Nonexponential Length Dependence of Molecular Conductance in Acene-Based Molecular Wires.

In the nonresonant regime, molecular conductance decays exponentially with distance, limiting the fabrication of efficient molecular semiconductors at the nanoscale. In this work, we calculate the conductance of a series of acene derivatives connected to gold electrodes using density functional theory (DFT) combined with the nonequilibrium Green's function (NEGF) formalism. We show that these systems have near length-independent conductance and can exhibit a conductance increase with molecular length depending on the connection to the electrodes. The analysis of the molecular orbital energies and transmission functions attribute this behavior to the dramatic decrease of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap with length, which shifts the transmission peaks near the Fermi level. These results demonstrate that the anchoring configuration determines the conductance behavior of acene derivatives, which are optimal building blocks to fabricate single-molecule devices with tunable charge transport properties.

Quantum interference enhances rectification behavior of molecular devices.

We present a theoretical and computational work and demonstrate that cross-conjugated molecules with electron-donating groups are efficient rectifiers with high conductance. The rectification ratios obtained are up to one order of magnitude at an applied bias voltage of 0.3 V. The use of electron-withdrawing groups to form donor-bridge-acceptor triads gives rectification ratios of the order of 102. We found that the high rectification results from localizing the Highest Occupied Molecular Orbital (HOMO) at one end of the molecular device. When the HOMO is localized, quantum interference effects substantially enhance rectification. Our observations rely on transport calculations of linearly-conjugated and cross-conjugated molecules using Non-Equilibrium Green's Function Technique and Density Functional Theory (NEGF-DFT). Analysis of transmission functions suggests a dependency of the rectification ratio on the anti-resonance peak position near the Fermi level of the electrode, allowing the possibility to modulate molecular rectification through electrochemical gating.