Nano-resolution reveals the true nature of graphene
In short: Graphene grown on the basal planes of silicon carbide is considered a most promising route for carbon-based nano-electronics. Two nonequivalent faces of silicon carbide can be used for this purpose, the carbon-face and the silicon-face. It was claimed that these two faces result in graphene with fundamentally different electronic properties. Here we reveal the actual similarity between graphene layers on the two faces by experiments on a nanometer scale. Moreover, the apparent difference previously seen in standard experiments can now be explained as the collective effect of microscopic grains of graphene formed on the carbon-face.
Graphene grown on the basal planes of silicon carbide is considered a most promising route for carbon-based nano-electronics. Two nonequivalent faces of silicon carbide can be used for this purpose, the carbon-face and the silicon-face. It has been claimed that these two faces produce graphene layers with fundamentally different electronic properties. While the electronic structure significantly changes with the number of stacked graphene layers on the silicon-face, single layer behavior has been reported for multilayer graphene on the carbon face.
Electrons in graphene exist only within certain energy bands, with a structure determined by the relation between the electron’s energy and its momentum. Isolated layers of graphene exhibit at the Fermi level a linear energy-momentum relation – a peculiar property shared with mass-less particles in the theory of relativity - giving the material unique and outstanding electronic properties. Additional graphene layers stacked on top of the first will break the symmetry of the atomic arrangement and cause splitting in the band structure. The single energy band splits into two or three bands for two or three stacked layers of graphene, respectively.
Graphene grown on the silicon-face of silicon carbide do exhibit the linear band structure of a single layer that splits into two and three bands for two and three layers, respectively. On the contrary, experiments performed on layers grown on the carbon-face apparently revealed single layer characteristics, even if it was tens of layers thick. It was therefore proposed that the stacking sequence of the graphene involves alternating 0° and 30° rotations of the layers relative to the silicon carbide substrate, in such a way that the symmetry as well as the single band structure is maintained.
Here we demonstrate that no such fundamental difference in the stacking sequence between silicon- and carbon-face graphene does exists. However, unlike silicon-face graphene, electron microscopy, diffraction and spectroscopy experiments show the formation of micron-sized grains (crystallographic domains) of graphene on the carbon-face. These grains can be single or multilayer graphene with different azimuthal orientations, but no rotational disorder between adjacent graphene layers within the grains. In order to measure the band structure of individual grains, we performed angle resolved photoelectron spectroscopy down to about 100-nanometer length scale (nano-ARPES), provided by the ANTARES beam line at the Soleil synchrotron radiation laboratory in France. The experimental results, displayed in the figure, show that multilayer graphene exhibits multiple bands and single layer graphene a single band - just like graphene grown on the silicon face. Moreover, a comparison of the details in the experimental data with a theoretical model reveals that the stacking sequence is identical for the two faces of the silicon carbide substrate. Thus, our findings imply a similar interaction between graphene layers on carbon-face and silicon-face silicon carbide, contrary to earlier claims.
Details of the research are described in Nature Scientific Reports 4 4157 (2014)
Leif I Johansson, professor emeritus
Phone: +46 (0)13 28 12 62
- European Science Foundation within the EuroGRAPHENE (EPIGRAT)
- Swedish Research Council (VR), including Linaeus Grant
- Swedish Foundation of Strategic Research (SSF)
- Swedish National Infrastructure for Computing (SNIC)
- Centre National de la Researche Scientifique (CNRS)
- Commissariat a l’Energie Atomique et aux Energies Alternatives (CEA)
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Last updated: 05/02/14