MoS₂ Nanosheets with Narrowest Excitonic Line Widths Grown by Flow-Less Direct Heating of Bulk Powders: Implications for Sensing and Detection

Developing techniques for the high-quality synthesis of mono and few-layered two-dimensional (2D) materials with lowered complexity and cost continues to remain an important goal, both for accelerating fundamental research and for application development. We present the simplest conceivable technique to synthesize micrometer-scale single-crystal, less than 1 nm thick, triangular monolayers of MoS2, i.e., by direct heating of bulk MoS2 powder onto proximally placed substrates. Room-temperature excitonic line width values of our samples are narrower and more uniform than those of 2D-MoS2 obtained by most other techniques reported in literature, and comparable to those of ultraflat boron–nitride (h-BN)-capped mechanically exfoliated samples, indicative of their high quality. Feature-rich Raman spectra absent in samples grown or obtained by most other techniques, also stand out as a testament of the high quality of our samples. A contact-growth mode facilitates direct growth of crystallographically wrinkled circular samples, which allows us to directly compare the optoelectronic properties of flat vs wrinkled growth from the same growth runs. Our method allows, for the first time, to quantitatively compare the impact of wrinkle on excitonic and Raman peak positions on identically synthesized 2D-MoS2. Wrinkles lead to average red shifts of ∼30 meV in the A-exciton position and ∼2 cm–1 in the E2g1 Raman peak in these samples. Our findings open up several possibilities that expand 2D material research. By eliminating the need for carrier-gas flow, mechanical motion, or chemical reactions, our method can be either miniaturized for substantially low-cost, high-quality scientific research or potentially scaled-up for the mass-production of 2D crystals for commercial purposes. Moreover, we believe that this technique can also be extended to other transition metal dichalcogenides and other layered materials, providing scientists and engineers a low-cost platform for advancing research in sensing, detection, and fundamental discoveries.