Raman spectra of (PbS)1.18(TiS2)2 misfit compound

A Raman study on the (PbS)1.18(TiS2)2 semiconductor structure with incommensurate layers (misfit) is reported. The different bands observed in the spectra are attributed to both the TiS2 host layers, at 219 (Eg) and 333 (A1g) cm−1, and the intercalated PbS layers: the LO(Γ), 2LO and 3LO phonons at 203, 412 and 634 cm−1, respectively. A phonon peak probably related to the superposition of TiS2 and PbS vibrations, was observed near 286 cm−1. The decrease of the phonon wave numbers (of the modes located at 203, 333, 412, and 634 cm−1) with increasing laser power pointed out negative temperature coefficients of these modes. A peak appearing with laser power near 151 cm−1 is discussed. For higher laser powers the formation of oxides and oxysulfates complexes (PbO, TiO2, PbO2, PbSO4, “PbO·PbSO4”) at the misfitʹs surface was observed from the Raman spectra. In the low-frequency region the peaks at 19, 73 and 95–96 cm−1 have been revealed associated presumably with vibrations of the PbS layers.