24 novembre 2021

[Material] – Appearance and origin of oxidation in 2D nanomaterials akin to tree rings.

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Transition metal dichalcogenides (TMDs) represent a large family of layered semiconductor materials of the type MX2, with M a transition metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. Many TMDs exhibit tunable band gaps that can undergo a transition from an indirect band gap in bulk crystals to a direct band gap in two-dimensional (2D, i.e. monolayer) nanosheets. (e.g., MoS2, WSe2, WS2, MoTe2).

Due to their extraordinary optical and electrical properties, these 2D TMDs have emerged as a promising class of atomically thin semiconductors for a new generation of electronic and optoelectronic devices. For instance, TMDs possess optical properties that could be used to make computers run a million times faster and store information a million times more energy-efficiently.

Recent studies also have shown the impressive photo-current conversions of these atomically thin semiconductors, such as MoS2 nanosheets, making them great candidates for next-generation visible light photodetectors.

However, many 2D materials with atomic-scale thicknesses suffer from oxidation and degradation effects under ambient conditions, which is one of the biggest obstacles in their practical applications. Given all the excitement around 2D TMDs, one particular issue that has garnered interest in the TMDs research community is the long-term stability of these materials in different conditions.

They must, for instance, be entirely shielded from light, with even short exposure causing oxidation severe enough to damage electrical contacts and completely destroy optical characteristics.

In particular, the edges of TMD crystal flakes are susceptible to degradation if they are not protected by other layers.

« When assembling van der Waals heterostructures, it is very useful to take the layer order and location of protected and unprotected edges in the TMDs flake into consideration, when for instance placing contacts or planning experiments involving multiple thicknesses, » Peter Bøggild, a professor in the Department of Physics at Technical University of Denmark (DTU), tells Nanowerk.

In their new work published in Nanoscale (« Long-term stability and tree-ring oxidation of WSe2 using phase-contrast AFM »), Bøggild and his colleagues show that phase-contrast atomic force microscopy (AFM) imaging allows to precisely distinguish covered from exposed edges in multilayer flakes.

What is remarkable in this work is the fact that the research team’s investigation covered a period of 75 months – so far the longest degradation study performed on any layered material. This enabled them to collect a unique data set of natural oxidation data in ambient conditions spanning this long time period of more than six years.

« It was a bit of a coincidence that we started studying this flake over so many years, » says Lene Gammelgaard, the paper’s first author. « First, we wanted to verify the correlation between the optical contrast and the thickness of the flake. A staircase-like flake with multiple regions with different number of layers was ideal for this. However, when we observed the quite striking difference in the phase contrast for some of the steps we got excited and decided to look further into it; so we made sure to put the flake into dark storage for further study. »

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