Varying Anisotropy in Oxidation of Two-Dimensional Transition Metal Dichalcogenides

In-plane anisotropy of two-dimensional crystals is not only intriguing but also of potential use for many applications. In this work, we studied how the structural anisotropy influences the thermal oxidation of single and few-layer transition metal dichalcogenides (TMDs) in the form of MX₂, where M = Mo and W; X = S and Se. The reaction was initiated from randomly distributed reaction centers and led to triangular oxides (TOs) and triangular etch pits (TEs) of varying roundness that were revealed by AFM phase and height images, respectively. To quantify a given triangular object's roundness, we determined an isotropy index (R) defined as the radius ratio between its inscribed and circumscribed circles. TOs of WX₂ showed larger isotropy values than TEs and TOs in MoX₂, indicating that more anisotropic reactions occur in the latter. The material-dependent anisotropy was verified with the reaction rates at edges that increased in the order of M-zigzag, armchair, and X-zigzag. The shape evolution of the triangular objects was modeled by kinetic Wulff construction based on the experimental rates for the edges.