"Nanocomposites and Nanomaterial "

The Spectral Manifestation of Nonlinear Resonant Wave Interactions in Vibrational Spectra of Dichalcogenides MeX2
Kornienko N.E.1, Naumenko A.P.1, Gubanov V.O.1, 
Fedorov S.E.2, Artemkina S.B.2
1 Taras Shevchenko National University of Kyiv, 64/13, Volodymyrs’ka Str., 01601, Kyiv, Ukraine 
E-mail: nikkorn@univ.kiev.ua
2 A.V. Nikolaev Institute for Inorganic Chemistry, 3, Acad Lavrentieva Prospect, Novosibirsk, Russian Federation.

Layered transition metal dichalcogenides MeX2 (Me=Mo, W, Te; X=S, Se) due to the existence of the band gap Eg ~ 2 eV and the exciton states A1, B1, as well as the role of d states are richer than carbon nanostructures. The vibrations are active in infrared (IR) and Raman (RS) spectra. Moreover, the Raman spectra (RS) in the MoX2 family can be obtained under both non-resonant and resonant conditions, which makes it possible to move from spontaneous to stimulated type of scattering upon excitation with continuous lasers, which is not taken into account in a great many publications. We studied the RS of one layer (1L) and several layers of MoS2 on Si, nanoparticles (NPs) with a thickness of 6 – 8 layers and sizes (4 – 5) x 20 (40 – 50) nm, nanocrystals (NC) 50x250 nm and bulk single crystals of MoS2, as well as RS MoSе2, MoTe2 and WS2 excited by laser radiation λL = 632.8 nm (He-Ne) and 488 nm (Ar+). Additionally, the IR spectra of ATR and diffuse scattering were studied. It was shown that the ν1(E1g) line dominated in the RS of the MoS2 NPs, and the ν2(E2g) and ν3(A1g) bands are fairly weak. But in single crystals and monolayers, the ν1 line often disappears, and the ν2.3 bands are strong (see Figure 1).
Anomalous changes in the ratio of the intensities of the ν1 and ν3 bands (~ 102 – 103 times), as well as ν2, ν3 are explained by the effect of several types of nonlinear resonant wave interactions (NRWI). As a result of the NRWI of thermally excited vibrational modes, nonequilibrium populations of the vibrational levels ni are established. In MoS2, this is facilitated by a number of resonant conditions: 2ν(ZA) ≈ ν1; ν(ZA) + ν(LA) ≈ 2ν(TA) ≈ ν2; ν(TA) + ν(LA) ≈ ν3, ν2 + ν0 ≈ ν3, where ν(ZA) = 146 cm-1, ν(LA) ≈ 179 – 194 cm-1, ν(LA) ≈ 232 – 236 cm-1, ν0 ≈25-32 cm-1
Changes in differences in population Δni affect the growth rates of Stokes waves ESi in Raman processes ~ AΔni|EL|2ESi. In particular, if the populations of the ground state and ν1 are equal ES1 ≈ 0, which is often observed. An excessive population of the ν3 n3 level leads to a weakening of the ν3 band in the RS comparing to ν2 (Figure, spectrum 2). Significant changes in the populations of vibrational states are confirmed by the detection of negative polarity bands for the overtones 3ν2 and 3ν3 in the IR absorbance spectrum (diffuse scattering) of the MoS2 nanocrystals, that is illustrated by Figure 2. There are two spectra (1, 2) are shown under changes of optical dencity ΔD<0 in comparison with ordinary spectrum 3. 

Figure 1. The comparison of fundamental bands ν1, ν2, ν3 in RS spectra onf nano and macrocrystals MoS2	Figure 2. The manifestation of bands with negative polarity in spectra of diffusion scattering of microcrystals MoS2 (1,2) and ordinary spectrum 	
Other types of NRWI are associated with stimulated Raman scattering processes and parametric nonlinear processes ωL = ωSi + ωPi; 2ωL = ωSi + ωAi. At the same time, the growth rate of ESi ~ BELAPi*; ESi ~ CEL2EAi*, where APi and EAi are the amplitudes of the phonon ωPi and anti-Stokes ωAi = ωL + ωPi waves. The influence of parametric processes is confirmed by the difference in the shifts of ωS,A waves relative to ωL. According to the literature data, changes in the ratio of the intensities of the ν2.3 bands are also observed when a voltage of ~ 2V is applied, deformation and a pressure of ~ 10 – 50 GPa are applied and when the center of inversion disappears. In these cases, three-wave parametric processes ωSi = ωL – ωPi are manifested, for which the collective nature of vibrational modes is important and the efficiency of formation ωSi is proportional to APi. This is observed when the anomalous amplification of the bands 2ν(LA) = 466 cm–1 at λL = 632.8 nm and ν1 and ν3 in the NPs and at high pressures, respectively. The increase in the vibrational nonlinearity in MeX2 is manifested in a huge increase in the intensity of overtones. For MoSe2 at λL = 488 nm, a series of high overtones 5ν3 – 10ν3 was observed. At the same time, the intensity of bands of 5 – 8 orders of magnitude reached 16 – 22% I3, which is characterized by giant anharmonicity. However, in MoTe2, overtones are suppressed, which characterizes a radical change in the properties of phonons and chemical bonds.