Thickness-dependent variations of atomic vibration, band-edge excitonic emission, and valleytronic response in layered Mo_0.55W_0.45S₂ ternary compounds

Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDs) are highly attractive due to their novel phenomena and potential device applications, but most of the studies are focused on binary TMDs except a few reports on ternary TMDs. Here, we report thickness-dependent mono-to-multi-layer transition behaviors of mechanically-exfoliated ternary TMD Mo_0.55W_0.45S₂. Dependences of Raman modes and their peak intervals on thickness (d) are divided into three regions (1.5 ≤ d ≤ 9.2, 9.2 < d ≤ 34, and > 34 nm), mostly resulting from the increase of van der Waals force and long-range Coulomb force in interlayer interactions at larger d. Photoluminescence (PL) peak and its intensity/full width at half maximum show similar three-region behaviors, which are associated with direct-to-indirect-bandgap transition as d increases. In particular, critical changes in the Raman scattering and PL occur at d = 9.2 nm (8–13 layers), meaning a 2D-to-3D transition. The effect of the valley polarization is observed up to d = 8.1 nm (7–11 layers), resulting from random mixing-induced suppression of the inversion symmetry in the multilayer. These results suggest that the critical layer number for the 2D-to-3D dimensional crossover in ternary Mo_0.55W_0.45S₂ is considerably larger than in binary TMDs, thereby exhibiting the direct-bandgap nature in an extended range of layer number, more useful for their optoelectronic device applications.