  Using in situ low-energy electron microscopy and density functional theory,
we studied the growth structure and work function of bilayer graphene on
Pd(111). Low-energy electron diffraction analysis established that the two
graphene layers have multiple rotational orientations relative to each other
and the substrate plane. We observed heterogeneous nucleation and simultaneous
growth of multiple, faceted layers prior to the completion of second layer. We
propose that the facetted shapes are due to the zigzag-terminated edges
bounding graphene layers growing under the larger overlying layers. We also
found that the work functions of bilayer graphene domains are higher than those
of monolayer graphene, and depend sensitively on the orientations of both
layers with respect to the substrate. Based on first-principles simulations, we
attribute this behavior to oppositely oriented electrostatic dipoles at the
graphene/Pd and graphene/graphene interfaces, whose strengths depend on the
orientations of the two graphene layers.
