Understanding the length dependence of molecular junction thermopower

Thermopower of molecular junctions is sensitive to details in the junction and may increase, decrease, or saturate with increasing chain length, depending on the system. Using McConnell's theory for exponentially suppressed transport together with a simple and easily interpretable tight binding model, we show how these different behaviors depend on the molecular backbone and its binding to the contacts. We distinguish between resonances from binding groups or undercoordinated electrode atoms, and those from the periodic backbone. It is demonstrated that while the former gives a length-independent contribution to the thermopower, possibly changing its sign, the latter determines its length dependence. This means that the question of which orbitals from the periodic chain that dominate the transport should not be inferred from the sign of the thermopower but from its length dependence. We find that the same molecular backbone can, in principle, show four qualitatively different thermopower trends depending on the binding group: It can be positive or negative for short chains, and it can either increase or decrease with length

Benchmark density functional theory calculations for nano-scale conductance

We present a set of benchmark calculations for the Kohn-Sham elastic transmission function of five representative single-molecule junctions. The transmission functions are calculated using two different density functional theory (DFT) methods, namely an ultrasoft pseudopotential plane wave code in combination with maximally localized Wannier functions, and the norm-conserving pseudopotential code Siesta which applies an atomic orbital basis set. For all systems we find that the Siesta transmission functions converge toward the plane-wave result as the Siesta basis is enlarged. Overall, we find that an atomic basis with double-zeta and polarization is sufficient (and in some cases even necessary) to ensure quantitative agreement with the plane-wave calculation. We observe a systematic down shift of the Siesta transmission functions relative to the plane-wave results. The effect diminishes as the atomic orbital basis is enlarged, however, the convergence can be rather slow.Comment: 10 pages, 7 figure

Complex band structure and electronic transmission

The function of nano-scale devices critically depends on the choice of materials. For electron transport junctions it is natural to characterize the materials by their conductance length dependence, $\beta$. Theoretical estimations of $\beta$ are made employing two primary theories: complex band structure and DFT-NEGF Landauer transport. Both reveal information on $\beta$ of individual states; i.e. complex Bloch waves and transmission eigenchannels, respectively. However, it is unclear how the $\beta$-values of the two approaches compare. Here, we present calculations of decay constants for the two most conductive states as determined by complex band structure and standard DFT-NEGF transport calculations for two molecular and one semi-conductor junctions. Despite the different nature of the two methods, we find strong agreement of the calculated decay constants for the molecular junctions while the semi-conductor junction shows some discrepancies. The results presented here provide a template for studying the intrinsic, channel resolved length dependence of the junction through complex band structure of the central material in the heterogeneous nano-scale junction.Comment: 7 pages, 6 figure

Quantum Interference in Off-Resonant Transport through Single Molecules

We provide a simple set of rules for predicting interference effects in off-resonant transport through single-molecule junctions. These effects fall in two classes, showing respectively an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same, or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diag- onalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions, interference causes no transmission nodes, but argue that it may lead to a non-standard gate-dependence of the zero-bias Kondo resonance.Comment: 12 pages, 7 figure

Non-magnetic and magnetic thiolate-protected Au25 superatoms on Cu(111), Ag(111) and Au(111) surfaces

Geometry, electronic structure, and magnetic properties of methylthiolate-stabilized Au$_{25}$L$_{18}$ and MnAu$_{24}$L$_{18}$ (L = SCH$_3$) clusters adsorbed on noble-metal (111) surfaces have been investigated by using spin-polarized density functional theory computations. The interaction between the cluster and the surface is found to be mediated by charge transfer mainly from or into the ligand monolayer. The electronic properties of the 13-atom metal core remain in all cases rather undisturbed as compared to the isolated clusters in gas phase. The Au$_{25}$L$_{18}$ cluster retains a clear HOMO - LUMO energy gap in the range of 0.7 eV to 1.0 eV depending on the surface. The ligand layer is able to decouple the electronic structure of the magnetic MnAu$_{24}$L$_{18}$ cluster from Au(111) surface, retaning a high local spin moment of close to 5 $\mu_{B}$ arising from the spin-polarized Mn(3d) electrons. These computations imply that the thiolate monolayer-protected gold clusters may be used as promising building blocks with tunable energy gaps, tunneling barriers, and magnetic moments for applications in the area of electron and/or spin transport.Comment: 5 pages, 4 figures, 1 tabl

Conductance of Sidewall-Functionalized Carbon Nanotubes: Universal Dependence on Adsorption Sites

We use density functional theory to study the effect of molecular adsorbates on the conductance of metallic carbon nanotubes (CNT). The five molecules considered (NO2, NH2, H, COOH, OH) lead to very similar scattering of the electrons. The adsorption of a single molecule suppresses one of the two available transport channels at the Fermi level while the other is left undisturbed. If more molecules are adsorbed on the same sublattice, the remaining open channel may or may not be blocked, depending on the relative position of the adsorbates. If the relative positions satisfy a simple geometric condition, this channel remains fully open independently of the number of adsorbed molecules.Peer reviewe