A2Bn–1PbnI3n+1 (A = BA, PEA; B = MA; n = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators

Quantum-well (QW) hybrid organic–inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications.


■ INTRODUCTION
Hybrid organic−inorganic perovskite (HOIP) crystals have attracted significant attention due to their remarkable properties, including long carrier diffusion length, low defect density, possibly high absorption coefficient, medium exciton binding energy, and tunable bandgap. 1−7 Such properties make them promising materials for next-generation optoelectronic applications. 8−13 In addition, the presence of the heavy lead (Pb) element is favorable to the absorption of high-energy X-rays and γ-rays. Despite their widespread applications and belowoven-furnace temperature processability, commercialization of these devices is hindered by their poor environmental stability. Quantum-well (QW) HOIP crystals, A 2 PbX 4 , 14 have shown remarkable environmental and thermal stability compared to their three-dimensional (3D) counterparts, BPbX 3 (B = methylammonium (MA)), while preserving significant optical and scintillation properties toward targeted applications. 14 −18 These materials consist of inorganic perovskite slabs intercalated with bulky organic cations that act as spacers between these layers, adopting the crystal structure of the Ruddlesden−Popper (RP) type. In particular, QW HOIP crystals, as direct bandgap materials, have demonstrated high potentialities as scintillating materials for fast timing applications in security, medical diagnosis, industrial sectors, high-energy physics, and materials sciences. 19−22 They exhibit high light yields >10 photons/keV and short scintillation decay times <15 ns, leading to good coincidence timing resolutions (CTR) < 150 ps. 21,23,10,8 However, the absorption lengths of typical QW HOIP are >2 times longer than that of the commercial CsI:Tl scintillators, 24 making the crystals less attractive for high-energy excitation applications, such as timeof-flight positron emission tomography (TOF-PET), which is operating at 511 keV. 20 In addition, so far QW HOIP scintillators have been reached with bromide of (Br) variants with 3 eV bandgaps (e.g., phenethylammonium lead bromide (PEA) 2 PbBr 4 and n-butylammonium lead bromide (BA) 2 PbBr 4 ), still possess scintillation light yields about half from 66 photons/keV of commercial CsI:Tl scintillator. 20 To improve the yields, one can look to the lower bandgap materials such as 3D HOIP or iodide (I) crystals with 2 eV bandgaps. They may provide higher light yields although so far the light yields were recorded at low temperature. 19 To achieve better absorption lengths and light yields, one should get higher density materials and smaller bandgaps, respectively. 20 For the faster decay time, one has to search iodide (I − ) crystals of QW HOIP, as most crystals exhibit decay times of <2 ns but lower light yield. 16 This requires a modulation of the crystals properties between those of QW and 3D HOIP structures, which can be synthetically achieved exploiting the versatility of the RP compositions A 2 B n−1 Pb n I 3n+1 (where n is an integer). Such materials allow the full control of the optoelectronic properties either by compositional engineering or by structural modulation exploiting different levels of quantum and dielectric confinement in materials with different dimensionalities (n). Among the numerous reported perovskite-based optoelectronic devices, including solar cells, 7,25−27 field-effect transistors (FETs), 28 light-emitting diodes (LEDs), 29,30 and photodetectors, 31 many are based on powders and thin films. Particularly, PEA 2 MA n−1 Pb n I 3n+1 compounds, which exhibit multiple quantum-well structures, are extensively investigated for application light-emitting diodes (LEDs) due to their excellent photoluminescence (PL) properties. However, there have been hitherto no reports available on the A 2 B n−1 Pb n I 3n+1 materials utilized as scintillators.
