In-situ Raman study of laser-induced graphene oxidation

We present in-situ Raman measurements of laser-induced oxidation in exfoliated single-layer graphene. By using high-power laser irradiation, we can selectively and in a controlled way initiate the oxidation process and investigate its evolution over time. Our results show that the laser-induced oxidation process is divided into two separate stages, namely tensile strain due to heating and subsequent $p$-type doping due to oxygen binding. We discuss the temporal evolution of the $D/G$-mode ratio during oxidation and explain the unexpected steady decrease of the defect-induced $D$ mode at long irradiation times. Our results provide a deeper understanding of the oxidation process in single-layer graphene and demonstrate the possibility of sub-$\mu$m patterning of graphene by an optical method.Comment: 5 pages, 4 figures [submitted as IWEPNM 2015 conference publication to pss(b)

Two-dimensional analysis of the double-resonant 2D Raman mode in bilayer graphene

By computing the double-resonant Raman scattering cross-section completely from first principles and including electron-electron interaction at the $GW$ level, we unravel the dominant contributions for the double-resonant 2D-mode in bilayer graphene. We show that, in contrast to previous works, the so-called inner processes are dominant and that the 2D-mode lineshape is described by three dominant resonances around the $K$ point. We show that the splitting of the TO phonon branch in $\Gamma-K$ direction, as large as 12 cm$^{-1}$ in $GW$ approximation, is of great importance for a thorough description of the 2D-mode lineshape. Finally, we present a method to extract the TO phonon splitting and the splitting of the electronic bands from experimental data.Comment: 5 pages Main + 6 pages Supplementary Material. Accepted for publication in Physical Review Letter

Graphene Grown on Ge(001) from Atomic Source

Among the many anticipated applications of graphene, some - such as transistors for Si microelectronics - would greatly benefit from the possibility to deposit graphene directly on a semiconductor grown on a Si wafer. We report that Ge(001) layers on Si(001) wafers can be uniformly covered with graphene at temperatures between 800{\deg}C and the melting temperature of Ge. The graphene is closed, with sheet resistivity strongly decreasing with growth temperature, weakly decreasing with the amount of deposited C, and reaching down to 2 kOhm/sq. Activation energy of surface roughness is low (about 0.66 eV) and constant throughout the range of temperatures in which graphene is formed. Density functional theory calculations indicate that the major physical processes affecting the growth are: (1) substitution of Ge in surface dimers by C, (2) interaction between C clusters and Ge monomers, and (3) formation of chemical bonds between graphene edge and Ge(001), and that the processes 1 and 2 are surpassed by CH$_{2}$ surface diffusion when the C atoms are delivered from CH$_{4}$. The results of this study indicate that graphene can be produced directly at the active region of the transistor in a process compatible with the Si technology

Doppelresonante Raman-Streuung in Graphen, mehrlagigem Graphen und Kohlenstoffnanoröhren

This works presents an analysis of double-resonant two-phonon and phonon-defect Raman modes in graphene, few-layer graphene and carbon nanotubes. First, the double-resonant 2D mode in Bernal-stacked bilayer graphene is analyzed. It is shown that the previously assumed assignment between scattering processes and spectral contributions is incorrect. The results demonstrate the correct interpretation of the complex 2D-mode lineshape and point out the importance of quantum interference effects in the double-resonant Raman scattering process. Furthermore, it is shown how the splitting between the transverse optical phonon branches and the splitting between the electronic bands can be derived from the experimental data. The second part of this thesis is devoted to the analysis of double-resonant phonon-defect Raman modes. First, a theoretical model is introduced to analyze the double-resonant scattering process in arbitrary carbon nanotubes. In the following, this model is applied to investigate the dependence of the D-mode frequency on the tube diameter and optical transition energy. The results finally harmonize the different previous experimental observations. Subsequently, the D'' mode in graphene and carbon nanotubes is analyzed. It is shown that this mode results from scattering of defects with longitudinal acoustic phonons from the Gamma-K high-symmetry direction, explaining the experimentally observed asymmetric lineshape. Finally, the laser-induced oxidation process in single-layer graphene is analyzed. As could be shown, oxidation in graphene can be divided into two subsequent steps, namely, tensile strain due to laser-induced heating and subsequent p-type doping due to oxygen binding.Die vorliegende Arbeit untersucht doppelresonante Raman-Streuung in Graphen, mehrlagigem Graphen und Kohlenstoffnanoröhren. Zunächst wird hierbei die 2D-Mode in zweilagigen Graphen-Schichten untersucht. Es kann gezeigt werden, dass die zuvor angenommene Zuordnung zwischen den Streuprozessen und den verschiedenen spektralen Komponenten inkorrekt ist. Die vorgestellten Ergebnisse erläutern schließlich die komplexe Linienform der 2D-Mode und betonen die Bedeutung von Interferenzeffekten zwischen verschiedenen doppelresonanten Streuprozessen. Ebenso kann gezeigt werden, dass die Aufspaltung zwischen den transversal optischen Phononenzweigen und die Aufspaltung der elektronischen Bänder aus den experimentellen Daten gewonnen werden kann. Der zweite Teil der vorliegenden Arbeit befasst sich mit doppelresonanter Raman-Streuung an Defekten. Hierbei wird zunächst ein theoretisches Modell etabliert um doppelresonante Raman-Streuung in beliegenden Kohlenstoffnanoröhren systematisch zu untersuchen. Im Folgenden wird dieses Modell angewandt um die Frequenz der D-Mode in Abhängigkeit vom Röhrendurchmesser und von der Energie des optischen Übergangs zu untersuchen. Weiterhin wird die D''-Mode in Graphen und Kohlenstoffnanoröhren untersucht, wobei gezeigt werden kann, dass diese Mode durch die Streuung von longitudinal akustischen Phononen aus der Gamma-K-Richtung mit Defekten entsteht. Abschließend wird die Laser-induzierte Oxidierung von Graphen betrachtet. Hierbei kann gezeigt werden, dass die Oxidierung in zwei aufeinanderfolgende Schritte aufgeteilt werden kann: tensile Verspannung durch Laser-induziertes Aufheizen der Probe und anschließend Dotierung durch das Binden von Sauerstoff an die Graphen-Schicht