Dimensional Control And Application Of All-inorganic Perovskite

All-inorganic perovskite,the Cs-Pb-Br system in specific,has become popular materials in many advanced fields,including low-threshold laser,solar cell,flexible display and even quantum communication.The[PbBr6]^4-octahedron is the basic structural unit,and its connection modes determine the dimension of perovskite.For example,the zero-dimensional（0D）Cs₄PbBr₆ and the three-dimensional（3D）CsPbBr₃ are the two extreme cases in terms of dimensionality,where in the former case octahedrons are disconnected with each other,in any dimension while in the latter case octahedrons are connecting with each other in all three dimensions.Although the structure is quite different,the two perovskites can be transformed into each other,and the synthesis methods are almost identical,which is easy to control the dimensionality.Interestingly,Cs₄PbBr₆ has a band gap of～3.9 e V due to the non-coupling of octahedral electrons,but actually emits green light（～2.4 e V）similar to CsPbBr₃.Hence,the mystery of the photoluminescence（PL）of 0D perovskites quickly aroused intense debate,where CsPbBr₃ played a key role in explaining the PL mechanism.In addition,radiation detection that converts high-energy photons into low-energy photons（X-ray imaging）or electric charge（X-ray detection）is essential for medical diagnosis,computed tomography and other applications.CsPbBr₃,which has excellent scintillation properties due to its chemical composition containing heavy atoms such as Cs and Pb,gradually showed its obvious advantages in the field of X-ray imaging and detection.In this thesis,two perovskites including Cs₄PbBr₆ and CsPbBr₃,and their optical properties are the main focus of the research.The PL mechanism of Cs₄PbBr₆ was discussed in Chapter 1,and new evidence for the origin of luminescence was proposed in Chapter 2 and 3;through the control of self-assembly behavior of CsPbBr₃ nanosheets,high-resolution X-ray imaging based on the CsPbBr₃ scintillation screen was realized in Chapter 4 and 5.The research content was detailed as follows:First,the controversy over the origin of fluorescence of 0D perovskites was partially due to the lack of real-time observation during crystal formation.In Chapter 2,by using a home-made in-situ observation system combined with a fluorescence microscope,the growth of crystals and the generation of emitting centers could be tracked in real time.After in-situ experiments,the cooling rate was the key factor to control the density of the luminescence center,and there was a delay effect in the generation of the luminescence center;the luminescence center was found distributing along specific direction of the c axis.The condition of ultra-slow cooling rate was designed by the hydrothermal method,and the full-body emitting crystals with photoluminescence quantum yield（PL QY）up to 83%was obtained.Second,although the novel properties of the crystal emitting center were found by in-situ observation of the crystallization process of Cs₄PbBr₆,it was still unable to learn its true PL origin.In Chapter 3,the theoretical analysis,and experiment were combined to explore the exciton structure of emission.The use of advanced characterization technology,such as neutron diffraction,resonance Raman technology and et al.,to eliminate the possibility of CsPbBr₃impurities and Br vacancy defects.The Johnson-Mehl-Avrami（JMA）model was used to simulate the evolution of the PL spectra.According to the linear relationship between radiation lifetime and temperature,the property of exciton was determined.According to the Rosales model,we proposed that the nature of the luminescence center was a two-dimensional phase of disordered Csn+1Pbn Br₃n+1.Although the direct evidence of emitter phase remains elusive,this work provided many new insights into the PL origin of Cs₄PbBr₆.Third,to solve the mystery of the luminescence of the 0D perovskite,the 3D perovskite CsPbBr₃ has to be taken seriously.In Chapter 4,the basic research on CsPbBr₃ has matured day by day,whereupon scientists are increasingly concerned that some of its special properties to use.Therefore,this work mainly reported the self-assembly of CsPbBr₃ nanosheets into a film for X-ray detection/imaging.An improved co-precipitation method was used to obtain nanosheet colloidal solution with a PL QY of more than 90%.A flat and crack-free large-area film used for scintillation imaging screen was prepared by simple drop-coating technique.The external PL QY of the scintillation screen reached 75%,which was amongst the highest reported values.The self-assembly characteristics of nanosheets ensured the excellent performance of the scintillation screen,and the dynamic process of self-assembly was analyzed through real-time XRD measurement.Fourth,the excellent performance of the CsPbBr₃ scintillation screen promised its practical use in high-resolution X-ray imaging field.In Chapter 5,time-resolved spectroscopy measurement showed that there was a F（?）rster resonance energy transfer effect（FRET）with efficiency up to 81%between the self-assembled nanosheets.The high PL QY and X-ray stability could be attributed to the FRET effect.The detection limit of the scintillation screen（0.05μGy/s）was only 1%of that required for routine medical testing.Importantly,the screen with an optimal thickness of 15μm exhibited unprecedented spatial resolution（26μm）when used for X-ray photography,and its performance was improved by an order of magnitude.In Chapter 6,the conclusion of this thesis was briefly summarized,and the future direction of all-inorganic perovskite was projected.