Giant Effects of Interlayer Interaction on Valence-Band Splitting in Transition Metal Dichalcogenides

Understanding the origin of valence band maxima (VBM) splitting in transition metal dichalcogenides (TMDs) is important because it governs the unique spin and valley physics in monolayer and multilayer TMDs. In this work, we present our systematic study of VBM splitting (Δ) in atomically thin MoS2 and WS2 by employing photocurrent spectroscopy. We found that VBM splitting in monolayer MoS2 and WS2 depends strongly on temperature, which contradicts the theory that spin–orbit coupling solely determines the VBM splitting in a monolayer TMD. We also found that the rate of change of VBM splitting with respect to temperature ( m = ∂ Δ ∂ T ) is the highest for monolayer (−0.14 meV/K for MoS2) and the rate decreases as the layer number increases (m ≈ 0 meV/K for 5 layers MOS2). Our density functional theory (DFT) and the GW with Bethe–Salpeter Equation (GW-BSE) simulations agree with the experimental observations and demonstrate that the temperature dependence of VBM splitting in monolayer and multilayer TMDs originates from the changes in the interlayer coupling strength between the neighboring layers and substrates. We also found that VBM splitting depends on the layer numbers and the type of transition metals.