  Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin
direct-gap semiconductors with potential applications in nanoelectronics,
optoelectronics, and electrochemical sensing. Recent theoretical and
experimental efforts suggest that they are ideal systems for exploiting the
valley degrees of freedom of Bloch electrons. For example, Dirac valley
polarization has been demonstrated in mechanically exfoliated monolayer MoS2
samples by polarization-resolved photoluminescence, although polarization has
rarely been seen at room temperature. Here we report a new method for
synthesizing high optical quality monolayer MoS2 single crystals up to 25
microns in size on a variety of standard insulating substrates (SiO2, sapphire
and glass) using a catalyst-free vapor-solid growth mechanism. The technique is
simple and reliable, and the optical quality of the crystals is extremely high,
as demonstrated by the fact that the valley polarization approaches unity at 30
K and persists at 35% even at room temperature, suggesting a virtual absence of
defects. This will allow greatly improved optoelectronic TMDC monolayer devices
to be fabricated and studied routinely.
