In recent years, two-dimensional (2D) materials have attracted enormous attention due to their peculiar optical and electronic properties. Of particular interest are certain 2D transition metal dichalcogenides (TMDCs) such as MoS2 and WS2 which are semiconductors with an optical gap in the visible regime. Alloying of Mo and W in (Mo,W)S2, makes it possible to tune the band gap for specific applications. The characterization of alloys, such as their composition and atom mixing, is of fundamental importance for their applications. Raman spectroscopy is a quick nondestructive characterization method and one of the most widely used techniques for the characterization of 2D materials. Alloying leads to a change of the dominant peaks in the Raman spectra as one changes the composition from pure MoS2 towards pure WS2. For traditional alloyed semiconductors, two typical cases are known: the “one-mode behaviour” where one observes a continuous shift of the peak with nearly same intensity for all compositions and the “two-mode behaviour” where one observes splitting of peaks with intensities proportional to composition while the frequencies of the peaks stay close to their parent peaks.
In this master thesis presentation, I will talk about an unusual multi-mode behavior of the dominant A1g Raman peak in (Mo,W)S2 alloys. Experiments have shown that it can be deconvoluted into several Lorentzian peaks. We have performed semi-empirical and ab initio simulations of the phonon modes and their Raman intensities. We explain the origin of the multi-mode behavior as due to the presence of four different bonding environments for sulphur atoms in the random alloys.