  We have fabricated a high-quality monolayer WSe2 film on bilayer graphene by
epitaxial growth, and revealed the electronic states by spin- and
angle-resolved photoemission spectroscopy. We observed a direct energy gap at
the Brillounin-zone corner in contrast to the indirect nature of gap in bulk
WSe2, which is attributed to the lack of interlayer interaction and the
breaking of space-inversion symmetry in monolayer film. A giant spin splitting
of ~0.5 eV, which is the largest among known monolayer transition-metal
dichalcogenides, is observed in the energy band around the zone corner. The
present results suggest a high potential applicability of WSe2 to develop
advanced devices based with the coupling of spin- and valley-degrees of
freedom.
