Development of a four-tip scanning tunneling microscope and study on surface electrical conduction at mamometer scales

我们研制了一种独立驱动的四探针扫描隧道显微镜，它可以在纳米尺度上进行四探针电导率测量。(1)我们测量了Si（111）-7 7清洁表面和Si（111）<3><3>× Ag清洁表面的电阻，探针间距从1μm到1 mm。结果表明，将探针间距减小到10μm以内，可大大提高电阻测量的表面灵敏度。从电阻与探针间距的关系可以看出，7×7表面具有三维导电性，而10 <3>× 10<3>-Ag表面具有二维导电性. (2)我们设计了一种探测二维导体各向异性的“正方形四探针法”，其中四个探针呈正方形排列。用这种方法，我们测量了准一维金属表面Si（111）-4 1-In的电导率，发现其各向异性高达70。(3)我们已经测量了单个碳纳米管的电导，通过选择和拾取它们的四个尖端中的两个。结果显示了电导的一个有趣的特征。

We have developed an independently driven four-tip scanning tunneling microscope in ultrahigh vacuum, which enables four-probe electrical conductivity measurements at nanometer scales.(1) We have measured the electrical resistance of Si(111)-7×7 clean and -√<3>×√<3>-Ag surfaces by changing the probe spacing from 1μm to 1mm. It has turned out that the surface sensitivity in resistance measurements greatly increases by reducing the probe spacing less than 10μm. From the probe-spacing dependence of the resistance, it is found that the 7×7 surface exhibits a three-dimensional electrical conduction, while the √<3>×√<3>-Ag surface has a two-dimensional character.(2) We have devised a 'square-four-point probe method' to detect the anisotropy in two-dimensional conductors, in which the four tips are arranged in square. With this method, we have measured the conductivity of a quasi-one-dimensional metallic surface, Si(111)-4×1-In, revealing that the anisotropy is as large as 70.(3) We have measured the conductance of single carbon nanotubes, by selecting and picking up them by two of the four tips. The results show an interesting feature of conductance.