Long-term stability and tree-ring oxidation of WSe2 using phase-contrast AFM

Lene Gammelgaard, Patrick R. Whelan, Timothy J. Booth, Peter Boggild*

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review


In this work, we use atomic force microscopy (AFM) to investigate the long-term evolution of oxidative defects of tungsten diselenide (WSe2) in ambient conditions over a period of 75 months, which is the longest such study performed on any layered material. In particular, we find that phase-imaging AFM of mechanically exfoliated WSe2 crystals provides convenient, direct identification of exposed and covered step-edges, and together with topographic thickness measurements allows complete determination of the layer arrangement in a multilayer flake. Step-edges with low or no phase-contrast consistently exhibit long-term stability in ambient conditions, indicating that they are covered and effectively protected by above-lying WSe2 layers. On the contrary, step-edges with initial high phase-contrast are clearly degraded after medium- to long-term exposure to ambient conditions (up to six months), indicating that these are not covered by other layers. Similar behaviour was observed for MoTe2 and MoS2. The correlation between phase-contrast and step order was confirmed by cross-sectional transmission electron microscopy. By comparing the phase-contrast line-traces in different locations and at different times, we find that long-term storage in ambient conditions led to evolution of a distinct ring-like pattern resembling the tree-lines arising from seasonal changes. Indeed the phase-contrast showed correlation with the average amount of sun-hours registered at the storage location. Storage in darkness slowed down the evolution of the tree-ring lines, in accordance with this explanation. Our work provides a unique dataset on long-term degradation of one of the most stable transition metal dichalcogenides, as well as insights into the conditions causing acceleration or inhibition of the degradation process.
Original languageEnglish
Issue number45
Pages (from-to)19238-19246
Publication statusPublished - 2021


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