The Melting Growth And Properties Of CsPbBr₃ Single Crystals For Room Temperature Nuclear Radiation Detection Use

As a typical representative of all-inorganic lead halide perovskite materials,Cesium lead bromine?CsPbBr₃?has been widely studied in the field of optoelectronic application in recent years because of the remarkably high carrier mobility and large diffusion length.Compared to the organic-inorganic hybrid halide perovskites,CsPbBr₃ not only has the similar optoelectronic performance and application fields,but also has a better stability.However,the overwhelming majority studies of CsPbBr₃ perovskite devices are focusing on polycrystalline thin films,nanocrystals,nanowires,nano-sheet and nano-cures,therefore,the potential optoelectronic applications of CsPbBr₃ devices are markedly limited.Furthermore,the CsPbBr₃ single crystals had the better optoelectronic properties.This dissertation mainly studied the powder synthesis,the crystal growth,the intrinsic point defects analysis,and the fabrication and characterization of CsPbBr₃ photoelectric detectors.The main content was as follow:Firstly,the synthesisofCsPbBr₃ polycrystallinepowdersbychemical co-precipitation method and high-temperature solid sintering method was investigated.And the optimum synthesis conditions were obtained by characterizing the phase,the elemental ratio and the particle size of the synthesized polycrystalline powders.Furthermore,the basic physical and chemical properties of CsPbBr₃ polycrystalline powders were characterized.The results of property characterization showed that:the melting point and the solidifying point were 567°C and 550°C,respectively.The band-gap was 2.252 eV,with the valence band maximum?VBM?and the conduction band maximum?CBM?locating at-6.36 eV and-4.11 eV,the work function was 5.32 eV.The crystal structure transforms from room temperature orthorhombic phase?Pbnm,equal to Pnma?to tetragonal phase?P4/mbm?at 88°C and then to cubic phase?Pm-3m?at 130°C.This part of investigation built the foundation for subsequent crystal growth,defect analysis and device fabrication.Secondly,the growth of CsPbBr₃ single crystal was conducted by a creative electronic dynamic gradient?EDG?method.The crystal structure and the elemental distribution in the crystal were characterized emphatically.The optimum crystal growth parameters of EDG method were obtained by characterizing integrity,transparency,growth direction and optical properties of the crystals.The crystal structure was investigated systematically by X-ray diffraction techniques and electron diffraction techniques.X-ray diffraction techniques were used to identify phase and investigate phase transition rules,and electron diffraction techniques were used to investigate crystal micro-structure.The results indicated that the grown CsPbBr₃ crystals were of pure room-temperature orthorhombic phase with high crystal quality.The elemental distribution along the axial and the radial in the crystals was studied by X-ray fluorescence?XRF?,energy dispersive spectrometer?EDS?,X-ray photo-electron spectroscopy?XPS?,respectively.Thirdly,intrinsic point defects in melt-grown CsPbBr₃ crystals were studied systematically.The thermal activation energy?E_T?,signature concentration?N_T?,and capture cross section??_n?of intrinsic point defects in CsPbBr₃ crystal were obtained by thermally stimulated current?TSC?technology and simultaneous multiple peak analysis?SIMPA?method.The types of defects were determined by combing the thermal activation energy with the first-principle calculations and the macroscopic performance.The results showed that:the total concentration of defects evidently increased along the grown direction from the tip to the tail in a same crystal,i.e.3.84×10¹44 cm^-33 for the tip,8.18×10¹44 cm^-33 for the middle,and 2.35×10¹55 cm^-33 for the tail,respectively.The total concentration of the defects had an obvious change in different stoichiometric?1%Pb-poor,stoichiometric,1%Pb-rich?crystals,i.e.8.18×10¹44 cm^-33 for 1%Pb-poor crystal,3.84×10¹44 cm^-33 for stoichiometric crystal,1.57×10¹77 cm^-33 for 1%Pb-rich crystals,respectively.Furthermore,the total defect types and the major defect types also changed in different situations.The systematic description and quantitative characterization of intrinsic defects in melt-grown CsPbBr₃ crystals were realized.Based on the study of intrinsic point defects,the element doping scheme for crystal performance improvement was proposed by combining with the first-principle calculations.Lastly,the optical and electrical properties of CsPbBr₃ single crystals,the fabrication and property of CsPbBr₃ photoelectric detectors were studied.Simple planar CsPbBr₃photoelectric detectors were fabricated with single crystal,and the detection response of CsPbBr₃ photoelectric detectors to different signals?325 nm pulsed laser,X-ray,gamma ray?were characterized.The results of characterization showed that CsPbBr₃ crystal had excellent optical properties with the average infrared transmittance over 70%at whole spectrum,average UV-vis transmittance over 50%,and the PL emission peak at 550 nm with a FWHM of 37 nm.The volt-current characteristic curve showed that a good ohmic contact formed between the Ag electrodes and wafer surfaces,and the crystal calculated resistivity was in the range of 10⁹¹0¹⁰·cm.I-t curves showed that the crystals had obvious polarization effect.The impedance characteristic curve showed that the relative dielectric constant was about 30 and the crystal resistivity was only 10⁶¹0⁷·cm at high frequency,which was 3⁴ orders of magnitude lower than that under the DC bias voltage.This phenomenon can be attributed to the strong ion-induced polarization effect in crystals.CsPbBr₃ photoelectric detectors had obvious detection response to 325 nm pulsed laser.CsPbBr₃ photoelectric detectors had a respond signal to noise was over 3⁵ and faster response?0.105 s?and recovery speed?0.181 s?.CsPbBr₃ photoelectric detectors had sensitive detection ability and response speed to X-rays,and the response sensitivity to different parts of a same crystal?tip,middle,and tail?was 948,846 and 738?C Gy^-1_airir cm^-2,respectively.However,CsPbBr₃ photoelectric detectors had a weak detection response to?-ray(²⁴¹Am,59.54 keV)radioactive sources,which can only be observed from the high-frequency oscilloscope and cannot be collected the distinguishable multichannel spectrum.This can mainly be attributed to the low resistivity of the grown crystals,and it might also be attributed to the low radiant energy and low radiation dose of²⁴¹Am radioactive sources.