In this article, we synthesize four different RP iodide-based QW HOIP crystals with n = 1, 2 as n is the number of 3D structures sandwiched between QW layers. We investigate the crystal structure, optical, and scintillation properties of (PEA) 2 PbI 4 , (BA) 2 PbI 4 , and the corresponding n = 2 RP phases (PEA) 2 MAPb 2 I 7 and (BA) 2 MAPb 2 I 7 . (PEA) 2 PbI 4 was previously discussed by our group in a short report by comparing different methods in crystal fabrications for light yield optimization. 32 Here we present the global trend in optical and scintillation properties with other unreported properties from other three iodide structures. We show that (BA) 2 PbI 4 and (PEA) 2 PbI 4 crystals exhibit green and red emission with the fastest PL decay time. We further find that n = 2 layered perovskite iodide scintillators exhibit a mass density of >3.0 g/cm 3 and tunable rather small bandgaps <2.1 eV resulting from quantum and dielectric confinement due to the dimensional reduction of the perovskite spacer layers compared to 3D structures. Among these, we observe that only (PEA) 2 MAPb 2 I 7 shows emission under X-and γ-ray excitations. From all these iodide crystals, we find light yields at room temperature (RT) (1−2 photons/keV) considerably lower than those of (BA) 2 PbBr 4 and (PEA) 2 PbBr 4 (10−40 photons/keV), while at 10 K, the light yields are comparable (∼10 photons/keV). 16 Thus, applications at low temperature are envisaged when a shorter radiation absorption length (∼3 cm at 511 keV) and a faster decay time component (∼0.5 ns) are foreseen. Such results may provide a new pathway for further improvements of these materials toward fast-timing applications.
Synthesis of QW HOIP Crystals. The QW A 2 PbI 4 crystals were synthesized using a method previously reported by Kowal et al. 32 The QW A 2 PbBr 4 crystals were synthesized using a modified version of the previously reported method. 14 where it was kept for 24 h, and during this time the temperature decreased from 100 to 20°C, allowing the growth of red crystals for PEA-based and dark red crystals for BAbased A 2 MAPb 2 I 7 , respectively. These were collected by filtration and dried at 100°C under vacuum. The obtained perovskite crystals were stored in the glovebox under an inert atmosphere.
X-ray Diffraction. A Bruker D8 Advance AXS diffractometer was used for measuring powder X-ray diffraction (XRD) spectra of the synthesized compounds. 32 The device used Cu Kα radiation with 1.5418 Å wavelength. Measurements were conducted at RT, under Bragg−Brentano geometry, 5 s/step scanning velocity, and 0.02°step size. FullProf Suite software was then used to analyze the acquired data.
PL, TRPL, and Absorption. For PL measurements the samples were excited with the use of picosecond laser diode with repetition rate 30 MHz, 375 and 532 nm peak wavelengths (Master Oscillator Fiber Amplifier, PicoQuant GmbH, Berlin, Germany), pulse duration 50 ps, and 10 mW average power. A microscopic objective with numerical aperture (NA) 0.4 and magnification 20× (Nikon Corporation, Tokyo, Japan) was used for excitation focusing and signal collection. The filtered PL signal was acquired by a highsensitivity visible light spectrometer (Ocean Optics, Orlando, FL). For TRPL measurements, the repetition rate was reduced to 10 MHz, and the PL signal, selected by bandpass filter 532 ± 25 nm, was coupled to a single-photon avalanche photodiode (APD). The timing response was analyzed by time-correlated single-photon counting electronics (Hydra-Harp 400, PicoQuant, Germany). A tungsten halogen light source (Ocean Optics LS-1) and same visible light spectrometer as for the PL experiments were used to measure the absorption of the samples in the transmission mode. All measurements were conducted at RT. The Journal of Physical Chemistry C pubs.acs.org/JPCC Article RL, TL, and Afterglow Curves. The X-ray excitation was provided by an Inel XRG3500 X-ray generator Cu-anode tube (45 kV/10 mA). For recording the optical signal, we used an Acton Research Corporation SpectraPro-500i monochromator, a Hamamatsu R928 photomultiplier tube (PMT), and an APD Cryogenic Inc. closed-cycle helium cooler. The crystals were exposed to X-ray radiation for 10 min, and the afterglow curve was recorded at temperature of 10 K. Then, TL glow curves were measured from 10 to 350 K by increasing the temperature, with 0.14 K/s heating rate. Finally, the RL signal was measured from 350 to 10 K by cooling the sample back. The measurement started from the highest temperature as to avoid thermal release of charge carriers which could possibly contribute to the emission yield.
