Wednesday, June 29, 2005
The Aviram and Ratner Rectifier
The simplest component of the integrated circuit that it is necessary to try and reproduce on the molecular scale is the rectifier, also known as the diode or semiconductor p-n junction. A rectifier can be defined as one or more diodes arranged for converting alternating current (AC) to direct current (DC). When only a single diode is used to rectify AC (resulting in only half wave rectification) the difference between the term diode and the term rectifier is merely one of semantics.
The first molecular rectifier was proposed in 1974 by Aviram and Ratner (original paper available here free), who proposed a model consisting of a donor π-system and an acceptor π-system, separated by a σ-bonded tunnelling bridge. This system could then, theoretically, be attached at both ends to metallic electrodes thereby completing an electric circuit. Aviram and Ratner predicted that, for the molecular rectifier to work, its properties must be equivalent to those of the bulk p-n junction.
In the donor-insulator-acceptor (D-σ-A) structure, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are confined to two different parts of the rectifier, D and A respectively. The insulating σ bridge prevents the orbitals from “spilling over” to the other part. If such a molecule were to be constructed and placed between two metal electrodes, the current-voltage characteristic of the junction would be expected to be highly asymmetric, i.e. molecular rectification can be defined as the absence of inversion symmetry,
I(V)=-I(-V) in the system, where I and V are the measured current and the applied voltage.
The Aviram and Ratner model can be thought of as occurring in two distinct stages:
1: At a particular value of applied voltage in the positive direction, the Fermi level of the electrode on A (cathode) aligns with the LUMO allowing electron tunnelling from the Fermi level to the LUMO. Simultaneously, the D side aligns with the HOMO, resulting in electron transfer from the HOMO to the Fermi level of the electrode on D (anode). In terms of the charge, the molecule has gone from D-σ-A to D+-σ-A-.
2: At this voltage, the current rises sharply because the electrons can now be loaded onto the LUMO, then tunnel inelastically through the σ-bridge to the HOMO and then escape into the second electrode. This returns the molecule from its excited, ionised state to the ground state of the system D-σ-A. In the opposite direction, a similar process does not occur until a much higher applied voltage.
The Aviram and Ratner model has now become the standard explanation for observed rectification from a molecular monolayer. In my next post I will discuss some of the evidence that has arisen over the last decade in support of the model, including my own paper (subscription required) that has been published recently in the Journal of materials chemistry, by the Royal Society of Chemistry.