Excitation mechanism of A1g 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 E-2g(1) and A(1g) modes are observed simultaneously for mono, bi- and multilayer MoS2 with 514.5 nm laser, but only the Raman peak of E-2g(1) 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 E-2g(1) 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 E-2g(1) 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. (C)2016 Optical Society of America