Scintillation Properties of Perovskite Single Crystals | The Journal of

Article Cite This: J. Phys. Chem. C 2019, 123, 17449−17453

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Scintillation Properties of Perovskite Single Crystals Yang Li,† Wenyi Shao,† Xiaoping Ouyang,*,†,‡,§ Zhichao Zhu,∥ Hang Zhang,† Xiao Ouyang,‡ Bo Liu,∥ and Qiang Xu*,† †

Department of Nuclear Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China § Northwest Institute of Nuclear Technology, Xi’an 710024, China ∥ School of Physics Science and Engineering, Tongji University, Shanghai 200092, China Downloaded via KEAN UNIV on July 18, 2019 at 07:39:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Low-cost solution-processed scintillation radiation detection materials are pursued urgently in ionization applications. Scintillators are key conversion materials that absorb high-energy X(γ) photons and convert them into UV−visible photons. Bulk mix organic−inorganic hybrid perovskites (CH3NH3PbCl3−XBrX) have been employed to detect X(γ) photons using a photon−photon conversion method. Optical properties were carefully investigated by photoluminescence (PL) spectra, PL lifetime, and X-ray excited luminescence (XEL) spectra at room temperature, which can be tuned with dopant Br concentration. Two XEL peaks centered at around 456 and 479 nm have been observed for the CH3NH3PbCl2.85Br0.15 single crystal scintillator. These two peaks originated from the near band-to-band emission and selfabsorption-induced re-emission. Thick crystals are able to enhance the self-absorption induced re-emission, while suppressing the near band-to-band emission.

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INTRODUCTION Organic−inorganic methylammonium lead halide perovskite materials CH3NH3PbX3 (MAPbX3, where MA is CH3NH3, X = Br, I, or Cl) have been demonstrated as one of the most promising materials for ionization radiation applications.1−7 This photocurrent detection method is mainly attributed to heavy atoms (Pb, Br, and I) and excellent electrical properties. The existence of heavy atoms can strongly absorb ionizing photons below 1 MeV;8 and electrical properties with large electron−hole diffusion lengths (>175 μm)9 and low trap density (108 cm−3)10 can enhance the transportation of electron−holes generated by ionization in crystals. In addition, other unique properties such as high absorption coefficient, tunable direct optical band gap (1.6−3.0 eV),11−13 and lowcost solution-growth method are also important advantages for optoelectronic applications. Recently, several research studies have focused on how to increase the sensitivity of X-ray detectors based on MAPbBr3 SCs for X-ray imaging.14,15 An X-ray detector based on methylammonium lead tribromide perovskite single crystals has been demonstrated with a sensitivity of 80 μC Gy−1 cm−2.16 The X-ray detector with surface Schottky barrier structure is able to greatly suppress current leakage and enhance charge collection; therefore, the sensitivity has been up to 359 μC Gy−1 cm−2 for 50 keV X-ray irradiation.17 However, its X-ray detection performance is still far away from that of the all-inorganic perovskite nanocrystal scintillator that is working under the photo-to-photon model.18 However, for applications, they are limited by the relatively low X-ray photon attenuation performance.19 Bulk crystals are enabled to stop high-energy incident photons and produce a large number of UV−visible photons. Therefore, we © 2019 American Chemical Society

investigated the scintillation performance of organic−inorganic methylammonium lead halide perovskite materials. In this study, we have grown high-quality Br-doped CH3NH3PbCl3 single crystals, by using an inverse solutiongrowth method at low temperature. We have investigated the luminescence properties of the perovskite single crystals and demonstrated the tunable emission spectra with various Br doping concentrations. The X-ray excited scintillation spectra were recorded for perovskite crystals at different thickness. The results indicated a strong self-absorption and re-emission photon phenomenon.

