Andrew WEE

National University of Singapore, Singapore

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Title: Epitaxial Graphene: Growth, Structure and Molecular Interactions

The discovery of graphene has opened up a new paradigm in nanoelectronics that could offer better performance than conventional semiconductor devices owing to its unusual Dirac fermion behaviour of its electrons that gives rise to superior mobility and the unique anomalous quantum Hall effect [1-3]. One of the key challenges is the growth of high quality epitaxial graphene, and the main approaches are the chemical vapour deposition of hydrocarbons on metal surfaces and thermal decomposition of silicon carbide (SiC).

In this talk, I describe our studies using in situ scanning tunnelling microscopy (STM), synchrotron photoemission (PES) and density functional theory (DFT) calculations to investigate the structure of the various reconstructions of 6H-SiC(0001) prior to its thermal decomposition to form epitaxial graphene (EG) [4]. Using a cobalt-decoration technique coupled with STM, the evolution of epitaxial graphene was found to preferentially begin at step edges of the silicon carbide surface and occurs with the loss of Si and breakdown of the C-rich (√6 x √6)R30° template, which provides the C source for graphene growth [5]. A new C-rich phase is formed at the interface which acts as a buffer layer for graphene from the underlying SiC substrate. STM reveals that graphene lies 2.3 ±0.3 Å above the buffer layer, larger than sp3 C-C bond length (1.54 Å) but shorter than graphite interlayer separation (3.37 Å), suggesting a pseudo van der Waals interfacial interaction. We show that the transition from monolayer EG to trilayer EG adopts a bottom-up growth mechanism [6]. With the increase in annealing temperature, the fluorescence yield of Si K-edge NEXAFS indicates an increase in disorder of Si atoms in the SiC substrate beneath the surface due to out-diffusion of Si atoms to the surface forming increased Si vacancies [7]. The interaction of EG and the SiC substrate is critical to its electronic and physical properties. Raman spectroscopy was used to study the structure of EG and its interaction with SiC substrate [8]. All the Raman bands of EG are blueshifted from that of bulk graphite and graphene made by micromechanical cleavage, and this is attributed to compressive strain induced by the substrate. We demonstrate that the electronic structures of EG grown on Si- and C-terminated SiC substrates are quite different.

We demonstrate that EG thermally grown on 6H-SiC(0001) can be p-doped via a novel surface transfer doping scheme of modifying the surface with the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) [9]. PES reveals that electron transfer from graphene to adsorbed F4-TCNQ is responsible for the p-type doping of graphene. This surface transfer doping scheme by surface modification with appropriate molecular acceptors represents a simple and effective method to non-destructively dope EG for future nanoelectronics applications. Preliminary molecular self-assembly studies on EG will also be discussed.

[1] K. S. Novoselov, A. K.Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, A. A. Firsov, Nature 438, 197 (2005).
[2] Y. Zhang, Y.-W. Tan, H. L. Stormer, P. Kim, Nature 438, 201 (2005).
[3] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First, W. A. de Heer, Science 312, 1191 (2006)
[4] W. Chen, H. Xu, L. Liu, X. Y. Gao, D. C. Qi, G. W. Peng, S. C. Tan, Y. P. Feng, K. P. Loh, A. T. S. Wee, Surf, Sci. 596, 176 (2005).
[5] S. W. Poon, W. Chen, E. S. Tok, A. T. S. Wee, Appl. Phys. Lett. 92, 104102 (2008).
[6] H. Huang, W. Chen, S. Chen, A. T. S. Wee, ACS Nano 2, 25 (2008)
[7] X. Y. Gao, S. Chen, T. Liu, W. Chen, A. T. S. Wee, T. Nomoto, S. Yagi, K. Soda, J. Yuhara, Phys. Rev. B 78, 201404 (2008).
[8] Z. H. Ni, W. Chen, X. F. Fan, J. L. Kuo, T. Yu, A. T. S. Wee, Z. X. Shen, Phys. Rev. B 77, 115416 (2008).
[9] W. Chen, S. Chen, D. C. Qi, X. Y. Gao, A. T. S. Wee, J. Am. Chem. Soc. 129, 10418 (2007).