Can weakly interlayer-coupled transition metal dichalcogenides open up a new prospect of two-dimensional family?

Abstract

Two-dimensional (2D) layered materials have been far more enriched in the two decades, among which interlayer coupling gradually has become the central factor in material design and mechanism studies. Beyond strongly interlayer-coupled 2D materials with thickness-dependent electron energy dispersion, weakly interlayer-coupled 2D materials are paid less attention due to their passivated band structures, especially for weakly interlayer-coupled transition metal dichalcogenides (TMDCs). In this perspective, we first introduce three energy parameters as the direct but superficial criterion to distinguish weakly and strongly interlayer-coupled TMDCs, including binding energy, in-plane translational energy, and cleavage energy. By collecting electron structure data of typical TMDCs, we then propose band-gap alteration percentage as a precise standard for semiconducting weakly interlayer-coupled TMDCs, which can be even extrapolated to semimetallic and metallic TMDCs by using energy shifting percentage as reference. We comprehensively discuss the underlying mechanisms of the unique layer-independent characteristics in the aspects of electron structures, which may commonly originate from localized intralayer charge distribution that results from various factors including orbital symmetry, crystal-field-induced energy level splitting, and stacking-determined interlayer polarization. As a consequence, the weak interlayer coupling enables low out-of-plane conductivity, stable optical properties, and low out-of-plane thermal conductivity, which provides ideal opportunities for temperature-insensitive integrated circuits, high-performance emitters, and thermally insulated devices. After the statement of recent advances, challenging fields, and future direction, we finally delineate a blueprint for 2D-material-based application including programmable 2D architecture and functionally integrated processing-in-memory all-2D chips. We believe that TMDCs with weak interlayer coupling can open up a new prospect in fundamental studies and practical applications based on the 2D family.