Double resonance Raman modes in monolayer and few-layerMoTe2

Abstract

We study the second-order Raman process of mono-and few-layer MoTe2, by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using 532, 633 and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the M point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van-Hove singularity of electronic density of states. Double resonance Raman process with inter-valley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy dependent Raman shifts in the experiment.

The structural phases and vibrational properties of Mo 1−x W x Te 2 alloys

“…0.09 alloys which we assign as a double-resonance Raman mode originating from the scattering of two longitudinal acoustic phonons from the M-point or an ( ) E M g 1 and a transverse acoustic mode, both also from the M point [38]. This feature has only been observed in few-layer 2H-MoTe 2 under resonant excitation [38], and its appearance in the bulk alloy samples is believed to originate from an enhancement in → ≠ q 0 Raman scattering processes by compositional disorder in the lattice.…”

“…This feature has only been observed in few-layer 2H-MoTe 2 under resonant excitation [38], and its appearance in the bulk alloy samples is believed to originate from an enhancement in → ≠ q 0 Raman scattering processes by compositional disorder in the lattice.…”

Raman scattering of few-layers MoTe 2

“…[16][17][18] Important information on the electron-phonon interactions can also be provided by resonant Raman spectroscopy [19][20][21][22] in which the resonances between excitation or scattered photons and electronic excitations is exploited. Motivated by a recent observation by Guo et al 23 that resonance of this type may be anticipated in a monolayer MoTe 2 at the energy of 2.07 eV, we have investigated the Raman scattering in few-layer MoTe 2 using He-Ne laser light with a wavelength of λ=632.8 nm for excitation. The energy of such excitation light, equal to 1.96 eV, matches the energy of the vertical electronic excitation from the first valence band to the second lowest conduction band at the M point of the MoTe 2 first Brillouin zone.…”

“…In our opinion the (i) peak couples with the electronic resonance while the other mode, (j), does not. The energy of the exciting photons (1.96 eV) coincides with the maximum of the electronic density of states (2.07 eV in the bulk) in the highest valence band and the second lowest conduction band at the M point of the Brillouin zone 23 . As it can be appreciated in Ref.…”

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

“…The LBM has B 2g symmetry in the bulk and is silent. As expected, we do not observe any interlayer mode for N = 1.In the mid-(100 − 200 cm −1 ) and high-frequency (200 − 300 cm −1 ) ranges, the Raman spectra of bilayer MoTe 2 displays four one-phonon features, which have previously been identified as originating from the following intralayer displacements: (i) the in-plane, outof-phase vibration of the Te planes, with E 1g symmetry in the bulk (iX mode, near 120 cm −1 ), (ii) the out-ofplane, out-of-phase vibration of the Te planes, with A 1g symmetry in the bulk (oX mode, near 170 cm −1 ), (iii) the in-plane vibration of the Mo and Te planes against each other, with E 2g symmetry in the bulk (iMX mode, near 230 cm −1 ), and (iv) the out-of-plane vibration of the Mo and Te planes against each other, with B 2g symmetry in the bulk, (oMX mode, near 290 cm −1 ) [13,20,39]. The bulk iX and oMX modes are predicted to be Raman inactive in a backscattering geometry and silent, respectively.…”