We report the first concurrent determination of conductance (G) and thermopower (S) of single-molecule junctions via direct measurement of electrical and thermoelectric currents using a scanning tunneling microscope-based break-junction technique. We explore several amine-Au and pyridine-Au linked molecules that are predicted to conduct through either the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO), respectively. We find that the Seebeck coefficient is negative for pyridine-Au linked LUMO-conducting junctions and positive for amine-Au linked HOMO-conducting junctions. Within the accessible temperature gradients (<30 K), we do not observe a strong dependence of the junction Seebeck coefficient on temperature. From histograms of thousands of junctions, we use the most probable Seebeck coefficient to determine a power factor, GS(2), for each junction studied, and find that GS(2) increases with G. Finally, we find that conductance and Seebeck coefficient values are in good quantitative agreement with our self-energy corrected density functional theory calculations.

GW quasiparticle energies were computed as a first-order correction to GGA-PBE, 1 with starting DFT-PBE eigenvectors and eigenvalues taken from the Quantum Espresso DFT package, 2 which is compatible with the BerkeleyGW implementation. 3 Within this approach, the frequencydependence of the dielectric function is obtained via the generalized Plasmon-pole (GPP) model. 1Troullier-Martins norm-conserving pseudopotentials 4 were employed to represent the core electrons and nuclei, with default core radii of 1.5 au for C and 1.3 au for H. The molecular structure of crystalline pentacene was taken from Ref. 5 and the lattice vectors from the experimentallydetermined solution-processed (S-) phase. 6 The number of unoccupied states used to build the dielectric function and self-energy operator were 4192 and 582 for the molecule and solid, respectively, spanning an energy range of ∼ 40 eV above the highest occupied state. The dielectric * To whom correspondence should be addressed

We demonstrate a new method of achieving rectification in single molecule devices using the high-bias properties of gold-carbon bonds. Our design for molecular rectifiers uses a symmetric, conjugated molecular backbone with a single methylsulfide group linking one end to a gold electrode and a covalent gold-carbon bond at the other end. The gold-carbon bond results in a hybrid gold-molecule "gateway" state pinned close to the Fermi level of one electrode. Through nonequilibrium transport calculations, we show that the energy of this state shifts drastically with applied bias, resulting in rectification at surprisingly low voltages. We use this concept to design and synthesize a family of diodes and demonstrate through single-molecule current-voltage measurements that the rectification ratio can be predictably and efficiently tuned. This result constitutes the first experimental demonstration of a rationally tunable system of single-molecule rectifiers. More generally, the results demonstrate that the high-bias properties of "gateway" states can be used to provide additional functionality to molecular electronic systems.