Three-fold diffraction symmetry in epitaxial graphene and the SiC substrate

Three-Fold Diﬀraction Symmetry in Epitaxial Graphene and the SiC Substrate D.A. Siegel, 1, 2 S.Y. Zhou, 1, 2 F. El Gabaly, 3 A. K. Schmid, 2 K. F. McCarty, 3 and A. Lanzara 1, 2 Department of Physics, University of California, Berkeley, CA 94720, USA Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Sandia National Laboratories, Livermore, California 94551, USA (Dated: November 15, 2009) The crystallographic symmetries and spatial distribution of stacking domains in graphene ﬁlms on 6H-SiC(0001) 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 or rhombohe- dral) 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. The past few years have witnessed a growing need to identify new methods for the synthesis of graphene ﬁlms for both basic research and industrial applications. Of all the methods explored so far, substrate growth methods seem the most promising due to the ease and reliability of growth of large-scale ﬁlms 1–3 . However, the presence of a substrate can often impose non-bulk-like structures on overlayer ﬁlms that alter their properties; one well- known example is the case of thin magnetic ﬁlms 4 . In graphene, the choice of substrate can have a major im- pact on the properties of the ﬁlm as well. For example, substrates that break the A-B sublattice symmetry (or equivalently the 6-fold rotational symmetry) of graphene, such as hexagonal boron nitride or AB-stacked bilayer graphene, result in the opening of a bandgap and the de- struction of the Dirac behavior of the quasiparticles in graphene 5–10 . On the other hand, substrates where this symmetry is preserved, such as graphene grown on the C face of SiC (which possesses azimuthal rotations between layers) can retain this Dirac behavior even for multilayer ﬁlms 3,11–13 . Therefore, understanding how graphene grows on top of a substrate is fundamental to engineering new graphene sheets with controlled properties. Here we will focus on epitaxial graphene grown on SiC(0001), one of the most studied graphene systems because of its potential for industrial application due to the pres- ence of a bandgap in measured as well as calculated spectra 9,10 . The mechanism behind this gap opening is still under debate, so understanding the precise structure of the graphene/buﬀer layer system remains an impor- tant issue 14,15 . One way to answer questions about the structure of epitaxial graphene might be through low energy elec- tron diﬀraction (LEED), which is a more direct probe of the crystal symmetry than STM 14–16 provided that the diﬀraction can be performed with a spatial resolu- tion that is smaller than the structural domains of the crystal. For example, LEED from a single Ru(0001) ter- race has the 3-fold symmetry of the hcp layer stacking, while LEED from a region containing multiple terraces has an averaged 6-fold symmetry 17 . Similar measure- FIG. 1: (Color online) (a) A LEED image (43eV) of an epitaxially-grown graphene sample. The region of intensity to the left of center is due to secondary electrons. (b) A car- toon of the LEED image in (a) that shows the relevant sets of diﬀraction spots more clearly. In both panels, two SiC spots are marked with stars, two graphite spots are marked with triangles, two 6 × 6 spots are marked with squares, and the (0,0) spot is marked with a circle. ments can be performed with greater spatial resolution by using dark-ﬁeld low-energy electron microscopy (LEEM) imaging. Dark-ﬁeld LEEM images are real-space images derived from higher order diﬀraction spots. This diﬀers from bright ﬁeld LEEM, where the images are obtained from the specularly reﬂected beam, the (0,0) diﬀraction spot. Thus, dark ﬁeld LEEM can be viewed as a tool comparable to LEED, where the dark ﬁeld LEEM im- age is a map of the intensity of a single LEED spot as a function of sample position. Combining several such im- ages obtained on inequivalent diﬀraction spots, one can determine direct evidence of asymmetries in the LEED diﬀraction peaks as a function of position in the LEEM image. Here we characterize the crystallographic structure of graphene/SiC ﬁlms and the spatial distribution of stack- ing domains by high resolution dark-ﬁeld LEEM imaging. We ﬁnd that the 6-fold symmetry is broken for the 1x1 SiC LEED spots, and for the 1x1 graphite LEED spots of multilayer (≥2 graphene layers in addition to the buﬀer layer) graphene. On the contrary, the apparent 6-fold symmetry of the graphite LEED spots is preserved in the buﬀer layer and single-layer graphene, showing that the stacking between these two layers diﬀers from that