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

1.低温四探针STM（绿色功能STM）的设计和构造：我们已经构造了显微镜，具有使机器在世界范围内独一无二的以下特征。(a)它在超高真空中工作，借助扫描电子显微镜，能够以纳米精度定位四个尖端。(b)它冷却样品并将其保持20小时降至7 K。(c)在STM中，每个针尖都能达到原子级的分辨率。(d)它通过一个PC控制器以系统的方式驱动四个尖端。（e）它的多功能驱动器可以实现三种测量模式，通常的STM/S，四点探针电导率测量和双尖端STS（绿色功能）模式。金属覆盖的碳纳米管尖端的开发：我们已经成功地制造了直径为10 nm的导电碳纳米管尖端，通过用PtIr薄层涂覆它们。这使得四探针测量中的最小针尖间距可达20 nm左右，这是非常小的 关于我们 r比通常的金属提示。当碳纳米管尖端被铁磁（NiFe）薄层覆盖时，尖端变成铁磁尖端，这对于磁力显微镜和自旋极化STM是有用的。应用：我们已经开始使用低温四尖STM进行各种测量。我们已经测量了一个薄的CoSi 2纳米线的电阻约40 nm的直径在室温下。结果表明，在20 nm的探针间距的导电仍然是扩散。测量了准一维金属表面Si（111）-4 × 1-In的电导率随温度的变化关系。结果表明，载流子在链向和链间的传导机制不同。沿着链的输运表现为Peierls跃迁引起的金属-绝缘体转变，而沿链的输运则是由于Si衬底的作用。我们现在试图获得两个尖端之间的一个信号，这是为了测量绿色的功能。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