Abstract
Using resonant Raman spectroscopy with 632.8 nm (1.96 eV) laser, we describe the Raman spectra of single crystals of transition metal dichalcogenides with nominal composition MoSe(2−x)Sx for x = 0.2. Changes in Raman spectra at some regions of the sample indicated non-stoichiometry and, in particular, chalcogenide vacancies. At low temperatures around 77 K, we observed unusual temperature dependent enhancement in the intensity of non-zone center modes as well as overtones and combination modes in Raman spectra. This enhancement in the intensity is correlated to the resonance achieved in the non-stoichiometric regions of the crystal at low temperatures. Observed resonance is attributed to modification in the electronic structure due to defects. Energy dispersive x-ray spectroscopy measurements confirmed chalcogenide vacancies in the crystals. The change in the electronic structure due to defects is also corroborated by photoluminescence spectroscopy measurements.
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