Excitation mechanism of A_1g mode and origin of nonlinear temperature dependence of Raman shift of CVD-grown mono- and few-layer MoS_2 films

Abstract

MoS₂ films are grown on SiO₂/Si substrates by chemical vapor deposition. The vibrational properties of optical phonons of mono-, bi- and multilayer MoS₂ are studied by Raman scattering spectroscopy over temperature range from 90 to 540 K with 514.5 nm and 785 nm lasers. The Raman peaks of E2g1 and A_1g modes are observed simultaneously for mono-, bi- and multilayer MoS₂ with 514.5 nm laser, but only the Raman peak of E2g1 mode is seen for monolayer MoS₂ as 785 nm laser is used, revealing electron-phonon exchange excitation mechanism of A_1g mode for the first time. The Raman shifts of E2g1 and A_1g modes present obvious nonlinear temperature dependence. A semi-quantitative model is used to fit the nonlinear temperature dependence of Raman shifts which matches well to experimental data. Meanwhile, the fitting results reveal that the nonlinear temperature dependence of Raman shifts of E2g1 mode mainly originates from three-phonon anharmonic effect, while one of A_1g mode is contributed by stronger three- and weaker four-phonon anharmonic effects cooperatively but two contributions are comparable in intensity.

Work Function Modulation of Molybdenum Disulfide Nanosheets by Introducing Systematic Lattice Strain

“… where, ω 0 and ω are the initial and shifted wave number respectively. We have taken γ = 0.21 and γ (A 1g ) = 0.42 27 , 28 . As previously discussed, both and A 1g modes in MoS 2 are very sensitive to fluence of ion beam irradiation.…”

Narrow photoluminescence and Raman peaks of epitaxial MoS 2 on graphene/Ir(1 1 1)

“…The temperature dependent phonon frequency is phenomenologically described as ∆ω = χ∆T where ∆ω is the frequency shift and χ is a phonon shift per Kelvin. For MoS 2 there is a consensus in the literature that χ ∼ −0.01 cm −1 /K [42,53,54] (see Table 1). Interestingly, this number is largely independent of the substrate and holds also for freestanding layers [44].…”

Anharmonicity in Raman-active phonon modes in atomically thin MoS2

“…concluded that the threephonon process are irrelevant in explaining the observed phonon-frequency shifts of Raman active modes, whereas Yang et-al. found the three-phonon processes to be more dominant [32,33]. Lanzillo et-al.…”

“…Even after intense research for more than a decade, an arXiv:2009.04206v1 [cond-mat.mes-hall] 9 Sep 2020 in-depth, combined experimental and theoretical study of the temperature evolution of the phonon-modes; in single-and few-layer MoS 2 is missing. Previous studies employed semi-quantitative models for the calculation of the temperature dependent Raman shift [32,33]. Two contradictory conclusions have been reported by fitting the experimental data to such models for 300 K< T <500 K. Najmei et-al.…”