Inorganic, Organic, and Perovskite Halides with Nanotechnology for High-Light Yield X- and γ-Ray Scintillators

Trends in scintillators that are used in many applications, such as medical imaging, security, oil-logging, high energy physics and non-destructive inspections are reviewed. First, we address traditional inorganic and organic scintillators with respect of limitation in the scintillation light yields and lifetimes. The combination of high–light yield and fast response can be found in Ce 3+ , Pr 3+ and Nd 3+ lanthanide-doped scintillators while the maximum light yield conversion of 100,000 photons/MeV can be found in Eu 3+ doped SrI 2 . However, the fabrication of those lanthanide-doped scintillators is inefficient and expensive as it requires high-temperature furnaces. A self-grown single crystal using solution processes is already introduced in perovskite photovoltaic technology and it can be the key for low-cost scintillators. A novel class of materials in scintillation includes lead halide perovskites. These materials were explored decades ago due to the large X-ray absorption cross section. However, lately lead halide perovskites have become a focus of interest due to recently reported very high photoluminescence quantum yield and light yield conversion at low temperatures. In principle, 150,000–300,000 photons/MeV light yields can be proportional to the small energy bandgap of these materials, which is below 2 eV. Finally, we discuss the extraction efficiency improvements through the fabrication of the nanostructure in scintillators, which can be implemented in perovskite materials. The recent technology involving quantum dots and nanocrystals may also improve light conversion in perovskite scintillators

Lithium-Doped Two-Dimensional Perovskite Scintillator for Wide-Range Radiation Detection

Two-dimensional lead halide perovskites have demonstrated their potential as high-performance scintillators for X- and gamma-ray detection, while also being low-cost. Here we adopt lithium chemical doping in two-dimensional phenethylammonium lead bromide (PEA)2PbBr4 perovskite crystals to improve the properties and add functionalities with other radiation detections. Li doping is confirmed by X-ray photoemission spectroscopy and the scintillation mechanisms are explored via temperature dependent X-ray and thermoluminescence measurements. Our 1:1 Li-doped (PEA)2PbBr4 demonstrates a fast decay time of 11 ns (80%), a clear photopeak with an energy resolution of 12.4%, and a scintillation yield of 11,000 photons per MeV under 662 keV gamma-ray radiation. Additionally, our Li-doped crystal shows a clear alpha particle/gamma-ray discrimination and promising thermal neutron detection through 6Li enrichment. X-ray imaging pictures with (PEA)2PbBr4 are also presented. All results demonstrate the potential of Li-doped (PEA)2PbBr4 as a versatile scintillator covering a wide radiation energy range for various applications

