Three-Fold Diffraction Symmetry in Epitaxial Graphene and the SiC   Substrate
  The crystallographic symmetries and spatial distribution of stacking domains in graphene films on SiC have been studied by low energy electron diffraction (LEED) and dark field imaging in a low energy electron microscope (LEEM). We find that the graphene diffraction spots from 2 and 3 atomic layers of graphene have 3-fold symmetry consistent with AB (Bernal) stacking of the layers. On the contrary, graphene diffraction spots from the buffer layer and monolayer graphene have apparent 6-fold symmetry, although the 3-fold nature of the satellite spots indicates a more complex periodicity in the graphene sheets. 
David A. Siegel, Shuyun Zhou, Farid El Gabaly, Andreas K. Schmid,   Kevin F. McCarty, and Alessandra Lanzara
cond-mat.mtrl-sci