Gel Growth and Characterization of Perovskite Single Crystals for Ionizing Radiation Detection

Abstract

Metal halide perovskites are promising candidates for ionizing radiation detection due to their tunable bandgap, high quantum yield, strong light absorption, and excellent charge transport properties. Among these, bismuth-based perovskites are gaining attention as non-toxic alternatives to lead-based perovskites. In this study, bismuth-based perovskite single crystals were successfully grown using a novel gel growth technique. The gel growth method has been widely applied to functional crystals such as nonlinear optical (NLO) materials, pharmaceutical crystals, nanosheets, and nanoparticles. Traditional methods like melt growth are commonly used for inorganic perovskites due to their chemical stability, while low-temperature solution growth is favored for hybrid perovskites due to the instability of organic compounds at higher temperatures. The gel growth technique offers a simpler alternative, allowing for controlled diffusion of reagents and the growth of high-quality single crystals at room temperature. The objective of this study was to demonstrate the gel growth of perovskite crystals and investigate the factors influencing the method. Gel growth offers several advantages, such as minimizing equilibrium defects, reducing non-equilibrium defects, and operating under ambient conditions. Silica gel, a well-established medium for high-quality crystal growth, was selected as the gel type. Sodium meta-silicate mixed with deionized water created a solution with a desired density of 1.06 g/cm3 , which was combined with an acidic solution for silica gel formation. Early trials with HBr as the acid yielded Cs3Bi2Br9 crystals, confirming that the gel and acid type did not contribute to crystal inclusions. HCl was then used in subsequent experiments due to its lower acidity and denser gel formation, which reduced nucleation sites and slowed diffusion. The perovskite crystals grown in this study included Cs3Bi2Br9 , Cs2AgBiBr6 , Cs3Bi2 I9 , MA3Bi2 I9 , FA3Bi2 I9 , and doped Cs3Bi2 (Brx I1−x )9 (0.3 < x < 0.6). A three port U-tube beaker arrangement with a 7-day gel aging period was used. Larger crystals were observed for hybrid perovskites (MA3Bi2 I9 and FA3Bi2 I9 ), with MA3Bi2 I9 crystals occasionally exhibiting hollowness due to sensitivity to environmental vibrations. Cs2AgBiBr6 crystals were successfully grown only when CsBr, AgBr, and BiBr3 were diffused separately, with variations in color indicating Bi-rich or Bi-poor regions due to diffusion dynamics. Notably, Cs3Bi2 I9 crystals exhibited more crystal faces than other perovskites, sparking interest in doping and additives to improve growth uniformity and reduce multi-faceted formations. Doping trials with mixed halide anions (Br and I) resulted in larger, more uniform Cs3Bi2 (Brx I1−x )9 crystals compared to the parent perovskites. The addition of choline bromide (CB), previously successful in enhanc- ing CsPbBr3 crystal morphology in solution growth, was tested in the gel method. While CB improved Cs3Bi2Br9 crystal growth and reduced nucleation, no significant effects were observed for hybrid perovskites, and Cs2AgBiBr6 crystals failed to form under these conditions. Material and optical characterizations confirmed the intrinsic structure and properties of the gel-grown crystals. X-ray diffraction (XRD) was used for structural confirmation, and ultraviolet-visible (UV-vis) spectroscopy estimated the bandgap values. Room-temperature photoluminescence (PL) spectra confirmed bandgaps and revealed potential defect or impurity peaks. Raman spectroscopy was employed to analyze the vibrational modes of the doped perovskites, with shifts in wavenumber confirming the incorporation of mixed halides into the crystal lattice. The bandgaps were around 2 eV for Cs3Bi2 I9 , MA3Bi2 I9 , FA3Bi2 I9 , and doped Cs3Bi2 (Brx I1−x )9 , approximately 2.2 eV for Cs2AgBiBr6 , and 2.54 eV for Cs3Bi2Br9 . Surface roughness and treatment were assessed using confocal laser scanning microscopy, revealing that gel-grown FA3Bi2 I9 crystals had significantly smoother surfaces than solution-grown counterparts (0.416 µm vs. 5.233 µm). Isopropanol was identified as the optimal solvent for surface cleaning, with no observed surface damage. Device fabrication for I-V measurements involved applying fast-drying silver paste to the crystals. High resistivity (109 to 1013 Ω · cm) was measured, crucial for minimizing leakage current and reducing electronic noise in radiation detectors. X-ray response tests, conducted using a glass reference sample to account for air ionization effects, indicated promising performance for all devices except Ag/Cs3Bi2 I9/Ag, which displayed indistinguishable responses from the glass reference. Hecht equation fitting estimated the mobility-lifetime (µτ) product, yielding values between 10−5 and 10−3 cm2 /V, consistent with previous studies. Linearity in X-ray response, important for radiation dosimetry, was observed in all devices except Ag/Cs2AgBiBr6/Ag, likely due to ion migration. Sensitivity ranged from 0.1 to 1.2 µCGy−1 a i r cm−2 , and detection limits between 6 and 45 µGy/s, slightly higher than the recommended threshold for medical diagnostic exposure (5.5 µGy/s). Despite the non-vacuum conditions, the results demonstrated reliability, and long-term stability was confirmed over 90 days. However, doped crystals exhibited increased dark current and reduced resistivity, likely due to environmental degradation. The gel growth method shows great potential for producing high-quality perovskite single crystals for radiation detection and optoelectronic applications.