  The monolayer WSe2 is interesting and important for future application in
nanoelectronics, spintronics and valleytronics devices, because it has the
largest spin splitting and longest valley coherence time among all the known
monolayer transition-metal dichalcogenides (TMDs). To obtain the large-area
monolayer TMDs' crystal is the first step to manufacture scalable and
high-performance electronic devices. In this letter, we have successfully
fabricated millimeter-sized monolayer WSe2 single crystals with very high
quality, based on our improved mechanical exfoliation method. With such
superior samples, using standard high resolution angle-resolved photoemission
spectroscopy, we did comprehensive electronic band structure measurements on
our monolayer WSe2. The overall band features point it to be a 1.2eV direct
band gap semiconductor. Its spin-splitting of the valence band at K point is
found as 460 meV, which is 30 meV less than the corresponding band splitting in
its bulk counterpart. The effective hole masses of valence bands are determined
as 2.344 me at Gamma, and 0.529 me as well as 0.532 me at K for the upper and
lower branch of splitting bands, respectively. And screening effect from
substrate is shown to substantially impact on the electronic properties. Our
results provide important insights into band structure engineering in monolayer
TMDs. Our monolayer WSe2 crystals may constitute a valuable device platform.
