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

1. 低温四尖STM（格林函数STM）的设计和构造：我们已经构造了显微镜，具有以下特点，使机器在世界范围内独一无二。(a)在超高真空环境下工作，借助扫描电子显微镜，使四个针尖的定位达到纳米级精度。(b)将样品冷却，并保持20小时，使样品降至7 K。(c)每个尖端在STM中达到原子分辨率。它的多功能前置放大器支持三种模式的测量，即通常的STM/S，四点探头电导率测量和双尖端STS（格林函数）模式。金属覆盖碳纳米管尖端的发展：我们已经成功地制造了直径为10纳米的导电碳纳米管尖端，并在其表面涂上了PtIr薄层。这使得四点探针测量的最小尖端间距减小到约20nm，这比通常的金属尖端要小得多。当碳纳米管尖端被铁磁薄层覆盖时，尖端成为铁磁尖端，可用于磁力显微镜和自旋极化STM.3。应用：我们已经开始使用低温四尖端STM进行各种测量。我们测量了直径约为40纳米的细CoSi2纳米线在室温下的电阻。结果表明，在探针间距为20nm时，导电仍然是扩散的。测量了准一维金属表面Si(111)-4 × 1- in的电导率与温度的关系。结果表明，链向和链间方向的载流子传导机制不同。沿链的输运表现为金属-绝缘体的佩尔斯转变，而跨链的输运则是由于Si衬底。我们现在正在尝试获得两个针尖之间的跨导信号，目的是测量格林函数。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