基于扫描探针表面原子识别的前期实验研究

【作者】 魏征；

【导师】 徐克尊； 陈向军；

【作者基本信息】 中国科学技术大学 ， 原子分子物理， 2007， 博士

【摘要】 如今扫描隧道显微镜（STM）已经成为表面科学中一种极其重要的测量分析手段，用于对固体表面形貌的测量和费米面附近电子态的探测，但是它无法直接识别表面原子的种类。本文介绍了将扫描探针与电子能谱技术相结合以便在微区表面识别原子的预研究工作，包括两个相关实验：超快电压脉冲阈值实验和扫描探针俄歇谱仪（Scanning Probe Auger Electron Spectrometer—SPAES）中鼓型电子能量分析器（Toroid）的搭建和调试。第一章介绍了基于扫描探针的表面原子识别中的一些基本概念，STM的工作原理以及扫描探针俄歇谱仪的研究进展。在第二章中，我们通过在一台STM的Pt针尖上施加单个超快电压脉冲，系统地研究了固定针尖样品间距下8ns-10μs脉冲宽度范围内的单个电压脉冲破坏石墨样品的阈值电压幅度V_th与脉宽T_p的关系，发现V_th=Aarcsinh（B／T_p）。结合超快电压脉冲经过I／V变换器后的畸变波形模拟，提出了一个场蒸发原子链-电迁移-导致石墨表面相变无定型化的模型来解释这一正比关系。利用阈值曲线的拟合参数和提出的物理模型，估算出与超快脉冲作用过程相关的一些有意义的物理量：针尖区域Pt晶面的平均大小、石墨对单超快电压脉冲所响应的杨氏模量以及阈值脉冲在隧道结上产生的电流密度，从而较好地解释了超快电压脉冲通过Pt针尖破坏石墨表面的整个物理过程。在第三章，我们描述了在前一章实验过程中发现的单个超快电压脉冲在石墨表面诱导的超大周期结构。这种结构在产生的初始阶段是不断变化的，这表明脉冲诱导的超大周期结构的顶层石墨内存在较强的应力。而在另一次实验中发现的石墨解理过程中产生的超大周期结构是极为稳定的，即便施加单个电压脉冲也没有破坏该超大周期结构。第四章提出将STM与一种新型的几何探测效率极高的鼓型能量分析器相结合的扫描探针俄歇谱仪（SPAES）方案。介绍了Toroid调试装置的搭建与构成，并对Toroid的电子光学特性进行了研究。初步调试出Toroid的能量分辨为0.6eV，并在此基础上测量了惰性气体Ar的L_2，3M_2，3M_2，3俄歇电子能谱和能损区间为11～14eV的价电子能损谱；发现了铜制气体喷嘴弹性峰低能端的3eV和8eV的能损峰，以及60eV附近的M_2，3M_4，5M_4，5俄歇峰。更多还原

【Abstract】 Nowadays scanning probe technique has been one of the most importance analytical tools in surface science. It could detect the topography and the electronic structure near the Fermi level of the surface. However the principle of scanning probe technique prevents it form identifying the surface atoms. This thesis focus on two relevant experiments aiming at combining the scanning probe with electron spectroscopy technique to identify the surface atoms: one is to decide the threshold amplitudes of single ultrafast voltage pulses applied to Pt tip which will destroy the graphite surface at a fixed tunneling impedance, the other describes the buildup and adjustment of a toroidal electron energy analyzer（TEEA）, which is the key component in the future Scanning Probe Auger Electron Spectrometer （SPAES） of our laboratory.Chapter 1 describes some basic concepts in the identification of surface atoms with scanning probes, the principle of the scanning tunneling microscope and recent progresses in the scanning probe electron spectrometers.In chapter 2, the threshold amplitudes of single ultrafast voltage pulses with different durations that destroy the graphite surfaces at a fixed tip sample distance are systematically investigated by applying single voltage pulses to the Pt tip of a STM. The threshold amplitude V_th vary with the duration of the pulses according to the relationship: V_th = Aarcsinh（B/T_p） . The behavior of the ultrafast pulse through the I/V converter of the STM is also simulated with Laplace transformation method. Based on the experimental data and the simulations, a theoretical model consisting of field evaporation-electromigration-disorder of graphite surfaces is proposed, which could explain the modification process quite well. According to the fitting parameters A, B derived from the experiments and the proposed model, we could estimate the average size of the Pt domains near the STM tips, the Young’s module of the graphite under the action of the single pulses and the current density passing through the tunneling junction.We also find that single ultrafast voltage pulse could induce superstructure on the graphite surface, as described in chapter 3. The pulse induced superstructure keep on changing at the beginning, indicating that strong intra-layer strain exist in the pulse induced superstructure. In contrast, another superstructure due to the cleaving process of the graphite is much more stable and will not be destroyed even after single ultrafast pulse applied on it.Chapter 4 describes the key problems which should be resolved in the project of SPAES. It also presents the whole setup of the TEEA and the results obtained after one year’s adjustments. With energy resolution of 0.6eV, we measure the L_2,3M_2,3N_2,3 Auger spectrum and the valence electron energy loss spectrum of Ar gas. The M_2,3M_4,5M_4,5 Auger spectrum and energy loss spectrum of Cu jet inlet is also found.更多还原