Charging-induced asymmetry in molecular conductors

Abstract

We investigate the origin of asymmetry in various measured current-voltage (I-V) characteristics of molecules with no inherent spatial asymmetry, with particular focus on a recent break junction measurement. We argue that such asymmetry arises due to unequal coupling with the contacts and a consequent difference in charging effects, which can only be captured in a self-consistent model for molecular conduction. The direction of the asymmetry depends on the sign of the majority carriers in the molecule. For conduction through highest occupied molecular orbitals (i.e. HOMO or p-type conduction), the current is smaller for positive voltage on the stronger contact, while for conduction through lowest unoccupied molecular orbitals (i.e. LUMO or n-type conduction), the sense of the asymmetry is reversed. Within an extended Hückel description of the molecular chemistry and the contact microstructure (with two adjustable parameters, the position of the Fermi energy and the sulphur-gold bond length), an appropriate description of Poisson's equation, and a self-consistently coupled non-equilibrium Green's function (NEGF) description of transport, we achieve good agreement between theoretical and experimental I-V characteristics, both in shape as well as overall magnitude.

Theory of High Bias Coulomb Blockade in Ultrashort Molecules

“…5(a), (b), and (c) are fits obtained for experiments [11], [12]. In case of molecular asymmetries [11], only positive bias is considered [9], the I-V asymmetries themselves being attributed to polarization effects [7]. In obtaining an experimental fit for [12], in Fig.…”

“…Though fairly successful in describing many aspects of single molecule conduction [3]- [7], there have been important discrepancies between theory and experiment [8]. The most common ones include poor match between theoretical and experimental current levels and zero-bias currents [3], [4], [8].…”

Coulomb repulsion effect in two-electron nonadiabatic tunneling through a one-level redox molecule

“…49,50 Equation ͑30͒ shows that rectification is impossible for symmetric vacuum tunneling contacts even when electron-phonon interactions are included. However, in …”

Molecular Transport Junctions: Asymmetry in Inelastic Tunneling Processes

“…The latter can be caused by geometric asymmetry, different coupling strength of the molecule to the contacts, weak links, and charging. 4,5 In such cases, change of the electronic structure of the molecule under bias reversal is responsible for the rectification.In inelastic electron tunneling spectroscopy (IETS), one follows the onset of inelastic effects on the tunneling current that are most pronounced in the second derivative of current vs voltage. 6 Here we consider the possible asymmetry of d 2 I/dV 2 under a similar voltage reversal.…”