Pulse Height and Scintillation Decay Measurements. For source of the γ-rays, a 137 Cs (662 keV, 210 kBq) radioisotope was used, and the converted photons were detected by a PMT (Hamamatsu R878) with 1.25 kV applied voltage. The output was integrated with a charge-sensitive preamplifier (Canberra 2005), and then it fed a spectroscopic amplifier (Canberra 2022) with a shaping time of 2 μs and a TUKAN-8K-USB multichannel analyzer. In the pulse height spectrum, the position of the photopeak was compared with the position of the mean value of the single electron response to obtain the photoelectron yield. The actual light yield for the radiation conversion in photons per MeV was obtained by taking into consideration the spectral matching of the sample luminescence to the PMT characteristics. Scintillation decay measurements were performed by the delayed coincidence single photon counting method. 16 A 137 Cs radioactive source, two Hamamatsu photomultiplier tubes (R1104 and R928 for "starts" and "stops", respectively), a Canberra 2145 time toamplitude converter, and a TUKAN-8K-USB multichannel analyzer were used.  6 } octahedral units. Moreover, PEA + and MA + organic cations were so disordered that the benzene ring and MA + cation could be hardly distinguished. Significant disorder exists in the interlayer cations of (BA) 2 MAPb 2 I 7 crystal, particularly for the CH 3 CH 2 − tail of butylammonium (the ligand head, NH 3 CH 2 CH 2 − is relatively stable), causing the atoms to move and destabilize the refinement. 37 Powder X-ray diffraction (XRD) patterns of ground perovskite crystals are shown in Figure 2a. The prominent low-angle diffraction peaks of A 2 PbX 4 are indicative of their (002) preferential orientation, while the preferential orientation of A 2 MAPb 2 I 7 (A = PEA, BA) occurs in the lattice planes of (2−14) and (111), respectively, as displayed in Figure 2a. The XRD patterns of the four crystals were analyzed with the Rietveld refinement method using the FullProf software, 38−41 and the results are shown in Table 1. Diffractograms of (PEA) 2 PbI 4 , (PEA) 2 MAPb 2 I 7 , (BA) 2 PbI 4 , and (BA) 2 MAPb 2 I 7 including photographs of the corresponding crystals are shown in Figure S1. The triclinic phase was found with P1 space group for (PEA) 2 PbI 4 and P1 for (PEA) 2 MAPb 2 I 7 . The (PEA) 2 MAPb 2 I 7 single crystals mostly represented a triclinic lattice structure at RT. 40,41 On the other hand, the orthorhombic phase can be found with primitive centrosymmetric Pbca space group for (BA) 2 PbI 4 42 and Cc2m space group for (BA) 2 MAPb 2 I 7 . 42,37,43 Due to the larger size of iodide than bromide, the (PEA) 2 PbI 4 crystal shows 209.60 Å 3 and the (BA) 2 PbI 4 crystal shows 219.14 Å 3 larger volume compared to their corresponding bromide crystal. The volume for (PEA) 2 MAPb 2 I 7 crystal is 378.95 Å 3 larger than the (BA) 2 MAPb 2 I 7 crystal, which is due to the larger size of PEA compared to the BA cation. Figure 2b presents the calculated absorption lengths 44 for photon energies up to 511 keV, and the insets correspond to the magnified X-ray spectral region at 50 keV and γ-ray spectral region at 511 keV. The mass density (ρ) of (PEA) 2 PbI 4 , (PEA) 2 MAPb 2 I 7 , (BA) 2 PbI 4 , and (BA) 2 MAPb 2 I 7 crystals are calculated as 2.59, 3.00, 2.73, and 3.14 g cm −3 , respectively. 32,45 Iodide (I − ) HOIP crystals have higher ρ than (PEA) 2 PbBr 4 (2.28 g cm −3 ) or (BA) 2 PbBr 4 (2.36 g cm −3 ) crystals, respectively. 46 The larger atomic size of iodide compared to bromide shows high mass density. In our works, ρ values of (PEA) 2 MAPb 2 I 7 and (BA) 2 MAPb 2 I 7 crystals are similar in comparison to the reported 3.00 g cm −3 40 and 3.16 g cm −3 , 37 respectively. As a result, all absorption lengths ( ) at 50 keV of those iodide crystals are at least 50% shorter than those of bromides ( of (BA) 2 PbBr 4 is 0.099 cm). 