Halide lead perovskites for ionizing radiation detection

Multi-Dopant Engineering in Perovskite Cs2SnCl6: White Light Emitter and Spatially Luminescent Heterostructure

Bi³⁺/Te⁴⁺ co-doped Cs₂SnCl₆ with dual emission spectrum (i.e., 450 and 575 nm) was achieved by a modified solution method, which can overcome the phase separation in the previous method for Cs₂SnCl₆ crystal growth. The two emission peaks arising from the two dopants Bi³⁺ and Te⁴⁺ have distinct photoluminescence (PL) lifetimes. Thus, the control of dopant ratio or PL delay time will regulate the PL intensity ratio between 450 and 575 nm peaks leading to adjustable emission color. The energy transfer between the two emission centers, which is confirmed by the optical spectra and PL lifetime, has a critical distance around 7.8 nm with a maximum of 50% transfer efficiency. The Bi³⁺/Te⁴⁺ co-doped Cs₂SnCl₆ with superior stability in water and aqua regia was fabricated into a single-phase white light-emitting diode. In the meantime, various luminescent heterostructures were obtained by epitaxial Cs₂SnCl₆ crystal growth with different dopants, which can broaden the study of composition engineering in halide perovskites.

Optical Probing of Crystal Lattice Configurations in Single CsPbBr3 Nanoplatelets

Quantum-confined nanostructures of CsPbBr₃ with luminescence quantum efficiencies approaching unity have shown tremendous potential for lighting and quantum light applications. In contrast to CsPbBr₃ quantum dots, where the fine structure of the emissive exciton state has been intensely discussed, the relationship among lattice orientation, shape anisotropy, and exciton fine structure in lead halide nanoplatelets has not yet been established. In this work, we investigate the fine structure of the bright triplet exciton of individual CsPbBr₃ nanoplatelets by polarization-resolved micro- and magnetophotoluminescence spectroscopy at liquid helium temperature and find a large zero-field splitting of up to 2.5 meV. A unique relation between the crystal structure and the photoluminescence emission confirms the existence of two distinct crystal configurations in such nanoplatelets with different alignments of the crystal axes with respect to the nanoplatelet facets. Polarization-resolved experiments eventually allow us to determine the absolute orientation of an individual nanoplatelet on the substrate purely by optical means.

Sequential Growth of 2D/3D Double-Layer Perovskite Films with Superior X-Ray Detection Performance

Perovskite materials in different dimensions show great potential in direct X-ray detection, but each with limitations stemming from its own intrinsic properties. Particularly, the sensitivity of two-dimensional (2D) perovskites is limited by poor carrier transport while ion migration in three-dimensional (3D) perovskites causes the baseline drifting problem. To circumvent these limitations, herein a double-layer perovskite film is developed with properly aligned energy level, where 2D (PEA)2 MA3 Pb4 I13 (PEA=2-phenylethylammonium, MA=methylammonium) is cascaded with vertically crystallized 3D MAPbI3 . In this new design paradigm, the 3D layer ensures fast carrier transport while the 2D layer mitigates ion migration, thus offering a high sensitivity and a greatly stabilized baseline. Besides, the 2D layer increases the film resistivity and enlarges the energy barrier for hole injection without compromising carrier extraction. Consequently, the double-layer perovskite detector delivers a high sensitivity (1.95 × 104 μC Gyair -1 cm-2 ) and a low detection limit (480 nGyair s-1 ). Also demonstrated is the X-ray imaging capacity using a circuit board as the object. This work opens up a new avenue for enhancing X-ray detection performance via cascade assembly of various perovskites with complementary properties.

Antisolvent-assisted Crystallization of Centimeter-sized Lead-free Bismuth Bromide Hybrid Perovskite Single Crystals with X-ray Sensitive Merits

Hybrid bismuth halides perovskites have emerged as promising candidates for X-ray detection, due to the strong absorptivity of high-energy X-ray photons, high resistivity, large carrier diffusion length and low toxicity. However, the mostly investigated hybrid bismuth iodides single crystals are usually opaque and require a harsh synthesis process. Herein, novel one-dimensional (1D) pentamethylenediamine bismuth bromide (PDA)BiBr₅ single crystals were synthesized via an antisolvent-assisted crystallization method at room temperature. Bulk (PDA)BiBr₅ single crystals have sizes of 10×1.3×1.5 mm³ and high transparency. They are shown to have low density of defects of 2.0×10¹⁰  cm^-3 and obvious photoconductivity. Moreover, they exhibit large bulk resistivity of 2.13×10¹¹  Ω cm and good X-ray attenuation coefficient. Consequently, the vertical structured (PDA)BiBr₅ single crystal X-ray photoconductor produces a sensitivity of 3.8 μC Gy_air ^-1  cm^-2 . This study provides a facile strategy for synthesizing bulk hybrid bismuth bromides single crystals with potential X-ray detection application.

Optoelectronic Properties of Low-Bandgap Halide Perovskites for Solar Cell Applications

Riding on the coat tails of rapid developments in single-junction halide perovskite solar cells, all-perovskite multijunction solar cells have recently garnered significant attention, with the highest power-conversion efficiency already reaching 25.6%. Much of this progress has been fueled by the rapid rise in the photovoltaic performance of low-bandgap halide perovskite absorbers, materials, which, to date, have only been achievable by the partial or complete substitution of lead with tin. However, much room still exists to develop a more critical understanding of key material properties in these low-bandgap perovskites. Herein, the key optoelectronic properties of absorption, carrier generation, recombination, and transport in these tin-containing perovskites are discussed, showing that intrinsic doping distinctively impacts many of these properties, thereby rendering this class of halide perovskites unique within the family. Current understanding of the mechanisms that degrade optoelectronic performance in these materials and the corresponding devices are also summarized. These collective results highlight an important interplay between doping, defects, and degradation that will need to be controlled. Finally, the current gaps in understanding of these low-bandgap perovskites are outlined, thereby providing guidelines for further research, which will unlock their full potential for realizing a plethora of high-performance optoelectronic devices.