Solution-processed lead halide perovskite single crystal scintillators

Perovskite scintillators have been a rising research topic after the zeal on promoting perovskite solar cell efficiency. Heavy element Pb, low fabrication cost and relatively strong emission make some of perovskite scintillators (like (PEA)2PbBr4 and (BA)2PbBr4 in this thesis) potentially competitive in comparison to current commercial scintillators. In this thesis, the focus will be on the how different components, including organic cation, halide anion and dopant affect the scintillator performance. A series of characterizations were systematically carried out to correlate the perovskites’ properties to their behavior and performance. Based on these characterization results, corresponding conclusions and predictions will be provided. To begin with, we synthesized and characterized 3D perovskite MAPbX3 (X=Cl, Br or I) single crystals that have been well studied in solar cell and optoelectronic areas but relatively new in the scintillator field. My first work (Chapter 4) demonstrated the effect of halide anions on thermal quenching behavior of perovskite under X-ray. The thermal quenching activation energy and the ratio between the thermal quenching rate and the radiative transition rate are decreasing from MAPbCl3 to MAPbI3 in 3D MAPbX3 family. The conclusion could be reasonably extrapolated to 3D perovskites beside methylammonium lead halide perovskites. After mastering the synthesis of 3D perovskite crystals and the related characterization analysis, we further explored the 2D perovskite crystals, which were predicted to be better compared to 3D counterpart. In my second work (Chapter 5), we found that Li doping can enhance the scintillating performance of 2D perovskite (PEA)2PbBr4, such as higher light yield and smaller light yield difference caused by temperature change. In addition, for the first time we utilized the Li dopant strategy to extend the detection energy range by demonstrating the detection of alpha particle and thermal neutron with our Li-doped (PEA)2PbBr4 crystals. With such strategy, there are numerous chances to develop new functionalities based on available perovskite scintillators. Furthermore, we explored the similarities and differences in 2D perovskite crystals made of varied cations and anions as my last work (Chapter 6). We synthesized eleven 2D perovskites with different cations and anions to compare the influence they brought. Bromide perovskite crystals are better over the chloride and iodide ones in terms of high chemical stabilities in ambient as well as their emission wavelengths that match the high quantum efficiency ranges of Si-based detectors. Simple linear alkyl and small bulky ring-containing cations tend to introduce (100) type which typically exhibits narrow emission and more likely to show negative thermal quenching behavior (stronger emission upon higher temperature). Cations containing O and N elements (beside ammonium group) usually lead to (110)-orientated perovskites which show broad emissions in visible light range because of self-trapped excitons. We believe our conclusions in this thesis could shed light on similar scintillator design investigations. At last, based on my experimental skills and knowledge learnt from other perovskite scintillator references, I ended up my thesis with the comparison of 2D and 3D perovskite single crystal scintillators. 3D perovskite scintillators (especially thick crystal samples) are easy to obtain due to early synthesis investigations. This provides a good way to predict, modify and test the properties and performances of 3D perovskite scintillators. Ultimately, we should focus on 2D ones due to their theoretically higher performances although the current synthetic method is inefficient. 2D perovskite single crystal scintillators are potential candidates to compete with current commercial scintillators in the future market.Doctor of Philosoph

Cyclability of a Lithium–Oxygen Battery Containing N-Methyl-2-Pyrrolidone and a Vertically Aligned Carbon Nanotube Cathode

The lithium–air battery has a high specific capacity but suffers from a poor cyclability. The reason for its poor cycle life is still unclear and many explanations, such as the degradation of the solvent, the lithium salt, and the carbon cathode, have been suggested. Although N-methyl-2-pyrrolidone (NMP) was shown to be stable under superoxide anion radical conditions in stand-alone tests, lithium–air cells containing NMP still showed a very low reversibility in previous work. Here, a freestanding vertically aligned carbon nanotube is used as a binder-free electrode in a lithium–air battery cathode, and the results indicate that the cyclability of the cell is significantly improved. This research proves that the content of the carbon electrode is of pivotal importance for the stability of lithium–air batteries

A Tough and High-Performance Transparent Electrode from a Scalable and Transfer-Free Method

Conductive metal films are patterned into transparent metal nanowire networks by using electrospun fibers as a mask. Both the transmittance and sheet resistance (6 Ω/□ at 83% transmittance and 24 Ω/□ at 92% transmittance) of the metal nanowire-based electrode out-perform commercial indium doped tin oxide (ITO) electrodes. The metal nanowire-based transparent electrodes were fabricated on both rigid glass and flexible polyethylene terephthalate (PET) substrates. In addition to state of art performance, the transparent electrodes also exhibit outstanding toughness. They can withstand repeated scotch tape peeling and various bending tests. The method for making the metal nanowire is scalable, and a touch screen on flexible substrate is demonstrated

Current oscillations and intermittent emission near an electrode interface in a hybrid organic-inorganic perovskite single crystal

