Jan Král

CsPbBr₃ nanocomposites are prospective luminescent materials with great potential for many light-emitting applications. However, embedding CsPbBr₃ nanocrystals into some potentially interesting matrices proves challenging in terms of maintaining their properties and good transparency of the final nanocomposite, e.g., high temperature polymerization of widely used and radiation-hard polystyrene or employment of more polar polymers such as polyurethanes often leads to nanocrystal degradation. Here, we present a functionalization strategy using ammonium hexafluorosilicate in combination with functional organosilanes for the fabrication of CsPbBr₃ nanocomposites, which provides improved thermal stability of surface passivation up to 110 °C. Moreover, it enables for the first time concomitant surface modification to enhance compatibility between nanocrystals and the matrix, leading to improved transparency of nanocomposites. We demonstrate that the process can be tailored by preparing polystyrene (PS) and polyurethane (PU) nanocomposites. This opens the way for easier processability of popular high cure temperature polymers such as polystyrene, as well as for efficient embedding of NCs into novel matrices with the aim of exploiting crucial properties for desired applications, for example, radiation hardness or flexibility.

Cesium lead halide nanocrystals (NCs) and their nanocomposites have attracted a lot of attention in the field of radiation detection thanks to their excellent luminescent properties, especially their potential for fast timing. However, most research on their nanocomposites focuses on low loadings (around 1 wt%) which is insufficient for detecting high-energy X-rays or γ-rays. There have been only few reports exploring materials with higher loadings but all with limited success in terms of transparency of the final material. In our work, we present nanocomposites of cesium lead halide NCs with loading up to 40 wt%. We employ innovative surface functionalization of the NCs to enhance their dispersion within the matrix, thereby improving the final material transparency. We display the NC dispersion within the matrix using confocal photoluminescence microscopy and we then characterise the radioluminescent properties of nanocomposites of cesium lead bromide (CPB) and cesium lead bromochloride NCs with varying chloride content. To asses their performance as radiation detectors, we measure their timing capabilities under X-rays. Our findings reveal a significant improvement in time resolution under X-rays from previously published 300 ps for CPB polymer nanocomposites to 215 ps. When combined with increased stopping power of high NC content, this advancement holds great promise for practical applications, e.g. in time-of-flight positron emission tomography and computed tomography or high energy physics.

Time‐of‐flight (TOF) technique, traditionally used in high energy physics (HEP) and positron emission tomography (PET), is now being explored for lower energy applications like computed tomography (CT). Regardless of the application, pushing the current boundaries in time resolution calls for novel technologies and materials exhibiting ultra‐fast time response. Semiconductor nanocrystals like cesium lead halide perovskites (CsPbBr₃), benefiting from quantum confinement effects, feature ultra‐fast decay and, when combined with a suitable bulk scintillator following a heterostructure concept, can also provide the necessary stopping power. In this work, thin films of CsPbBr₃ on top of BGO, LYSO:Ce, and GAGG:Ce,Mg wafers are fabricated to test their impact on the single crystal scintillator time resolution under soft X‐rays excitation (about 10 keV). It is demonstrated that the CsPbBr₃ layer significantly improves the overall time resolution in all cases, achieving up to a tenfold improvement with BGO and GAGG:Ce,Mg. Under 511 keV γ‐rays, a proof‐of‐concept of the heterostructure design for TOF‐PET using CsPbBr₃ thin film deposited on GAGG:Ce,Mg bulk crystal is successfully tested. Shared events depositing energy in both materials are identified, resulting in more than twofold improved coincidence time resolution: 118 ± 4 ps full‐width‐at‐half‐maximum (FWHM) compared to the 272 ± 8 ps of solely GAGG:Ce,Mg.

Lead halide perovskite nanocrystals of the formula CsPbBr₃ have recently been identified as potential time taggers in scintillating heterostructures for time-of-flight positron emission tomography (TOF-PET) imaging thanks to their ultrafast decay kinetics. This study investigates the potential of this material experimentally. We fabricated CsPbBr₃ thin films on scintillating GGAG:Ce (Gd_2.985Ce_0.015Ga_2.7Al_2.3O₁₂) wafer as a model structure for the future sampling detector geometry. We focused this study on the radioluminescence (RL) response of this composite material. We compare the results of two spin-coating methods, namely the static and the dynamic process, for the thin film preparation. We demonstrated enhanced RL intensity of both CsPbBr₃ and GGAG:Ce scintillating constituents of a composite material. This synergic effect arises in both the RL spectra and decays, including decays in the short time window (50 ns). Consequently, this study confirms the applicability of CsPbBr₃ nanocrystals as efficient time taggers for ultrafast timing applications, such as TOF-PET.

CsPbBr₃ nanocrystals have been identified as a highly promising material for various optoelectronic applications. However, they tend to coexist with Cs₄PbBr₆ phase when the reaction conditions are not controlled carefully. It is therefore imperative to understand how the presence of this phase affects the luminescence performance of CsPbBr₃ nanocrystals_. We synthesized a mixed CsPbBr₃-Cs₄PbBr₆ sample, and compared its photo- and radioluminescence properties, including timing characteristics, to the performance of pure CsPbBr₃ nanocrystals. The possibility of energy transfer between the two phases was also explored. We demonstrated that the presence of Cs₄PbBr₆ causes significant drop in radioluminescence intensity of CsPbBr₃ nanocrystals, which can limit possible future applications of Cs₄PbBr₆-CsPbBr₃ mixtures or composites as scintillation detectors.