Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Metal Dichalcogenides
The effect of air exposure on WSe2/HOPG, MoS2/HOPG and SnSe2/SnSe/HOPG were determined via scanning tunneling microscopy. WSe2 and SnSe2/SnSe/HOPG were grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and, afterwards, a Se adlayer cap was deposited in-situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. Conversely, MoS2 was grown via chemical vapor deposition on HOPG, and transferred into UHV chamber without capping. After annealing of WSe2 at 773 K to remove (decap) 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 de-capped WSe2. The air-induced adsorbates can be removed by annealing at 523 K. Air exposure caused the edges of the WSe2 to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra (STS) 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 edge dangling 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 towards the valence band. However, after air exposure, the band gap of the oxidized WSe2 edges became larger (about 1.08 eV greater) than the band gap of the WSe2 terraces, resulting in the electronic passivation of the WSe2 step edges In case of CVD grown MoS2 on HOPG, before air exposure, triangular islands of MoS2 were observed on HOPG and a 2.3 eV band gap on the MoS2 monolayer (ML) was observed in STS. However, after air exposure for one day, massive amount of hydrocarbon was observed on MoS2/HOPG, mostly near MoS2 step edges. Although the band gap of MoS2 ML terrace was similar to bare MoS2 ML, the band gap at step edges of MoS2 was mixed with large band gap (oxide formation) and narrow band gap (hydrocarbon) regions. Therefore, when MoS2 is employed for single domain devices, air induce contaminants are only a problem at step edges.
MBE grown SnSe2 layer was not stable in ambient air contrast to MBE grown WSe2 and CVD grown MoS2. After decapping Se adlayer on SnSe2/HOPG at 250 °C for 15 min, atomically flat SnSe2 layer was observed in the STM images. Band gaps of 1.7 eV on SnSe2 ML and 1.1 eV on SnSe2 BL were observed by STS. However, after air exposure for 1 day, the entire SnSe2 layer appear disordered in STM (decomposed) and STS displayed very narrow band gap. Therefore, SnSe2 requires the capping layer or a passivation method for the device fabrication.