Resonance Raman scattering in bulk 2H-MX₂ (M = Mo, W; X = S, Se) and monolayer MoS₂

Abstract

We have performed a comparative study of resonance Raman scattering in transition-metal dichalcogenides 2H-MX 2 semiconductors (M ¼ Mo, W; X ¼ S, Se) and single-layer MoS 2. Raman spectra were collected using excitation wavelengths 633 nm (1.96 eV), 594 nm (2.09 eV), 532 nm (2.33 eV), 514 nm (2.41 eV), and 488 nm (2.54 eV). In bulk-MoS 2 and WS 2 , the resonant energies appear to coincide with their exciton excitations. The resonance can be fine tuned by varying sample temperatures, which confirms its excitonic origin in both MoS 2 and WS 2. Temperature dependence of Raman intensities is analyzed in the context of resonance Raman theory, which agrees well with the existing absorption data. While in WSe 2 , the resonance has been observed in a wider range of excitations from 633 to 514 nm, which cannot be explained with its excitonic energies of 1.6 and 2.0 eV. It is considered that additional excitonic bands induced by band splitting are involved in the inter-band transitions and substantially extend the resonance energy range. The Raman resonance energy range remains unchanged in single-layer MoS 2 compared with that in the bulk sample. However, most phonon modes in single-layer MoS 2 are significantly broadened or strongly suppressed under resonance conditions. This change could be related to the modification of acoustic modes by the substrate. V

Recent Advancement on the Optical Properties of Two-Dimensional Molybdenum Disulfide (MoS2) Thin Films

“…The resonance Raman spectra show that multiple modes are shifted from the first-order Raman frequencies, which suggests that the electronic transitions are strongly coupled with phonon modes. There are several reports of this resonant Raman scattering phenomenon in MoS2 [78,79,86,91,93,[98][99][100]. All of these studies are in general agreement on the attribution of the peaks observed in the resonant Raman scattering spectrum, which suggests coupling between vibrational modes and longitudinal acoustic phonon modes.…”

“…Finally, the intensity of resonance Raman modes have an observed temperature dependence in some studies [85,[98][99][100]106]. For example, J. M. Chen and C. S. Wang had detected both the dependence of the second-order Raman modes at room temperature (300K) and liquid nitrogen temperature (80K).…”

“…There are several possible reasons for this result. The most reasonable possibility is that the long-range acoustic modes in monolayer MoS2 are strongly affected by interactions with the substrate [99,104]. In the bulk case, only a relatively tiny amount of atoms in the whole sample directly interact with the substrate, and the substrate therefore has little impact on the lattice vibrations.…”

Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material

“…However, the resonant excitation energy will be different because of the different band structures. For example, Fan et al found that the Raman resonance in bulk WS 2 covers a large energy range from 1.96 (633 nm) to 2.41 (514 nm) eV, 101 which is attributed to the broad features of its B exciton (Fig. S4, ESI).…”

Second harmonic generation in nanoscale films of transition metal dichalcogenide: Accounting for multipath interference

“…The Fabry-Perot (FP) cavity can be tuned for the used wavelength to enhance different optical effects in a single layer, from visualization 13 by color contrast 14 to observation of luminescence 1 and Raman scattering. 15,16 In order to get quantitative information from such measurements, direct calculations of transmissivity and reflectivity with interference taken into account are required.…”