X-ray detector is widely used in security inspection, medical imaging, industrial flaw detection, nuclear power plant, scientific research and other fields. Its working mode is mainly divided into the following two types: direct detection of converting X-ray to electrical signal, and indirect detection of converting X-ray to visible light and then to electrical signal through optical detector. Because X-ray is harmful to human body, it is very important to find a kind of X-ray detector material with high sensitivity and low detection limit. Traditional X-ray detector material such as direct detection material or indirect detection material scintillator has the disadvantages of complex preparation (such as CZT), poor stability (such as a-Se), low sensitivity (such as CsI: Tl). Professor Jiang Tang's group is devoted to the development of high sensitive X-ray conversion materials and high-performance detectors. Recently, two consecutive X-ray detection articles have been published on Advanced Materials, and breakthroughs have been made in both direct and indirect detection fields.
In the field of indirect detection of scintillator, the light yield of scintillator is one of the important indexes to determine the X-ray conversion efficiency and detection contrast. A large Stokes shift and a high photoluminescence quantum yield (PLQY) are the prerequisites for high light yield. Using RbBr and CuBr as precursors, a new type of non-lead halide scintillator Rb2CuBr3 with one-dimensional crystal structure was synthesized by the cooling crystallization process. The excitation wavelength is 300nm, the emission wavelength is 385nm, the Stokes Shift is 85nm (0.91ev), and the self absorption effect is low. Its one-dimensional crystal structure and easily distorted lattice characteristics make it easy to form self-trapped excitons (STE), and the corresponding exciton binding energy is 758.9mev, which ensures the high radiation recombination efficiency. The quantum yield is 98.6%, which provides the necessary conditions for high light yield. The scintillation characteristics show that Rb2CuBr3 has the same X-ray absorption capacity as the traditional scintillator (CsI: Tl or LYSO), and the emission wavelength has a good matching with silicon photomultiplier tube (SiPM) or photomultiplier tube (PMT). Finally, the high light yield of 91056photons/MeV is achieved (as shown in Figure 1)
Figure 1. a. Crystal structure of Rb2CuBr3；b. The PLQY spectra of Rb2CuBr3 crystals；c. Radioluminescence spectrum under 30 keV X-ray excitation；d. The comparison of light yield between Rb2CuBr3 and several well-acknowledged scintillators.
In the field of direct detection, lead-based perovskite semiconductor has made significant progress in the field of X-ray detection due to its advantages of high X-ray absorption coefficient and high carrier collection efficiency, which proves the great potential of perovskite in the field of X-ray detection. However, there are still some problems in the preparation of thick films by solution process, such as solvent evaporation, small grain size, low crystallinity, poor stability of organic-inorganic hybrid perovskite and so on. One of the bottlenecks in X-ray imaging using perovskite materials is the realization of perovskite thick film with quasi single crystal and high sensitivity. The quasi single crystal ensures that the polycrystalline film has the single crystal transmission property, which is conducive to improving the carrier collection efficiency, the thick film is conducive to the full absorption of X-ray, and the high sensitivity is conducive to the realization of high contrast imaging.
In order to solve this bottleneck problem, Jiang Tang’s group proposed to melt CsPbBr3 into liquid at high temperature, disperse it on conductive glass substrate, and then the film was formed by hot-pressing using quartz. The CsPbBr3 prepared by this method has high crystallinity and uniform orientation, with a thickness of 240 μm and columnar crystal. In terms of electrical properties, CsPbBr3 thick film has carrier mobility (38 cm2v-1s-1) and μτ value (1.32 × 10-2 cm2v-1) comparable to that of single crystal. Theoretical and experimental results also show that CsPbBr3 is easy to form a shallow bromine vacancy defect in the process of thick film preparation, which is beneficial to photoconductivity gain. Subsequently, the researchers prepared the CsPbBr3 quasi single crystal thick film as a photoconductive X-ray detector, and the device sensitivity reached 55684 μC Gyair-1cm-2 @ 5.0 V mm-1 (as shown in Figure 2).
Figure 2. a. Preparation scheme of CsPbBr3 thick film by four-step hot-pressed method；b. Cross-sectional SEM image of slowly cooled CsPbBr3 thick film；c. Photocurrent-dose rate curves of the detector under different electric fields
The researchers believe that the new metal halide scintillators Rb2CuBr3 and CsPbBr3 with the above excellent performance will show great potential in the field of X-ray imaging. Two papers were published online on Advanced Materials (DOI: 10.1002/adma.201904711; Doi: 10.1002/adma.201904405). Prof. Jiang Tang and associate researcher Guangda Niu are co-corresponding authors of these two papers.
