Nanoscience Studies of Carbon Nanotubes and Bismuth Nanowires

该项目致力于碳纳米管和铋纳米线的纳米科学研究。对碳纳米管的研究受到Dresselhaus小组最近对孤立的单壁碳纳米管的拉曼光谱的观察的推动。这使得研究纳米管的振动结构成为可能，并通过共振拉曼效应提供了有关电子结构的详细信息。它还通过电子水平上的三角翘曲效应，提供了对单个纳米管的结构的独特确定，包括其直径和手性，或碳六边形相对于纳米管轴的取向，以两个整数(n；m)表示。这种非破坏性地确定单个纳米管的(n；m)的能力为碳纳米管的研究开辟了新的研究领域，因为纳米管的许多物理性质以及预测和解释物理现象的理论计算都强烈依赖于这些(n；m)指数。建议的碳纳米管研究专门针对孤立的碳纳米管的共振拉曼光谱研究。这些拉曼光谱的每个已知光谱特征都将被深入研究，包括径向呼吸模式、切向G带、D带以及这些特征的泛音和组合模式。我们将在单纳米管水平上研究光谱对纳米管直径、手性、半导体与金属行为、温度和激光功率水平的依赖关系，并将其与对单壁碳纳米管束的观测和理论预测进行比较。铋纳米线系统，由于晶体铋的有效质量小、L点带隙小和带隙重叠小，为研究目前由Pi‘s小组可以制备的线径(~50 nm)纳米线上的新量子现象提供了机会。该方法将重点放在纳米线直径减小导致的半金属到半导体的转变上，并利用光学和传输技术，建立在使用与温度相关的电阻测量所进行的观察的基础上。此外，还将探索等电子的Bi(1-x)SBX合金体系中电子结构随纳米线直径和Sb浓度的变化。将特别注意导线直径和Sb浓度，其中10个不同的载流子口袋(1个T点、3个L点和6个H点)都被预测具有简并的空穴子带边缘，从而产生非常大的电子态密度。与这种异常高的态密度相关的物理现象将通过与温度相关的输运和光学性质来探索，并将使用模型计算来解释这些测量结果。%该项目解决了具有高度技术相关性的材料科学主题领域的基础研究问题。该计划的一个重要特点是通过在一个具有根本和技术意义的领域培训学生和博士后研究人员，将研究和教育结合起来。该项目通过高度协作的前沿研究环境带来的独特教育体验，帮助学生发展技术、沟通和组织/管理技能。*

This project addresses nanoscience studies of carbon nanotubes and bismuth nanowires. The research on carbon nanotubes is motivated by the Dresselhaus group's recent observation of the Raman spectrum from an isolated single wall carbon nanotube. This allows study of the vibrational structure of one nanotube and also provides detailed information on the electronic structure through the resonance Raman effect. It also provides, through the trigonal warping effect on the electronic levels, a unique determination of the structure of a single nanotube, including its diameter and chirality, or the orientation of the carbon hexagons with respect to the nanotube axis, expressed in terms of two integers (n; m). This ability to determine (n; m) for an individual nanotube non-destructively opens new research areas in carbon nanotube research, since many of the physical properties of a nanotube and the theoretical calculations which predict and explain physical phenomena are strongly dependent on these (n; m) indices. The proposed carbon nanotube studies are aimed specifically at resonance Raman spectroscopy studies of an isolated carbon nanotube. Each of the known spectral features of these Raman spectra will be studied in depth, including the radial breathing mode, the tangential G-band, the D-band, and overtones and combination modes of these features. The dependence of the spectra on nanotube diameter, chirality, semiconducting vs. metallic behavior, temperature, and laser power level will be investigated at the single nanotube level, from which comparisons will be made to observations on single wall carbon nanotube bundles and to theoretical predictions. The bismuth nanowire system, because of the small effective masses, small L-point band gap and small band overlap of crystalline bismuth, offers opportunities to study novel quantum phenomena on nanowires with wire diameters (~ 50 nm) that can presently be fabricated by the PI's group. The approach is to focus on the semimetal to semiconductor transition induced by decrease in nanowire diameter, and to use optical and transport techniques, building on the observations already made using temperature dependent resistance measurements. Also, changes in the electronic structure as a function of nanowire diameter and antimony concentration in the isoelectronic Bi(1-x) Sbx alloy system will be explored. Particular attention will be given to wire diameter and Sb concentration where 10 different carrier pockets (1 T-point, 3 L-point and 6 H-point) are all predicted to have a degenerate hole subband edge, thereby giving rise to a very large electronic density of states. Physical phenomena associated with this unusually high density of states will be explored by temperature dependent transport and optical properties and the interpretation of these measurements will be made using model calculations.%%% The project addresses basic research issues in a topical area of materials science with high technological relevance. An important feature of the program is the integration of research and education through the training of students and postdoctoral researchers in a fundamentally and technologically significant area. The project assists development of technical, communication, and organizational/management skills in students through unique educational experiences made possible by a highly collaborative forefront research environment.***