Anomalous Lattice Vibrations of Single and Few-Layer MoS2

TL;DR

This study investigates the Raman spectral features of single and few-layer MoS2, revealing anomalous vibrational behaviors linked to Coulombic interactions and stacking effects, which aid in precise layer thickness determination.

Contribution

It introduces a detailed analysis of thickness-dependent Raman shifts in MoS2, highlighting effects beyond van der Waals coupling, and proposes a reliable method for atomic-level layer identification.

Findings

Raman modes E12g and A1g show opposite frequency shifts with layer thickness.

Frequency shifts are influenced by Coulombic interactions and stacking-induced intralayer changes.

The method enables precise determination of MoS2 layer number.

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 3-dimensional to the 2-dimensional regime.