Issue 45, 2021

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

Abstract

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.

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

Supplementary files

Article information

Article type
Paper
Submitted
17 8 2021
Accepted
04 11 2021
First published
04 11 2021

Nanoscale, 2021,13, 19238-19246

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

L. Gammelgaard, P. R. Whelan, T. J. Booth and P. Bøggild, Nanoscale, 2021, 13, 19238 DOI: 10.1039/D1NR05413A

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