Excitons in CsPbBr3 Halide Perovskite

Excitons in Bridgman grown halide perovskite CsPbBr₃ single crystals were examined using photoluminescence (PL) spectroscopy to determine the nature of the electronic states. The photoluminescence intensity was strongly temperature-dependent and depended upon the specific exciton band. At low temperatures intrinsic disorder and its related shallow below bandgap tail states determine the emission properties. Photoluminescence at low temperature revealed the presence of several strong bands at the band edge that is attributed to free or trapped/bound excitons. This PL emission results from strong electron-phonon coupling with an average phonon energy E_ph of 6.5 and 27.4 meV for the emissions, comparable to that observed in other perovskites. The Huang-Rhys parameter S was calculated to be 3.81 and 1.51, indicating strong electron-phonon coupling. The interactions between electrons and phonons produce small polarons that tend to bind charge carriers and result in trapped/bound excitons. The transient photoluminescence response of each specific band was studied, and the results indicated a multiphonon recombination process. Average PL lifetimes of ∼17 ns for free excitons and ∼38 ns for trapped/bound excitons were determined. The observed edge states could be associated with native defects such as vacancies and interstitials, as well as twinning due to the cubic-to-tetragonal phase transition in CsPbBr₃. Elimination of the trapping sites for binding excitons could lead to improved charge transport mobilities, carrier lifetimes, and detector properties in this system.

Halide perovskites scintillators: unique promise and current limitations

The widespread use of X- and gamma-rays in a range of sectors including healthcare, security and industrial screening is underpinned by the efficient detection of the ionising radiation. Such detector applications are dominated by indirect detectors in which a scintillating material is combined with a photodetector. Halide perovskites have recently emerged as an interesting class of semiconductors, showing enormous promise in optoelectronic applications including solar cells, light-emitting diodes and photodetectors. Here, we discuss how the same superior semiconducting properties that have catalysed their rapid development in these optoelectronic devices, including high photon attenuation and fast and efficient emission properties, also make them promising scintillator materials. By outlining the key mechanisms of their operation as scintillators, we show why reports of remarkable performance have already emerged, and describe how further learning from other optoelectronic devices will propel forward their applications as scintillators. Finally, we outline where these materials can make the greatest impact in detector applications by maximally exploiting their unique properties, leading to dramatic improvements in existing detection systems or introducing entirely new functionality.

Determination of X-ray detection limit and applications in perovskite X-ray detectors

X-ray detection limit and sensitivity are important figure of merits for perovskite X-ray detectors, but literatures lack a valid mathematic expression for determining the lower limit of detection for a perovskite X-ray detector. In this work, we present a thorough analysis and new method for X-ray detection limit determination based on a statistical model that correlates the dark current and the X-ray induced photocurrent with the detection limit. The detection limit can be calculated through the measurement of dark current and sensitivity with an easy-to-follow practice. Alternatively, the detection limit may also be obtained by the measurement of dark current and photocurrent when repeatedly lowering the X-ray dose rate. While the material quality is critical, we show that the device architecture and working mode also have a significant influence on the sensitivity and the detection limit. Our work establishes a fair comparison metrics for material and detector development.

Heterojunction structures for reduced noise in large-area and sensitive perovskite x-ray detectors

Polycrystalline perovskites can be readily fabricated into large areas using solution depositions; however, they suffer from large dark currents that are tens to hundreds times higher than industrially relevant values, limiting their application in low-dose x-ray detection. Here, we show that the application of a heterojunction structure into polycrystalline films significantly reduces the dark current density by more than 200 times to subnanoampere per square centimeter without reducing the sensitivity of the detectors. The heterojunction perovskite films are formed by laminating several membrane films filled with perovskites of different bandgaps. A gradient bandgap is formed during annealing. The detectors have a lowest detectable dose rate of 13.8 ± 0.29 nGy_air s⁻¹ for 40-keV x-ray and can conduct dynamic x-ray imaging at a low-dose rate of 32.2 nGy_air s⁻¹. Simulation and experimental analysis show that the heterojunction is tolerant of halide diffusion and can be stable for over 15 years.

Sb2Se3 film with grain size over 10 µm toward X-ray detection

Direct X-ray detectors are considered as competitive next-generation X-ray detectors because of their high spatial resolution, high sensitivity, and simple device configuration. However, their potential is largely limited by the imperfections of traditional materials, such as the low crystallization temperature of α-Se and the low atomic numbers of α-Si and α-Se. Here, we report the Sb₂Se₃ X-ray thin-film detector with a p-n junction structure, which exhibited a sensitivity of 106.3 µC/(Gy_air·cm²) and response time of < 2.5 ms. This decent performance and the various advantages of Sb₂Se₃, such as the average atomic number of 40.8 and μτ product (μ is the mobility, and τ is the carrier lifetime) of 1.29 × 10^-5 cm²/V, indicate its potential for application in X-ray detection.

Hybrid halide perovskite neutron detectors

Interest in fast and easy detection of high-energy radiation (x-, γ-rays and neutrons) is closely related to numerous practical applications ranging from biomedicine and industry to homeland security issues. In this regard, crystals of hybrid halide perovskite have proven to be excellent detectors of x- and γ-rays, offering exceptionally high sensitivities in parallel to the ease of design and handling. Here, we demonstrate that by assembling a methylammonium lead tri-bromide perovskite single crystal (CH₃NH₃PbBr₃ SC) with a Gadolinium (Gd) foil, one can very efficiently detect a flux of thermal neutrons. The neutrons absorbed by the Gd foil turn into γ-rays, which photo-generate charge carriers in the CH₃NH₃PbBr₃ SC. The induced photo-carriers contribute to the electric current, which can easily be measured, providing information on the radiation intensity of thermal neutrons. The dependence on the beam size, bias voltage and the converting distance is investigated. To ensure stable and efficient charge extraction, the perovskite SCs were equipped with carbon electrodes. Furthermore, other types of conversion layers were also tested, including borated polyethylene sheets as well as Gd grains and Gd₂O₃ pellets directly engulfed into the SCs. Monte Carlo N-Particle (MCNP) radiation transport code calculations quantitatively confirmed the detection mechanism herein proposed.