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METHODS Synthesis of Single Crystals. Various Br-doped CH3NH3PbCl3 single crystals are synthesized by an inverse solution-growth method in two steps. First, high purity CH3NH3X (X = Br, Cl) powder is synthesized. The chemicals methyl amine (CH3NH2, 33 wt % in ethanol)/HX (HBr, 48 wt % in water, HCl, 38 wt % in water) with a molar ratio of 1.2:1 are mixed at 0 °C by using an ice bath. After reacting for about 2 h, the mixed solution is evaporated in a rotary evaporator at 60 °C to obtain raw CH3NH3X powder. The raw CH3NH3X powder was dissolved in diethyl ether and ethanol for purification and recrystallization at least three times. The purified powder is evaporated in a rotary evaporator to obtain dry CH3NH3X powder, and then, CH3NH3PbX3 (X = Br, Cl) single crystals are synthesized. CH3NH3PbCl3 single crystals were harvested from the dimethyl sulfoxide (DMSO, ≥99.0%) Received: June 3, 2019 Revised: June 15, 2019 Published: June 24, 2019 17449

DOI: 10.1021/acs.jpcc.9b05269 J. Phys. Chem. C 2019, 123, 17449−17453

Article

The Journal of Physical Chemistry C

structural distortion without changing the cubic structure of perovskite.23 Figure 2a,b shows the energy-dispersive spectral (EDS) mapping of Br-doped CH3NH3PbCl3 single crystals. It is found

solution (molar ratio of 1.2:1). To synthesize CH3NH3PbBrxCl3−x single crystals, various molar ratios of CH3NH3Cl, PbCl2, CH3NH3Br, and PbBr2 powder were dissolved into DMSO and dimethylformamide mixed solution and stirred continuously for 12 h at 60 °C. The precursor solution was filtered and kept at 80 °C for several days. Then, high-quality crystals were harvested from the solution. Characterization. Powder X-ray diffraction (XRD) patterns were measured using a diffractometer (Rigaku Ultima IV diffractometer) equipped with a Cu Kα X-ray tube (λ = 0.15406 nm). Homemade X-ray fluorescence (XRF) measurement consisted of an X-ray generator, a silicon drift detector (AMPTEK Inc. Model X-123SDD, America), and a multichannel analyzer (AMPTEK Inc. Model DP5). X-rays are generated from an Ag anode target that is coupled with a 7.8 μm beryllium window and collimated by a 2 mm diameter copper collimator. Scanning electron microscopy and energydispersive spectrometer (EDS) mapping were carried out with an Oxford (X-ACT) scanning electron microscope. Steadystate photoluminescence (PL) spectra and X-ray excitation spectra were excited by a 266 nm laser (CNI. Model MPL-F266 nm, China) and 80 keV X-ray irradiation (from Ag target), respectively. These two emission photon spectra were collected by a spectrometer (Shanghai Fu Heng Optics Model PG-2000Pro). The photoluminescence lifetime curve was measured using a transient state spectrophotometer (Edinburgh FLS 980, England).

Figure 2. (a, b) X-ray energy-dispersive spectral (EDS) mapping of CH3NH3PbCl3−XBrX crystals; the green and red points show the distribution of Br and Cl elements, respectively. (c) XRF pattern of thin CH3NH3PbCl3−XBrX crystals in air with different Br compositions.

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RESULTS AND DISCUSSION A series of Br-doped CH3NH3PbCl3 single crystals have been grown by a solution-processed method, as shown in Figure 1a−d. While the doped Br concentration is increasing, the

that the Br and Cl elements are uniformly distributed in the crystals. To accurately measure the dopant concentration of Br, the X-ray fluorescence (XRF) measurement has been employed to analyze these crystals.24 The XRF pattern is shown in Figure 2c, where the signals ascribed to Cl (Kα1, Kβ1) and Pb (Lα1, Kβ1) have been observed clearly. Two peaks located at around 11.9 and 13.26 keV are assigned to Br Kα1 and Kβ1, respectively. Based on the relative XRF intensity of Pb, Cl, and Br, the accurate Br dopant concentrations of these crystals are CH3NH3PbCl2.92Br0.08, CH3NH3PbCl2.85Br0.15, and CH3NH3PbCl1.7Br1.3, respectively. In addition, these measured results are consistent with the theoretically calculated results from the XRD pattern.25,26 Figure 3 shows the photoluminescence (PL) spectra of CH3NH3PbCl3−XBrX single crystals that were excited by a 266 nm laser and acquired with a spectrometer in air at room

