  The ability to synthesize high-quality samples over large areas and at low
cost is one of the biggest challenges during the developmental stage of any
novel material. While chemical vapor deposition (CVD) methods provide a
promising low-cost route for CMOS compatible, large-scale growth of materials,
it often falls short of the high-quality demands in nanoelectronics and
optoelectronics. We present large-scale CVD synthesis of single- and few-
layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us
to obtain extremely high-quality single-crystal monolayer MoS2 samples with
field-effect mobility exceeding 30 cm2/Vs in monolayers. These samples can be
readily synthesized on a variety of substrates, and demonstrate a high-degree
of optoelectronic uniformity in Raman and photoluminescence mapping over entire
crystals with areas exceeding hundreds of square micrometers. Owing to their
high crystalline quality, Raman spectroscopy on these samples reveal a range of
multi-phonon processes through peaks with equal or better clarity compared to
past reports on mechanically exfoliated samples. This enables us to investigate
the layer thickness- and substrate-dependence of the extremely weak phonon
processes at 285 cm-1 and 487 cm-1 in 2D MoS2. The ultra-high,
optoelectronic-grade crystalline quality of these samples could be further
established through photocurrent spectroscopy, which clearly reveal excitonic
states at room temperature, a feat that has been previously demonstrated only
on device fabricated with mechanically exfoliated that were artificially
suspended across trenches. Our method reflects a big step in the development of
atomically thin, 2D MoS2 for scalable, high-quality optoelectronics.
