Long-term stability of mechanically exfoliated MoS 2 flakes
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Research Letter
Long-term stability of mechanically exfoliated MoS2 flakes Prachi Budania, Paul Baine, and John Montgomery, School of Electronics, Electrical Engineering and Computer Science, Queen’s University, Belfast BT9 5AH, UK Conor McGeough, and Tony Cafolla, School of Physical Sciences, Dublin City University, Glasnevin, Dublin D09 NR58, Ireland Mircea Modreanu, Tyndall National Institute, University College Cork, Cork T12 R5CP, Ireland David McNeill, and Neil Mitchell, School of Electronics, Electrical Engineering and Computer Science, Queen’s University, Belfast BT9 5AH, UK Greg Hughes, School of Physical Sciences, Dublin City University, Glasnevin, Dublin D09 NR58, Ireland Paul Hurley, Tyndall National Institute, University College Cork, Cork T12 R5CP, Ireland Address all correspondence to Prachi Budania at [email protected] (Received 8 July 2017; accepted 19 September 2017)
Abstract The long-term stability of mechanically exfoliated MoS2 flakes was compared for storage in the air and storage under vacuum. Significant changes in MoS2 flakes were observed for samples stored in the air, whereas similar flakes on samples stored in vacuum underwent no change. Small speckles were observed to appear on the surface of flakes stored in the air, followed by thinning and eventual decomposition of MoS2 flakes. The speckles are suspected to be formed by oxidation of MoS2 in the presence of atmospheric oxygen and water molecules, resulting in the formation of hydrated MoO3.
Introduction Two-dimensional (2D) transition metal dichalcogenides (TMDs) have received attention as potential materials for fabricating ultrathin electronic devices, owing to their inherent semiconductor band gap which is absent in the graphene.[1–3] However, the atmospheric stability of semiconducting TMDs is a crucial parameter, which could significantly affect the performance of TMD-based devices. Previously reported work emphasized the degradation of TMDs under specific conditions such as high-temperature annealing, elevated humidity or exposure to electron beam irradiation.[4–6] Compared with other TMDs, MoS2 is considered to be fairly stable under ambient conditions. However, recent studies on MoS2 electronic devices have suggested that atmospheric conditions may significantly affect device properties.[7,8] Recently, Gao et al. presented a study on the ambient stability of MoS2 and WS2 layers grown by chemical vapor deposition (CVD).[9] They observed cracks develop on the surface of ultra-thin MoS2 and WS2 flakes over the 1-year period of study. These changes were attributed primarily to the sulfur vacancies, which are inherently present in TMD films grown by CVD. Mechanically exfoliated flakes have always been considered of superior quality in comparison with CVD grown layers.[10,11] The pristine quality and crystallinity of exfoliated films are maintained because they are exfoliated directly from the parent crystal. In this work, the long-term stability of mechanically exfoliated MoS2 flakes is compared for samples stored in the air and under vacuum
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