stm001

Nanoscale characterisation of graphene and quasi-free standing graphene on SiC(0001)

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 [1] that the structural and electronic coupling of graphene with the SiC substrate can be relieved by intercalating hydrogen under the interfacial-reconstructed carbon layer ( buffer layer). The resulting quasi-free standing monolayer graphene (QFMLG) 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.

As shown in Fig. 1, STM images of the buffer layer show a superstructure modulation of the hexagonal lattice due to the covalent bonds between the buffer layer and the silicon carbide substrate. Intercalation of atomic hydrogen between the buffer layer and the silicon carbide substrate breaks the covalent bonds between the buffer layer and the substrate by forming hydrogen-silicon bonds which passivate the surface, returning the carbon rich buffer layer to a state of sp2-bonded carbon, i.e. quasi-free standing monolayer 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.

Fig. 1 – STM images of (a, b) the buffer layer and (d) QFMLG. Panel (a) shows the long-range periodicity imposed on the buffer layer by the substrate. The solid and dashed diamond designates the quasi-(6 x 6) and 6sqrt(3). unit cell, respectively. Images in panel (a) were taken with a sample bias of +1.7 V. Under optimal tunneling conditions (main image in panel a) as opposed to earlier stages (inset in (a)) the atomic lattice superimposed on the quasi-(6 x 6) periodicity is revealed. Panels (b, d) are zoomed-in images of the buffer layer and QFMLG imaged with a sample bias of .0.223 V and +0.103 V, respectively. The upper insets in (b, d) present the structural models of the buffer layer and QFMLG, respectively. The lower insets in panels (b) and (d) are zoomed in 2D Fast Fourier Transforms (2DFFT) of one of the (1 x 1) spots of the graphene lattice with the quasi-(6 x 6) satellite spots visible only on the buffer layer. Scale bar 0.58 / nm.. Panel (c) shows atomically resolved STM images taken on the buffer layer and QFMLG and the corresponding line profiles along the graphene periodicity. The STM images in panel (c) have been filtered to remove noise. All measurements were taken in constant-current mode with the current set to 0.3 nA.

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 [2] that monolayer graphene stripes are present close to the steps of the SiC terraces, separated by buffer layer regions. On the former the presence of a composed 2D Raman band near the steps of the SiC terraces confirmed the evolution of buffer layer 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.

References

C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, U. Starke, Phys. Rev. Lett. 103, 246804 (2009).
T. Ohta et al., Phys. Rev. B 81, 121411(R), 2010.

Publications

S. Goler, C. Coletti, V. Piazza, P. Pingue, F. Colangelo, V. Pellegrini, K. E. Emtsev, S. Forti, U. Starke, F. Beltram, and S. Heun: Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene, Carbon 51 (2013) 249 – 254.
RHK Image of the Month - April 2013.

Presented at

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]
Sarah Goler: Scanning tunnelling microscopy to image the zerolayer of epitaxial graphene grown on SiC(0001): the building block of all graphene and graphene derivatives of SiC(0001), Columbia University, December 2011. [Abstract] [Talk]