MoC/MAPbI3 hybrid composites for efficient photocatalytic hydrogen evolution

Metal halide perovskites, such as iodine methylamine lead (MAPbI₃), have received extensive attention in the field of photocatalytic decomposition of HI for hydrogen evolution, due to their excellent photoelectric properties. In this paper, a new MAPbI₃-based composite, MoC/MAPbI₃, was synthesized. The results show that 15 wt% MoC/MAPbI₃ has the best hydrogen production performance (38.4 μmol h^-1), which is approximately 24-times that of pure MAPbI₃ (1.61 μmol h^-1). With the extension of the catalytic time, the hydrogen production rate of MoC/MAPbI₃ reached 165.3 μmol h^-1 after 16 h due to the effective separation and transfer of charge carriers between MoC and MAPbI₃, showing excellent hydrogen evolution rate performance under visible light. In addition, the cycling stability of MoC/MAPbI₃ did not decrease in multiple 4 h cycle tests. This study used the non-precious metal promoter MoC to modify MAPbI₃, and provides a new idea for the synthesis of efficient MAPbI₃-based composite catalysts.

Solution Epitaxy of Halide Perovskite Thin Single Crystals for Stable Transistors

Halide perovskites hold promise for energy and optoelectronic applications due to their fascinating photophysical properties and facile processing. Among various forms, epitaxial thin single crystals (TSCs) are highly desirable due to their high crystallinity, reduced defects, and easy epitaxial integration with other materials. However, a cost-effective method for obtaining TSCs with perfect epitaxial features remains elusive. Here, we demonstrate a direct epitaxial growth of high-quality all-inorganic perovskite CsPbBr₃ TSCs on various substrates through a facile solution process under near-ambient conditions. Structural characterizations reveal a high-quality epitaxy between the obtained perovskite TSCs and substrates, thus leading to efficiently reduced defects. The resultant TSCs display a low trap density (∼10¹¹ cm^-3) and a long carrier lifetime (∼10.16 ns). Top-gate/top-contact transistors based on these TSCs exhibit high on/off ratios of over 10⁵, an optimal hole mobility of 3.9 cm² V^-1 s^-1, almost hysteresis-free operation, and high stability at room temperature. Such a facile approach for the high-yield production of perovskite epitaxial TSCs will enable a broad range of high-performance electronic applications.

Embedding of Ti3 C2 Tx Nanocrystals in MAPbI3 Microwires for Improved Responsivity and Detectivity of Photodetector

Organic-inorganic hybrid MAPbI₃ microwires show unique optoelectronic properties for high performances of photodetectors (PDs). However, the defects-assisted nonradiative recombination is harmful for carrier transport, which limits the performances improvement of MAPbI₃ microwires PDs. Traditional organic passivation agents effectively combine the surface defects of microwires and also reduce the mobility of overall film based on the perovskite microwires. Therefore, the improvement of internal carrier transport of microwires and the mobility of integrated film simultaneously is a particular challenge for fabrication of performances enhanced perovskite microwires PDs. Here, the Ti₃ C₂ T_x NCs are fabricated by nonfocus laser irradiation in liquid environment, and hybrids the high conductive NCs in the MAPbI₃ microwires. The presence of Ti₃ C₂ T_x NCs renders defects passivation, enhancement of crystalline orientation, charge transport, and carrier extraction for MAPbI₃ microwires, and boots the mobility of microwires based film, leading to about tenfolds enhancement of performances of PDs than that of the control. The maximum responsivity and the detectivity of the Ti₃ C₂ T_x NCs embedded MAPbI₃ microwires PDs reach to 1.70 A W^-1 and 7.0 × 10¹¹ Jones in visible window, respectively. The findings suggest that the laser generated high conductive Ti₃ C₂ T_x NCs is an effective additive for perovskite microwires to fabricate performances enhanced optoelectronics.

Highly Resolved and Robust Dynamic X-Ray Imaging Using Perovskite Glass-Ceramic Scintillator with Reduced Light Scattering

All-inorganic perovskite quantum dots (QDs) CsPbX3 (X = Cl, Br, and I) have recently emerged as a new promising class of X-ray scintillators. However, the instability of perovskite QDs and the strong optical scattering of the thick opaque QD scintillator film imped it to realize high-quality and robust X-ray image. Herein, the europium (Eu) doped CsPbBr3 QDs are in situ grown inside transparent amorphous matrix to form glass-ceramic (GC) scintillator with glass phase serving as both matrix and encapsulation for the perovskite QD scintillators. The small amount of Eu dopant optimizes the crystallization of CsPbBr3 QDs and makes their distribution more uniform in the glass matrix, which can significantly reduce the light scattering and also enhance the photoluminescence emission of CsPbBr3 QDs. As a result, a remarkably high spatial resolution of 15.0 lp mm-1 is realized thanks to the reduced light scattering, which is so far a record resolution for perovskite scintillator based X-ray imaging, and the scintillation stability is also significantly improved compared to the bare perovskite QD scintillators. Those results provide an effective platform particularly for the emerging perovskite nanocrystal scintillators to reduce light scattering and improve radiation hardness.

Exploring Transport Behavior in Hybrid Perovskites Solar Cells via Machine Learning Analysis of Environmental-Dependent Impedance Spectroscopy

Hybrid organic-inorganic perovskites are one of the promising candidates for the next-generation semiconductors due to their superlative optoelectronic properties. However, one of the limiting factors for potential applications is their chemical and structural instability in different environments. Herein, the stability of (FAPbI₃ )_0.85 (MAPbBr₃ )_0.15 perovskite solar cell is explored in different atmospheres using impedance spectroscopy. An equivalent circuit model and distribution of relaxation times (DRTs) are used to effectively analyze impedance spectra. DRT is further analyzed via machine learning workflow based on the non-negative matrix factorization of reconstructed relaxation time spectra. This exploration provides the interplay of charge transport dynamics and recombination processes under environment stimuli and illumination. The results reveal that in the dark, oxygen atmosphere induces an increased hole concentration with less ionic character while ionic motion is dominant under ambient air. Under 1 Sun illumination, the environment-dependent impedance responses show a more striking effect compared with dark conditions. In this case, the increased transport resistance observed under oxygen atmosphere in equivalent circuit analysis arises due to interruption of photogenerated hole carriers. The results not only shed light on elucidating transport mechanisms of perovskite solar cells in different environments but also offer an effective interpretation of impedance responses.

