Showing papers by "Heiko B. Weber published in 2008"

Atomic and electronic structure of few-layer graphene on SiC(0001) studied with scanning tunneling microscopy and spectroscopy

Abstract: Epitaxial growth of graphene on SiC surfaces by solid state graphitization is a promising route for future development of graphene based electronics. In the present work, we have studied the morphology, atomic scale structure, and electronic structure of thin films of few-layer graphene (FLG) on SiC(0001) by scanning tunneling microscopy and spectroscopy (STS). We show that a quantitative evaluation of the roughness induced by the interface is a tool for determining the layer thickness of FLG. We present and interpret thickness dependent tunneling spectra, which can serve as an additional fingerprint for the determination of the layer thickness. By performing spatially resolved STS, we find evidence that the charge distribution in bilayer graphene is inhomogeneous.

Charge Transport Through a Cardan‐Joint Molecule

Abstract: The charge transport through a single ruthenium atom clamped by two terpyridine hinges is investigated, both experimentally and theoretically. The metal-bis(terpyridyl) core is equipped with rigid, conjugated linkers of para-acetyl-mercapto phenylacetylene to establish electrical contact in a two-terminal configuration using Au electrodes. The structure of the [Ru(II)(L)(2)](PF(6))(2) molecule is determined using single-crystal X-ray crystallography, which yields good agreement with calculations based on density functional theory (DFT). By means of the mechanically controllable break-junction technique, current-voltage (I-V), characteristics of [Ru(II)(L)(2)](PF(6))(2) are acquired on a single-molecule level under ultra-high vacuum (UHV) conditions at various temperatures. These results are compared to ab initio transport calculations based on DFT. The simulations show that the cardan-joint structural element of the molecule controls the magnitude of the current. Moreover, the fluctuations in the cardan angle leave the positions of steps in the I-V curve largely invariant. As a consequence, the experimental I-V characteristics exhibit lowest-unoccupied-molecular-orbit-based conductance peaks at particular voltages, which are also found to be temperature independent.

Charge Transport through Molecular Rods with Reduced π-Conjugation

Abstract: A series of oligophenylene rods of increasing lengths is synthesized to investigate the charge-transport mechanisms. Methyl groups are attached to the phenyl rings to weaken the electronic overlap of the π-subsystems along the molecular backbones. Out-of-plane rotation of the phenyl rings is confirmed in the solid state by means of X-ray analysis and in solution by using UV/Vis spectroscopy. The influence of the reduced π-conjugation on the resonant charge transport is studied at the single-molecule level by using the mechanically controllable break-junction technique. Experiments are performed under ultra-high-vacuum conditions at low temperature (50 K). A linear increase of the conductance gap with increasing number of phenyl rings (from 260 meV for one ring to 580 meV for four rings) is revealed. In addition, the absolute conductance of the first resonant peaks does not depend on the length of the molecular wire. Resonant transport through the first molecular orbital is found to be dominated by charge-carrier injection into the molecule, rather than by the intrinsic resistance of the molecular wire length.

Analysis of interface trap parameters from double-peak conductance spectra taken on N-implanted 3C-SiC MOS capacitors

Abstract: 3C-SiC/SiO2 capacitors are fabricated by over-oxidation of an implanted Gaussian nitrogen (N) profile and investigated by conductance spectroscopy. An unexpected double peak structure is observed in the conductance spectra indicating two types of independent traps at different energy positions in the bandgap, which change their charge state with identical time constant. A theoretical model is developed assuming two independent distributions of interface traps in the bandgap of 3C-SiC with different trap parameters. The experimental G /ω – V and C – V spectra are simulated and the trap parameters are determined on the basis of this model. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Atmospheric pressure graphitization of SiC(0001)- A route towards wafer-size graphene layers

Abstract: We have investigated epitaxial graphene films grown on SiC(0001) by annealing in an atmosphere of Ar instead of vacuum Using AFM and LEEM we observe a significantly improved surface morphology and graphene domain size Hall measurements on monolayer graphene films show a carrier mobility of around 1000 cm^2/Vs at room temperature and 2000 cm^2/Vs at 27K The growth process introduced here establishes the synthesis of graphene films on a technologically viable basis

Ionization Energies of Phosphorus Donors in 6H-SiC

Abstract: 6H-SiC was doped with phosphorus (P) donors by neutron transmutation. IR transmission spectra were taken in the temperature range from 6 K to 300 K. A great number of absorption lines is observed at temperatures below 140 K; these lines are attributed to transitions of the donor electron between ground states and bound excited states of P-related donors. Based on Faulkner's theory, three series of effective-mass-like states (P1, P2, P3) could be identified. The corresponding ground state energies are: E(P1) = EC - 91.5 meV, E(P2) = EC - 81.8 meV, E(P3) = EC - 73.5 meV.

Atmospheric Pressure Graphitization of SiC(0001) - A Route Towards Wafer-Size Graphene Layers.

Abstract: We have investigated epitaxial graphene films grown on SiC(0001) by annealing in an atmosphere of Ar instead of vacuum. Using AFM and LEEM we observe a significantly improved surface morphology and graphene domain size. Hall measurements on monolayer graphene films show a carrier mobility of around 1000 cm/Vs at room temperature and 2000 cm/Vs at 27K. The growth process introduced here establishes the synthesis of graphene films on a technologically viable basis.