46 However, those values become at least 5% shorter at 511 keV. 46 The (BA) 2 MAPb 2 I 7 has the shortest absorption length of all the studied crystals as can be seen from Figure 2b. At 50 keV, which is widely used in X-ray imaging, of (PEA) 2 PbI 4 is 0.050 cm, 16% longer than of (BA) 2 PbI 4 of 0.042 cm, and for (PEA) 2 MAPb 2 I 7 is 0.039 cm, just 11% longer than of (BA) 2 MAPb 2 I 7 of 0.035 cm. The effect of the PEA cation on the density leads to a 20% longer for A 2 PbI 4 and 6% shorter for A 2 MAPb 2 I 7 compared to their BA cation crystals at 511 keV, and the results are summarized in Table 1. For TOF PET, longer values of (PEA) 2 PbI 4 among the four crystals are still 44% longer at 50 keV and 50% longer at 511 keV than that of a commercial scintillator, CsI:Tl. 20,24 The ab initio components for the scintillation efficiencies can be studied through their band structures. Therefore, we employ the density functional theory (DFT) method to calculate the density of states (DOS) and determine the optical bandgap (E g ). The band structures with their total (black) and   27 PL measurements were performed on bulk crystals of (PEA) 2 PbI 4 , (PEA) 2 MAPb 2 I 7 , (BA) 2 PbI 4 , and (BA) 2 MAPb 2 I 7 (Figure 3b). 49 In addition, absorption and PL spectra excited at 375 nm with a logarithmic scale of y-axis recorded at RT, decay curves, and photographs of the corresponding (PEA) 2 PbBr 4 and (BA) 2 PbBr 4 crystals are shown in Figure  S3. PL spectra recorded at RT in Figures 3b,c exhibit two different emission origins depending on the excitation wavelength. On one hand, exciting the samples with 375 nm wavelength produces an emission band at 532 nm (green) for (PEA) 2 PbI 4 and (BA) 2 PbI 4 and 577 nm (yellow) for (PEA) 2 MAPb 2 I 7 , and (BA) 2 MAPb 2 I 7 crystals which is much more intense than the band at 620 nm (red). On the other hand, when using the longer wavelength excitation of 532 nm, only one red emission band is observed at 620 nm (red) for (BA) 2 PbI 4 and a broad band at 660 nm (red) for (PEA) 2 PbI 4 . For (PEA) 2 MAPb 2 I 7 crystals, the red emission at 620 nm at its bandgap energy (2.0 eV) with an appreciable PL emission broad band at 748 nm can be originated from the edges of the exfoliated layers of perovskite crystal, as reported by Blancon et al. 50 The broadband of the PEA 2 PbI 4 crystal at 660 nm is possibly due to the radiative path of electron capture at a positive iodide vacancy with a subsequent hole capture. 51 There is no emission band observed for the (BA) 2 MAPb 2 I 7 crystal at 532 nm excitation wavelength. The green emission band has a full width at half-maximum (FWHM) equal to 19 nm for (PEA) 2 PbI 4 and (BA) 2 PbI 4 and 22 nm for (PEA) 2 MAPb 2 I 7 and (BA) 2 MAPb 2 I 7 while the red band is broad (110 nm) for (PEA) 2 PbI 4 and narrow (32 nm) for (BA) 2 PbI 4 and (PEA) 2 MAPb 2 I 7 . The PL characteristics are observed for (PEA) 2 PbI 4 and (PEA) 2 MAPb 2 I 7 crystals since they were synthesized using PbI 2 precursor and additionally treated with PEAI. 38 The origin of the green emission band is the characteristic excitonic emission from inorganic PbI 2 layers while the red emission is associated with the in-plane iodide vacancy causing surface states. 38 We probe the spectral origin of the emitting states observed using PL and time-resolved PL (TRPL) spectroscopy. The TRPL decay curves of perovskite crystals were fitted by exponential decay functions which are shown in Figures 4a−d. Most decay times in iodide QW HOIP crystals are faster than those in bromide QW HOIP crystals (see Figure S3), as they can affect the scintillation decay times. Such <1 ns fast decay components of iodide QW HOIP crystals were also observed in previous observations of QW HOIPs. 16,22,32 The microsecond decay components were also observed under twophoton excitation. 52 To do the analysis, we also present RT decay curves excited at 532 nm monitoring 620 nm emission in Figure S4. The decay components for the 532 nm emission band of the (PEA) 2 PbI 4 crystal are 0.1, 0.5, and 4.1 ns, and they are associated with free-exciton emission. The average lifetime value τ avg PL of 1.0 ns is similar to those reported in ref 38. In the same crystal, the decay components at 620 nm emission are 3.6 and 37.7 ns, while τ avg PL is 36.6 ns. This is 36.6 times slower than τ avg PL at 532 nm (see Figure S4). The fastest decay time of 0.3 ns was observed at 532 nm and 0.4 ns at 620 nm for the (BA) 2 PbI 4 crystal, which are about 4 and 99 times faster than τ avg PL at 532 and 620 nm of the (PEA) 2 PbI 4 crystal, respectively. On the other hand, the decay components for the The term absorption length is denoted as .
