Davydov Splitting and Excitonic Resonance Effects in Raman Spectra of Few-Layer MoSe₂

Abstract

Raman spectra of few-layer MoSe2 were measured with 8 excitation energies. New peaks that appear only near resonance with various exciton states are analyzed, and the modes are assigned.The resonance profiles of the Raman peaks reflect the joint density of states for optical transitions, but the symmetry of the exciton wave functions leads to selective enhancement of the A1g mode at the A exciton energy and the shear mode at the C exciton energy. We also find Davydov splitting of intra-layer A1g, E1g, and A2u modes due to inter-layer interaction for some excitation energies near resonances. Furthermore, by fitting the spectral positions of inter-layer shear and breathing modes and Davydov splitting of intra-layer modes to a linear chain model, we extract the strength of the inter-layer interaction. We find that the second-nearest-neighbor inter-layer interaction amounts to about 30% of the nearest-neighbor interaction for both in-plane and out-of-plane vibrations. 2 KEYWORDSMoSe2, Molybdenum diselenide, Raman spectroscopy, Davydov splitting, excitonic resonance 3 Few-layer semiconducting transition metal dichalcogenides (TMD's) are studied intensively owing to bandgap energies in the range of near infrared to visible wavelengths which make them suitable for various electronic and optoelectronic applications. 1,2 Monolayer MoSe2 shows a luminescence peak at ~1.6 eV which is suitable for applications in deep red or near infrared regions of the spectrum. MoSe2 is also used in TMD heterostructures such as WSe2/MoSe2 3,4 or MoS2/MoSe2 5,6 which exhibit interesting physical properties due to unique band alignment between these atomically thin semiconductors. Raman spectroscopy is a powerful tool to characterize 2-dimensional materials such as graphene or TMD's. For TMD materials, Raman spectroscopy is used to characterize the number of layers, 7-9 the stacking order, 10-12 strain, 13,14 or doping density. 15 However, it has been reported that the Raman spectrum of a TMD material varies greatly depending on the excitation laser used, which is attributed to excitonic resonance effects. [16][17][18][19] Because of reduced dielectric screening in 2-dimensional materials, the excitons in TMD's are known to have very large binding energies, [20][21][22][23][24] and so tightly localized wave functions. Resonance with such exciton states greatly modifies the Raman scattering process to result in extraordinary resonance Raman effects. In the Raman spectra of MoTe2 and WS2, Davydov splitting of some of the main Raman peaks have been reported. [25][26][27] Davydov splitting, also known as factor-group splitting, is the splitting of bands in the electronic or vibrational spectra of crystals due to the presence of more than one equivalent entity in the unit cell. 28,29 Since this is due to breaking of degeneracy by interactions of each entity, it is an important probe to investigate interactions between each system. In TMD's, the weak inter-layer interaction causes splitting of the intra-layer vibration modes. Since this s...

Comparative investigation of the vibrational properties of bulk 2H–MoS2 and its exfoliated nanosheets under high pressure

“…These two modes increase very rapidly at the beginning of compression, and then the increase is gradually slowed down after 17 GPa. The B 1u mode is often explained as the Davydov pair of the A 1g mode . Our results demonstrated that the B 1u peak appears on the high‐wavenumber side of the A 1g mode at 0 GPa, consistent with the literature, but contrary with .…”

“…The B 1u mode is often explained as the Davydov pair of the A 1g mode. [29,30] Our results demonstrated that the B 1u peak appears on the high-wavenumber side of the A 1g mode at 0 GPa, consistent with the literature, [23] but contrary with. [24] The details of B 1u in two samples at 0 GPa can be found in Fig.…”

Direct Exfoliation of High‐Quality, Atomically Thin MoSe2 Layers in Water

“…The variation in the E 2g 1 mode arises from the dielectric screening of the long-range Coulomb interaction and the surface effects. [29] Note that the absence of additional peaks on the low-frequency side of the A 1g mode, which is assigned to the Davydov splitting, [30] as well as the absence of any sharp peak at ≈353 cm −1 , which is related to the interlayer interactions, confirms that the MoSe 2 layers obtained in our experiments are mostly mono-and bilayers.…”

Atomic‐Level Customization of 4 in. Transition Metal Dichalcogenide Multilayer Alloys for Industrial Applications

“…Figure f demonstrates the Raman spectrum obtained from the MoSe 2 layers, indicating that the fingerprints of MoSe 2 (A 1g , E 2g , and B 2g phonon modes) are identified. The three discernible Raman features containing 1) the presence of B 2g modes, 2) the intensity ratio of the A 1g to E 2g modes (≈5), and 3) an asymmetric line shape in the A 1g modes, as presented in close inspection of the A 1g mode in Figure f, correlated well with evidence for Davydov splitting, suggesting the synthesis of multilayer MoSe 2 . The sulfide‐based TMD layered materials exhibit strong in‐plane phonon mode intensity, whereas the selenide‐based materials usually possess relatively weak in‐plane phonon modes, in good agreement with earlier studies .…”