Observation of valence band fine structures in R-type MoS2 by interlayer coupling

Interlayer coupling offers a symmetry-sensitive pathway for engineering the electronic structure and optical response of atomically thin two-dimensional semiconductors. Here, we combine temperature-dependent spectroscopic ellipsometry with first-principles density functional theory (DFT) calculations to investigate large-area monolayer and bilayer 3R-phase MoS2 grown on Al2O3 substrates. Unlike monolayer-MoS2, bilayer 3R-MoS2 exhibits a fourfold valence-band fine structure at the K/K′ points. DFT calculations show that this splitting originates from interlayer-coupling-induced band separation that is further resolved by spin–orbit coupling under broken inversion symmetry. By comparison, bilayer-2H MoS2 preserves spin degeneracy because of inversion symmetry, highlighting the decisive role of stacking symmetry in controlling the valley-resolved band structure. Spectral-weight analysis further indicates an enhanced optical response in bilayer-3R MoS2, consistent with strengthened band nesting in the Γ–M region. These findings establish interlayer coupling and stacking symmetry as key parameters for tuning excitonic transitions, valley electronic structure, and light–matter interactions in two-dimensional transition metal dichalcogenides.