Symmetry, delocalization, and molecular conductance

Abstract

Molecules bonded between two metal contacts form the simplest possible molecular devices. Coupled by the molecule, the left and right contact-based states form symmetric and antisymmetric pairs near the Fermi level. We relate the size of the resulting energy splitting ⌬E to the symmetry and degree of delocalization of the coupling molecular orbital. Qualitative trends in molecular conductances are then estimated from the variations in ⌬E. We examine benzenedithiol and other molecules of interest in transport.

Electrical transport in saturated and conjugated molecular wires

“…The applied source-drain and the gate bias are incorporated as an external electrical (dipolar) field 44,47,49 in the electronic Hamiltonian of the quantum chemistry code using boundary conditions for an isolated molecule. The MO's are recomputed at the equilibrium geometry in presence of the external electrical field.…”

“…Including the external field in the computation of the electronic structure is also important because quasi-degenerate MO's in the absence of external field, as is the case for the HOMO and HOMO À 1 in saturated alkane bridges, can be coupled when an electrical field is applied. 24,47…”

“…The device is therefore in a short circuit configuration, where the edges of the Fermi window defined by the source-drain bias are given by E F þ eV sd /2 and E F À eV sd /2. In our model as in others, 20,[43][44][45][46][47] the Au atoms that are perturbed by the bonding to the sulfur atoms of the molecular bridge are included in an extended system, Au n -S-bridge-S-Au n . This system is then coupled to the metallic gold electrodes (Fig.…”

“…The electronic structure of the extended system is computed using a molecular approach with the Gaussian 03 package 48 including external fields in the Hamiltonian. In this approach, 20,43,44,47,49 the external field is applied over a finite region much larger than the molecule so that the boundary conditions remain that of an isolated molecule. The current is obtained using time independent scattering theory formalism 50 which computes the steady state current for a system displaced from equilibrium by the coupling between the interface and the bulk electrode.…”

Theoretical study of the electronic conduction through organic nanowires

“…For a molecule to act as a molecular wire it should principally possess a fully delocalized low lying vacant molecular orbital that should be spread all over the molecule. This can serve as a channel of conduction in the electron transfer process, that is to say, if the delocalization is higher, the electron transfer is faster [35,36]. A comparison of energy levels in the HOMO and the LUMO and HLG for nine free molecules is given in Table 1 (cis (a), trans (b)).…”