Implanting germanium into graphene

Incorporating heteroatoms into the graphene lattice may be used to tailor its electronic, mechanical and chemical properties. Direct substitutions have thus far been limited to incidental Si impurities and P, N and B dopants introduced using low-energy ion implantation. We present here the heaviest impurity to date, namely $^{74}$Ge$^+$ ions implanted into monolayer graphene. Although sample contamination remains an issue, atomic resolution scanning transmission electron microscopy imaging and quantitative image simulations show that Ge can either directly substitute single atoms, bonding to three carbon neighbors in a buckled out-of-plane configuration, or occupy an in-plane position in a divacancy. First principles molecular dynamics provides further atomistic insight into the implantation process, revealing a strong chemical effect that enables implantation below the graphene displacement threshold energy. Our results show that heavy atoms can be implanted into the graphene lattice, pointing a way towards advanced applications such as single-atom catalysis with graphene as the template.Comment: 20 pages, 5 figure

Friction Drag on a Particle Moving in a Nematic Liquid Crystal

The flow of a liquid crystal around a particle does not only depend on its shape and the viscosity coefficients but also on the direction of the molecules. We studied the resulting drag force on a sphere moving in a nematic liquid crystal (MBBA) in a low Reynold's number approach for a fixed director field (low Ericksen number regime) using the computational artificial compressibility method. Taking the necessary disclination loop around the sphere into account, the value of the drag force anisotropy (F_\perp/F_\parallel=1.50) for an exactly computed field is in good agreement with experiments (~1.5) done by conductivity diffusion measurements. We also present data for weak anchoring of the molecules on the particle surface and of trial fields, which show to be sufficiently good for most applications. Furthermore, the behaviour of the friction close to the transition point nematic isotropic and for a rod-like and a disc-like liquid crystal will be given.Comment: 23 pages RevTeX, including 3 PS figures, 1 PS table and 1 PS-LaTeX figure; Accepted for publication in Phys. Rev.

Symmetry-breaking supercollisions in Landau-quantized graphene

Recent pump-probe experiments performed on graphene in a perpendicular magnetic field have revealed carrier relaxation times ranging from picoseconds to nanoseconds depending on the quality of the sample. To explain this surprising behavior, we propose a novel symmetry-breaking defect-assisted relaxation channel. This enables scattering of electrons with single out-of-plane phonons, which drastically accelerate the carrier scattering time in low-quality samples. The gained insights provide a strategy for tuning the carrier relaxation time in graphene and related materials by orders of magnitude

Controllable Growth of Vertically Aligned Graphene on C-face SiC

We investigated how to control the growth of vertically aligned graphene on C-face SiC by varying the processing conditions. It is found that, the growth rate scales with the annealing temperature and the graphene height is proportional to the annealing time. Temperature gradient and crystalline quality of the SiC substrates influence their vaporization. The partial vapor pressure is crucial as it can interfere with further vaporization. A growth mechanism is proposed in terms of physical vapor transport. The monolayer character of vertically aligned graphene is verified by Raman and X-ray absorption spectroscopy. With the processed samples, d0 magnetism is realized and negative magnetoresistance is observed after Cu implantation. We also prove that multiple carriers exist in vertically aligned graphene