Solvent Free Laminated Fabrication of Lead Halide Perovskites for Sensitive and Stable X-ray Detection

The halide perovskite X-ray detector can meet the urgent needs of low-dose medical imaging by X-rays. However, there is still a pressing challenge in lacking robust methods for large-scale fabrication of high-quality perovskite films with tunable thickness. Here we report a laminated fabrication of polycrystalline MAPbI₃ by using solvent-free liquid perovskite molten-salt (PMS), that offers reduced toxic issue, scalable fabrication, and highly tunability in film thickness. Nylon membrane was chosen as a scaffold for the infiltration of PMS, which simultaneously acts as a physical barrier to suppress the ionic migration in the MAPbI₃-nylon composite (denoted as MAPbI₃-LLP). The enhanced material properties result in good stability and high performance of X-ray detectors that show low detection limit and high sensitivity. Additionally, single gamma-ray photon detection was realized by MAPbI₃-LLP detectors. The promising performance characteristics of such polycrystalline detectors can accelerate the adoption of polycrystalline perovskites in X-ray imaging and gamma-ray detection.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Oriented Halide Perovskite Nanostructures and Thin Films for Optoelectronics

Oriented semiconductor nanostructures and thin films exhibit many advantageous properties, such as directional exciton transport, efficient charge transfer and separation, and optical anisotropy, and hence these nanostructures are highly promising for use in optoelectronics and photonics. The controlled growth of these structures can facilitate device integration to improve optoelectronic performance and benefit in-depth fundamental studies of the physical properties of these materials. Halide perovskites have emerged as a new family of promising and cost-effective semiconductor materials for next-generation high-power conversion efficiency photovoltaics and for versatile high-performance optoelectronics, such as light-emitting diodes, lasers, photodetectors, and high-energy radiation imaging and detectors. In this Review, we summarize the advances in the fabrication of halide perovskite nanostructures and thin films with controlled dimensionality and crystallographic orientation, along with their applications and performance characteristics in optoelectronics. We examine the growth methods, mechanisms, and fabrication strategies for several technologically relevant structures, including nanowires, nanoplates, nanostructure arrays, single-crystal thin films, and highly oriented thin films. We highlight and discuss the advantageous photophysical properties and remarkable performance characteristics of oriented nanostructures and thin films for optoelectronics. Finally, we survey the remaining challenges and provide a perspective regarding the opportunities for further progress in this field.

Solid-State Neutron Detection Based on Methylammonium Lead Bromide Perovskite Single Crystals

Perovskite-based semiconductors, such as methylammonium and cesium lead halides (MPbX₃: M = CH₃NH₃⁺ or Cs⁺; X = I^-, Br^-, or Cl^-), have attracted immense attention for several applications, including radiation detection, due to their excellent electronic and optical properties.^1,2,3,4,5,6 In addition, the combination of perovskites with other materials enables unique device structures. For example, robust and reliable diodes result when combined with metal oxide semiconductors. This device can be used for detection of nonionizing and ionizing radiation. In this paper, we report a unique perovskite single-crystal-based neutron detector using a heterojunction diode based on single-crystal MAPbBr₃ and gallium oxide (Ga₂O₃) thin film. The MAPbBr₃/Ga₂O₃ diodes demonstrate a leakage current of ∼7 × 10^-10 A/mm², an on/off ratio of ∼10², an ideality factor of 1.41, and minimal hysteresis that enables alpha particle, gamma-ray, and neutron detection at a bias as low as (-5 V). Gamma discrimination is further improved by 85% by optimizing the thickness of the perovskite single crystal. The MAPbBr₃/Ga₂O₃ diodes also demonstrate a neutron detection efficiency of ∼3.92% when combined with a ¹⁰B neutron conversion layer.

Polarons and Charge Localization in Metal-Halide Semiconductors for Photovoltaic and Light-Emitting Devices

Metal-halide semiconductors have shown excellent performance in optoelectronic applications such as solar cells, light-emitting diodes, and detectors. In this review the role of charge-lattice interactions and polaron formation in a wide range of these promising materials, including perovskites, double perovskites, Ruddlesden-Popper layered perovskites, nanocrystals, vacancy-ordered, and other novel structures, is summarized. The formation of Fröhlich-type "large" polarons in archetypal bulk metal-halide ABX₃ perovskites and its dependence on A-cation, B-metal, and X-halide composition, which is now relatively well understood, are discussed. It is found that, for nanostructured and novel metal-halide materials, a larger variation in the strengths of polaronic effects is reported across the literature, potentially deriving from variations in potential barriers and the presence of interfaces at which lattice relaxation may be enhanced. Such findings are further discussed in the context of different experimental approaches used to explore polaronic effects, cautioning that firm conclusions are often hampered by the presence of alternate processes and interactions giving rise to similar experimental signatures. Overall, a complete understanding of polaronic effects will prove essential given their direct influence on optoelectronic properties such as charge-carrier mobilities and emission spectra, which are critical to the performance of energy and optoelectronic applications.

Visible colorimetric dosimetry of UV and ionizing radiations by a dual-module photochromic nanocluster

Radiation dosimeters displaying conspicuous response of irradiance are highly desirable, owing to the growing demand of monitoring high-energy radiation and environmental exposure. Herein, we present a case of dosimetry based on a discrete nanocluster, [Th6(OH)4(O)4(H2O)6](TPC)8(HCOO)4∙4DMF∙H2O (Th-SINAP-100), by judiciously incorporating heavy Th6 polynuclear centers as radiation attenuator and organic linkers as photo-responsive sensor. Interestingly, dual-module photochromic transitions upon multiple external stimuli including UV, β-ray, and γ-ray are integrated into this single material. The striking color change, and more significantly, the visible color transition of luminescence in response to accumulating radiation dose allow an on-site quantitative platform for naked-eye detection of ionization radiations over a broad range (1-80 kGy). Single crystal X-ray diffraction and density functional theory calculations reveal that the dual-module photochromism can be attributed to the π(TPC) → π*(TPC) intermolecular charge transfer driven by enhanced π-π stacking interaction between the adjacent TPC moieties upon irradiation.

