Analytical Model for Molecular-Scale Charge Transport

Abstract

We demonstrate a simple analytical model for elastic charge transport on the nanometer scale within a single molecule. It is based on the assumptions usually employed for quantum chemical calculations, except that of equilibrium between the currents flowing in opposite directions. It allows the possibility of a nonuniform potential distribution and imbalance between the population of states of opposite momentum. We analyze two implementable configurations and find the predictions to be consistent with recent experimental results.

Polarization of Charge-Transfer Bands and Rectification in Hexadecylquinolinium 7,7,8-Tricyanoquinodimethanide and Its Tetrafluoro Analog

“…2 We confirmed unimolecular rectification in hexadecyl-γ-quinolinium 7,7,8-tricyanoquinodimethanide, C 16 H 33 γQ-3CNQ ( Figure 1, structure 1), sandwiched first between Al electrodes at room temperature, 4,5 and between 105 and 370 K, 6 then at room temperature between Au electrodes. 7,8 The spectroscopy of 1 9,10 and theoretical calculations [11][12][13][14][15] confirmed conclusively that its ground state is the very polar zwitterion D + -π-A -(D + is the quinolinium moiety, and A -is the tricyano-quinodimethanide moiety), while the first excited state has the less polar structure D 0 -π-A 0 .…”

Mechanism of Rectification in Tunneling Junctions Based on Molecules with Asymmetric Potential Drops

“…The result is that the donor-bridge-acceptor part of the molecule is positioned asymmetrically inside the junction. 41,42,43,44 Figure 3F shows an example of such a molecular rectifier reported by Metzger et al 44 Thus, these experiments fail to identify the mechanism of rectification because they cannot distinguish between that described by Williams 1 and Baranger, 2 involving asymmetric potential drops and a single molecular orbital, and that described by Aviram and Ratner, involving two molecular orbitals. 3 Other factors that may complicate the potential landscape of these junctions include the presence of ions in the junction and incomplete localization of the HOMO and LUMO levels.…”

“…3A). Though many investigators have reported rectification using this class of compounds, 4,13,43,48,41 the mechanism of rectification remains obscure for five reasons. i) These junctions have often incorporated electrodes of different materials, but analysis of them has not considered the difference of electrode materials as a source of rectification.…”

Electrical transport across a structurally ordered phthalocyanine film: Role of defect states

“…24 In a simple picture, one would expect these energy levels to correspond to the energy difference between the chemical potential and the highest occupied molecular orbital ͑HOMO͒ and/or the lowest unoccupied molecular orbital ͑LUMO͒. However, for a given temperature, the energy difference between the conductance peak bias value at positive bias voltage and that of the shoulder at negative bias voltage is significantly smaller than the transport gap of E g ϳ 2.3 eV that is usually measured for phthalocyanines.…”