Figure 1. (a−d) Top view photograph of CH3NH3PbCl3−XBrX crystals with various Br concentrations. (e) Powder XRD pattern of CH3NH3PbCl3−XBrX crystals with various Br concentrations.

color of CH3NH3PbCl3 single crystals turns from transparent to light yellow or golden. The structures of these crystals were carefully checked with powder XRD measurement, as shown in Figure 1e. Typical XRD peaks ascribed to Br-doped CH3NH3PbCl3 SCs have been observed, indicating that these materials are cubic phase.20 It is worth noticing that the ratio of relative intensity between diffraction planes (200) and (210) is decreasing with the doped Br concentration, demonstrating that Br− substitutes for Cl− in the atomic site of the crystal structure.21,22 In addition, all diffraction peaks slightly shift toward a smaller angle with the increase of doped Br concentration. This is because the ionic radius of Br is larger than that of Cl, which induces the dilatational strain in all of the crystals. It can be inferred that doping leads to a slightly

Figure 3. PL spectra of Br-doped CH3NH3PbCl3 single crystals. The PL excitation wavelength is 266 nm. 17450

DOI: 10.1021/acs.jpcc.9b05269 J. Phys. Chem. C 2019, 123, 17449−17453

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The Journal of Physical Chemistry C temperature. Single emission peaks of CH 3NH3PbCl 3, CH 3 NH 3 PbCl 2 . 9 2 Br 0 . 0 8 , CH 3 NH 3 PbCl 2 . 8 5 Br 0 . 1 5 , and CH3NH3PbCl1.7Br1.3 located at around 401, 405, 418, and 490 nm have been observed. These peak positions exhibited an obvious red shift with an increase of doped Br concentration. The phenomenon that tunable PL spectra are shifting from near-violet to near-red by simply changing the Br dopant concentration is consistent with previously reported results.27−29 The symmetrical and small full width of halfmaximum (FWHM) of these CH3NH3PbCl3−XBrX excitation peaks indicates that these crystals are of high-quality and with fewer defects.30 It is worth noticing that the FWHMs of these samples are increasing with the increase of doped Br concentration from 12 nm of CH3NH3PbCl3 to 23 nm of CH3NH3PbCl1.7Br1.3. The PL lifetime is a required characteristic of scintillation performance. Figure 4 shows the PL lifetime of two samples

Figure 5. XEL emission spectra of CH3NH3PbCl3−XBrX crystals excited by 80 keV X-ray irradiation.

thin crystals are in the range of 20−32 nm, and these values are much smaller than that of the commercial scintillator GSO/Ge of 60 nm.34 The small values of FWHM are mainly ascribed to the highly crystalline quality of our samples. After comparison with the PL and X-ray excited luminescence (XEL) spectra, all of the XEL emission peaks are slightly red-shifted and the values of FWHM increase a little. This is because PL spectra are coming from the surface area of the materials, whereas due to the high-penetration performance of X-rays, the XEL spectra are generated from the internal area of the crystals. The internal light emissions are easily absorbed by internal traps, defects, or grain boundaries, which induces a strong selfabsorption effect and decreases the scintillation performance.35 To evaluate the thickness-dependent scintillation performance, the XEL spectra of CH3NH3PbCl2.85Br0.15 with various thicknesses have been studied. From Figure 6a, broad and unsymmetrical peaks have been observed. The emission peaks became sharp and the peak position shifted from 458 to 452 nm, whereas the crystal thickness decreased from 2.8 to 1.0 mm. These broad peaks have been divided into two emission peaks centered at around 456 and 479 nm, respectively, while fitting by two Gaussian equations. Short-wavelength excitation peaks are assigned to near-band recombination emission, whereas the long-wavelength excitation peaks are related to self-absorption induced re-emission. It is worth noticing that this similar photon recycling phenomenon has been demonstrated in perovskite materials such as MAPbI 3 or MAPbBr3.36,37 In addition, we have integrated the intensity of the individual section of near-band recombination emission (PLN) and self-absorption-induced re-emission (PLR). As shown in Figure 6b, the results reveal that internal near band gap recombination emission dominates a large section of light emission for thin layer materials, and the ratio is decreasing with the increase of crystal thickness. These phenomena agreed with the previously reported most successful scintillators.38−41

