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

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 E2g(1) 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.

Raman Spectroscopy of Shear Modes in a Few-Layer MoS2

“…The ω x,N subbranch corresponds to previously observed S1 shear mode in MoS 2 and WSe 2 [10] which is also referred to as C 2 [14]. Using the value of ω x = 22.6 cm −1 one can also find the in-plane (shear) force constant K x = 2.7 × 10 19 N/m 3 which perfectly matches the previously reported value [10] and which is smaller than the value for WSe 2 (3.1 × 10 19 N/m 3 ) [13]. The most intriguing property of the observed Raman spectrum is the peak at 27.5 cm −1 observed in 6 ML sample (see Fig.…”

“…Assuming that the force constant K x does not change significantly as N changes from 2 to N −→ ∞ [13] one can predict the value of ω x = 22.6 cm −1 . The energy of the shear mode in bilayer predicted by the model corresponds to the energy of one of the modes observed in our experiment at 22.5 cm −1 .…”

Spectroscopic Signatures for Interlayer Coupling in MoS2–WSe2 van der Waals Stacking

“…[35][36][37] It is observed that the Raman peaks for either MoS2 or WSe2 region show no obvious change after the thermal annealing at 300 o C as shown in Figure S2(a), which implies that the annealing process does not cause significant structural defects or modification. For the hetero-junction before annealing, we find that the Raman characteristic peaks are closely coinciding with the individual WSe2…”

Raman scattering of few-layers MoTe 2

“…This property makes this method a valuable complement to atomic force microscopy in determining the number of layers constituting thin flakes of TMD crystals under study. [16][17][18] Important information on the electron-phonon interactions can also be provided by resonant Raman spectroscopy [19][20][21][22] in which the resonances between excitation or scattered photons and electronic excitations is exploited. Motivated by a recent observation by Guo et al 23 that resonance of this type may be anticipated in a monolayer MoTe 2 at the energy of 2.07 eV, we have investigated the Raman scattering in few-layer MoTe 2 using He-Ne laser light with a wavelength of λ=632.8 nm for excitation.…”