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

近年来，STM中的原子转移引起了人们的广泛关注。然而，由于实验条件并不总是恒定的，STM中原子转移的机理尚未揭示；针尖的原子结构影响针尖与样品之间邻近区域的感应强场和通过电流。然后，我们构建了一种新的UHV-STM/FEM，它可以承受10kV的高压，并将STM头的尖端温度提高到1500 C。该设计使得利用热场蒸发的STM尖端的清洁和积聚成为可能。尖端可以通过一个三维机械平台定位在表面上，该平台由惯性压电马达螺钉驱动，步长为30nm。STM和FEM工作模式切换方便；尖端可以对准STM样品支架的锥形小孔，靠近样品，然后从尖端场发射电子的图案可以投射到样品后面的屏幕上。从而可以对原子转移实验前后的尖端尖端进行评价。我们通过STM/STS研究了Si (111)7x7，使用了[111]定向W尖端，该尖端是通过在施加正高压的同时加热而形成的。从-3到3v扫描的STS测量结果导致隧道光谱不稳定。在STS测量后，FEM图像立即显示，顶点急剧变化，中心变暗。结果表明，扫描电压将Si原子转移到尖端，Si在尖端的吸附增加了尖端的功函数。这是STS测量中硅原子转移的证据。

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.