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极大兴趣。然而，STM中原子转移的机制尚未被揭示，因为实验条件并不总是恒定的;在针尖和样品之间的邻近区域中的感应高场和通过电流受针尖顶点的原子结构的影响。在此基础上，我们构建了一种新的UHV-STM/FEM，其耐受性为在STM头中施加10 kV的高压并将尖端温度提高到1500 ℃。该设计使得利用热场蒸发清洁和建立STM针尖成为可能。针尖可以使用三维机械平台定位在表面上，该平台由具有30 nm步长的惯性压电螺钉驱动。STM和FEM操作模式可以很容易地切换;尖端可以对准STM样品保持器的锥形小孔，靠近样品，然后来自尖端的场发射电子的图案可以投射在样品后面的屏幕上。我们用一个[111]取向的W针尖，在正高压下加热而成，用STM/STS研究了Si（111）7 × 7。STS测量扫描从-3至3 V导致不稳定的隧穿光谱。STS测量后，FEM图像显示顶点急剧变化，中心变暗。这一结果表明，扫描电压将Si原子转移到针尖顶端，并且Si在针尖顶端的吸附增加了顶端的功函数。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.