Interlayer resonant Raman modes in few-layerMoS2

Abstract

We report two first-order Raman modes in the spectra of few-layer MoS 2 at 286 and 471 cm −1 that are only observed at excitation energies above 2.4 eV. We show that these normally not observed modes are interlayer modes related to symmetry-forbidden modes of the single layer. Based on group theory, we provide a general treatment and systematic classification of all phonon modes in few-layer crystals with inversion symmetry and/or horizontal reflection symmetry. The results can thus be applied to different materials like few-layer graphene, transition-metal dichalcogenides, or BN. Moreover, the few-layer specific Raman modes are strongly resonant with the C optical transition in MoS 2 . We conclude that the corresponding exciton wave function is extended over all layers of the few-layer MoS 2 , in contrast to the A and B excitons.

Unified Description of the Optical Phonon Modes in N-Layer MoTe2

“…3(c)), but no appreciable splitting can be resolved. However, the iMX feature downshifts as N increases, consistently with previous reports on N -layer MX 2 [8-10, 13, 16, 26, 29-31, 37].The splitting of a bulk phonon mode in a N -layer 2H-MX 2 can be understood based on a group theory analysis [9][10][11][12][13][14][15][16]28]. Bulk MoTe 2 belongs to the D 6h nonsymmorphic space group and crystals with odd or even N belong to the D 3h and D 3d symmorphic space groups, respectively.…”

“…Indeed, although bulk MX 2 exhibit indirect bandgaps, monolayer MX 2 are direct bandgap semiconductors [3,4] with remarkable spin, valley [5] and optoelectronic properties [6]. More Generally, N -layer MX 2 crystals provide an ideal platform to uncover the impact of symmetry breaking and interlayer interactions on the electronic, optical and vibrational properties, from the bulk (three-dimensional) to the monolayer (quasi two-dimensional) limit.In particular, in N -layer MX 2 , interlayer interactions result in a splitting of all the monolayer phonon modes [7][8][9][10][11][12][13][14][15][16] (see Table I). The latter effect is known as the Davydov splitting [17] and is closely related to the force constants that govern the vibrational properties of MX 2 [9].…”

Spectroscopic Signatures of AA′ and AB Stacking of Chemical Vapor Deposited Bilayer MoS2

“…The E g symmetry band in the AA 0 stacked bilayer corresponds to the E 2g symmetry band in the 2H bulk MoS 2 . 30 The E g peak represents vibration of Mo and S atoms in the basal plane, whereas in the A 1g vibrational configuration, Mo atoms remain fixed and only S atoms vibrate along the c-axis. The new Raman peaks, which are not commonly observed with the 514 or 532 nm laser lines, are assigned based on resonant Raman of bulk MoS 2 .…”

Raman characterization of platinum diselenide thin films

“…In MoS 2 , for example, these single atomic layers typically consist of two hexagonal planes of sulfur atoms arranged around an interstitial hexagonal plane of Mo atoms in a trigonal prismatic, or 2H, arrangement. This geometry and its resulting symmetry leads to several interesting properties, such as an increased direct bandgap in the monolayer 1 , and an evolution of Raman spectra with increasing layer thickness 6 .…”