Fig. 3

The change of intra-layer phonon frequencies \( {\omega}_{A_1^{\hbox{'}}}\approx \)403 cm⁻¹ and \( {\omega}_{E_1^{\hbox{'}}}\approx \)384 cm⁻¹ with increasing of layer numbers. Studied monolayer phonon frequencies are transformed into bulk ones ω(A _1g ) ≈408 cm⁻¹ and \( \omega \left({E}_{2 g}^1\right)\approx \)382 cm⁻¹ correspondingly; thick dots are experimental dependences and circles corresponds to difference between frequencies ω(A _1g ) and \( \omega \left({E}_{2 g}^1\right) \) for each layer; solid curves describe the theoretical dependences; fitting parameters (in cm⁻¹) for ω(A _1g ), \( {V}^{s_0,{s}_0} \) = 8, \( {V}^{s_0,{s}_1} \) = \( {V}^{s_1,{s}_0} \) = 6, \( {V}^{s_1,{s}_1} \) = 3; for \( \omega \left({E}_{2 g}^1\right) \), \( {V}^{s_0,{s}_0} \) = 5, \( {V}^{s_0,{s}_1} \) = \( {V}^{s_1,{s}_0} \) = 4, \( {V}^{s_1,{s}_1} \) = 2; (experimental data are taken from [4])