Hybrid organic-inorganic lead perovskites have a great potential in optoelectronic device applications because of their high stability, narrow band emission, and strong luminescence. Single crystals with few defects are the best candidates to disclose a variety of interesting and important properties for light-emitting devices. Here, we investigate a single-crystalline CH3NH3PbBr3 perovskite for its transport and electroluminescence properties. A simple fabrication method was used to obtain a 10 ± 2 μm channel between two gold wire electrodes, which showed bright intermittent electroluminescence near the interface of one wire after cooling down with a constant biasing voltage. The active region of the perovskite single crystal was pristine, well isolated from surroundings through fabrication to the characterization process. Our presented sample provided an ideal condition to study bulk ionic-electronic properties of hybrid halide perovskites. At constant 6 V bias, the current through the sample shows temperature-dependent oscillation with Arrhenius behavior, suggesting a thermally activated process. The light emission from the sample experiences an intermittent emission rate once every 26 ± 6 min. Here, we envisage that the current oscillations and intermittent emission are caused by ion-mediated negative differential resistance and conductive filament formation, respectively. The latter observation inspires future applications of the material from neuromorphic computing to the development of electroluminescence devices.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Library of Two-Dimensional Hybrid Lead Halide Perovskite Scintillator Crystals

International audienceTwo-dimensional (2D) hybrid lead halide perovskites are potential candidates for high light yield scintillators as they have small band gaps between 3 and 4 eV and large exciton-binding energy. Here, we discuss the scintillation properties from a total of 11 organic/inorganic hybrid perovskite crystals with two already reported crystals, (PEA)2PbBr4 and (EDBE)PbBr4. Their photoluminescence and X-ray luminescence (XL) spectra are dominated by narrow and broad band emissions, and they correspond to free exciton and self-trapped exciton, respectively. The lifetimes derived from time-resolved XL strongly vary from 0.6 to 17.0 ns. These values make this type of compound among the fastest scintillators. For the light yield derived from the XL, we found that only (PEA)2PbBr4, (EDBE)PbBr4, and (BA)2PbBr4 crystals have light yields between 10,000 and 40,000 photons/MeV. The mechanisms for thermal quenching and afterglow are discussed in order to optimize the light yields. With gamma-ray excitation, we reported the best energy resolution of 7.7% at 662 keV with excellent proportionality. Finally, this study paves the way toward the ultimate high light yield and fast scintillators for medical and homeland security applications

Spin correlated-plasmons at room temperature driven by electronic correlations in lead-free 2D hybrid organic-inorganic perovskites

Hybrid organic-inorganic perovskites (HOIPs) have emerged to the forefront of optoelectronic material advancements for the past few years. However, our understanding on electronic structure and correlations are still lacking. Herewith, by simultaneously analyzing complex dielectric function, loss functions, and reflectivity directly obtained from spectroscopic ellipsometry and supported with theoretical calculations, we report new spin correlated-plasmons with low loss in (MA)2CuCl4. Photoluminescence and time-resolved photoluminescence measurements show a broadband emission band originating from the self-trapped emission excitons. Through X-ray absorption spectroscopy and resonant photoemission spectroscopy measurements at the C K-edge, a resonance enhancement peak is observed and unravels a charge transfer event due to the opening of an extra autoionization channel. Our result shows the importance of coupling between spin correlated-plasmons and electron-hole pairs together with spin-dependent exchange interaction in determining electronic structure and optical properties of HOIPS.Ministry of Education (MOE)National Research Foundation (NRF)This work is supported by the Singapore Ministry of Education (MOE) AcRF Tier-2 (MOE2017 T2-1-135, MOE2018-T2-1- 088, MOE2018-T2-2-117, and MOE2019-T2-1-163), MOE AcRF Tier-3 (MOE2014-T3-1-004), MOE-AcRF Tier-1 (R144-000-423-114, R-144-000-398-114, R-144-000-379-114 and R-144-000-368-112), the Singapore National Research Foundation under its Competitive Research Funding (No. R-398-000- 087-281) and under its Medium Sized Centre Program (Centre for Advanced 2D Materials and Graphene Research Centre), NUS YIA, and the 2015 PHC Merlion Project. NTU authors acknowledge the MOE-AcRF Tier-2 (MOE2016-T2-1-052). The authors acknowledge the Singapore Synchrotron Light Source (SSLS) for providing the facility necessary for conducting the research. The Laboratory is a National Research Infrastructure under the National Research Foundation Singapore