarifications regarding the isolated\nsingle peaks and spectral features are satisfactory. I have no further concerns.\nOur response. We thank Reviewer #3 for his/her positive evaluation of our work and the\nexplicit recommendation for publication."}]

Decision letter