Comparative optical response and structural assessment of MoS₂ and MoSe₂ single crystals grown via iodine-assisted chemical vapor transport

Transition metal dichalcogenides (TMDs) of molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂) have been of specific interest over the past few years due to their fascinating optical and electronic behavior, especially at their two-dimensional phases. The compounds have strong light matter interaction absorption and thus are promising candidates for next-generation optoelectronic devices like photodetectors, light-emitting devices, and flexible electronic devices. In the current work, we present the optical properties of single crystals of MoS₂ and MoSe₂ grown by the Chemical Vapor Transport (CVT) method using iodine as the transport agent. The CVT process allows the growth of high-purity, large-area, phase-pure crystals of high crystallinity. The CVT process entailed the encapsulation of stoichiometric weights of molybdenum and sulfur/selenium powders with iodine in evacuated quartz ampoules followed by growth due to the thermal gradient over a time period of ten days. Optical characterization was performed using photoluminescence (PL), Raman spectroscopy, and UV-Visible absorption spectroscopy. PL spectra indicated sharp excitonic peaks corresponding to direct transitions in monolayer-like areas, along with defect-related emissions in thicker samples. Raman spectral analysis indicated characteristic E₂g¹ and A₁g phonon modes for MoS₂, along with A₁ and E modes for MoSe₂, which reflect good crystallinity and interlayer interactions. UV-Vis measurements confirmed the direct bandgap nature of the two materials with estimated values of around 1.80 eV for MoS₂ and 1.55 eV for MoSe₂. The observations reflect the tunable optical behavior of CVT-grown TMD crystals and their integration into various optical and electronic devices. This comparative report provides valuable insights into the role of variation of chalcogen on modulation of optoelectronic responses, thus enabling material-specific engineering in layered semiconductor devices.