Figure 4. PL lifetime of Br-doped CH3NH3PbCl3 single crystals. The PL excitation wavelength is 375 nm.

under a 375 nm pulsed laser with 5 mW cm−2 excitation power. The PL lifetime curve can be described by the biexponential function31 ij −t yz ij −t yz I(t ) = A1 expjjj zzz + A 2 expjjj zzz jτ z jτ z (1) k 1{ k 2{ where I is the normalized PL intensity at time t, A1 and A2 are the constants, and τ1 and τ2 are the fast lifetime and slow lifetime, respectively. The average photoluminescence lifetime can be fitted with the expression32

τav = ((A1 × τ12) + (A 2 × τ22))/((A1 × τ1) + (A 2 × τ2)) (2)

CH3NH3PbCl3 has a slightly longer average lifetime of 5.85 ns (fast lifetime: 1.16 ns, slow lifetime: 7.54 ns), whereas CH3NH3PbCl2.92Br0.08 has an average lifetime of 3.53 ns (fast lifetime: 1.22 ns, slow lifetime: 5.02 ns). The biexponential lifetime behavior stems from two decay pathways, where the fast lifetime is attributed to the nonradiative recombination of defects and the slow lifetime is assigned to the radiative recombination of charge carriers.33 The scintillation spectra of the crystals were excited by 80 keV (Ag target) at room temperature. As shown in Figure 5, the emission peaks of CH3NH3PbCl3, CH3NH3PbCl2.92Br0.08, CH3NH3PbCl2.85Br0.15, and CH3NH3PbCl1.7Br1.3 are located at 438, 448, 456, and 536 nm, respectively. The FWHM of these

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CONCLUSIONS We have synthesized high-quality direct-band-gap Br-doped CH 3 NH 3 PbCl 3 single crystals with various doped Br concentrations by using a solution-processed method. The scintillation spectra can be easily tuned in the visible range by changing the doped Br concentration due to the tuning of the band gap. The photoluminescence lifetime curve revealed that the Br-doped CH3NH3PbCl3 single crystal has a short lifetime. In addition, a strong self-absorption and re-emission phenomena have been observed, which is mainly associated 17451

