scanning tunneling microscopy
Graphene grown on SiC(0001) surfaces is an attractive material for future carbon based electronics, as it combines the exciting properties of exfoliated graphene with a manufacturing-friendly planar structure. Moreover, it has been recently demonstrated  that the structural and electronic coupling of graphene with the SiC substrate can be relieved by intercalating hydrogen under the interfacial-reconstructed carbon layer (zerolayer). The resulting quasi-free standing graphene on SiC(0001) is an even more appealing candidate for electronic applications. We use scanning tunneling microscopy (STM) to investigate and compare the atomic structure of graphene and quasi-free standing graphene layers. Combined micro-Raman and AFM analysis performed on hydrogenated and non-hydrogenated samples provide spatially resolved information on their thickness and quality.
STM images of zero- and monolayer graphene show a superstructure modulation of the hexagonal lattice due to the covalent bonds between the zerolayer and the silicon carbide substrate. Intercalation of atomic hydrogen between the zerolayer and the silicon carbide substrate breaks the covalent bonds between the zerolayer and the substrate by forming hydrogen-silicon bonds which passivate the surface, returning the carbon rich zerolayer to a state of sp2-bonded carbon, i.e. quasi-free standing graphene. STM analysis confirms that this material no longer yields the superstructure corrugation due to the interaction with the substrate. Atomically resolved images of the honeycomb lattice of both hydrogenated and non-hydrogenated samples indicate a low density of defective sites.
Correlated investigations using AFM in topography, phase imaging and lateral force mode, as well as micro-Raman were performed on hydrogenated and non-hydrogenated samples. On the latter our analysis confirmed  that monolayer graphene stripes are present close to the steps of the SiC terraces, separated by zerolayer regions. On the former the presence of a composed 2D Raman band near the steps of the SiC terraces confirmed the evolution of zerolayer regions into quasi-free-standing monolayers and showed that the monolayer regions found on non-hydrogenated samples turn into quasi-free-standing bilayer graphene after hydrogenation. Moreover, Raman spectroscopy measurements confirm that the quasi-free standing monolayer graphene investigated in this work does not have a higher density of defect sites than as-grown monolayer graphene, as the D peak obtained for hydrogenated and non-hydrogenated samples are comparable.
S. Goler, C. Coletti, V. Piazza, P. Pingue, F. Colangelo, V. Pellegrini, K. V. Emtsev, U. Starke, S. Heun, F. Beltram: Nanoscale characterisation of graphene and quasi-free standing graphene on SiC(0001), Quantum Phenomena in Graphene, other low-dimensional Materials, and optical Lattices, Erice (Sicily, Italy), July 26 - August 7, 2011 [Abstract] [Poster]
Dr. Stefan Heun
NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore
Piazza San Silvestro 12
56127 Pisa, Italy
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