Study On The Photophysical Properties Of All-inorganic Halide Perovskite Hexagonal Nanocrystals

In recent years,halide perovskite materials have demonstrated excellent promise in photovoltaic and luminescent applications.Halide perovskite is a promising optoelectronic material that has received extensive attention and researches.Halide perovskite materials have the advantages of high absorption coefficients,tunable band gaps,narrow emission wavelength,high fluorescence quantum yields,high carrier mobility and fewer defect states.Halide perovskite materials include organic-inorganic hybrid and all-inorganic halide perovskite materials,where the poor stability of organic-inorganic hybrid perovskite becomes a key issue,while all-inorganic halide perovskites such as CsPbBr₃ exhibit better stability and show great potential in various optoelectronic applications.CsPbBr₃ perovskite materials include nanorods,spherical quantum dots,cube nanocrystals and other different morphologies,the properties of which can vary greatly depending on the morphology,and the differences in properties can have an impact on their applications.Therefore,it is necessary to study the influence of morphology on the properties of perovskite materials.In this paper,we have synthesized CsPbBr₃ cubic nanocrystals and CsPbBr₃nanocrystals of hexagonal morphologies combining thermal injection and ultrasonic-assisted synthesis of water-oil interface transformation method.Using CsPbBr₃ cubic nanocrystals as a reference,in order to investigate the properties of CsPbBr₃hexagonal nanocrystals,the following studies were carried out:1.Transmission electron microscopy experiments are carried out on the two morphological samples and the images show that the materials are uniform in size and evenly distributed.The X-ray diffraction experiments are carried out and the results are compared with standard cards,and both samples are found to be in the orthogonal phase structure.2.Steady-state absorption and steady-state emission spectroscopy experiments show that the absorption and emission peaks of the CsPbBr₃ nanocrystals with different morphologies are similar.CsPbBr₃ hexagonal nanocrystals have a narrower full width at half maxima of the emission peak,which have better monochromatic properties.Variable-temperature steady-state emission experiments reveal that the exciton activation energies of CsPbBr₃ hexagonal nanocrystals and cubic nanocrystals are all greater than the thermal interference energy at room temperature.The longitudinal optical phonon energy of CsPbBr₃ hexagonal nanocrystals was 13.9 me V,which is smaller than that of cubic nanocrystals at 17.8 me V,indicating that the hot carriers in CsPbBr₃ hexagonal nanocrystals cool more slowly.3.Through the transient absorption,the carrier kinetic characteristics of CsPbBr₃hexagonal nanocrystals are analysed at different photon fluxes and different temperatures and compared with CsPbBr₃ cubic nanocrystals.The increasing of photon fluxes and the decreasing of temperatures speed up the trap state capture and electron-hole recombination processes and slow down the hot carrier cooling process.Compared to CsPbBr₃ cubic nanocrystals,CsPbBr₃ hexagonal nanocrystals have longer hot carrier cooling time and shorter trap state capture time and electron-hole complexation time,thanks to the larger absorption cross section and fewer defect states in CsPbBr₃ hexagonal nanocrystals.4.The circular polarized transient absorption study of CsPbBr₃ hexagonal nanocrystals is carried out.Two kinetic curves are obtained when the pump light and the probe light are co-circular and counter-circular polarized,and the new curve obtained by differencing the two curves is fitted to obtain the spin relaxation life.CsPbBr₃ hexagonal nanocrystals have a longer spin relaxation lifetime than CsPbBr₃cubic nanocrystals.5.The space-dependent PL tests of CsPbBr₃ hexagonal nanocrystals and CsPbBr₃ cubic nanocrystals colloidal solutions in optical fiber microcavities have been carried out,which reveals significant optical waveguide phenomena.The better optical waveguide performance of CsPbBr₃ hexagonal nanocrystals compared to CsPbBr₃ cubic nanocrystals is attributed to the higher fluorescence quantum yield of CsPbBr₃ hexagonal nanocrystals.