High-Temperature Stable FAPbBr3 Single Crystals for Sensitive X-ray and Visible Light Detection toward Space

Organolead trihalide perovskite single crystals (SCs) offer unprecedented opportunity for X-ray and visible light detection. Nevertheless, it remains a challenge to keep simultaneous high-performance and stability at a high-temperature working mode. Herein, formamidinium lead bromide (FAPbBr₃) SCs are developed to successfully address these issues. Low-temperature crystallized induced FAPbBr₃ SCs possess an excellent mobility-lifetime product and an ultralow surface charge recombination velocity, thus exhibiting an X-ray dose rate as low as 0.3 μGy_air s^-1 as a sensitive radiation detector. Furthermore, it also contributes a specific detectivity as high as 3.5 × 10¹² cm Hz^1/2 W^-1, keeping stable at high-temperature of 460 K as a photodetector. A prototype of an imaging system with diffuse reflection mode is constructed using detectors as receivers, enabling defined scanning images in a high temperature environment. The bifunctional FAPbBr₃ SC detectors will motivate new strategies for stable detection in an extreme space environment.

Tunable Broad Light Emission from 3D "Hollow" Bromide Perovskites through Defect Engineering

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called "hollow" perovskites. In this work, we engineered a new family of 3D highly defective yet crystalline "hollow" bromide perovskites with general formula (FA)_1-x(en)_x(Pb)_1-0.7x(Br)_3-0.4x (FA = formamidinium (FA⁺), en = ethylenediammonium (en²⁺), x = 0-0.44). Pair distribution function analysis shed light on the local structural coherence, revealing a wide distribution of Pb-Pb distances in the crystal structure as a consequence of the Pb/Br-deficient nature and en inclusion in the lattice. By manipulating the number of Pb/Br vacancies, we finely tune the optical properties of the pristine FAPbBr₃ by blue shifting the band gap from 2.20 to 2.60 eV for the x = 0.42 en sample. A most unexpected outcome was that at x> 0.33 en incorporation, the material exhibits strong broad light emission (1% photoluminescence quantum yield (PLQY)) that is maintained after exposure to air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite, demonstrating that meticulous defect engineering is an excellent tool for customizing the optical properties of these semiconductors.

Unraveling the Charge Transport Mechanism in Mechanochemically Processed Hybrid Perovskite Solar Cell

The long-term operation of organic-inorganic hybrid perovskite solar cells is hampered by the microscopic strain introduced by the multiple thermal cycles during the synthesis of the material via a solution process route. This setback can be eliminated by a room temperature synthesis scheme. In this work, a mechanochemical synthesis technique at room temperature is employed to process CH₃NH₃PbI₂Br films for fabricating perovskite solar cell devices. The solar cell device has produced a 957 mV V_oc, a 16.92 mA/cm² short circuit current density, and a 10.5% efficiency. These values are higher than the published values on mechanochemically synthesized CH₃NH₃PbI₃. The charge transport properties of the devices are studied using DC conductivity and AC impedance spectroscopy, which show a multichannel transport mechanism having both ionic and electronic contributions. A much smaller defect density in the mechanochemically synthesized hybrid perovskite material is confirmed. A polarization assisted recombination mechanism is observed to have a dominant effect on the overall charge transport mechanism. However, no obvious grain boundary and intralayer lattice defect related responses are found in the perovskite layer. Interfacial charge transport and recombination are found to show major effects on both the temperature dependent and illumination dependent impedance spectra.

Lone-Pair-Induced Structural Ordering in the Mixed-Valent 0D Metal-Halides Rb23BiIII x SbIII 7-x SbV 2Cl54 (0 ≤ x ≤ 7)

Mixed-valent metal-halides containing ns² lone pairs may exhibit intense visible absorption, while zero-dimensional (0D) ns²-based metal-chlorides are generally colorless but have demonstrated promising optoelectronic properties suitable for thermometry and radiation detection. Here, we report solvothermally synthesized mixed-valent 0D metal-halides Rb₂₃Bi^III _x Sb^III _7-x Sb^V ₂Cl₅₄ (0 ≤ x ≤ 7). Rb₂₃Sb^III ₇Sb^V ₂Cl₅₄ crystallizes in an orthorhombic space group (Cmcm) with a unique, layered 0D structure driven by the arrangement of the 5s² lone pairs of the Sb^IIICl₆ octahedra. This red material is likely the true structure of a previously reported monoclinic "Rb_2.67SbCl₆" phase, the structure of which was not determined. Partially or fully substituting Sb^III with isoelectronic Bi^III yields the series Rb₂₃Bi^III _x Sb^III _7-x Sb^V ₂Cl₅₄ (0 < x ≤ 7), which exhibits a similar layered 0D structure but with additional disorder that yields a trigonal crystal system with an enantiomorphic space group (R32). Second harmonic generation of 532 nm light from a 1064 nm laser using Rb₂₃Bi^III ₇Sb^V ₂Cl₅₄ powder confirms the noncentrosymmetry of this space group. As with the prototypical mixed-valent pnictogen halides, the visible absorption bands of the Rb₂₃Bi^III _x Sb^III _7-x Sb^V ₂Cl₅₄ family are the result of intervalent Sb^III-Sb^V and mixed-valent Bi^III-Sb^V charge transfer bands (CTB), with a blueshift of the absorption edge as Bi^III substitution increases. No PL is observed from this family of semiconductors, but a crystal of Rb₂₃Bi^III ₇Sb^V ₂Cl₅₄ exhibits a high resistivity of 1.0 × 10¹⁰ Ω·cm and X-ray photoconductivity with a promising μτ product of 8.0 × 10^-5 cm² s^-1 V^-1. The unique 0D layered structures of the Rb₂₃Bi^III _x Sb^III _7-x Sb^V ₂Cl₅₄ family highlight the versatility of the ns² lone pair in semiconducting metal-halides, pointing the way toward new functional 0D metal-halide compounds.

