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

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

Effect of commensurate lithium doping on the scintillation of two-dimensional perovskite crystals

Two-dimensional (2D) hybrid lead bromide perovskites are candidates for high light yield, fast scintillators. In this paper, we discuss the effect of commensurate Lithium (Li)-doping (1 : 1 Li : Pb precursor ratio) on the scintillation properties of two previously reported high light yield 2D-perovskite crystals, PEA2PbBr4 and BA2PbBr4. The effect of Li, e.g. light yield enhancement, is more prominent in PEA2PbBr4 compared to BA2PbBr4. Both perovskite crystals show a broadening of the radioluminescence spectrum and a slightly longer afterglow, with residual scintillation below 2% after 5 s for both materials. The effects of Li on the negative thermal quenching exhibited by the perovskites are also discussed. Li doping increases the light yield of PEA2PbBr4 by 78% and both perovskite materials show an improvement in their energy resolutions, with a record of 7.7% at 662 keV for the Li-doped PEA2PbBr4. Both perovskite crystals also show very fast gamma-ray excited scintillation decays, with average times of 12.9 ns and 8.0 ns for PEA2PbBr4 and BA2PbBr4, respectively. This work shows that Li doping brings a significant improvement of the perovskite performance, making the perovskites more competitive for fast, high light yield applications in the medical, security or industrial sectors.Ministry of Education (MOE)The authors acknowledge financial supports from the Singapore Ministry of Education (MOE2019-T1-002-087) and Thales-CINTRA Funding

Thermal Quenching and Dose Studies of X‑ray Luminescence in Single Crystals of Halide Perovskites

Temperature- and dose-dependent measurements of X-ray luminescence (XL) in various perovskite single crystals are reported. For methylammonium lead halide perovskites (MAPbX₃, MA = methylammonium, X = Cl, Br, or I), the quenching temperature of XL intensities shifts to lower temperatures in the sequence from Cl to I. This quenching is strongly affected by the decrease of the thermal activation energy Δ<i>E</i>_q from 53 ± 3 to 6 ± 1 meV. We replace MA in MAPbBr₃ with Cs and observe that the quenching temperature even shifts to lower temperature. But unlike the MAPbX₃ perovskites, the quenching in CsPbBr₃ is now affected by the increase of the ratio between the thermal quenching rate and the radiative transition rate (Γ₀/Γ_v) from 15 ± 1 to 66 ± 14. The same influence was observed if we dope MAPbBr₃ with Bi³⁺, Γ₀/Γ_v increases to 78 ± 18 for crystal with Bi/Pb ratio of 1:10 in precursor solution. For larger dose of X-ray, we observe that the XL intensities are still linear without saturation. Unlike temperature-dependent measurements, we do not observe the line width narrowing in dose-dependent XL spectra. Thus, this scintillator is still stable with the large X-ray dose in comparison with the variation in the temperature