Experimental and Theoretical Spectroscopy of Functionalized Graphene – ETSFG

Graphene, a single layer of graphite, is a material that is currently receiving a lot of attention. The linear crossing of the valence and conduction bands at the Fermi level makes it a material that is conceptually interesting ("Dirac electron behaviour") and that is promising for construction of electronic devices (e.g., ambipolar field-effect transistors). The two main production methods are mechanical exfoliation from graphite crystals ("the scotch tape method") and epitaxial growth on silicon carbide. With the first method one can produce high quality samples (evidenced by a large electron mobility) but the size remains rather limited. With the second method, large samples can be produced but the electron mobility is lower. In this project we will investigate alternative routes towards the production of large and high-quality decoupled graphene layers and we will assess its electronic and optical properties in different environments. On the one hand we will use the catalytic growth of graphene on metal substrates (in particular nickel) with subsequent decoupling from the substrate (e.g., by intercalation). On the other hand, we will explore the decoupling of the different layers in graphite single crystals. This can be achieved, e.g., by intercalation of alkali, alkali earth, or rare earth atoms. In these systems, the intercalation can lead to a complete decoupling of the sheets and thus to a linear band-structure of the pi-bands around the Fermi level. Different spectroscopic methods will be used to probe the structural, electronic, and vibrational properties of graphene and its interaction with the environment. This requires a combined experimental and theoretical/computational approach. Angular resolved photo-emission spectroscopy (ARPES) resolves the electronic-band structure. Raman spectroscopy and inelastic X-ray scattering resolve the vibrational properties. High resolution electron energy loss spectroscopy (EELS) will yield both information on the momentum-dependent dielectric properties (e.g. plasmons) and on the phonon dispersion. The electronic and vibrational properties are expected to sensitively depend on the interaction of the graphene layer (band hybridization by substrate interaction, doping, impurity formation, etc.). These changes will be monitored in detail by our experiments. The spectroscopy measurements will be accompanied by detailled ab-initio calculations. We will calculate the structural properties of graphene on different substrates (orientation, buckling, charge transfer, ...) and the vibrational properties (phonons) with the methods of density-functional theory. Interpretation of photo-emission spectra will require detailled calculations of the quasi-particle band-structure with the methods of many-body perturbation theory. Part of these calculations can be done with existing ab-initio codes. However, in order to investigate very complex systems (requiring many atoms in the unit cell) where many-body perturbation theory is computationally too demanding, conceptual and computational development is necessary. We will use density-functional theory with the new generation of hybrid functionals for calculation of the quasi-particle band structure. A part of this project will thus be devoted to the implementation of these functionals in a public ab-initio code for solid state calculations and to testing/adapting functionals for graphitic systems. The long term goal of the project is to understand and controll the interaction of graphene with the substrate or with other layers (in intercalation compounds). This will eventually enable us to "peel off" single layers of graphene in order to deposit them on an insulating substrate. And it will enable us to determine the optimal conditions for application in transport, electronics, and optoelectronics.