Metal Halide Perovskite/2D Material Heterostructures: Syntheses and Applications

The past decade has witnessed the great success achieved by metal halide perovskites (MHPs) in photovoltaic and related fields. However, challenges still remain in further improving their performance, as well as, settling the stability issue for future commercialization. Recently, MHP/2D material heterostructures that combining MHPs with the low-cost and solution-processable 2D materials have demonstrated unprecedented improvement in both performance and stability due to the distinctive features at hetero-interface. The diverse fabrication techniques of MHPs and 2D materials allow them to be assembled as heterostructures with different configurations in a variety of ways. Moreover, the large families of MHPs and 2D materials provide the opportunity for the rational design and modification on compositions and functionalities of MHP/2D materials heterostructures. Herein, a comprehensive review of MHP/2D material heterostructures from syntheses to applications is presented. First, various fabrication techniques for MHP/2D material heterostructures are introduced by classifying them into solid-state methods and solution-processed methods. Then the applications of MHP/2D heterostructures in various fields including photodetectors, solar cells, and photocatalysis are summarized in detail. Finally, current challenges for the development of MHP/2D material heterostructures are highlighted, and future opportunities for the advancements in this research field are also provided.

Perovskite-inspired materials for photovoltaics and beyond-from design to devices

Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed.

Ligand assisted growth of perovskite single crystals with low defect density

A low defect density in metal halide perovskite single crystals is critical to achieve high performance optoelectronic devices. Here we show the reduction of defect density in perovskite single crystals grown by a ligand-assisted solution process with 3‐(decyldimethylammonio)‐propane‐sulfonate inner salt (DPSI) as an additive. DPSI ligands anchoring with lead ions on perovskite crystal surfaces not only suppress nucleation in solution, but also regulate the addition of proper ions to the growing surface, which greatly enhances the crystal quality. The grown CH₃NH₃PbI₃ crystals show better crystallinity and a 23-fold smaller trap density of 7 × 10¹⁰ cm⁻³ than the optimized control crystals. The enhanced material properties result in significantly suppressed ion migration and superior X-ray detection sensitivity of CH₃NH₃PbI₃ detectors of (2.6 ± 0.4) × 10⁶ µC Gy⁻¹air cm⁻² for 60 kVp X-ray and the lowest detectable dose rate reaches (5.0 ± 0.7) nGy s⁻¹, which enables reduced radiation dose to patients in medical X-ray diagnostics.

The performance of a metal halide perovskite single crystal is governed by the defect density. Here, the authors report a high quality single crystal perovskite grown by a ligand-assisted solution process with DPSI achieving 23-fold smaller trap density than that without DPSI.

Polyhydroxy Ester Stabilized Perovskite for Low Noise and Large Linear Dynamic Range of Self-Powered Photodetectors

Solution-processed perovskites as emerging semiconductors have achieved unprecedented milestones in sensor optoelectric devices. Stability along with the device noise issues are the major obstacle for photodetectors to compete with the traditional devices. Here, we demonstrated that l-ascorbic acid (l-AA) as a polyhydroxy ester can coordinate with the amino group of formamidine cations (FA⁺) through multiple hydrogen bond interactions to stabilize the perovskite, which protect the FA⁺ ions from nucleophile attack and effectively suppress the degradation of FA⁺ ions, improving the perovskite stability and suppressing the device noise to below 0.3 pA Hz^-1/2 with a large linear dynamic range of 239 dB. The dual functions of l-AA enable the perovskite photodetector to have a high detectivity of 10¹² Jones. The self-powered device works with no energy consumption and maintains an undegraded performance over 1200 h of inspection at ambient conditions, which is promising for infrastructure construction, signal sensing, and real-time information delivery.

State-of-the-Art Mobile Radiation Detection Systems for Different Scenarios

In the last decade, the development of more compact and lightweight radiation detection systems led to their application in handheld and small unmanned systems, particularly air-based platforms. Examples of improvements are: the use of silicon photomultiplier-based scintillators, new scintillating crystals, compact dual-mode detectors (gamma/neutron), data fusion, mobile sensor networks, cooperative detection and search. Gamma cameras and dual-particle cameras are increasingly being used for source location. This study reviews and discusses the research advancements in the field of gamma-ray and neutron measurements using mobile radiation detection systems since the Fukushima nuclear accident. Four scenarios are considered: radiological and nuclear accidents and emergencies; illicit traffic of special nuclear materials and radioactive materials; nuclear, accelerator, targets, and irradiation facilities; and naturally occurring radioactive materials monitoring-related activities. The work presented in this paper aims to: compile and review information on the radiation detection systems, contextual sensors and platforms used for each scenario; assess their advantages and limitations, looking prospectively to new research and challenges in the field; and support the decision making of national radioprotection agencies and response teams in respect to adequate detection system for each scenario. For that, an extensive literature review was conducted.

Triple-Cation and Mixed-Halide Perovskite Single Crystal for High-Performance X-ray Imaging

Low ionic migration is required for a semiconductor material to realize stable high-performance X-ray detection. In this work, successful controlled incorporation of not only methylammonium (MA⁺ ) and cesium (Cs⁺ ) cations, but also bromine (Br^- ) anions into the FAPbI₃ lattice to grow inch-sized stable perovskite single crystal (FAMACs SC) is reported. The smaller cations and anions, comparing to the original FA⁺ and I^- help release lattice stress so that the FAMACs SC shows lower ion migration, enhanced hardness, lower trap density, longer carrier lifetime and diffusion length, higher charge mobility and thermal stability, and better uniformity. Therefore, X-ray detectors made of the superior FAMACs SCs show the highest sensitivity of (3.5 ± 0.2) × 10⁶ μC Gy_air ^-1 cm^-2 , about 29 times higher than the latest record of 1.22 × 10⁵ μC Gy_air ^-1 cm^-2 for polycrystalline MAPbI₃ wafer under the same 40 keV X-ray radiation. Furthermore, the FAMACs SC X-ray detector shows a low detection limit of 42 nGy s^-1 , stable dark current, and photocurrent response. Finally, it is demonstrated that high contrast X-ray imaging is realized using the FAMACs SC detector. The effective triple-cation mixed halide strategy and the high crystalline quality make the present FAMACs SCs promising for next-generation X-ray imaging systems.

