Type of presentation: Poster

MS-2-P-1536 Mapping electronic states in Graphene

Löffler S.1,2, Pardini L.3, Hambach R.4, Kaiser U.4, Schattschneider P.1,2, Draxl C.3
1Institute of Solid State Physics, Vienna University of Technology, Austria, 2University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Austria, 3Department of Physics, Humboldt University Berlin, Germany, 4Electron Microscopy Group of Materials Science, Ulm University, Germany
stefan.loeffler@tuwien.ac.at

Graphene and similar carbon-based materials are currently the focus of much research. In particular, their peculiar electronic properties are arousing a lot of interest. At the same time, the question of the influence of defects – such as vacancies or dopant atoms – is of particular practical importance. Recently, it has been reported that different dopant atom configurations change the charge distribution [1] and, therefore, give rise to different electron energy-loss spectrometry (EELS) signals [2]. Likewise, introducing vacancies changes the local crystal structure [3] and, hence, also the local charge distribution and EELS signal.

This gives rise to the hope to map the electronic states using energy-filtered transmission electron microscopy (EFTEM) [4]. In this work, we present predictions regarding the possibility of direct mapping of electronic states in both ideal, pristine Graphene and Graphene with defects using EFTEM. To that end, calculations of the electronic states of Graphene, with and without defects, were carried out using the full-potential all-electron density functional theory (DFT) package "exciting" [5]. Subsequently, its output was used to calculate the inelastic scattering kernels which, combined with elastic scattering calculations, ultimately result in EFTEM images.

The EFTEM images were calculated for a variety of acceleration voltages and lens aberration functions to simulate realistic conditions, as well as investigate the optimal experimental conditions. Fig. 1 shows EFTEM simulations for Graphene with and without core-hole effects included in the DFT calculations. It is clearly visible that this greatly alters the expected EFTEM images. Fig. 2 goes a step further and shows the images to be expected when mapping a vacancy. This demonstrates that EFTEM at high spatial resolution could become an invaluable tool for the study of electronic states in carbon-based materials.

[1] Meyer et al., Nat Mater 10 (2011) 209
[2] Zhou et al., PRL 109 (2012) 206803
[3] Meyer et al., Nano Lett 8 (2008) 3582
[4] Löffler et al., Ultramicroscopy 131 (2013) 39
[5] http://exciting-code.org/

The authors acknowledge financial support by the FWF (I543-N20), the DPG and the MWK Baden-Württemberg.

Fig. 1: Simulated EFTEM images for 40 keV electrons for pristine multi-layer Graphene without (left) and with (middle) core-hole effects under ideal imaging conditions. In both cases, an energy loss of 6.5 eV above the C K-edge onset was used. Additionally, the partial density of states is shown (right).

Fig. 2: Simulated EFTEM images for Graphene with a vacancy. The images show π* (left) and σ* (right) states. The circle marks the vacancy. Both images were simulated for a beam energy of 80 keV and ideal imaging conditions.