Molecular Rectifiers: A New Design Based on Asymmetric Anchoring Moieties

Abstract

The quest for a molecular rectifier is among the major challenges of molecular electronics. We introduce three simple rules to design an efficient rectifying molecule and demonstrate its functioning at the theoretical level, relying on the NEGF-DFT technique. The design rules notably require both the introduction of asymmetric anchoring moieties and a decoupling bridge. They lead to a new rectification mechanism based on the compression and control of the HOMO/LUMO gap by the electrode Fermi levels, arising from a pinning effect. Significant rectification ratios up to 2 orders of magnitude are theoretically predicted as the mechanism opposes resonant to nonresonant tunneling.

HOMO–LUMO coupling: the fourth rule for highly effective molecular rectifiers

“…27 HLG reduction of organic molecules in metallic junctions has been previously attributed to Fermi level pinning of the molecular states when attached to metal electrodes. 10,22 Likewise, the HLG reduction of 4TPA-C 60 is attributed to Fermi level pinning of the HOMO and LUMO at the Au/thiolate and C 60 /Au interfaces respectively, which aligns the HOMO and LUMO independently of the isolated gas phase energy gap. The electronic response of the molecule to finite biases was then calculated within the NEGF-DFT framework (see Methology for details).…”

“…It is therefore advantageous to identify U-type systems which have rectifying properties inherent in their asymmetric molecular structure.160) utilizing both the U and S mechanisms. 10,11 Simulations showed that the HOMO and LUMO of their proposed molecule was asymmetrically coupled to opposing electrodes. The HOMO-LUMO gap (HLG) could thus be tuned by application of a potential difference across the molecule.…”

“…5,7 The results prove that a high rectification ratio can be obtained using twisted π Donor-Acceptor systems despite suggestions that a saturated bridge is required to fulfil the third rule. 10 The twisted π geometry also allows 4TPA-C 60 to achieve a high zero-bias conductance of 0.021 G 0 due to the absence of an insulating bridge. Conductance through similar geometries have been shown to be higher by an order of magnitude as compared to σ -linked bridges.…”

“…The results show that a saturated bridge is not required to obtain high rectification ratios via control of molecular orbitals, in contrast to previous suggestions in the literature. 10,22 The asymmetric response of the HOMO and LUMO is attributed to unidirectional charge transfer from donor to acceptor within the molecule, which prevents HOMO-LUMO crossing at higher forward biases. This HOMO-LUMO pinning effect results in the molecule being able to switch between a conductive state under forward bias and an insulating state under reverse bias.…”

Reconciling perturbative approaches in phonon-assisted transport junctions

“…The metal-molecule hybridization energy should be made small though, so as not to conceal this asymmetry by level broadening. This situation parallels with design principles for standard tunneling diodes [71][72][73] . We apply the voltage in a symmetric manner, and for simplicity, we keep the levels fixed, independent of bias.…”