Identification of individual and few layers of WS2 using Raman Spectroscopy

Abstract

The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.

Monolayer behaviour in bulk ReS2 due to electronic and vibrational decoupling

“…Again in contrast, the Raman spectrum is known to be highly sensitive to the number of layers in MoS 2 ( Fig. 4d) and other sTMDs [12][13][14] . For example, going from the bulk to monolayer sTMDs, the E 2g mode in MoS 2 typically stiffens by B5 cm À 1 , and the A 1g mode typically softens by a similar amount.…”

MoS2/WS2 Heterojunction for Photoelectrochemical Water Oxidation

“…41,42 Similarly, Raman characterization of 60 nm WS 2 films shows an energy difference between the in-plane E 1 2g and the out-of-plane A 1g modes comprised between 66±0.2 cm -1 and 68 ±0.2 cm -1 (Figure 2a and S5d, f and h), suggesting the formation of a nanosheets network with electronic coupling comparable to mono-and a few-layers. 43,44 Analogous to bilayer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 and ~ 526 nm in WS 2 , overall suggesting a predominant contribution from few -layered nanosheets. 37,48 The absorption peak positions were extracted by fitting the UV-Vis spectra, which were corrected to the scattering due to the thickness of the films (~ 60 nm) and fitted using a combination of Lorentzian functions ( Figure …”

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.…”