Adjusting the electron transport of graphene structures
E.K. Athanassiou, F.M. Koehler, R.N. Grass, W.J. Stark
Istitute for Chemical and Bioengineering, ETH Zurich, CH
Keywords: carbon, tunelling, graphene, electron
Abstract:Graphene electronic properties have attracted considerable interest in the past years. In order to develop novel two- and three dimensional structures as interesting alternatives for electronic solutions reliable and pattern-controlled strategies to adjust the electronic properties and transport of graphene are needed. Latter is illustrated by using two approaches, where the electronic properties of graphene could be engineered either by covalent chemistry or variation of their layers thickness. Graphene layer thickness. Graphene is a typical zero gap semiconductor. Still their electronic properties strongly change by varying the thickness of graphene layers. Here we present the production of core/shell type materials consisting of a conductive metal copper core and a protective graphene shell. The influence of the layer thickness on their electron transport could be shown by a simple macroscopic property such as the electrical conductivity. Depending on the number of graphene layers the composite exhibits either a highly sensitive pressure and temperature depending electrical resistivity (bi-trilayer graphene) or conductive properties with no sensitivity (multilayer graphene). The thin (bi-, trilayer) graphene shell exhibits an insulating behavior inducing a high energy band gap. In contrast multilayer (5-7 layers) graphene behaves like a thin graphite film resulting in a material with no sensitive electrical properties. The conduction mechanism of such trilayer graphene coated copper materials is predominately based on a tunneling mechanism. whereas the multilayer graphene composite follows a VRH conduction mechanism. Since the quantum phenomenon tunneling is highly sensitive, the here described large scale assembly of tunneling barriers can be adjusted just by the thickness of the applied graphene layers and described by the electrical resistivity. Adjustment of surface potential by chemistry. By combining well established radical chemistry at ambient conditions and classical lithography, we find that the structure and electronic properties of graphene-like carbon structures can be permanently altered through covalent chemical functionalization. Depending on the covalently attached chemical groups, the surface potential of graphene changes by injecting or removing the electron density from the graphene surface. We further demonstrate how the classic Hammett concept and the Linear Free Enthalpy Relationship from organic chemistry can be used to predict the surface potential shifts in graphene-like carbon surfaces. References:  E.K. Athanassiou, R.N. Grass and W.J. Stark, Nanotechnology, 17, 1668, 2006.  E.K. Athanassiou, C. Mensing and W.J. Stark, Sens. Actuators A, 138, 120, 2007.  E.K. Athanassiou, et al. Phys. Rev. Lett., 101, 166804, 2008.  F.M. Koehler, et al. Angew. Chem. Int. Ed., 47, 1, 2008.