Elucidating the Role of Seeding Promoters on MoSe₂ Synthesis and its Impact on Optoelectronic Properties

Monolayer transition metal dichalcogenides (TMDs) are attractive for future optoelectronic and valleytronic applications due to their direct band gap and preferential valley filling in response to circularly polarized light. Despite this promise, few processes exist to synthesize single crystals of primarily monolayers over a large area. Chemical vapor deposition (CVD) is an attractive option to produce TMDs at the research scale; however, selenide-based TMDs (e.g., MoSe₂ and WSe₂) are particularly difficult to synthesize. To promote lateral growth, large organic molecules are frequently deposited onto the substrate prior to growth. The role of these promoters during synthesis and their subsequent effect on TMD properties is unclear. In this work, we synthesize monolayer MoSe₂ on SiO₂ with and without the seeding agent, PTAS, and find significant differences in their growth behavior. Our results suggest that PTAS acts as a reservoir to increase the local concentration of Mo and Se precursors, thereby promoting MoSe₂ growth. We explain the diminished PL in the PTAS-assisted MoSe₂ as a consequence of the shorter nucleation time, which results in exposure to the effects of high temperature. These findings serve as important steps towards the scalable growth of monolayer TMDs.