Theory and Spectroscopy on functionalized graphene

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 behavior") and that is promising for the 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 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 with the intrinsic 2D Dirac electrons (linear dispersion at K) as a starting point. We will engineer its electronic and optical properties by modifying the different environments in a controlled manner. This point will be also addressed in parallel to step one after a detailed understanding of the influence of the different types of interaction on this archetypical 2D electron system. Experimentally we will use the catalytic growth of graphene on metal substrates (in particular nickel) with subsequent decoupling from the substrate (e.g., by intercalation). In addition, 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 ions. In these systems, the intercalation can lead to a complete decoupling of the sheets and thus to a linear band-structure of the p-bands around the Fermi level. Different complementary spectroscopic methods will be used to probe the structural, electronic, and vibrational properties of graphene as a function of its interaction with the environment. We will use inelasic x-ray scattering (IXS), electron energy loss spectroscopy (HREELS) and Raman spectroscopy to study the phonon dispersion as a function of doping, as well as angle resolved photoemission (ARPES) in order to elucidate the electronic quasiparticle dispersion. The spectroscopy measurements will be accompanied by detailed ab-initio calculations using existing codes and for very complex systems (requiring many atoms in the unit cell) where many-body perturbation theory is needed, we will use hybrid functionals for the quasiparticle band interaction. This will require new conceptual developments and their implementation in existing ab-initio codes. A part of this project will thus be developed to the conceptual development and testing of these functionals for graphitic systems. The aim of the joined project is to clearly elucidate the different size of the interactions to substrates as well as to dopands as well as the understanding of the underlying coupling mechanism. We will aim at controlling the size of the interaction in order to produce, on the one hand intrinsic graphene in order to study the Dirac electrons and on the other hand graphene with a tunable energy gap. This will be achieved by a combined experimental and theoretical approach i.e. calculating the interaction of graphene on Ni as a function of different intercalants. This will also help us to choose the right dopands in order to peel off big individual graphene layers from the Ni and transfer it to transparent substrates for the optical and EELS experiments. The success of the decoupling and the interplay between charge transfer and hybridisation will be monitored by ARPES and x-ray absorption spectroscopy (XAS). Spectroscopic measurements and calculations will thus guide the way towards a new production route of single graphene layers. In addition, we elucidate in detail the underlying coupling mechanism by determining the electron-electron, electron-hole and electron-phonon interaction as a function of environment. These are key parameters for understanding electron transport and optical properties and provide important input for accessing the application potential of functionalized graphene systems in nano- and optoelectronics. This detailed assessment is the final goal of our joined project period.

Graphene, a single layer of graphite, is a material that is currently receiving a lot of attention because of its unique structural and electronic properties which make them interesting for different applications for instance in composites, Nano- and optoelectronics. Individual nanoelectronic devices based on functionalized graphene show exceptionally good and promising properties, but a reproducible integration into classical semiconductor technology still has to be achieved. The major problem hindering this application is the absence of a large scale atomically exact control of the interfacial interaction of graphene with the environment and the incomplete basic understanding of e.g. the interaction between graphene and its substrate as well as between functional centers and graphene. In this joined cooperation project we had important contributions to address these open issues. Firstly, we have produced large area electronically isolated graphene with its relativistic Dirac electrons. Then we have tailored these intrinsic properties via modifying the local environment. This controlled modification of the electronic properties was performed by substitution reactions with H and N, via intercalation doping using K, Li, Ca und Ba as well as Au and Ge, and via deposition of pentacene. We have used a synergetic approach combining cutting edge spectroscopy (Vienna) and ab-initio theoretical description (Lille) of these complex systems to gain insight into the different influence of the interaction with substrates, ions and other functional centers on the intrinsic properties of graphene. As a highlight of the project we have used a combined Raman spectroscopical and ab-initio theoretical study to unravel the complex interplay between the influence of charge transfer and strain in different graphite intercalation compounds yielding the analysis on how to directly determine the local strain in doped graphene on an absolute scale via a contact free optical method. This allows for the first time to measure the interfacial strain in for instance graphene composites and devices. Additional important results are the controlled H-doping with the production of a H-defect band, the control of the bonding environment in N-doped graphene, the control of the graphene pentacene interface and the corresponding assessment of the application potential in organic electronics as well as the analysis of the electron-phonon coupling based superconducting coupling constant in alkaline(earth) graphite intercalation compounds and Ba doped graphene. As a final result of the project we could for the first time control the interface between graphene and classical semiconductors like Ge in a large area on an atomistic scale. This result is an important step forward towards the integration of nanoelectronic devices based on graphene into existing semiconductor technology.