We investigate the electronic properties of graphene nanoflakes on Ag(111) and Au(111) surfaces by means of scanning tunneling microscopy and spectroscopy as well as density functional theory calculations. Quasiparticle interference mapping allows for the clear distinction of substrate-derived contributions in scattering and those originating from graphene nanoflakes. Our analysis shows that the parabolic dispersion of Au(111) and Ag(111) surface states remains unchanged with the band minimum shifted to higher energies for the regions of the metal surface covered by graphene, reflecting a rather weak interaction between graphene and the metal surface. The analysis of graphene-related scattering on single nanoflakes yields a linear dispersion relation E(k), with a slight p-doping for graphene/Au(111) and a larger n-doping for graphene/Ag(111). The obtained experimental data (doping level, band dispersions around EF, and Fermi velocity) are very well reproduced within DFT-D2/D3 approaches, which provide a detailed insight into the site-specific interaction between graphene and the underlying substrate.

To assist the design of efficient molecular junctions, a precise understanding of the charge transport mechanisms through nanoscaled devices is of prime importance. In the present contribution, we present time-and space-resolved electron transport simulations through a nanojunction under time-dependent potential biases. We use the driven Liouville−von Neumann approach to simulate the time evolution of the one-electron density matrix under nonequilibrium conditions, which allows us to capture the ultrafast scattering dynamics, the electronic relaxation process, and the quasi-stationary current limit from the same simulation. Using local projection techniques, we map the coherent electronic current density, unraveling insightful mechanistic details of the transport on time scales ranging from atto-to picoseconds. Memory effects dominate the early time transport process, and they reveal different current patterns on short time scales in comparison to those in the long-time regime. For nanotransistors with high switching rates, the scattering perspective on electron transport should thus be favored.

In this contribution, we aim at supporting theoretical transistor material design using a combination of electronic structure theory, transport simulations, and local current analysis. Our effort focuses on defective zigzag graphene nanoribbons (ZGNRs) to design molecular junctions with an atomically precisely controlled degree of defect dilution. Electronic structure calculations within a periodic density functional theory (DFT) framework yield information about the band structures. These serve as a guide for constructing a transport model of the nanojunctions composed of a defective ZGNR scattering region connected to pristine ZGNR leads. Performing nonequilibrium Green's function simulations on selected systems of interest, their transport properties in the quasi-stationary limit are revealed. Following a recent procedure, associated current densities are mapped on a real-space representation. The presence of defects leads to concentrated current flow in the middle region, which is close to the defect edges. The degree of defect dilution as well as the width of the nanojunction have strong influences on the local current densities.

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