Scanning probe microscopy is a powerful tool to probe low-dimensional systems. The local information provided by scanning probe microscopy is invaluable for studying effects such as interactions and scattering. Using this approach, we have probed the local electronic properties of graphene. The honeycomb lattice in graphene creates a unique linear dispersion relation and the charge carriers behave as massless fermions near the Dirac point. We have studied the effect of charged impurities and the underlying substrate on the local density of states. We find that long‑range scattering from charged impurities locally shifts the charge neutrality point leading to electron and hole doped regions. By using boron nitride as a substrate, we observe an improvement in the electronic properties of the graphene as well as a moire pattern due to the misalignment of the graphene and boron nitride lattices [1]. We find that the periodic potential due to the boron nitride substrate creates a set of 6 new superlattice Dirac points in graphene [2]. The ultraflat and clean nature of graphene on boron nitride devices allows for the observation of scattering from buried step edges, which is used to map the dispersion relation [3]. More complicated graphene heterostructures can be created by adding additional layers or other two-dimensional materials. Our latest results with trilayer graphene and rotated bilayers will also be discussed.

[1] J. Xue et al., Nature Materials 10, 282 (2011).

[2] M. Yankowitz et al., Nature Physics 8, 382 (2012).

[3] J. Xue et al., Phys. Rev. Lett. 108, 016801 (2012).

Based on the unique electronic properties and high chemical stability of graphene, a number of exciting applications are generated. However, since graphene films are only one or a few atomic layers thick, chemical analysis is hampered by the much larger information depth of most techniques. This leaves a gap which can be filled by Low Energy Ion Scattering (LEIS). LEIS is known for its extreme surface sensitivity. It gives a quantitative analysis of the outer atomic layer of a surface as well as in-depth information [1]. It can thus be used to selectively analyze a graphene layer. In general [2], the atomic sensitivities are independent of the neighboring atoms (no matrix effects), so a quantitative elemental characterization is feasible. Since carbon is a common contamination in surface analysis, it is crucial to distinguish such contamination from the carbon in graphene.

In an earlier LEIS study [3] of carbon segregation in C-doped rhenium it was shown that by varying the temperature the carbon species can be altered reversibly into a carbidic or graphitic state. The graphitic layer was one or a few atoms thick (depending on the temperature). A strong matrix effect was observed in LEIS, which is believed to be typical for graphene. The sp-band of graphitic carbon is so wide that it extends to the He 1s level. This gives a strong quasi-resonance neutralization of the scattered He⁺ ions.

In the present study graphene layers were grown on copper foils and then transferred to an oxidized Si wafer [4]. The samples were analyzed with a dedicated high-sensitivity LEIS instrument. As reference for a carbon containing material without a wide conduction band, silicon rubber was chosen.

It was found that the neutralization of He⁺ by graphene is much stronger than by the rubber. For example, the neutralization of 1.5 keV He⁺ ions is almost 100x more effective for graphene.

It will be shown how it is possible to quantify the amount of carbon contamination (hydrocarbons, alcohols, etc.) on graphene using the differences in neutralization. Thus LEIS cannot only be used to verify the closure of the graphene layer, its thickness and purity, but also to check its (wide band) electronic structure.

[1] H.H. Brongersma, Low-Energy Ion Scattering, in: Characterization of Materials, J. Wiley & Sons (2012). DOI: 10.1002/0471266965.com144

[2] H.H. Brongersma, M. Draxler, M. de Ridder, P. Bauer, Surf. Sci. Repts 62 (2007) 63-109.

[3] L.C.A. van den Oetelaar, S.N. Mikhailov, H.H. Brongersma, Nucl. Instrum. Meth. B85 (1994) 420.

[4] Xuesong Li et al., Science 324, 1312 (2009). DOI: 10.1126/science.1171245