Anomalous Lattice Vibrations of Single- and Few-Layer MoS₂

TLDR

The study exfoliated single‑ and few‑layer MoS₂ onto SiO₂/Si and used Raman spectroscopy together with atomic force microscopy to characterize layer thickness. Raman measurements revealed that the E₁₂g mode softens while the A₁g mode stiffens with increasing thickness, a behavior attributed to Coulombic interactions and stacking‑induced intralayer bonding changes, enabling atomic‑level thickness determination and illustrating structural evolution from 3D to 2D.

Abstract

Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.