Identifying the Length Dependence of Orbital Alignment and Contact Coupling in Molecular Heterojunctions

Abstract

Transport in metal-molecule-metal junctions is defined by the alignment and coupling of molecular orbitals with continuum electronic states in the metal contacts. Length-dependent changes in molecular orbital alignment and coupling with contact states were probed via measurements and comparisons of thermopower (S) of a series of phenylenes and alkanes with varying binding groups. S increases linearly with length for phenylenediames and phenylenedithiols while it decreases linearly in alkanedithiols. Comparison of these data suggests that the molecular backbone determines the length dependence of S, while the binding group determines the zero length or contact S. Transport in phenylenes was dominated by the highest occupied molecular orbital (HOMO), which aligns closer to the Fermi energy of the contacts as approximately L(-1), but becomes more decoupled from them as approximately e(-L). In contrast, the decreasing trend in S for alkanedithiols suggests that transmission is largely affected by gold-sulfur metal induced gap states residing between the HOMO and lowest unoccupied molecular orbital.

Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

“…[13][14][15][16]23,65,66 In most cases the evidence for hole tunneling is based on transport across relatively short alkyl chains, up to C12, and is indirect. An exception is the measurement of thermo-power (Seebeck coefficient), where the positive coefficient was taken as direct evidence for hole / HOSO-mediated tunneling.…”

Thermoelectric effects in molecular quantum dots with contacts

“…where A (1) and g (1)< dd are the electronic dot spectral function and response function. In the steady state regime our approximation conserves the particle current: J L + J R = 0.…”

Molecular dynamics study of the thermopower of Ag, Au, and Pt nanocontacts

“…9 Thus, electron and hole conduction can be distinguished 10 and the alignment of molecular levels with respect to the Fermi energy can be determined. 9,11,12 It turns out that the experiments are described by a combination of electronic structure and transport calculations. 13,14 For instance, the linear increase of the thermopower with molecule length in the typical off-resonant transport situation has been found in both experiment 9,12,15 and theory.…”