Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Molecular Beam Epitaxy Grown WSe2 Monolayers and Bilayers

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• 作者：Jun Hong Park ; Suresh Vishwanath ; Xinyu Liu ; Huawei Zhou ; Sarah M. Eichfeld ; Susan K. Fullerton-Shirey ; Joshua A. Robinson ; Randall M. Feenstra ; Jacek Furdyna ; Debdeep Jena ; Huili Grace Xing ; Andrew C. Kummel
• 刊名：ACS Nano
• 出版年：2016
• 出版时间：April 26, 2016
• 年：2016
• 卷：10
• 期：4
• 页码：4258-4267
• 全文大小：866K
• 年卷期：0
• ISSN：1936-086X

文摘

The effect of air exposure on 2H-WSe₂/HOPG is determined via scanning tunneling microscopy (STM). WSe₂ was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited in situ on WSe₂/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe₂ layers nucleate at both step edges and terraces of the HOPG. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe₂ surface; however, the band gap of the terraces remained unaffected and nearly identical to those on decapped WSe₂. The air-induced adsorbates can be removed by annealing at 523 K. In contrast to WSe₂ terraces, air exposure caused the edges of the WSe₂ to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra compared to the terraces of air-exposed WSe₂ monolayers. The preferential oxidation at the WSe₂ edges compared to the terraces is likely the result of dangling edge bonds. In the absence of air exposure, the dangling edge bonds had a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level toward the valence band. However, after air exposure, the band gap of the oxidized WSe₂ edges became about 1.08 eV larger than that of the WSe₂ terraces, resulting in the electronic passivation of the WSe₂.