Millimeter-Scale Continuous Film of MoS₂ Synthesized Using a Mo, Na, and Seeding Promoter-Based Coating as a Solid Precursor

Abstract

While the chemical vapor deposition technique can be used to fabricate 2D materials in a larger area, materials like MoS2 have limited controllability due to their lack of self-controlling nature. This article presents a new technique for synthesizing a void-free millimeter-scale continuous monolayer MoS2 film through the diffusion of a well-controlled Mo, Na, and seeding promoter-based coating under a low-pressure N2 atmosphere. Compared to the conventional method, this technique provides precise control of solid precursors, where MoS2 grows next to the coating. At 800 °C, the synthesized MoS2 showed a uniform single-layer MoS2 film; however, a Na-free coating showed nanoscale voids and poor crystal quality, which are attributed to a higher edge-attachment barrier that slows down the MoS2 lateral growth. The synthesized MoS2 with Na-containing solution showed an intense PL peak with a 1.86 eV band gap. Even at the relatively low temperature of 700 °C, compared to the Na-excluded condition, MoS2 showed almost two times higher area coverage with a comparatively larger crystal size. This finding may assist in the future development of MoS2-based electronic and optoelectronic devices such as transistors and photodetectors.

Development of a novel theory of pressure-induced nucleation in supercritical carbon dioxide

“…Nucleation driving force, Δg PI-SCPN , was defined as the reduction of Gibbs energy caused by the phase transformation of single solute molecules. Substituting eqn (8) and (13) into eqn (4),…”

“…8,9 Taking MoS 2 as an example, a related study 10 suggested that utilizing the "Seeding Promoter" on the substrate could lower the free energy for the nucleation, and benefit the layer growth by increasing the surface adhesive energy; meanwhile, the growth temperature was reduced and the film size, crystallinity and continuity were effectively increased. [11][12][13] Supercritical fluid deposition (SCFD) provides a flexible alternative to CVD. 14,15 Supercritical carbon dioxide (scCO 2 ), as a commonly used supercritical fluid (SCF), 16 has low viscosity, 17 high diffusivity and almost zero surface tension.…”

“…8,9 Taking MoS 2 as an example, a related study 10 suggested that utilizing the “ Seeding Promoter ” on the substrate could lower the free energy for the nucleation, and benefit the layer growth by increasing the surface adhesive energy; meanwhile, the growth temperature was reduced and the film size, crystallinity and continuity were effectively increased. 11–13…”