  Transition metal dichalcogenides (TMDs) are van der Waals layered materials
with sizable and tunable bandgaps, offering promising platforms for
two-dimensional electronics and optoelectronics. To this end, the bottleneck is
how to acquire high-quality single crystals in a facile and efficient manner.
As one of the most widely employed method of single-crystal growth,
conventional chemical vapor transport (CVT) generally encountered problems
including the excess nucleation that leads to small crystal clusters and slow
growth rate. To address these issues, a seed crystal is introduced to suppress
the nucleation and an inner tube is adopted as both a separator and a flow
restrictor, favoring the growth of large-size and high-quality TMD single
crystals successfully. Three examples are presented, the effective growth of
millimeter-sized MoSe2 and MoTe2 single crystals, and the greatly shortened
growth period for PtSe2 single crystal, all of which are synthesized in high
quality according to detailed characterizations. The mechanism of seeded CVT is
discussed. Furthermore, a phototransistor based on exfoliated multi-layered
MoSe2 displays excellent photoresponse in ambient conditions, and considerably
rapid rise and fall time of 110 and 125 us are obtained. This work paves the
way for developing a facile and versatile method to synthesize high-quality TMD
single crystals in laboratory, which could serve as favorable functional
materials for potential low-dimensional optoelectronics.

Materials Science