Felix Herziger

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 the 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 line shape is described by three dominant resonances around the K point. We show that the splitting of the transversal optical (TO) phonon branch in the Γ-K direction, as large as 12  cm(-1) in the GW approximation, is of great importance for a thorough description of the 2D-mode line shape. Finally, we present a method to extract the TO phonon splitting and the splitting of the electronic bands from experimental data.

Understanding the growth mechanism of graphene on Ge/Si(001) surfaces

The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30◦ with respect to each other. The growth mode is attributed to the mechanism when small graphene “molecules” nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene “molecules” nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.

Beyond double-resonant Raman scattering: Ultraviolet Raman spectroscopy on graphene, graphite, and carbon nanotubes

The study of Raman scattering processes in the ultraviolet excitation energy range is not common in carbon-based materials. This paper reports that the widely known double-resonant Raman scattering, which is dominant in the visible excitation energy in these materials, is suppressed in the UV range. This is an important development because one can now measure two-phonon densities of states that were not accessible before. The results demonstrate the high potential of UV Raman spectroscopy for research on graphitic nanocarbons.

Molecular beam epitaxy of graphene on mica

Realization of graphene devices is often hindered by the fact that the known layer growth methods do not meet the requirements of the device fabrication in silicon mainstream technology. For example, the relatively straightforward method of decomposition of hexagonal SiC is not CMOS-compatible due to the high-thermal budget it requires [Moon et al., IEEE Electron Device Lett. 31, 260 (2010)]. Techniques based on layer transfer are restricted because of the uncertainty of residual metal contaminants, particles, and structural defects. Of interest is thus a method that would allow one to grow a graphene film directly in the device area where graphene is needed. Production of large area graphene is not necessarily required in this case, but high quality of the film and metal-free growth on an insulating substrate at temperatures below 1000 degrees C are important requirements. We demonstrate direct growth of defect-free graphene on insulators at moderate temperatures by molecular beam epitaxy. The quality of the graphene was probed by high-resolution Raman spectroscopy, indicating a negligible density of defects. The spectra are compared with those from graphene flakes mechanically exfoliated from native graphite onto mica. These results are combined with insights from density functional theory calculations. A model of graphene growth on mica and similar substrates is proposed.

Graphene-based electro-absorption modulator integrated in a passive polymer waveguide platform

A graphene-based electro-absorption modulator has been integrated into a passive polymer waveguide platform for the first time. The opto-electronic properties of the structure are investigated with numerical simulations and measurements of a fabricated device. The graphene layers transferred to the polymer substrate were analyzed by means of Raman spectroscopy and the results indicate a high crystalline quality of the two-dimensional material. The voltage-dependent transmission through a 25 µm long device has been measured in the telecommunications-relevant wavelength range between 1500 nm and 1600 nm yielding an extinction ratio of 0.056 dB/µm.

Understanding double-resonant Raman scattering in chiral carbon nanotubes: Diameter and energy dependence of the

We present a theoretical model to describe the double-resonant scattering process in arbitrary carbon nanotubes. We use this approach to investigate the defect-induced

D

D

mode

mode in CNTs and unravel the dependence of the

Breakdown of Far-Field Raman Selection Rules by Light–Plasmon Coupling Demonstrated by Tip-Enhanced Raman Scattering

D

Graphene grown on Ge(0 0 1) from atomic source

-mode frequency on the CNT diameter and on the energy of the resonant optical transition. Our approach is based on the symmetry of the hexagonal lattice and geometric considerations, hence the method is independent of the exact model that is chosen to describe the electronic band structure or the phonon dispersion. We finally clarify the diameter dependence of this Raman mode that was controversely discussed in the past and demonstrate that, depending on the experimental conditions, in general two different dependencies can be measured. We also prove that carbon nanotubes with arbitrary chiral index can exhibit a