Raman study and lattice dynamics of single molecular layers of MoS2.

Abstract

A valence-force-field model has been used to study the lattice dynamics of molybdenum disulfide single molecular layers. A comparison between the room-temperature Raman spectra of aqueous suspensions and the calculated phonon-dispersion curves for single layers of ${\mathrm{MoS}}_{2}$ with trigonal prism and octahedral coordination indicates that ${\mathrm{MoS}}_{2}$ single layers adopt a structure in which the Mo atoms are octahedrally coordinated, as opposed to bulk ${\mathrm{MoS}}_{2}$ where the trigonal prism coordination is usually found. Besides the zone-center modes, strong Raman peaks have been observed at 156, 226, and 333 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The presence of these peaks is attributed to a zone-folding mechanism resulting from the formation of a 2${\mathit{a}}_{0}$\ifmmode\times\else\texttimes\fi{}${\mathit{a}}_{0}$ superlattice in the single layers of ${\mathrm{MoS}}_{2}$. The superlattice is, in turn, believed to correspond to the basal-plane atomic arrangement of a distorted octahedral structure. Lattice-dynamics calculations based on an octahedrally coordinated structure with a significant metal-metal interaction yield good agreement with the observed frequencies. When the single molecular layers restack and the sulfur-sulfur interaction between the layers is recovered, the layers convert back to the trigonal prismatic configuration of crystalline ${\mathrm{MoS}}_{2}$.