New First Order Raman-active Modes in Few Layered Transition Metal Dichalcogenides

Abstract

Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E′, E″ and E2g modes shift to lower frequencies, and the A′1 and A1g modes shift to higher frequencies. Second, new high frequency first order A′1 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A′1 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.

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].…”

“…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].…”

Atomically Thin Heterostructures Based on Single-Layer Tungsten Diselenide and Graphene

“…X-ray photoelectron spectroscopy (XPS) confirms there is no measureable reaction between graphene and WSe2 (Fig. 2a, b), and the integrals of high resolution spectra of the Se 3d and W 4f peaks leads an estimated Se:W ratio of Monolayer WSe2 is a direct-gap semiconductor, 23 which can be confirmed by photoluminescence (PL) measurements (Fig. 2d).…”

Vibrational and optical properties of MoS2: From monolayer to bulk

“…The Raman frequencies are correlated with the number of layers which allows their unequivocal identification. The trend of the Raman modes E 2g (in-plane mode) and A 1g (out-of-plane mode) with the number of layers has been intensively discussed, both theoretically [66,67,62,68] and experimentally [57,69,68]. The A 1g mode follows a predictable behavior.…”