The Journal of Physical Chemistry C pubs.acs.org/JPCC Article (PEA) 2 MAPb 2 I 7 crystal of 0.6 and 1.4 ns correspond to the exciton emission as the decay curve was measured for the 577 nm emission band and τ avg PL of 0.9 ns, while the decay components at the 620 nm emission peak position are 0.4 and 7.2 ns, and τ avg PL is 4.5 ns, which is 5 times slower than τ avg PL at 577 nm emission. The fast decay components for the (BA) 2 MAPb 2 I 7 crystal of 0.3 and 0.8 ns correspond to the exciton emission as the decay curve was measured for the 577 nm emission band only, and τ avg PL of 0.5 ns is observed. Although for (PEA) 2 PbI 4 and (PEA) 2 MAPb 2 I 7 crystals the decay times at 577 nm emission are fast, the presence of the emission for respective emission bands at 660 and 748 nm in Figure 3c can make the scintillation decay curves slower as the TRPL monitoring 620 nm emission exhibits slower decay components >8 ns (see Figure S4). As expected from the absence of the long wavelength band in the BA 2 PbI 4 crystal in Figure 3c, the decay time at 620 nm also yields a similar value as that at 532 nm (see Figures 4c and S4c for comparison).
The radioluminescence (RL) spectra at RT in Figure 5a are dominated by the red broadband emission, which resembles the red emission band in PL spectra excited with 532 nm wavelength in Figure 3a for (PEA) 2 PbI 4 and (PEA) 2 MAPb 2 I 7 crystals.On one hand, the red surface defect emission (630− 665 nm) dominates against the green exciton emission (520− 545 nm) for (PEA) 2 PbI 4 and (PEA) 2 MAPb 2 I 7 crystals as seen in their RL spectra at RT. This is due to self-absorption as it was observed in other previous QW HOIP crystals. 16,53 On the other hand, the green surface defect emission (560 nm) strongly dominates the red emission (700 nm) for the (BA) 2 PbI 4 crystal, and there is no emission excited by X-rays for the (BA) 2 MAPb 2 I 7 crystal. However, the (PEA) 2 PbI 4 crystal shows higher self-absorption compared to the (BA) 2 PbI 4 crystal, and overall the self-absorption of the (PEA) 2 MAPb 2 I 7 crystal is much more stronger among all the crystals due to presence of MAPbI 3 impurities. 48 Afterglow decays recorded after 10 min of X-ray irradiation at the 10 K curve of (PEA) 2 2 PbI 4 , which is 5 times faster than (PEA) 2 PbI 4 . The afterglow for A 2 PbI 4 crystals is faster than that observed in A 2 BPb 2 I 7 crystals due to low trap density. The presence of traps is directly related to the chain length of the organic cation. 54 We examine the presence of trap states in the investigated scintillators by performing thermoluminescence (TL) measurements. TL is the phenomenon of afterglow with temperature of a previously exposed materials by highenergy radiation. Originally the thermally activated afterglow is due to the phonon-assisted release of trapped charge carriers with temperature, leading to radiative recombination. 16,33 TL spectra and the corresponding fits are shown in Figures 5c−f, and TL parameters are given in Table S2 for (PEA) 2 PbI 4 , (PEA) 2 MAPb 2 I 7 , (BA) 2 PbI 4 , and (BA) 2 MAPb 2 I 7 . All samples show several glow peaks as the temperature of the sample is raised, indicating the presence of traps in the crystals. Unfortunately, those traps are mostly deeper, and they have more trapped charge carriers than those in (PEA) 2 PbBr 4 and (BA) 2 PbBr 4 . 