Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2

TL;DR

This study investigates the thickness-dependent lattice vibrations in mono- and few-layer WS2 and WSe2, revealing characteristic phonon shifts and multiphonon features useful for identifying monolayers.

Contribution

It provides the first detailed Raman spectral analysis of WS2 and WSe2 layers, highlighting their phonon behavior and resonance effects, expanding understanding beyond MoS2.

Findings

Phonon modes show stiffening and softening with increasing layers.

Multiphonon bands exhibit resonance-enhanced intensity and layer-dependent presence.

Monolayers have unique multiphonon spectral signatures.

Abstract

Thickness is one of the fundamental parameters that define the electronic, optical, and thermal properties of two-dimensional (2D) crystals. Phonons in molybdenum disulfide (MoS2) were recently found to exhibit unique thickness dependence due to interplay between short and long range interactions. Here we report Raman spectra of atomically thin sheets of WS2 and WSe2 in the mono- to few-layer thickness regime. We show that, similar to the case of MoS2, the characteristic and modes exhibit stiffening and softening with increasing number of layers, respectively, with a small shift of less than 3 cm-1 due to large mass of the atoms. Thickness dependence is also observed in a series of multiphonon bands arising from overtone, combination, and zone edge phonons, whose intensity exhibit significant enhancement in excitonic resonance conditions. Some of these multiphonon peaks are found to be absent only in monolayers. These features provide a unique fingerprint and rapid identification for monolayer flakes.