利用掃描穿隧顯微鏡觀察昆拉赫震盪及其應用

Abstract

在本實驗中，我們以掃描穿隧顯微鏡(STM)和掃描穿隧能譜術(STS)觀察金屬表面形貌以及昆拉赫振盪，昆拉赫振盪所觀察到的駐波態分佈會隨著表面材料及結構的改變而有所差異。在Cu(111)及重構的Au(111)表面上鍍上Ag薄膜，透過STS的量測，發現基板與薄膜的高能量駐波態會有ㄧ定量差值。此現象不僅在均勻薄膜中可見，在結構稍有差異的薄膜(Co/Cu(111)系統)及單原子薄膜系統中皆有相似現象。根據本文所提出的模型，此定量差值是基板與薄膜的功函數差值，且此功函數的差值與試片表面電場應無相依性。 在同ㄧ材質的表面(Ag/Cu(111)系統)上，也在不同結構上得到相異的駐波強度。此差異可視為是因結構不同而造成的電子穿透率變動，當穿透率較小時，反射造成的駐波強度會較為強烈。然而這些駐波強度會在能譜中呈現一種守衡的現象，可見同一物質的相異結構會在駐波態有不同程度的反應。在Z-V能譜量測過程中，當電壓逐漸上升，探針退後量亦會隨之增加。我們試著在探針遠離表面的過程中，對重構的Au(111)表面做影像分析，可得在探針距表面到60Å之遠時，擁有較1nm為佳的解析能力。如此的技術非常適用於奈米碳管或DNA的觀察及電性分析，遠距離的掃描可避免此類軟物質在掃描過程中被探針帶走，而高解析度亦能確保電性量測的精確性。

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are employed in this thesis to study the surface topography and the Gundlach Oscillation on metal surface. The separations of standing wave states between the films and substrates vary with the characteristics of the surface. Constant energy separations between the film and substrates exhibit in uniform and various thin film systems. We proposed a model to explain the energy separation, and it is considered as the work function difference between the film and substrates. According to our result, the energy separations at higher bias are field-independent. Our results also show that the intensity of Gundlach Oscillation varies with distinct structures. Higher intensity of Gundlach Oscillation occurs accompanied with lower transmissivity. However for the same material, the Gundlach oscillation intensities of distinct regions are conserved during Z-V measurement. It shows that different structures response distinctly at the energy of standing wave states. Observing reconstructed Au(111) surface with Z-V measurement, the resolution is better than 1.1nm even when the tip-sample displacement is large as 60Å. This technique is quite applicable in observing soft materials as carbon nanotubes or DNA. Generally soft materials tend to be dragged by the tip since the distance between the tip and sample is less than 10Å. Such dragging can be avoided if the scanning is operated with several nanometers as tip-sample distance.