  Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot
of attention recently, because of their interesting electronic, optical, and
mechanical properties. Among large numbers of TMDCs, monolayer of tungsten
diselenides (WSe2) is of particular interest since it possesses a direct band
gap and tunable charge transport behaviors, which make it suitable for a
variety of electronic and optoelectronic applications. Direct synthesis of
large domains of monolayer WSe2 and their growth mechanism studies are
important steps toward applications of WSe2. Here, we report systematical
studies on ambient pressure chemical vapor deposition (CVD) growth of monolayer
and few layer WSe2 flakes directly on silica substrates. The WSe2 flakes were
characterized using optical microscopy, atomic force microscopy, Raman
spectroscopy, and photoluminescence spectroscopy. We investigated how growth
parameters, with emphases on growth temperatures and durations, affect the
sizes, layer numbers, and shapes of as-grown WSe2 flakes. We also demonstrated
that transport properties of CVD-grown monolayer WSe2, similar to
mechanically-exfoliated samples, can be tuned into either p-type or ambipolar
electrical behavior, depending on the types of metal contacts. These results
deepen our understandings on the vapor phase growth mechanism of WSe2, and may
benefit the uses of these CVD-grown monolayer materials in electronic and
optoelectronics.