DOI: 10.1021/acs.jpcc.9b05269 J. Phys. Chem. C 2019, 123, 17449−17453

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Figure 6. (a) XEL emission spectra of CH3NH3PbCl2.85Br0.15 with different thicknesses. (b) The ratio of PLN/PLT and PLR/PLT with different thicknesses, where PLX (X = N, R, T) are the integrated intensity of assigned emission of near-band recombination emission, self-absorption induced re-emission, and total PL spectra, respectively. (7) Wei, H.; Huang, J. Halide Lead Perovskites for Ionizing Radiation Detection. Nat. Commun. 2019, 10, No. 1066. (8) Kasap, S. Photodetectors: Low-cost X-ray Detectors. Nat. Photonics 2015, 9, 420−421. (9) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron−Hole Diffusion Lengths >175 μm in SolutionGrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (10) Lian, Z.; Yan, Q.; Gao, T.; Ding, J.; Lv, Q.; Ning, C.; Li, Q.; Sun, J. Perovskite CH3NH3PbI3(Cl) Single Crystals: Rapid Solution Growth, Unparalleled Crystalline Quality, and Low Trap Density toward 108 cm−3. J. Am. Chem. Soc. 2016, 138, 9409−9412. (11) Prajongtat, P.; Thomas, D. Precipitation of CH3NH3PbCl3 in CH3NH3PbI3 and its Impact on Modulated Charge Separation. J. Phys. Chem. C 2015, 119, 9926−9933. (12) Zhang, Z.; Ren, L.; Yan, H.; Guo, S.; Wang, S.; Wang, M.; Jin, K. Bandgap Narrowing in Bi-Doped CH3NH3PbCl3 Perovskite Single Crystals and Thin Films. J. Phys. Chem. C 2017, 121, 17436−17441. (13) Butler, K. T.; Frost, J. M.; Walsh, A. Band Alignment of The Hybrid Halide Perovskites CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3. Mater. Horiz. 2015, 2, 228−231. (14) Büchel, P.; Moses, R.; Sandro, F. T.; Gebhard, J. M.; Genesis, N. A.; Rene, F.; Markus, B.; Wilhelm, M.; Samuele, L.; Oier, B.; et al. X-Ray Imaging with Scintillator-Sensitized Hybrid Organic Photodetectors. Nat. Photonics 2015, 9, 843−848. (15) Zhang, Y.; Sun, R.; Ou, X.; Fu, K.; Chen, Q.; Ding, Y.; Xu, L.; Liu, L.; Han, Y.; Malko, A. V.; et al. Metal Halide Perovskite Nanosheet for X-ray High-Resolution Scintillation Imaging Screens. ACS Nano 2019, 13, 2520−2525. (16) Wei, H.; Fang, Y.; Mulligan, P.; Chuirazzi, W.; Fang, H. H.; Wang, C.; Ecker, B. R.; Gao, Y.; Loi, M. A.; Cao, L.; et al. Sensitive Xray Detectors Made of Methylammonium Lead Tribromide Perovskite Single Crystals. Nat. Photonics 2016, 10, 333−339. (17) Xu, Q.; Shao, W.; Li, Y.; Zhang, X.; Ouyang, X.; Liu, J.; Liu, B.; Wu, Z.; Ouyang, X.; Tang, X.; et al. High-Performance Surface Barrier X-ray Detector Based on Methylammonium Lead Tribromide Single Crystals. ACS Appl. Mater. Interfaces 2019, 11, 9679−9684. (18) Chen, Q.; Wu, J.; Ou, X.; Huang, B.; Almutlaq, J.; Zhumekenov, A. A.; Guan, X.; Han, S.; Liang, L.; Yi, Z.; et al. AllInorganic Perovskite Nanocrystal Scintillators. Nature 2018, 561, 88− 93. (19) Almutlaq, J.; Yin, J.; Mohammed, O. F.; Bakr, O. M. The Benefit and Challenges of Zero-Dimensional Perovskites. J. Phys. Chem. Lett. 2018, 9, 4131−4138. (20) Liu, Y.; Yang, Z.; Cui, D.; Ren, X.; Sun, J.; Liu, X.; Zhang, J.; Wei, Q.; Fan, H.; Yu, F.; et al. Two-Inch-Sized Perovskite CH3NH3PbX3 (X = Cl, Br, I) Crystals: Growth and Chatacterization. Adv. Mater. 2015, 27, 5176−5183. (21) Kumawat, N. K.; Dey, A.; Kumar, A.; Gopinathan, S. P.; Narasimhan, K. L.; Kabra, D. Band gap Tuning of CH3NH3Pb(Br1‑xClx)3 Hybrid Perovskite for Blue Electroluminescence. ACS Appl. Mater. Interfaces 2015, 7, 13119−13124.

with the internal traps. The scintillation spectra consist of nearband recombination emission photons and self-absorptioninduced re-emission photons, which is correlated with the thickness of the crystals. While the thickness of the crystals is increasing, the self-absorption-induced re-emission photons contribute a high percentage of the emission photons. These results indicate that Br-doped CH3NH3PbCl3 single crystals may be provided as a choice for scintillation materials in ionization detection applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.O.). *E-mail: [email protected] (Q.X). ORCID

Qiang Xu: 0000-0002-4720-7477 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (Grant nos. 11705090, 11875166, and 11435010).

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published July 3, 2019. Figure 6 was incorrect and has been updated. The revised version re-posted on July 5, 2019.

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DOI: 10.1021/acs.jpcc.9b05269 J. Phys. Chem. C 2019, 123, 17449−17453