Optical properties of large-area ultrathin MoS₂ films: Evolution from a single layer to multilayers

We investigated the optical properties of ultrathin MoS₂ films (number of layers: N-=-1, 2, 4, and 12) using Raman spectroscopy, photoluminescence (PL) spectroscopy, and spectroscopic ellipsometry. We estimated the layer thicknesses based on Raman spectra. We characterized the microstructural properties of a single-layer MoS₂ film using atomic force microscopy. We measured the lowest-energy A and B excitons using PL spectroscopy. We measured the ellipsometric angles (Ψ and Δ) of MoS₂ thin films using spectroscopic ellipsometry, and obtained the dielectric functions as the films' thickness changed from a single layer to multi-layers. We determined the films' optical gap energies from the absorption coefficients. Applying the standard critical point model to the second derivative of the dielectric function (d²ε(E)/dE²), we determined several critical point energies. The d²ε(E)/dE² spectra showed doublet peaks around 3-eV corresponding to the C and D transitions, as well as doublet peaks around 2-eV corresponding to the A and B transitions. These doublet structures at 3-eV are attributed to the transitions in the Brillouin zone between the Γ and K points.