Characterizing edge and stacking structures of exfoliated graphene by photoelectron diffraction

The two-dimensional C 1s photoelectron intensity angular distributions (PIADs) and spectra of exfoliated graphene flakes and crystalline graphite were measured using a focused soft X-ray beam. Suitable graphene samples were selected by thickness characterization using Raman spectromicroscopy after transferring mechanically exfoliated graphene flakes onto a 90-nm-thick SiO2 film. In every PIAD, a Kagomé interference pattern was observed, particularly clearly in the monolayer graphene PIAD. Its origin is the overlap of the diffraction rings formed by an in-plane C-C bond honeycomb lattice. Thus, the crystal orientation of each sample can be determined. In the case of bilayer graphene, PIAD was threefold-symmetric, while those of monolayer graphene and crystalline graphite were sixfold-symmetric. This is due to the stacking structure of bilayer graphene. From comparisons with the multiple scattering PIAD simulation results, the way of layer stacking as well as the termination types in the edge regions of bilayer graphene flakes were determined. Furthermore, two different C 1s core levels corresponding to the top and bottom layers of bilayer graphene were identified. A chemical shift to a higher binding energy by 0.25 eV for the bottom layer was attributed to interfacial interactions. (author)