Low-frequency Raman modes and electronic excitations in atomically thin MoS2films

Abstract

Atomically thin MoS2 crystals have been recognized as a quasi-2D semiconductor with remarkable physical properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS2, especially in the low-frequency range (<50 cm −1 ). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS2 layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes the number of unit layers) trend. With the aid of first-principles calculations we assign the former as the shear mode E 2 2g . The latter is distinguished as the compression vibrational mode, similar to the surface vibration of other epitaxial thin films. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS2 films. In addition, we observe a broad feature around 38 cm −1 ( 5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.

Gate‐Tunable Resonant Raman Spectroscopy of Bilayer MoS2

“…However, a direct experimental evidence of such decoupling is unavailable: the interlayer breathing mode in bilayer MoS 2 excited by a 633 nm laser has not been resolved before due to a large resonance background. [21,22] Therefore, the measurement we show herein serves as an indirect method to probe the interlayer vibrational modes.…”

“…In contrast, a direct measurement of interlayer breathing modes is very challenging because of the large resonance background, especially in atomically thin layers. [21,22] …”

Three-band tight-binding model for monolayers of group-VIB transition metal dichalcogenides

“…∆ v SOC , but gives no spin splitting of the conduction band at K, denoted by ∆ c SOC . In fact, the conduction-band spin splitting (CBSS) is not zero but a finite small value, [33][34][35][36][37] and has been analyzed for MoS 2 by previous works. 31,55 Similar to the strong valley-spin coupling in the valence band, 18 the CBSS is also valley dependent due to the time-reversal symmetry and leads to weak valley-spin coupling.…”

Giant magneto-optical Raman effect in a layered transition metal compound

“…S1). The most likely origin of this effect is the anomalous behavior of the monolayer (30), both electronically and structurally. MoS 2 is an indirect-gap semiconductor from bilayer to bulk but exhibits a direct gap for the monolayer case.…”

“…Zeng et al (30) showed that integrated intensity of the A-mode gradually decreases with reduced thickness from the bulk but suddenly increases in a monolayer sample. This gives an apparently lower integrated intensity of A-mode in the bilayer than in the monolayer or bulk case (Fig.…”