Excitation mechanism of A_1g mode and origin of nonlinear temperature dependence of Raman shift of CVD-grown mono- and few-layer MoS₂ films.

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.