Thickness-driven evolution of optical transitions and dielectric function in few-layer MoS2

Abstract

MoS2 film thickness strongly influences light absorption and interaction, impacting its performance in layered optoelectronic devices. Earlier studies have mostly examined the optical features, dielectric response, and electronic structure separately, especially for large-area films grown by chemical vapor deposition (CVD). Here, we bring these together and follow how they evolve as CVD-grown MoS2 changes from bilayer to bulk-like thickness (1.32–5.14 nm), using UV–visible absorption, Raman spectroscopy, spectroscopic ellipsometry, and first-principles calculations. We resolve six optical transitions (A–F) between 1.7 and 5.5 eV. The A and B excitons come from spin–orbit-split valence bands at the K/K′ valleys, while the higher-energy C–F transitions arise from band-nesting and interband processes at several critical points. As the films thicken, the transitions broaden and shift, reflecting stronger interlayer coupling and dielectric screening, and the Raman mode separation tracks changes in the optical bandgap. Our calculations reproduce the overall ordering and thickness trends of the measured transitions and identify their origin in the band structure.