Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy

Abstract

Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work highlights the underlying physics of intervalley scattering of electrons by acoustic phonons, which is essential for valley depolarization in MoS2.

Temperature dependence of the double‐resonance Raman bands in monolayer MoS2

“…Moreover, the strong spin–orbit interaction splits the valence band, giving rise to the so‐called A and B excitons, at 1.89 and 2.06 eV, respectively . The strong coupling between A and B excitons with the acoustic phonons has been revealed by several studies of transition metal dichalcogenides (TMDs) specially with Raman spectroscopy …”

“…Moreover, the coupling between phonons and excitons leads to the scattering of excited electrons by phonons away from the center of the BZ, giving rise to the so‐called double‐resonance (DR) Raman process. New features related to the combination of two or more phonons appear in the Raman spectrum, and their positions depend on the laser excitation energy …”

“…Carvalho et al have carefully analyzed the dispersive behavior of the DR features in MoS 2 measured with different laser lines and concluded that they are ascribed to intervalley scattering of the excited electron from different valleys and different acoustic phonons . Some processes in MoS 2 are similar to the case of the 2D band in graphene, where the excited electron is scattered from a valley in the vicinity of the K point to another valley in the vicinity of the K′ point by an acoustic phonon with momentum close to the K point.…”

A Guide to and Review of the Use of Multiwavelength Raman Spectroscopy for Characterizing Defective Aromatic Carbon Solids: from Graphene to Amorphous Carbons

“…A bibliometric search with the key words "double resonance" + "graphite" + "Raman" in the abstract returns at present 122 papers, concerning not only graphene but also other allotropes like carbon nanotubes, meaning this double resonance mechanism is well established. In fact, it is so well established that it is now used on other isoelectronic and structural analogs of graphene such as 2D dichalcogenides (ME 2 , M = metal, E = S, Se or Te) to interpret multiwavelength Raman spectra [106][107][108].…”

NaCl-assisted one-step growth of MoS2–WS2in-plane heterostructures

“…[31]. In addition, the LA (M) mode is a defect-induced Raman mode, which arises from the M and K points in the Brillouin zone [32][33][34]. Compared to the sample grown at 850°C without NaCl reported by Xie [26], the addition of NaCl does not enhance this defect-induced Raman band, seen in SI 2, available online at stacks.iop.org/NANO/28/325602/mmedia.…”