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

TLDR

Thickness determines the electronic, optical, and thermal properties of 2D crystals, and phonons in MoS₂ exhibit a unique thickness dependence due to short‑ and long‑range interactions. The study reports Raman spectra of mono‑ to few‑layer WS₂ and WSe₂ sheets. Raman spectroscopy was performed on atomically thin WS₂ and WSe₂ sheets across mono‑ to few‑layer thicknesses. The spectra show that A1g and E2g¹ modes stiffen and soften with increasing layers, multiphonon bands display thickness‑dependent intensity and resonance enhancement, and certain peaks appear only in multilayers, providing a distinctive fingerprint for monolayer identification.

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 the interplay between short and long range interactions. Here we report Raman spectra of atomically thin sheets of WS2 and WSe2, isoelectronic compounds of MoS2, in the mono- to few-layer thickness regime. We show that, similar to the case of MoS2, the characteristic A1g and E2g1 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.