Davydov Splitting and Excitonic Resonance Effects in Raman Spectra of Few-Layer MoSe₂

Raman spectra of few-layer MoSe₂ were measured with eight excitation energies. New peaks that appear only near resonance with various exciton states are analyzed, and the modes are assigned. The resonance profiles of the Raman peaks reflect the joint density of states for optical transitions, but the symmetry of the exciton wave functions leads to selective enhancement of the A_1g mode at the A exciton energy and the shear mode at the C exciton energy. We also find Davydov splitting of <i>intra</i>layer A_1g, E_1g, and A_2u modes due to <i>inter</i>layer interaction for some excitation energies near resonances. Furthermore, by fitting the spectral positions of <i>inter</i>layer shear and breathing modes and Davydov splitting of <i>intra</i>layer modes to a linear chain model, we extract the strength of the <i>inter</i>layer interaction. We find that the second-nearest-neighbor interlayer interaction amounts to about 30% of the nearest-neighbor interaction for both in-plane and out-of-plane vibrations

Thickness-Dependent Phonon Renormalization and Enhanced Raman Scattering in Ultrathin Silicon Nanomembranes

We report on the thickness-dependent Raman spectroscopy of ultrathin silicon (Si) nanomembranes (NMs), whose thicknesses range from 2 to 18 nm, using several excitation energies. We observe that the Raman intensity depends on the thickness and the excitation energy due to the combined effects of interference and resonance from the band-structure modulation. Furthermore, confined acoustic phonon modes in the ultrathin Si NMs were observed in ultralow-frequency Raman spectra, and strong thickness dependence was observed near the quantum limit, which was explained by calculations based on a photoelastic model. Our results provide a reliable method with which to accurately determine the thickness of Si NMs with thicknesses of less than a few nanometers