  Developing techniques for high-quality synthesis of mono and few-layered 2D
materials with lowered complexity and cost continues to remain an important
goal, both for accelerating fundamental research and for applications
development. We present the simplest conceivable technique to synthesize
micrometer-scale single-crystal triangular monolayers of MoS2, i.e. by direct
heating of bulk MoS2 powder onto proximally-placed substrates. Room-temperature
excitonic linewidth 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 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-strained circular samples, which allows us to
directly compare the optoelectronic properties of flat vs. strained growth from
the same growth runs. Our method allows, for the first time, to quantitatively
compare the impact of strain on excitonic and Raman peak positions on
identically-synthesized 2D-MoS2. Strain leads to average Red-shifts of ~ 30 meV
in the A-exciton position, and ~ 2 cm-1 in the E12g Raman peak in these
samples. Our findings open-up several new 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
mass-production of 2D crystals for commercial purposes. Moreover, we believe
this technique can also be extended to other transition metal dichalcogenides
and other layered materials.