Highly efficient photoelectric effect in halide perovskites for regenerative electron sources

Electron sources are a critical component in a wide range of applications such as electron-beam accelerator facilities, photomultipliers, and image intensifiers for night vision. We report efficient, regenerative and low-cost electron sources based on solution-processed halide perovskites thin films when they are excited with light with energy equal to or above their bandgap. We measure a quantum efficiency up to 2.2% and a lifetime of more than 25 h. Importantly, even after degradation, the electron emission can be completely regenerated to its maximum efficiency by deposition of a monolayer of Cs. The electron emission from halide perovskites can be tuned over the visible and ultraviolet spectrum, and operates at vacuum levels with pressures at least two-orders higher than in state-of-the-art semiconductor electron sources.

Microscopic (Dis)order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites

Recent developments in the field of high efficiency perovskite solar cells are based on stabilization of the perovskite crystal structure of FAPbI₃ while preserving its excellent optoelectronic properties. Compositional engineering of, for example, MA or Br mixed into FAPbI₃ results in the desired effects, but detailed knowledge of local structural features, such as local (dis)order or cation interactions of formamidinium (FA) and methylammonium (MA), is still limited. This knowledge is, however, crucial for their further development. Here, we shed light on the microscopic distribution of MA and FA in mixed perovskites MA_1-x FA _x PbI₃ and MA_0.15FA_0.85PbI_2.55Br_0.45 by combining high-resolution double-quantum ¹H solid-state nuclear magnetic resonance (NMR) spectroscopy with state-of-the-art near-first-principles accuracy molecular dynamics (MD) simulations using machine-learning force-fields (MLFFs). We show that on a small local scale, partial MA and FA clustering takes place over the whole MA/FA compositional range. A reasonable driving force for the clustering might be an increase of the dynamical freedom of FA cations in FA-rich regions. While MA_0.15FA_0.85PbI_2.55Br_0.45 displays similar MA and FA ordering as the MA_1-x FA _x PbI₃ systems, the average cation-cation interaction strength increased significantly in this double mixed material, indicating a restriction of the space accessible to the cations or their partial immobilization upon Br^- incorporation. Our results shed light on the heterogeneities in cation composition of mixed halide perovskites, helping to exploit their full optoelectronic potential.

Achieving UV and X-ray Dual Photochromism in a Metal-Organic Hybrid via Structural Modulation

Rational design and synthesis of new photochromic sensors have been active research areas of inquiry, particularly on how to predict and tailor their properties and functionalities. Herein, two thulium 2,2':6',2''-terpyridine-4'-carboxylate (TPC)-functionalized metal-organic hybrids, Tm(TPC)₂(HCOO)(H₂O) (TmTPC-1) and Tm(TPC)(HCOO)₂ (TmTPC-2) with different photochromic response behaviors, have been successfully prepared, allowing for straightforward investigations of the structure-property correlation. Single-crystal X-ray diffraction and electron paramagnetic resonance analyses revealed that the incorporation of a unique dangling decorating TPC unit in TmTPC-1 offers a shorter and more accessible π-π interaction pathway between the adjacent TPC moieties than that in TmTPC-2. Such a structural feature leads to the production of radical species via a photoinduced intermolecular electron-transfer (IeMCT) process upon UV or X-ray irradiation, which ultimately endows TmTPC-1 with a rather unusual UV and X-ray dual photochromism. A linear relationship between the change of UV-vis absorbance intensity and X-ray dose was established, making TmTPC-1 a promising dosimeter for X-ray radiation with an extremely high energy threshold (30 kGy). To advance the development for real-world application, we have fabricated polyvinylidene fluoride (PVDF) membranes incorporating TmTPC-1 for functioning either as a UV imager or as an X-ray radiation indicator. Lastly, TmTPC-1 exhibits high thermal stability (up to 400 °C) and radioresistance (at least 900 kGy), and also excellent reversibility of photochromic transformation (at least 5 cycles).

Highly Sensitive and Stable X-ray Detector Based on a 0D Structural Cs4PbI6 Single Crystal

Lead halide perovskite single crystals with a high X-ray stopping power and large μτ product have been successfully used for X-ray detection. However, poor air stability and ionic migration lead to the degradation of the devices for long-term operations. Here, we report a solution-processed lead 0D bulk Cs₄PbI₆ single crystal, which have I atoms in isolated [PbI₆]^4-. This 0D Cs₄PbI₆ single crystal has a μτ product of 9.7 × 10^-4 cm² V^-1 and an activation energy of 321.28 meV. We have fabricated a photoconductor device with a symmetric Au/Cs₄PbI₆/Au sandwich structure, which exhibits a high sensitivity of 451.49 μC Gy^-1 cm^-2 under 30 keV X-ray irradiation at an applied voltage of 30 V. After storing the device in air at room temperature for three months, the device retains nearly the same sensitivity as the original. These results demonstrate that this 0D Cs₄PbI₆ perovskite single crystal has great potential for practical X-ray detection applications.

Ambient Fabrication of Organic-Inorganic Hybrid Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted significant attention in recent years due to their high-power conversion efficiency, simple fabrication, and low material cost. However, due to their high sensitivity to moisture and oxygen, high efficiency PSCs are mainly constructed in an inert environment. This has led to significant concerns associated with the long-term stability and manufacturing costs, which are some of the major limitations for the commercialization of this cutting-edge technology. Over the past few years, excellent progress in fabricating PSCs in ambient conditions has been made. These advancements have drawn considerable research interest in the photovoltaic community and shown great promise for the successful commercialization of efficient and stable PSCs. In this review, after providing an overview to the influence of an ambient fabrication environment on perovskite films, recent advances in fabricating efficient and stable PSCs in ambient conditions are discussed. Along with discussing the underlying challenges and limitations, the most appropriate strategies to fabricate efficient PSCs under ambient conditions are summarized along with multiple roadmaps to assist in the future development of this technology.

