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

我们研制了一台在超高真空中独立驱动的四探针扫描隧道显微镜，实现了纳米级的四探针电导率测量。(1)通过改变探针间距从1μm到1 mm，测量了Si(111)-7×7 CLEAN和-√&lt；3&gt；×√&lt；3&gt；-Ag表面的电阻。结果表明，当探头间距小于10μm时，表面灵敏度大大提高。从探头间距对电阻的依赖关系可以看出，7×7表面表现为三维导电，而√&lt；3&gt；×√&lt；3&gt；-Ag表面具有二维特性。(2)我们设计了一种四端呈正方形排列的正方形四点探针法来检测二维导体的各向异性。用这种方法测量了准一维金属表面Si(111)-4×1-In的电导，发现其各向异性高达70。(3)我们用四个尖端中的两个来选择和拾取单个碳纳米管，测量了它们的电导。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.