46 The glow peaks for the (PEA) 2 PbI 4 crystal are observed at temperatures 47, 101, and 144 K with trap densities of 1.2 × 10 4 , 1.3 × 10 4 , and 2.1 × 10 3 , respectively, as shown in Figure 5c. Significant and several glow peaks with less noisy result for (PEA) 2 MAPb 2 I 7 crystal are observed due to the high intensity and more traps as summarized in Table S2, including a deep trap over a long temperature range (up to 200 K), as shown in Figure 5d. On the other hand, negligible glow peaks are observed at temperatures 54 and 127 K with trap density of 2.9 × 10 3 and and 1.5 × 10 4 , respectively, for the (PEA) 2 PbI 4 crystal as shown in Figure 5e and at 39 and 78 K with trap densities of 1.1 × 10 4 and 5.5 × 10 3 , respectively, as shown in Figure 5f. A 2 MAPb 2 I 7 HOIP crystals and especially for the (PEA) 2 MAPb 2 I 7 crystal show more traps due to the presence of MAPbI 3 impurities 48 which has strong traps. 55 In Light yield is an important property of a scintillator, i.e., the efficiency of the scintillator to convert the energy of absorbed X-and γ-rays into visible photons. 20 RL measurements were used to determine the comparative values of light yields for perovskite scintillators. 19,56 The γ-ray pulse height method gives quantitative values for the light yield and also provides information on the energy resolution of the scintillator. We note that the pulse height method is integrated for light yields faster than 2 μs while the RL comparison is integrated over ∼1s longer time. In our current analysis, the light yield is the comparison of the photopeak signals in the pulse height spectra at certain energy of γ-ray radiation with the scintillator single electron response. 33 Figure 6a present the results for pulse height spectra recorded under γ-ray excitation. The pulse height spectrum in Figure 6a for (PEA) 2 PbI 4 , (BA) 2 PbI 4 , and (PEA) 2 MAPb 2 I 7 crystals exhibit structures of Compton scattering and photoelectric peak; however, the obtained energy resolution value at 662 keV from the 137 Cs source of γray excitation is above 32%, which is still far from beating the best energy resolution for a lithium-doped (PEA) 2 PbBr 4 of 7.7% at the same energy. 16 Figure 6b shows the temperaturedependent normalized light yield under 45 keV X-ray excitation.
Based on the integration of the RL intensities at each temperature, the temperature-dependent light yield was calculated. Under X-ray excitation, large amounts of charge carries are involved, leading to the large possibility of trapping. 16 As shown in Figure S5, the temperature-dependent RL 2D maps of some HOIP crystals illustrate their different patterns. Because of the negative thermal quenching behaviors,    (1) where S denotes the electron−hole transport efficiency to the exciton recombination and Q signifies the luminescence quantum efficiency of the exciton. There are also losses during the transport of the light in the detector due to internal scattering and reabsorption, so the actual light yield of the scintillator might be less than expected, depending on the geometry of the scintillator. 20 The light yields of all QW HOIPS are summarized in Table S3. (BA) 2 PbI 4 has the highest light yield of 2 photons/keV among all crystals while (PEA) 2 PbI 4 has 1 photon/keV at RT. The light yield of (BA) 2 PbI 4 has improved at both RT and 10 K compared to the reported 16 one as shown in Table S3 due to the different fabrication method. Because (BA) 2 PbI 4 has a slightly larger E g of 0.1 eV, we expect that the light yield is to be smaller, but it appears that (BA) 2 PbI 4 has a larger Q and/or S than (PEA) 2 PbI 4 . This can be seen in the trend between (BA) 2 PbBr 4 and (PEA) 2 PbBr 4 . 53,33 For (PEA) 2 MAPb 2 I 7 , the light yield is slightly improved than that of (PEA) 2 PbI 4 because the bandgap is smaller (2.10 eV). The smallest bandgap for (BA) 2 MAPb 2 I 7 is 1.80 eV and it shows a very small light yield at RT (see also Figure S5) as there is a strong quenching due to the bandgap being too small. 