Phase Transformation Of CsBr-PbBr₂ System And Synthesis And Optical Properties Of CsPbBr₃ Naonowires

The rise of halide perovskites as light harvesters has stunned the photovoltaic community.These perovskites can harvest the light very efficiently because their outstanding transport properties,highly optical absorption and tolerant to the material’s defects.The all-inorganic perovskite materials,developed in the last two years,display unique superiority in the luminescent devices.As going for application,further researches on the controllable preparation and comprehensive understanding of luminescence mechanism are urgently needed.For example,the inorganic perovskites of CsPbBr₃ synthesized by the solution method are often suffered from by-products such as Cs₄PbBr₆ and CSPb₂Br₅,thus understanding the formation mechanism of these three compounds is the prerequisite to fabricate a high performance device.Additionally,the nanocrystal is considered to be one of the excellent candidates for the next generation light source,due to its high radiation transition rate from the confined exciton,but the exactly underlying mechanism is still unclear.In this context,we carried out the systematic investigation in this dissertation,including the controllable preparation of CsPbBr₃ bulk crystals and nanostructures,the transformation and regulation between different perovskite crystal phases,and the dark-bright exciton energy splitting in CsPbBr₃ nanowires.The contents of the dissertation are outlined as follows.In chapter one,we first summarized the crystal structure,electronic structure,optical properties,synthesis methods,and optoelectronic device applications of halide perovskites.We then briefly introduced the motivation and main results of the dissertation.In chapter two,we have investigated thoroughly the solvent-related effect on the formation of CsPbBr₃,Cs₄PbBr₆ and CSPb₂Br₅.Based on the EXAFS of Pb L-Il edge in precursor solutions and single crystal X-ray diffraction measurement on the resultants,we identified that the coordination numbers of a plumbite oligomers in precursor solutions are the same as those of the corresponding resultants.The finding suggests that the coordination number of the product is controlled dominantly by the solvent.In addition,the phase transitions from Cs₄PbBr₆ to CsPbBr₃,amorphous state and CSPb₂Br₅ triggered by water vapor were also observed clearly.In chapter three,we successfully prepared CsPbBr₃ nanocubes,nanosheets,and nanowires through tuning the reaction temperature.More importantly,the ultra-fine nanowires with various diameters have be obtained for the first time.The results indicated that the reaction temperature is a decisive factor to determine the morphology and dimension of produced nanocrystals by adjusting the crystal phase of nucleation seeding（orthorhombic phase,tetragonal phase,cubic phase）.At the same time,we studied the roles of temperature and ligands in the growth of the nanowires.Finally,we estimated the bandgap and exciton binding energy of CsPbBr₃ nanowires through their photoluminescence peaks and the calculation with one dimensional infinite potential well model.In chapter four,we investigated the Rashba effect on dark-brig t exciton energy splitting in CsPbBr₃ nanowires.It was found that the strain has a great effect on the dark-bright exciton energy splitting,in which the generated（or relaxed）strain can be regulated by the rapid cooling down（or heating up）of temperature.The result implied that the increased strain can weaken Rashba effect and therefore enlarges the dark-bright exciton energy splitting.As a consequence,the photoluminescence quantum yield（PLQY）of CsPbBr₃ nanowires reduces.It is vice versa for the decreasing strain.Further analysis from the theoretical model demonstrates that,for the weak Rashba case,except for the decreasing of bright-dark exciton energy splitting,the wave function of bright exciton can be mixed with a small amount of the wave function from dark state,leading to the increase of fluorescence lifetime.On the contrast,for the strong Rashba case,the wave functions of dark state will be mixed with some wave function of bright state,resulting in the lowest level of exciton becomes ’lighten’.In chapter five,we presented briefly the current challenges of the inorganic perovskites and outlook for the ongoing work.