Development of Green's Function STM and Application to Study of Nano Electronic Transport Dynamics

1.低温四尖扫描隧道显微镜(Green‘s Function STM)的设计与制作：我们已经建造了这台显微镜，它具有以下特点，使这台机器在世界范围内独一无二。(A)它工作在超高真空中，借助扫描电子显微镜，能够以纳米级的精度定位四个尖端。(B)将样品冷却并倾倒至7K，保存20小时。(C)通过每个尖端达到STM中的原子分辨率。(D)通过一台PC控制器系统地驱动四个针尖(E)其多功能前置放大器支持三种测量模式，即通常的STM/S、四点探头电导率测量和双针尖STS(格林函数)模式。金属包覆碳纳米管尖的研制：我们通过在碳纳米管尖上涂覆一层PTIR薄膜，成功地制备出直径为10 nm的导电碳纳米管尖端。这使得四点探针测量中的最小针尖间距降到了大约20 nm，这远远小于…比通常的金属尖端的R更大。当碳纳米管尖端被一层铁磁性(NiFe)薄层覆盖时，尖端就变成了铁磁性尖端，这对磁力显微镜和自旋极化STM.应用：我们已经开始使用低温四尖扫描隧道显微镜进行各种测量。我们测量了直径约40 nm的CoSi2纳米线在室温下的电阻。结果表明，在探头间距为20 nm时，导电性仍然是扩散的。测量了准一维金属表面Si(111)-4x1-In的电导率与温度的关系。结果表明，载流子在链方向和链间方向的导电机理不同。链上的输运表现为金属-绝缘体的Peierls转变，而跨链的输运是由Si衬底引起的。我们现在正在尝试获得两个尖端之间的跨导信号，目的是测量格林函数。较少

1. Designing and Construction of a low-temperature four-tip STM (Green's function STM): We have constructed the microscope, having the following features which make the machine world-wide unique. (a) It works in ultra-high vacuum, with aid of scanning electron microscope, enabling positioning the four tips with nanometer precision. (b) It cools the sample and tips down to 7 K with keeping them for 20 hours. (c) It attains atomic-resolution in STM by each tip. (d) It drives the four tips in systematic ways by a single PC controller (e) Its multi-functional preamplifier enables three modes of measurements, usual STM/S, four-point-probe conductivity measurements, and double-tip STS (Green's function) modes.2. Development of metal-covered carbon nanotube tips: We have succeeded in fabricating conductive carbon nanotube tips of 10 nm in diameter, by coating them with PtIr thin layers. This makes the minimum tip spacing in four-point-probe measurement down to about 20 nm, which is far smalle … More r than by usual metal tips. When the carbon nanotube tips are covered by a ferromagnetic (NiFe) thin layer, the tip becomes a ferromagnetic tip which is useful for a magnetic force microscope and spin-polarized STM.3. Applications: We have started various measurements by using the low-temperature four-tip STM. We have measured the electrical resistance of a thin CoSi2 nanowire of about 40 nm in diameter at room temperature. The results show that the electrical conduction is still diffusive at 20 nm probe spacing. The temperature dependence of conductivity of a quasi-one-dimensional metallic surface, Si(111)-4x1-In, was measured. The results show that the carrier conduction mechanism is different between the chain-direction and inter-chain direction. The transport along the chains shows a metal-insulator transition by Peierls transition while that across the chains is due to the Si substrate. We are now trying to obtain a signal of transconductance between two tips, which is aiming the measurement of Green's function. Less