F. Hiebel

Electronic and structural characterization of divacancies in irradiated graphene

This work was supported by Spain’s MICINN under Grants No. MAT2010-14902, No. CSD2010-00024, and No. CSD2007-00050, and by Comunidad de Madrid under Grant No. S2009/MAT-1467. M.M.U., I.B., F.H, P.M, J.Y.V., and J.M.G.R. also acknowledge the PHC Picasso program for financial support (project No. 22885NH).M.M.U. acknowledges financial support from MEC under FPU Grant No. AP-2004-1896. I.B. was supported by a Ramon y Cajal project of the Spanish MEC. F.H. held a doctoral support from the Region Rhone-Alpes

Interface structure of graphene on SiC: an ab initio and STM approach

High temperature treatment of SiC surfaces is a well-established technique for producing graphene directly on top of an insulating substrate. In this domain an important question is the influence of the substrate on the atomic and electronic structure of the graphene layers. This requires a detailed investigation of the interactions at the graphene-SiC interface. Surface science techniques and ab initio calculations are well suited for that purpose. In this paper, we present a brief review of the recent investigations performed in this domain by scanning tunnelling microscopy (STM) and ab initio simulations. It is largely based on the work performed in our group, but it also provides a survey of the literature in these fields. Both the so-called Si and C face of the hexagonal 6H(4H)SiC{0 0 0 1} substrates will be considered, as they show markedly different types of behaviour.

Atomic and electronic structure of monolayer graphene on 6H-SiC(0001)(3 3) : a scanning tunneling microscopy study.

We present an investigation of the atomic and electronic structure of graphene monolayer islands on the 6H-SiC(0001)(3 3) (SiC(3 3)) surface reconstruction using scanning tunneling microscopy (STM) and spectroscopy (STS). The orientation of the graphene lattice changes from one island to the other. In the STM images, this rotational disorder gives rise to various superlattices with periods in the nm range. We show that those superlattices are moir e patterns (MPs) and we correlate their apparent height with the stacking at the graphene/SiC(3 3) interface. The contrast of the MP in STM images corresponds to a small topographic modulation (by typically 0.2 A) of the graphene layer. From STS measurements we nd that the substrate surface presents a 1 :5 eV wide bandgap encompassing the Fermi level. This substrate surface bandgap subsists below the graphene plane. The tunneling spectra are spatially homogeneous on the islands within the substrate surface gap, which shows that the MPs do not impact the low energy electronic structure of graphene. We conclude that the SiC(3 3) reconstruction eciently passivates the substrate surface and that the properties of the graphene layer which grows on top of it should be similar to those of the ideal material.

Multiscale investigation of graphene layers on 6H-SiC(000-1)

In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-μm scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the μm scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

Epitaxial graphene morphologies probed by weak (anti)-localization

We show how the weak field magneto-conductance can be used as a tool to characterize epitaxial graphene samples grown from the C or the Si face of silicon carbide, with mobilities ranging from 120 to 12 000 cm2/(V·s). Depending on the growth conditions, we observe anti-localization and/or localization, which can be understood in term of weak-localization related to quantum interferences. The inferred characteristic diffusion lengths are in agreement with the scanning tunneling microscopy and the theoretical model which describe the “pure” mono-layer and bilayer of graphene [MacCann et al., Phys. Rev. Lett. 97, 146805 (2006)].

Graphene on the carbon face of SiC: Electronic structure modification by hydrogen intercalation

It has been shown that the first C layer on the SiC(0001)(2{\times}2)C surface already exhibits graphene-like electronic structure, with linear pi bands near the Dirac point. Indeed, the (2{\times}2)C reconstruction, with a Si adatom and C restatom structure, efficiently passivates the SiC(0001) surface thanks to an adatom/restatom charge transfer mechanism. Here, we study the effects of interface modifications on the graphene layer using density functional theory calculations. The modifications we consider are inspired from native interface defects observed by scanning tunneling microscopy. One H atom per 4 {\times} 4 SiC cell (5 {\times} 5 graphene cell) is introduced in order to saturate a restatom dangling bond and hinder the adatom/restatom charge transfer. As a consequence, the graphene layer is doped with electrons from the substrate and the interaction with the adatom states slightly increases. Native interface defects are therefore likely to play an important role in the doping mechanism on the C terminated SiC substrates. We also conclude that an efficient passivation of the C face of SiC by H requires a complete removal of the reconstruction. Otherwise, at variance with the Si terminated SiC substrates, the presence of H at the interface would increase the graphene/substrate interaction.

Impact of local stacking on the graphene-impurity interaction: Theory and experiments

We investigate the graphene-impurity interaction problem by combining experimental - scanning tunneling microscopy (STM) and spectroscopy (STS) - and theoretical - Anderson impurity model and density functional theory (DFT) calculations - techniques. We use graphene on the SiC(000-1)(2x2)_C reconstruction as a model system. The SiC substrate reconstruction is based on silicon adatoms. Graphene mainly interacts with the dangling bonds of these adatoms which act as impurities. Graphene grown on SiC(000-1)(2x2)_C shows domains with various orientations relative to the substrate so that very different local graphene/Si adatom stacking configurations can be probed on a given grain. The position and width of the adatom (impurity) state can be analyzed by STM/STS and related to its local environment owing to the high bias electronic transparency of graphene. The experimental results are compared to Andersons model predictions and complemented by DFT calculations for some specific local environments. We conclude that the adatom resonance shows a smaller width and a larger shift toward the Dirac point for an adatom at the center of a graphene hexagon than for an adatom just on top of a C graphene atom.