  High quality WSe2 films have been grown on bilayer graphene (BLG) with
layer-by-layer control of thickness using molecular beam epitaxy (MBE). The
combination of angle-resolved photoemission (ARPES), scanning tunneling
microscopy/spectroscopy (STM/STS), and optical absorption measurements reveal
the atomic and electronic structures evolution and optical response of
WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor,
when integrated in a BLG-based heterostructure, thus shifting the
direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit,
WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the
largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The
exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value
that is orders of magnitude larger than that of conventional 3D semiconductors,
yet small as compared to other 2D transition metal dichalcogennides (TMDCs)
semiconductors. Finally, our finding regarding the overall modification of the
electronic structure by an alkali metal surface electron doping opens a route
to further control the electronic properties of TMDCs.
