利用掃描探針顯微鏡分析二硫化鉬氧化前後之表面形貌及電性結構

Abstract

本實驗利用掃描穿隧式電子顯微鏡 (scanning tunneling microcopy, STM)探討氧化前後之二硫化鉬 (molybdenum disulfide, MoS2)表面形貌及電性結構。本實驗所使用的樣品為二硫化鉬塊材，此塊材置放於摻雜氟之氧化錫(fluorine-doped tin oxide, FTO) 導電玻璃基板上，並以STM分析樣品氧化前後的差異。 實驗中觀察到二硫化鉬在氧化前之表面有自然缺陷，面積約為65 nm2，並利用掃描穿隧能譜測量，分析出二硫化鉬能隙大小約為1.2 eV。其後，在通有臭氧氣體的環境下，將二硫化鉬加熱至270℃，進行三小時的氧化製程，氧化後發現缺陷擴大形成三角形凹陷，深度約為0.68 nm (此為單層二硫化鉬之厚度值)，單一凹陷面積跨大至約200 nm2。掃描穿隧能譜測量出氧化後的樣品能隙改變，在能隙間有電子態產生使得能隙消失，轉變成類似金屬行為。此能帶結構轉變可能是臭氧的氧原子取代硫原子，在表面形成MoOx (x<3)，使得二硫化鉬的表面電性變化。 另一方面，為觀察金屬電極與氧化後之二硫化鉬接面的電性，先用熱蒸鍍在二硫化鉬表面鍍上一層厚度約2 nm的金薄膜，之後再將樣品置入同一溫度臭氧環境下氧化。由STM分析，顯示在覆蓋金的二硫化鉬條件下，氧化製程後，導電度有明顯提升。與製作成元件後的結果比較，氧化後二硫化鉬與金電極的接面電性改變，說明二硫化鉬氧化後可能形成MoOx產物而改變電性表現。

In this study, the surface structure and electrical properties of molybdenum disulfide (MoS2) is investigated by scanning tunneling microcopy (STM) before and after the MoS2 is annealed in ozone atmosphere. Here the bulk MoS2 sample is placed on the conducting substrate of fluorine-doped tin oxide coated glass. The bare MoS2 surface after mechanical exfoliation is investigated. The sample is then put in a chamber filled with ozone gas and heated up to 270℃for 3 hours. In the STM analysis, the surface of bare MoS2 exhibits many natural hole defects. The area of the hole defects is about 65 nm2. Using scanning tunneling microscope, the band gap of MoS2 is measured to be about 1.2 eV. After ozone treatment, the defects on the surface of MoS2 show triangle shape and the area of the hole defects expands. The area of defect holes is about 200 nm2. The depth of the triangle holes is about 0.68 nm, which is close to the thickness of monolayer MoS2. After ozone treatment, the band gap of MoS2 almost disappears and some impurity states form in the band gap energy regime. The size of defect holes on surface increases and the band gap of oxidized MoS2 decreases. It implies that some of S atoms are replaced by O atoms to form MoOx (x<3) on the MoS2 surface. In order to check the interface properties between the metallic electrode and the oxidized MoS2, we deposited 2-nm thick gold film on MoS2 and put the sample in ozone for oxidation. After inspected by STM, we observe a consistent result of increasing electron states in the band gap energy regime and an increase of electrical conductivity.