铋层状钙钛矿铁电材料的晶体生长与光电性能研究

Bismuth layer-structured ferroelectrics (BLSFs) have important application prospects in the field of photoelectric functional devices because of their unique characters such as natural superlattices, narrow band gaps and high Curie temperatures. How to modify the crystallization habits of the BLSFs to obtain bulk crystals is the key issue determining their applications in photoelectric domains. By choosing Bi2WO6 and Bi4Ti3O12 as the research objects, this project aims at developing high temperature solution growth technology to grow bulk crystals of the BLSFs. First, methods overcoming the flake growth habit of the BLSFs will be explored through investigating the growth morphology and microcosmic mechanism in different candidate solvents. Temperature shocking and local cooling methods will be adopted to control the number and position of spontaneous nucleation and then, gradually expand dimensions of the crystals. Second, top-seeded solution growth method will be used to grow centimeter-sized crystals by optimizing the growth parameters including thermal field, temperature gradient and rotating velocity. On the basis of grown crystals of BLSFs, further investigations on the crystal structure, domain states, optical and electrical anisotropy, photovoltaic effects etc., will be carried out. The internal relationship and regulations among the photoelectric properties, microscopic structure and ferroelectricity will be explored as well. The current project will be of great significance to lay a foundation for the practical applications of the BLSFs in optoelectronic devices.

铋层状钙钛矿铁电体因其天然的超晶格结构、较小的禁带宽度和较高的居里温度，在光电功能器件领域具有重要的应用前景。如何改善该类材料的结晶习性获得较大尺寸晶体是推进其光电应用的关键科学问题。本项目选取Bi2WO6和Bi4Ti3O12为研究对象，根据晶体特性，采用助熔剂法探索晶体生长工艺，研究不同助熔剂环境下晶体的生长形态和微观生长机制，探求改善晶体片状生长习性的方法和途径。利用温度震荡法和局部冷却法控制晶体成核数量和位置逐步扩大晶体尺寸。采用顶部籽晶法，通过优化生长温度、温度梯度、旋转速率等生长工艺参数，生长厘米级尺寸单晶。进而表征其晶体结构、电畴状态、光学和电学各向异性及光伏性能，深入探索晶体光电性能与微观结构及铁电性的内在关联及调控规律，为此类材料在光电器件中的实际应用奠定基础。

铁电材料是一类重要的信息功能材料，具有优异的介电、铁电、压电和热释电特性，而且具有重要的非线性光学、电光和光折变等效应。近些年来，随着新能源和太阳能利用方面需求的增长，铁电材料所表现出来的光电特性逐渐引起人们越来越多的关注。层状钙钛矿铁电晶体具有较小的禁带宽度、大的自发极化强度、丰富畴壁结构及良好的电学各向异性，使它在光驱动器、光传感器、光探测器及光电多功能复合光伏器件等领域表现出潜在的应用前景。如何改善晶体的结晶习性获得较大尺寸晶体是推进其光电应用的关键科学问题。本项目根据铋层状钙钛矿铁电晶体的生长特性，采用助熔剂法探索晶体生长工艺，研究不同助熔剂环境下晶体的生长形态和微观生长机制，成功克服了其层状生长习性，生长出了片状Bi2WO6和Bi4Ti3O12晶体。在此基础上，研究了铁电单晶样品的光学、电学和光电性能，发现此类晶体具有良好的非线性光学性能、丰富的电学特性和优异的光电响应，且具有宽的光谱响应范围、极快的光电响应速率(<0.1s)和大的开关转换比(>1000)，揭示了晶体光电性能与微观结构及铁电性的内在关联及调控规律。此外，还首次在中心对称BiVO4材料中发现了反常光伏效应，观察到了远超过带隙的光伏电压和稳态的光伏电流。相关研究成果对于拓展铁性材料在非线性光学、电学和光电领域的应用具有重要意义。