Epitaxial graphene grown on SiC substrate has been intensively studied over the past ten years, in view of its applications in nanoelectronics. The C-terminated face of the substrate offers a particular interest due to its rotational stacking, which decouples adjacent graphene sheets. A major issue is the microscopic surface heterogeneity giving, for example, domains with varying numbers of graphene layers with different electronic and chemical properties. Energy-filtered photoelectron emission microscopy (PEEM) can correlate real space chemical and work function mapping with reciprocal space imaging of the complete band structure of micron scale regions. Therefore, this technique allows a better understanding of the local properties – morphology, chemistry of the interface and electronic properties – which is still required for the development of new electronic devices.
The photoemission threshold is directly related to the work function (WF) of a material. By fitting the spectrum of each pixel of an image series, acquired over the photoemission threshold, it is therefore possible to map the work function value over the field of view (FoV) of the microscope. This procedure has been applied on an image series of epitaxial graphene on SiC(000-1) (figure 1a). The work function variation is thought to be due to the graphene thickness and the chemistry of the graphene/SiC interface. Similarly, the intensity map of the SiC component of C 1s core level can be used to estimate the thickness variation of the graphene, and can be correlated to the WF map. When the number of graphene layer increases, the SiC peak intensity decreases, relative to the graphene one. In this way, the local thickness of the graphene film can be estimated from the relative SiC and graphene C 1s intensity; within the FoV graphene domains between 1 and 3 monolayers thick are identified.
Figure 1: a) Work function map of a 53 μm FoV, using a photon energy of 654.3 eV. b) Intensity map (arbitrary units) of the SiC substrate component of C 1s spectrum after background subtraction for a FoV of 34 μm. c) Constant energy cut in the k-space below the Dirac point, acquired at a binding energy of 1.3 eV, for a graphene bilayer area.
Thanks to the easy switching between the real space and the reciprocal space imaging modes of the NanoESCA (Omicron Nanotechnology), it is possible to obtain the complete band structure of a given area down to 10 µm2, by inserting a field aperture (FA) to select the desired region. A constant energy cut of a graphene bilayer in the k-space below the Dirac point is presented in figure 1c. It shows the presence of two sets of Dirac cones, with the well-known sixfold symmetry. They are attributed to two commensurately rotated graphene sheets, with a rotation angle of 21.9°. A fainter feature presenting also a sixfold symmetry, which appears inside the radius of the most intense ones, is due to a diffraction resulting from the supercell (g1 and g2) formed by the commensurate rotations, as shown in the figure 1c. The reciprocal space imaging mode of the NanoESCA allows immediate visualization of these diffracted cones because it probes all wave vectors parallel to the sample surface.
Dark field imaging has been performed using focused He lamp (He I =21.2 eV), on such epitaxial graphene sample grown on a SiC(000-1) substrate. The real space image of the FoV at the photoemission threshold is shown in figure 2a. Figure 2b presents a constant energy cut of the band structure, averaged over the whole FoV. Six commensurate rotation angles are observed (a-f). The contrast aperture in the diffraction plane of the microscope has then been closed to a diameter of 0.11 Å-1 at each of the Dirac cones and the microscope has been switched back to the real space mode. The corresponding real space image shows the spatial origin of photoelectrons for a specific emission angle. By repeating this experiment for each rotational angle, a spatial distribution of the commensurate rotations has been determined (figure 2c). The Dirac cones at the K and K’ point of the first Brillouin zone, typical of graphene, can therefore be used to determine a real spatially-resolved map as function of the photoelectron wave vector.
Figure 2: a) Real space image of the FoV at the photoemission threshold. b) Constant energy cut in the k-space below the Dirac point showing the commensurately rotations of the graphene sheets. c) Reconstructed valence band image from the sum of the dark field images, acquired for the six commensurate rotation angles.
The PEEM study of epitaxial graphene on SiC(000-1) allows the correlation of spatial imaging, which links the WF variation to the number of graphene layer along with its interface chemistry, and to the local band structure, highlighting diffraction effects due to commensurate rotations of the graphene sheets. Dark field PEEM completes the information with a spatially resolved map of the commensurately rotated graphene sheets.
For more information, read the following paper:
Microscopic correlation between chemical and electronic states in epitaxial graphene on SiC(000-1), C. Mathieu, N. Barrett, J. Rault, Y. Mi, B. Zhang, W. A. de Heer, C. Berger, E. Conrad and O. Renault, Physical Review B 83, 235436 (2011).
Laboratory-based real and reciprocal space imaging of the electronic structure of few layer graphene on SiC(000-1) using photoelectron emission microscopy, N. Barrett, K.Winkler, B.Kromker, E.H.Conrad, Ultramicroscopy 130, 94 (2013).
Microscopic chemical and electronic structure of few layer graphene on SiC(000-1), C. Mathieu and N. Barrett, PICO, The Omicron Nanoscience Newsletter 17, 6-8 (2013)