Layer-Dependent Chemical Reactivity of MoS₂

Two-dimensional (2D) materials offer a unique platform to expand the horizons of various disciplines, including physics, chemistry, materials science and engineering. One intriguing aspect of these materials is their distinct properties compared to their bulk counterpart, largely due to the quantum confinement effect at atomically thin region. Particularly, the thickness dependent physical properties of 2D materials have been extensively studied in the past two decades. Yet, little study is performed on the thickness dependent chemical properties. In this work, we report on the layer-dependent chemical reactivity of MoS₂ in a nitridation reaction. Our findings show that the reactivity of MoS₂ decreases with the decrease of the number of layers. This behavior is attributed to the layer-dependent binding energy of NH₃ at sulfur vacancies of MoS₂, as supported by theoretical calculations. The reaction results in the formation of MoN crystals across all thicknesses of MoS₂. Detailed atomic-level investigations using transmission electron microscope (TEM) reveal distinct morphologies of the reaction products. Specifically, MoN nanoplates are observed from the reaction products of monolayer MoS₂, and the continuity of the MoN film increases with the increase of the number of layers. This work demonstrates 2D materials to be a powerful avenue for advancing our understanding of materials chemistry.