II-VI族轴向线/线型异质结纳米线的液相催化制备及界面结构的“蓄水池效应”研究

Axial heterojunctions of semiconductor nanowires exhibit unique band structures and optoelectronic performance, as well as wide applications in nanodevices. It has been shown that catalyst particles, remaining at the end of as-grown nanowires, are capable of continuing to catalyze the growth of other kinds of semiconductor nanowires and are effective for the atomic incorporation and lattice connection of the newly-grown nanowire with the former one along their axial orientation, thus leading to various axial wire/wire semiconductor heterojunctions. Instead of the traditional metal catalysts such as Au and Bi, our project will employ silver chalcogenides, i.e., Ag2SxSe1-x (x=0~1), as novel catalysts, and on the basis of solution-phase catalytic growth strategy, systematically conduct the design and controllable synthesis of several representative axial wire/wire-type heterojunction nanowires composed of group II-VI semiconductors of ZnS, CdS, ZnSe and CdSe. By regulating and altering the solubility of II-VI group compounds in the catalysts through the composition variations of catalytic particles (that is, S/Se ratios), which is called “Reservoir Effect” in the literature, this project is going to pay intensive attention to investigating and controlling the microscopic structural characteristics at wire/wire heterointerfaces, including transition region width, chemical composition switching and lattice structures, and further ascertain the dependency relationship of these structural features with the catalyst compositions (Ag2SxSe1-x, x=0~1). In the meanwhile, the optical properties, such as UV-Vis absorption and fluorescence excitation and emission spectra, of the as-obtained heterojunction nanowires will be measured, evaluated, and assigned to the materials components and electronic band structures after careful analyses.

纳米线基半导体轴向异质结具有独特的能带结构和光电性能以及器件方面的广泛应用。研究表明，保留在纳米线末端的催化剂颗粒，能够继续催化生长另一种半导体纳米线并使之与原有纳米线实现原子交融和晶格连接，易于形成轴向线/线异质结。不同于传统Au、Bi等金属催化剂的是，本课题拟选取银基硫属化合物Ag2SxSe1-x（x=0~1）为催化剂，基于溶液催化生长机理，设计并可控制备几种典型的由ZnS、CdS、ZnSe、CdSe等II-VI族半导体组成的轴向线/线型异质结纳米线。注重通过催化剂颗粒自身S/Se比例的差异，调节和改变II-VI族化合物在催化剂中的溶解度（即“蓄水池效应”），考察和控制线/线异质界面的微观结构特性，包括过渡区宽度、成分转变和晶格结构，探明这些结构特性与催化剂化学组成的依赖关系。同时，评估异质结纳米线的紫外可见吸收、荧光激发/发射等光谱学特征，分析并澄清与材料组成、能带结构的相互关联。

一维半导体纳米线/棒的可控制备、生长机理及性能研究是纳米科技领域十分活跃的研究课题之一。在单根纳米线上进行组分加工和异质结构建，如p-n结，type-I和type-II型半导体结等，具有广阔的物理和器件应用前景。本项目设计的思路是，通过催化剂颗粒连续催化生长两种半导体纳米线，同时在催化剂内部实现二者的界面连接，进而获得由两种（如II-VI族半导体）材料构成的轴向线/线型异质结纳米线。课题具有较强的创新性、前沿性，但也具有挑战和难度。同时，本项目以探索、发现具有普遍意义和推广价值的半导体纳米线/棒的制备路线，实现无机功能（纳米）材料的精准调控制备，发展无机固体与纳米合成化学为目标。项目执行中主要开展的研究内容包括：基于催化机理的II-VI族纳米线、轴向线/线型异质结纳米的线制备及催化剂/纳米线界面、线/线界面的结构研究，基于晶体各向异性结构驱使的V2-VI3族结晶纳米棒的制备、生长机理及光电性能研究，以及基于模板效应和阳离子交换反应的纳米棒/棒转化的研究。. 主要研究结果如下：1）以Ag2Se为催化剂，获得了ZnSe/CdSe轴向异质结纳米线（产率10-20%），HRTEM揭示了界面的ZB(111)晶面的外延连接生长习性；2）利用液相催化生长机理，成功制备出II-VI族纳米线/棒及其超细纳米线(~5.5 nm ZnSe、7.8 nm CdSe、8.4 nm CdS)；3）利用原位加热高分辩透射电子显微镜(in situ heating HRTEM)，首次观察和证实Ag2Se/ZnSe催化剂/纳米线界面倾向于由BCC-Ag2Se(110)/FCC-ZnSe(111)晶面构成；4）制备出Bi2S3、(SbxBi1-x)2S3、Sb2S3、Sb2Se3等V2-VI3族纳米棒，阐明了它们的各向异性生长机理，评估了光吸收、光电探测性能；5）实现了Bi2S3-AgBiS2纳米棒/纳米棒的化学转化，合理的揭示了形貌保持和阳离子扩散/交换反应的动力学、热力学因素。