X-ray stability and degradation mechanism of lead halide perovskites and lead halides

Lead halide perovskites have become a leading material in the field of emerging photovoltaics and optoelectronics. Significant progress has been achieved in improving the intrinsic properties and environmental stability of these materials. However, the stability of lead halide perovskites to ionising radiation has not been widely investigated. In this study, we investigated the radiolysis of lead halide perovskites with organic and inorganic cations under X-ray irradiation using synchrotron based hard X-ray photoelectron spectroscopy. We found that fully inorganic perovskites are significantly more stable than those containing organic cations. In general, the degradation occurs through two different, but not mutually exclusive, pathways/mechanisms. One pathway is induced by radiolysis of the lead halide cage into halide salts, halogen gas and metallic lead and appears to be catalysed by defects in the perovskite. The other pathway is induced by the radiolysis of the organic cation which leads to formation of organic degradation products and the collapse of the perovskite structure. In the case of Cs0.17FA0.83PbI3, these reactions result in products with a lead to halide ratio of 1 : 2 and no formation of metallic lead. The radiolysis of the organic cation was shown to be a first order reaction with regards to the FA+ concentration and proportional to the X-ray flux density with a radiolysis rate constant of 1.6 × 10-18 cm2 per photon at 3 keV or 3.3 cm2 mJ-1. These results provide valuable insight for the use of lead halide perovskite based devices in high radiation environments, such as in space environments and X-ray detectors, as well as for investigations of lead halide perovskites using X-ray based techniques.

Kilogram-Scale Crystallogenesis of Halide Perovskites for Gamma-Rays Dose Rate Measurements

Gamma-rays (γ-rays), wherever present, e.g., in medicine, nuclear environment, or homeland security, due to their strong impact on biological matter, should be closely monitored. There is a need for simple, sensitive γ-ray detectors at affordable prices. Here, it is shown that γ-ray detectors based on crystals of methylammonium lead tribromide (MAPbBr3) ideally meet these requirements. Specifically, the γ-rays incident on a MAPbBr3 crystal generates photocarriers with a high mobility-lifetime product, allowing radiation detection by photocurrent measurements at room temperatures. Moreover, the MAPbBr3 crystal-based detectors, equipped with improved carbon electrodes, can operate at low bias (≈1.0 V), hence being suitable for applications in energy-sparse environments, including space. The γ-ray detectors reported herein are exposed to radiation from a 60Co source at dose rates up to 2.3 Gy h-1 under ambient conditions for over 100 h, without any sign of degradation. The excellent radiation tolerance stems from the intrinsic structural plasticity of the organic-inorganic halide perovskites, which can be attributed to a defect-healing process by fast ion migration at the nanoscale level. The sensitivity of the γ-ray detection upon volume is tested for MAPbBr3 crystals reaching up to 1000 cm3 (3.3 kg in weight) grown by a unique crystal growth technique.

Crystal Size Effect on Carrier Transport of Microscale Perovskite Junctions via Soft Contact

To reduce the size of optoelectronic devices, it is essential to understand the crystal size effect on the carrier transport through microscale materials. Here, we show a soft contact method to probe the properties of irregularly shaped microscale perovskite crystals by employing a movable liquid metal electrode to form a self-adaptative deformable electrode-perovskite-electrode junction. Accordingly, we demonstrate that (1) the photocurrents of perovskite quantum dot films and microplatelets show profound differences regarding both the on/off ratio and the response time upon light illumination; and (2) small-size perovskite (<50 μm) junctions may show negative differential resistance (NDR) behavior, whereas the NDR phenomenon is absent in large-size perovskite junctions within the same bias regime. Our studies provide a method for studying arbitrary-shaped crystals without mechanical damage, assisting the understanding of the photogenerated carriers transport through microscale crystals.

Designing Ultraflexible Perovskite X-Ray Detectors through Interface Engineering

X-ray detectors play a pivotal role in development and advancement of humankind, from far-reaching impact in medicine to furthering the ability to observe distant objects in outer space. While other electronics show the ability to adapt to flexible and lightweight formats, state-of-the-art X-ray detectors rely on materials requiring bulky and fragile configurations, severely limiting their applications. Lead halide perovskites is one of the most rapidly advancing novel materials with success in the field of semiconductor devices. Here, an ultraflexible, lightweight, and highly conformable passively operated thin film perovskite X-ray detector with a sensitivity as high as 9.3 ± 0.5 µC Gy-1 cm-2 at 0 V and a remarkably low limit of detection of 0.58 ± 0.05 μGy s-1 is presented. Various electron and hole transporting layers accessing their individual impact on the detector performance are evaluated. Moreover, it is shown that this ultrathin form-factor allows for fabrication of devices detecting X-rays equivalently from front and back side.

Characteristics of a Hybrid Detector Combined with a Perovskite Active Layer for Indirect X-ray Detection

In this study, we investigated the characteristics of an organic-inorganic hybrid indirect-type X-ray detector with a CH₃NH₃PbI₃ (MAPbI₃) perovskite active layer. A layer with a thickness of 192 nm annealed at 100 °C showed higher absorption, higher crystallinity, and lower surface roughness than did perovskite layers made under different conditions. In the indirect X-ray detector, a scintillator coupled with the detector to convert X-ray photons to visible photons, and the converted photons were absorbed by the active layer to generate charge carriers. The detector with the optimized MAPbI₃ (192 nm thick and 100 °C annealing condition) active layer was coupled with a CsI(Tl) scintillator which consisted of 400 μm thick CsI and 0.5 mm thick Al, and achieved the highest sensitivity, i.e., 2.84 mA/Gy·cm². In addition, the highest short-circuit current density (J_SC), i.e., 18.78 mA/cm², and the highest mobility, i.e., 2.83 × 10⁻⁴ cm²/V·s, were obtained from the same detector without the CsI(Tl) scintillator.