Defect studies in transition metal dichalcogenide MoSe1.8S0.2 using resonant Raman spectroscopy

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.