AVS 63rd International Symposium & Exhibition
    2D Materials Focus Topic Monday Sessions
       Session 2D+MI+SA-MoM

Paper 2D+MI+SA-MoM1
Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Molecular Beam Epitaxy Grown WSe2 Monolayers and Bilayers

Monday, November 7, 2016, 8:20 am, Room 103B

Session: 2D Materials Characterization including Microscopy and Spectroscopy
Presenter: Andrew Kummel, University of California, San Diego
Authors: J.H. Park, Univeristy of California, San Diego
S. Vishwanath, Cornell University
X. Liu, University of Notre Dame
H. Zhou, Cornell University
S.M. Eichfeld, Pennsylvania State University
S.K. Fullerton-Shirey, University of Pittsburgh
J.A. Robinson, Pennsylvania State University
R. Feenstra, Carnegie Mellon University
J. Furdyna, University of Notre Dame
D. Jena, Cornell University
H.G. Xing, Cornell University
A.C. Kummel, University of California, San Diego
Correspondent: Click to Email
The effect of air exposure on 2H-WSe2/HOPG was determined via scanning tunneling microscopy. WSe2 was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterwards, 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. The grain boundaries and the step edges of WSe2 ML have a bias dependence in STM imaging, consistent with difference electronic states with the defect-free terraces. After exposure air for 1 day, although the edge of WSe2 is partially oxidized, the grain boundaries still maintain a defective electronic structure. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe2 surface; however, as shown for localized electronic structure measurement using scanning tunneling spectroscopy (STS), the bandgap of the terraces remained unaffected and nearly identical to those on de-capped 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 STS bandgap 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 bonds at step edges. In the absence of air exposure, the dangling edge bonds have a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level towards the valence band. However, after air exposure, the band gap of the oxidized WSe2 edges became larger about 1.08 eV that of the WSe2 terraces, resulting in the electronic passivation of the WSe2.