Spin splitting and strain in epitaxial monolayer WSe2 on graphene

We present the electronic and structural properties of monolayer WSe2 grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the WSe2 valence band at (K) over bar was Delta(so) = 0.469 +/- 0.008 eV, as determined by angle-resolved photoemission spectroscopy. Synchrotron-based grazingincidence in-plane x-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe2 to be a(wse2) = 3.2757 +/- 0.0008 angstrom. This indicates a lattice compression of -0.19% relative to bulk WSe2. By using the experimentally determined graphene lattice constant (a(MLG) = 2.4575 +/- 0.0007 angstrom), we found that a 3 x 3 unit cell of the slightly compressed WSe2 is perfectly commensurate with a 4 x 4 graphene lattice with a mismatch below 0.03%, which could explain why the monolayer WSe2 is compressed on MLG. From XRD and first-principles calculations, we conclude that the observed size of strain will affect Delta(so) only on the order of a few meV. In addition, angle-resolved, ultraviolet, and x-ray photoelectron spectroscopies shed light on the band alignment between WSe2 and MLG/SiC and indicate electron transfer from graphene to the WSe2 monolayer. As further revealed by atomic force microscopy, the WSe2 island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of WSe2 favors the weak van der Waals interactions with graphene, while it is perturbed by the influence of the SiC substrate and its carbon buffer layer.