| In recent years,halide perovskites have been drawn tremendous attentions of reaserchers due to their excellent physical properties,such as low defect density,long carrier lifetime（μτ）,high carrier mobility（μ）,long diffusion length,adjustable optical band gap,and so on.However,the toxicity of lead is one of the important factors limiting the extensive application of halide perovskite materials.The bismuth-based perovskite single crystals have the same excellent photoelectric properties as lead-based perovskite materials,and they are also friendly to the environment and human body.In addition,they have many other advantages,such as low trap density,no grain boundaries,and the intrinsic performance.Therefore,the bismuth-based perovskite single crystals show broad application prospects in the field of photoelectric detection and X-ray detection.The high quality and crack-freeCs3Bi2Br9 single crystal（diameter:12 mm,length:40 mm）was grown by Bridgman method for the first time.And the crystal quality,layered structure,optical and thermal properties of Cs3Bi2Br9 single crystal were systematically investigated.In addition,we evaluated the carriers’ transportation and photoresponse performance of（120）plane for Cs3Bi2Br9 single crystal.Finally,we investigate the influence of crystals quality on（001）plane and the impact of vertical and planar devices on the performance of X-ray detection for（120）plane,respectively.The content of the paper is divided into five chapters:In the first chapter,the crystal structure,crystal growth method and application are summarized.The basis,purpose and main research contents of the paper are put forward.In the second chapter,the synthesis and purification of Cs3Bi2Br9 polycrystals and the growth of Cs3Bi2Br9 single crystals were introducted.The polycrystalline Cs3Bi2Br9 was synthesized with BiBr3 and CsBr.Finally,the high quality and dark green polycrystalline Cs3Bi2Br9 was obtained.The high quality polycrystalline Cs3Bi2Br9 was purified by directional solidification process.The results showed that the quality of Cs3Bi2Br9 crystal after two purifications was higher.In addition,the optimum shape of the quartz ampoule was obtained by optimizing the top area of ampoule used for crystallization.The effects of cooling time,temperature gradient and growth rate onCs3Bi2Br9 single crystals were determined.Firstly,we found that the optimizing cooling time was 72 h duringCs3Bi2Br9 single crystals growth.Secondly,the best temperature gradient for Cs3Bi2Br9 crystals was 15℃ cm-1.Finally,the greatest growth rate was 0.5 mm h1.Therefore,high quality and crack-free Cs3Bi2Br9 single crystal（diameter:12 mm,length:～40 mm）was successfully grown,when the cooling time was 72 h,the temperature gradient was 15℃ cm-1 and the growth rate was 0.5 mm h1.The（001）and（120）planes of Cs3Bi2Br9 single crystal were obtained by XRD.The defect component measurement showed that the secondary phase of CsBr was formed in Cs3Bi2Br9 crystal during the crystal growth,the existence of the secondary phase will seriously affect the quality of Cs3Bi2Br9 crystalIn the third chapter,the density,hardness,optical and thermal properties,stability and crystal structure of Cs3Bi2Br9 single crystals were characterized for the first time.The density of Cs38i2Br9 single crystal was about 4.74 g cm-3.The Vickers hardness of Cs3Bi2Br9 single crystal was 54.57 Kg mm-2,and the corresponding Mohs hardness value was 2.56.Cs3Bi2Br9 single crystal was stable before 500℃,and no phase transition wasoccurd from-153℃ to 400℃.The specific heat of Cs3Bi2Br9 crystal is 0.205 J g-1 k-1.When the temperature increases from 25℃ to 200℃,the thermal diffusivity of Cs3Bi2Br9 crystals along the crystallographic b axis decreases from 0.247 to 0.169 mm2 s-1,and the thermal conductivity decreased from 0.238 W m-1 k-1 to 0.163 W m-1 k-1.The thermal expansion coefficient of Cs3Bi2Br9 crystals along the crystallographic b axis was 5.22×10-5 K-1.The cut-off edge of ultraviolet absorption and PL emission peak ofCs3Bi2Br9 crystal was 481 nm,corresponding to the band gap was 2.61eV.And the composition of Cs3Bi2Br9 single crystal did not changes when it was exposed in the air for 3 months,which showed the good stability of Cs3Bi2Br9 crystal.In the fourth chapter,the electrical properties,photoresponse and X-ray detection performance of Cs3Bi2Br9 crystal were characterized.The relative dielectric constant of（120）plane for Cs3Bi2Br9 single crystal was 7.5.The trap density for Cs3Bi2Br9 single crystal was 9.7×1010 cm-3,and the carrier mobility was 0.17 cm2 V-1 s-1.The mobility lifetime product of Cs3Bi2Br9 single crystal was 3.73×10-5 cm2 V-1.The maximum resistivity was～6.8×1011 Ωcm.The optimal excitation wavelength was 430 nm,and the highest on-off ratio of（120）plane was more than 150 times in this excitation wavelength.Cs3Bi2Br9 single crystals possessed stronger attenuation coefficient and higher X-ray absorption coefficient.The X-ray detection performances ofCs3Bi2Br9 single crystals were enhanced from 69.1 μC Gy-1 cm-2 to 99.3 μC Gy-1 cm-2,and the dark current drifts decreased form 7.3×10-5 pA cm-1 s-1 V-1 to 4.1×10-5 pA cm-1 s-1 V-1,which was attributed to the quality improving of the Cs3Bi2Br9 single crystals.In addition,compared with the dark current baseline of CsPbBr3 single crystal,the dark current baseline of Cs3Bi2Br9 single crystal was no drift,indicating that Cs3Bi2Br9 single crystal possessed the characteristics of weak ion migration.In addition,X-ray detection devices with vertical and planar structures were fabricated by Cs3Bi2Br9（120）plane.The sensitivity of 181μC Gy-1 cm-2 was obtained at 100 V.However,the sensitivity of X-ray detector with vertical device structure was 8.53 μC Gy-1cm-2 at 100 V.Therefore,the planar devices of（120）plane for Cs3Bi2Br9 single crystal was higher X-ray response performance than vertical structure devices.In the fifth chapter,the main conclusions and innovations of this paper were summarized,and the future work to be carried out was put forward. |
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