利用掃描穿隧顯微鏡研究氧化還原反應製備金平面的穿隧能譜及質量轉移

Abstract

我們以掃描穿隧顯微鏡(Scanning Tunneling Microscope, STM)研究四氯金酸(HAuCl4)加入乙二醇(ethylene glycol)溶液中，再利用苯胺 (aniline)進行氧化還原反應而形成的金平面。利用穿透電子顯微鏡(Transmission Electron Microscope,TEM)的原子影像及繞射圖形判斷其晶格常數與能量散射頻譜儀(Energy Dispersive Spectroscopy, EDS)確認樣品組成成分為金元素。利用掃描電子顯微鏡(Scanning Electron Microscope, SEM) 與原子力顯微鏡(Atomic Force Microscope, AFM)測得面積大於100 μm2，厚度約為40~80 nm的微米級金平面。利用掃描穿隧顯微鏡發現金平面具有兩種不同的結構，其一為單層金原子堆疊而成的層狀堆疊結構，另一為單層金原子但非層狀堆疊的單層金島嶼結構。我們利用掃描穿隧顯微鏡探討在這兩種相異的金平面下，穿隧能譜及樣品表面結構上有何不同。 利用掃描穿隧顯微鏡量測兩種不同形貌的金平面其穿隧能譜，發現層狀堆疊結構中，其穿隧能譜在樣品偏壓為-400 mV時，會有一小尖峰存在，其結果與文獻中金(111)面所量測的穿隧能譜吻合。而在單層島嶼結構並未發現此現象，故判斷此種平面並非金(111)平面。在掃描層狀堆疊結構的金平面時，發現最上層的金原子較容易受針尖的影響而導致平面面積逐漸縮減，而在第二或更下層的平面面積則沒有甚麼變化。我們將平面面積隨時間變化的結果用衰減公式y=y0+Ae-t/τ做擬合，發現針尖與樣品表面距離越近，上層的金原子受到針尖焦耳熱效應或電場的影響越劇烈，故金原子逐漸被探針轉移。

Scanning tunneling microscope (STM) is used to study surfaces of gold flakes, fabricated by redox reaction. In addition, Transmission electron microscope (TEM) is used to take atomic image and diffraction pattern for the evaluation of the lattice constant. The composition of gold flakes is confirmed, as well, by energy dispersive spectroscope (EDS). The area and thickness of the gold flakes are about 100μm2 and 40-80 nm, respectively, which are measured from scanning electron microscope (SEM) and atomic force microscope (AFM) images. According to the structure observed in AFM images, gold flakes are divided into two types. The Type I flake reveals well-defined layer structures and the Type II flake shows island structures. In this thesis, we investigate the mass-transfer phenomenon and the scanning tunneling spectroscopy (STS) on both Type I and Type II flakes. We observe a peak at -400 mV, which belongs to the surface state of Au (111), on Type II flakes while it is not observed on Type I flakes. In addition, we adjust the tip-surface distance to study the STM tip effect on the mass transport on both types of flakes. The change of island area as a function of scanning time is recorded. The physical properties on the two types of gold flakes are different.