Resonant quenching of Raman scattering due to out-of-plane A1g/A′1 modes in few-layer MoTe2

Abstract

Temperature-dependent (5 K-300 K) Raman scattering study of A 1g /A′ 1 phonon modes in mono-layer (1L), bilayer (2L), trilayer (3L), and tetralayer (4L) MoTe 2 is reported. The temperature evolution of the modes' intensity critically depends on the flake thickness. In particular with λ = 632.8-nm light excitation, a strongly non-monotonic dependence of the A 1g mode intensity is observed in 2L MoTe 2 . The intensity decreases with decreasing temperature down to 220 K, and the A 1g mode almost completely vanishes from the Stokes scattering spectrum in the temperature range between 160 K and 220 K. The peak recovers at lower temperatures, and at T = 5 K, it becomes three times more intense that at room temperature. Similar non-monotonic intensity evolution is observed for the out-of-plane mode in 3L MoTe 2 in which tellurium atoms in all three layers vibrate in-phase. The intensity of the other out-of-plane Raman-active mode (with vibrations of tellurium atoms in the central layer shifted by 180° with respect to the vibrations in outer layers) only weakly depends on temperature. The observed quenching of the Raman scattering in 2L and 3L MoTe 2 is attributed to a destructive interference between the resonant and non-resonant contributions to the Raman scattering amplitude. The observed "antiresonance" is related to the electronic excitation at the M point of the Brillouin zone in few-layer MoTe 2 .

Resonance Raman effects in transition metal dichalcogenides

“…In most cases, Davydov splitting is observed only for wavelength specific to the material. For example, it is most prominently observed for 1.96‐eV excitation in the case of MoTe 2 whereas it is observed in MoSe 2 only for excitation energies greater than 2.33 eV …”

“…Because this splitting is directly related to the inter‐layer interaction, Davydov splitting can be used to estimate the inter‐layer interaction as well as the number of layers . Davydov splitting has been observed in MoTe 2 , MoSe 2 , and WS 2 . In most cases, Davydov splitting is observed only for wavelength specific to the material.…”

“…Such excitation‐energy‐dependent relative enhancement of splits Raman modes has been ascribed to quantum interference effects . Recent measurements on the temperature dependence of the relative intensities of Davydov‐split peaks seem to indicate that the interplay between resonant and nonresonant contributions to the Raman scattering is playing an important role . For 3 and 4TL, the infrared active modes are not resolved even for resonant excitations, probably because they are overwhelmed by relatively stronger Raman active modes with similar frequencies , .…”

“…Polarized Raman spectroscopy is often a convenient tool to determine the crystallographic orientation of some TMDs that have anisotropic structures such as the distorted‐1T structure . For example, polarized Raman spectroscopy has been used to estimate the orientation of few‐layer ReS 2 , ReSe 2 , and WTe 2 crystals. However, it has been found that the polarization dependences of the main Raman peaks often exhibit peculiar excitation energy dependences .…”

Raman scattering from the bulk inactive out–of–plane $${{\bf{B}}}_{{\bf{2}}{\bf{g}}}^{{\bf{1}}}$$ mode in few–layer MoTe2

“…The impact of sample’s thickness on the lattice vibrations becomes even stronger when resonant excitation of RS is employed. Under such conditions, the resulting change in the electronic configuration of investigated TMD material leads to several effects which cannot be studied with non-resonant RS measurements, such as the enhancement of particular features in the RS spectrum 17 related to multiphonon processes 18 or quenching of Raman-active modes due to quantum interference 19,20 . The observation of Davydov splitting of phonon modes in few-layer TMD films, which arises from interlayer interactions, is also facilitated by resonant excitation of RS spectra 21,22 .…”