The study of dispersive 'b'-mode in monolayer MoS2 in temperature dependent resonant Raman scattering experiments

We report on resonant Raman scattering studies of monolayer MoS2 as a function of the excitation laser energy (1.959-2.033 eV) and temperature (T = 7-295 K). In complementary reflectivity contrast experiments we determined the temperature evolution of the A exciton and trion resonances. We focus our studies on the dispersive, second order 'b' mode related to the resonant two phonon Raman process of successive emissions of the acoustic LA and TA phonons at K points. We found that when excitation laser energy is tuned across the A exciton level this mode shifts almost linearly to lower frequency with the rate equal −83 and −71 cm−1/eV at T = 7 and 295 K, respectively, which is about two times higher rate than those reported in the previous studies of monolayer MoS2 but very close the relevant rate recorded for bulk MoS2. We interpret this effect as related to the difference of concentration of two dimensional electron gas. We also determined, using excitation with the He-Ne laser the temperature shifts of the Raman peaks of dispersive 'b' and dispersionless E′ and A1′ modes. We found that absolute value of the temperature coefficient of 'b' mode, equals 3.5 × 10−2 cm−1/K, is much higher than those of E′ and A1′ modes, equal 0.4 × 10−2 and 0.8 × 10−2 cm−1/K, respectively.