57 The maximum light yield of (PEA) 2 PbBr 4 crystal at RT is 10 photons/keV, 58,59 larger than the iodide-based QW HOIP crystal scintillators. On one hand, the light yield for the (PEA) 2 MAPb 2 I 7 crystal is 1.4 photons/ keV at RT, which is still low compared to the (BA) 2 PbI 4 HOIP scintillator of 2 photons/keV. On the other hand, the (PEA) 2 MAPb 2 I 7 scintillator has a peak position in the pulse height spectrum which is slightly higher than the position in that of the (PEA) 2 PbI 4 scintillator meaning higher light yield. Because the light yields at RT were observed in three crystals, the delay distribution and the coincidence timing resolution (CTR) measurement results for (PEA) 2 PbI 4 , (BA) 2 PbI 4 , and (PEA) 2 MAPb 2 I 7 crystals are shown in ref 32 and Figure S6, respectively. We obtain CTR values of 138 ± 5, 149 ± 10, and 207 ± 14 ps for (PEA) 2 PbI 4 , (BA) 2 PbI 4 , and (PEA) 2 MAPb 2 I 7 crystals, respectively, and all still have similar values. They still can be 2 times improved by increasing the light yield at low temperature, making them similar to or slightly better than those of bromide crystals. 32 The light yield stability measured from the pulse height spectra of the (PEA) 2 MAPb 2 I 7 crystal for 6 h and the derived values of the light yield were plotted with the normalized values at the initial time as shown in Figure S7, showing that the hygroscopicity is not as notorious as other iodides, e.g., LaI 3 :Ce 3+ . 57 We determine the decay at high energies by investigating the γ-ray excited scintillation decay curves recorded under γ-ray excitation at 662 keV presented in Figure 6c while we report the exponential fitting parameters for the decay curves in Table S4. From the decay curves in Figure  6c, we can immediately see that (BA) 2 PbI 4 scintillator leads to a faster decay compared to the (PEA) 2 PbI 4 scintillator, while the decay of (PEA) 2

■ CONCLUSIONS
In summary, we investigated the effects of PEA and BA cation on the crystal structure as well as optical and scintillation properties of both QW HOIP A 2 PbI 4 and A 2 MAPb 2 I 7 crystals based on the XRD analysis, absorption, PL, TRPL, temperature-dependent RL, and pulse height measurements. Based on the XRD results, we calculated the band structure and density of states using DFT analysis. From the PL measurement, we observed that A 2 PbI 4 crystals exhibit green and red emission with fastest PL decay time. Overall, we observed that (BA) 2 MAPb 2 I 7 scintillator exhibits the highest mass density, and smaller bandgap due to the increased thickness of the inorganic slabs. We conducted temperature-dependent RL measurements to explore the effects of temperature on the scintillation properties of the perovskites, including the effect on the afterglow. We observed comparable light yields for all iodide crystals at 10 K (∼10 photons/keV), while light yields at RT (between 1 and 2 photons/keV) are still much lower compared to QW HOIP bromide crystals. The biggest advantages of QW HOIP iodide scintillators compared to their bromide counterparts are shorter radiation (X-and γ-ray) absorption lengths (between 0.035 and 0.050 cm at 50 keV), faster decay time components (between 0.3 and 1.0 ns), and comparable light yields (between 7.5 and 10 photons/keV) at low temperature. Furthermore, our study of optical and scintillation properties of both A 2 PbI 4 and A 2 MAPb 2 I 7 crystals provides insight for further improvements on the radiation absorptions and emission rates toward high sensitivity and fast radiation detection applications.
■ ASSOCIATED CONTENT

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.3c00824. Rietveld refinement of crystal XRD diffractograms, absorption spectra with Elliot fits, TRPL recorded at 532 nm excitation monitoring 620 nm emission, RL spectra mapping at different temperatures, CTR results, and light yield stability of iodide-based QW HOIPS; PL and absorption spectra of (PEA) 2