Construction of an experimental apparatus for atom transfer between nano-regions

近年来，扫描隧道显微镜中的原子转移引起了人们的极大兴趣。然而，由于实验条件并不总是恒定的，STM中原子转移的机制尚未被揭示，针尖与样品之间邻近区域的感应强场和通过电流受针尖尖端的原子结构的影响。在此基础上，我们构建了一种新型的超高压扫描隧道显微镜/有限元分析系统，可以承受10KV的高压，并将扫描隧道显微镜头部的尖端温度提高到1500ﾟC。该设计使得利用热场蒸发来清洁和堆积STM针尖成为可能。可以使用三维机械工作台将尖端定位在表面上，该工作台由惯性压电螺丝以30 nm的步长驱动。可以很容易地切换STM和FE操作模式；针尖可以对准邻近样品的STM样品支架的锥形小孔，然后从针尖发射的场发射电子的图案可以投射到样品后面的屏幕上。我们用STM/STS方法研究了Si(111)7x7，采用[111]取向的W针尖，这种针尖是在正高压下加热形成的。STS测量从-3到3V扫描导致了不稳定的隧穿光谱。在STS测量后，有限元图像立即显示，尖端急剧变化，中心变得更暗。这一结果表明，在扫描电压作用下，Si原子转移到了尖端，并且Si在尖端的吸附增加了尖端的功函数。这是STS测量中Si原子转移的证据。

Recently, atom transfer in STM has attracted much interest. However, the mechanism of atom transfer in STM has not been revealed, because the experimental conditions are not always constant ; the induced high field and passing current in the proximity region between the tip and the sample are affected by the atomic structure of the tip apex. Then we have constructed a new UHV-STM/FEM with a tolerance of applying a high voltage of 10kV and increasing the tip temperature to 1500ﾟC in the STM head. The design makes possible the cleaning and build-up of the STM tip utilizing thermal field evaporation. The tip can be positioned on the surface using a 3-dimensional mechanical stage, which driven by inertia-piezomotor screws with a 30nm step. The STM and FEM operation modes can be easily switched ; the tip can be aligned to a tapered small hole of the STM sample holder, adjacent to the sample, and then the pattern of field emitted electrons from the tip can be projected on the screen behind the sample. Thus the tip apex can be evaluated before and after the experiments of atom transfer.We have studied Si (111)7x7 by STM/STS using a [111]-oriented W tip, which was built-up by heating it while applying a positive high voltage. STS measurements sweeping from-3 to 3 V resulted in unstable tunneling spectra. Immediately after the STS measurements, FEM images showed that the apex changed drastically to be darker at the center. This result indicates that the Si atoms were transferred to the tip apex by the sweeping voltages and the adsorption of Si on the tip apex increased the workfunction of the apex. This is the evidence of Si atom transfer in the STS measurements.