Air sensitivity of MoS2, MoSe2, MoTe2, HfS2 and HfSe2
McCarthy, Eoin K.
Povey, Ian M.
McCarthy, Melissa M.
Nagle, Roger E.
A surface sensitivity study was performed on different transition-metal dichalcogenides (TMDs) under ambient conditions in order to understand which material is the most suitable for future device applications. Initially, Atomic Force Microscopy and Scanning Electron Microscopy studies were carried out over a period of 27 days on mechanically exfoliated flakes of 5 different TMDs, namely, MoS2, MoSe2, MoTe2, HfS2, and HfSe2. The most reactive were MoTe2 and HfSe2. HfSe2, in particular, showed surface protrusions after ambient exposure, reaching a height and width of approximately 60 nm after a single day. This study was later supplemented by Transmission Electron Microscopy (TEM) cross-sectional analysis, which showed hemispherical-shaped surface blisters that are amorphous in nature, approximately 180–240 nm tall and 420–540 nm wide, after 5 months of air exposure, as well as surface deformation in regions between these structures, related to surface oxidation. An X-ray photoelectron spectroscopy study of atmosphere exposed HfSe2 was conducted over various time scales, which indicated that the Hf undergoes a preferential reaction with oxygen as compared to the Se. Energy-Dispersive X-Ray Spectroscopy showed that the blisters are Se-rich; thus, it is theorised that HfO2 forms when the HfSe2 reacts in ambient, which in turn causes the Se atoms to be aggregated at the surface in the form of blisters. Overall, it is evident that air contact drastically affects the structural properties of TMD materials. This issue poses one of the biggest challenges for future TMD-based devices and technologies.
Atomic force microscopy , Electron microscopy , Energy dispersive spectroscopy , Hafnium oxides , High resolution transmission electron microscopy , Molybdenum compounds , Scanning electron microscopy , Transition metals , Transmission electron microscopy , X ray spectroscopy
Mirabelli, G., McGeough, C., Schmidt, M., McCarthy, E. K., Monaghan, S., Povey, I. M., McCarthy, M., Gity, F., Nagle, R., Hughes, G., Cafolla, A., Hurley, P. K. and Duffy, R. (2016) ‘Air sensitivity of MoS2, MoSe2, MoTe2, HfS2, and HfSe2’, Journal of Applied Physics, 120, 125102 (9pp). doi:10.1063/1.4963290
© 2016, the Authors. This is an author-created, un-copyedited version of an article accepted for publication in the Journal of Applied Physics. The Version of Record is available online at http://dx.doi.org/10.1063/1.4963290