Forbidden and Second-Order Phonons in Raman Spectra of Single and Few-Layer MoS2 Close to C Exciton Resonance

Abstract

The complex Raman spectra of single- to few-layer MoS2 obtained by different methods and substrates are analyzed and compared to bulk. Depending on the Raman process that originates the phonons, these are sensitive to different variables such as the number of layers (N), the growth method, or the substrate. The behavior of forbidden 150 and 190 cm–1 peaks is explained by a double resonant mechanism close to C exciton assisted by intrinsic defects, and their strong dependence on N is due to electronic structure modifications. The band around 450 cm–1, in off-resonance conditions, is found to be little sensitive to the number of layers or to the obtaining method but strongly influenced by the coupling to the substrate and by temperature; it is thus conjectured to be an excellent indicator of strain. Several peaks are found to occur solely for vapor deposition methods, indicating the presence of specific defects.

Evaporation of Eco-Friendly MoS2 Nanofilm for Interface Engineering in Carbon-Based Sb2(S,Se)3 Solar Cell

“…As shown in Figure (b), the sample without MoS 2 displays the characteristic peaks at 194, 279, and 302 cm –1 corresponding to Sb 2 S 3 , and the weak Raman peaks at 120 and 156 cm –1 belong to Sb 2 Se 3 , indicating that the slight substitution of Se occurs on the surface of Sb 2 S 3 film. After depositing a MoS 2 nanofilm, the sample has peaks around 251 and 450 cm –1 assigned to MoS 2 , which agree well with the observed platelet MoS 2 nanoparticles prepared by the physical vapor transport (PVT) and chemical vapor deposition (CVD) processes …”

“…This preferred (00l) orientation of the as-deposited MoS 2 film is in line with that of the as-purchased MoS 2 powder, as displayed in the Figure S2. It is well-known that less-layered or monolayer MoS 2 can be easily stripped from the bulk because two-dimensional MoS 2 layers are held together only by van der Waals interactions in the vertical direction, which enables the possible deposition of (00l)-oriented MoS 2 . To further confirm the formation of MoS 2 on Sb 2 (S,Se) 3 , a Raman test is conducted.…”

“…It is well-known that less-layered or monolayer MoS 2 can be easily stripped from the bulk because two-dimensional MoS 2 layers are held together only by van der Waals interactions in the vertical direction, which enables the possible deposition of (00l)-oriented MoS 2 . 15 To further confirm the formation of MoS 2 on Sb 2 (S,Se) 3 , a Raman test is conducted. As shown in Figure 2(b), the sample without MoS 2 displays the characteristic peaks at 194, 279, and 302 cm −1 corresponding to Sb 2 S 3 , and the weak Raman peaks at 120 and 156 cm −1 belong to Sb 2 Se 3 , indicating that the slight substitution of Se occurs on the surface of Sb 2 S 3 film.…”

Edge Modes of MoS2 via Indirect Double Resonant Raman Spectroscopy

“…Defect induced features for MoS 2 in the 100–250 cm –1 region have been well reported in the literature as the sum of interactions with many different normally Raman-inactive acoustic phonons from across the BZ under specific resonance conditions. ,,− These modes appear as a consequence of elastic scattering by defects and a breakdown of the wave vector q ≈ 0 selection rule in crystalline materials. A saddle point in the LA branches of the phonon dispersion between M and K results in a high phonon density of states (pDOS) near 220 cm –1 , , and broad features in this region are usually the first identifiable ”defect modes”, which are generally regarded as an indication of poor crystallinity or small particle size (see Figure S7).…”

“…Double Resonance Raman (DRR) describes second-order Raman processes where the intermediate scattered state is also a real electronic state, rather than a virtual state as in standard second-order Raman processes . This provides a significant enhancement to scattering probabilities and is used extensively to account for the second-order resonant scattering processes in graphene and TMDs. − While momentum conservation ( q ≈ 0 selection rule) limits first-order Raman-active phonons to those near the Brillouin zone (BZ) center (Γ), higher-order scattering processes can involve phonons with larger wavevectors ( q i ≠ 0), provided the sum of those wavevectors throughout the scattering processes is zero . In graphene, a defect-activated DRR feature known as the ”D mode” allows sensitive defect detection and differentiation of armchair and zigzag edges due to real-space and reciprocal space backscattering conditions, making it a powerful tool for nanomaterials characterization. , While DRR modes are prolific in TMD materials, to the best of our knowledge no such resonant edge modes have yet been reported for MoS 2 or other TMDs.…”