機械剝離法前後二硒化鉬掃描穿隧式顯微術之研究

Abstract

二硒化鉬層狀半導體屬於過渡金屬二硫族化物（TMD）材料，為二維材料中的一種。由硫族元素與ⅣB、ⅤB、ⅥB、ⅦB族過渡元素鍵結產生一層平面，層與層之間再以較弱的凡德瓦力結合形成塊材。由於此特性，TMD材料往往能由塊材分離出穩定的二維單層結構。藉由掃描穿隧式顯微鏡（STM）進行研究，量測樣品表面與其局部特性，測得的資訊能讓我們更認識二硒化鉬在表面上的行為。 本次實驗在超高真空（UHV）環境下，以STM觀察二硒化鉬在四種情況的改變，分別為在大氣下曝露長時間的表面（Non-Fresh）、剛經機械剝離法處理完的表面（Fresh）、經機械剝離法處理過，又在大氣下放置27天下的表面（After Fresh 27 Days）、經機械剝離法處理過，特定曝露在氧氣之下的表面（Exposure to oxygen）。在機械剝離法前後的缺陷密度及種類都有著顯著的差異。再次經過27天曝大氣後，缺陷密度則有轉變回機械剝離法前的趨勢，此改變有可能造成樣品在大氣下的電性變化。特定曝氧氣，其表面電性更接近機械剝離法前的狀態，推測氧氣在表面電性上的改變扮演著重要的角色。本實驗比較缺陷密度、缺陷附近的能帶排列圖（Band alignment）及大尺度下的掃描穿隧能譜（STS）後，得出缺陷於大氣作用下的變化與其變化造成材料表面能帶結構的改變。Molybdenum diselenide layered semiconductors belong to the transition metal dichalcogenide (TMD) material, which is one of the two-dimensional materials. The bonding of chalcogen elements with IVB, ⅤB, ⅥB, and VIIB group transition elements produces a plane, and the layers are combined with a weak van der Waals forces to form a block. Because of this characteristic, TMD materials can often separate stable two-dimensional single-layer structures from blocks. Through the research of scanning tunneling microscope (STM), measuring the surface and local characteristics of the sample, the measured information can make us better understand the behavior of molybdenum diselenide on the surface. In this experiment, under ultra-high vacuum (UHV) environment, STM was used to observe the changes of molybdenum diselenide in four situations, which were exposed to the surface for a long time under the atmosphere (Non-Fresh), just after the mechanical peeling treatment The surface (Fresh), the surface-treated by mechanical peeling, and then placed under the atmosphere for 27 days (After Fresh 27 Days), the surface-treated by mechanical peeling, the surface exposed to oxygen specifically (Exposure to oxygen). There are significant differences in defect density and types before and after mechanical stripping. After 27 days of exposure to the atmosphere again, the defect density tends to change back to that before the mechanical stripping method. This change may cause the electrical properties of the sample under the atmosphere. For specific oxygen exposure, the surface electrical properties are closer to the state before the mechanical stripping method, and it is speculated that the change in oxygen surface electrical properties plays an important role. In this experiment, after comparing the defect density, the band alignment near the defect (Band alignment) and the large-scale scanning tunneling energy spectrum (STS), it is concluded that the change of the defect under the atmosphere and its change caused the surface energy band structure of the material change.