Porphyrin single molecule wires for nanoelectronics

Lead Research Organisation: Cardiff University
Department Name: School of Physics and Astronomy


The prime aspiration of molecular nanoelectronics is to fabricate and interconnect molecules that can replace, or at least augment, present silicon based technology, with the molecules functioning as interconnects, switches, transistors or even logic gates. Clearly big challenges exist if such technologies are ever to reach fruition. In this respect, one of the key scientific challenges is to synthesise and reliably connect single molecular wires which can transport charge over long distance and also perform other functions such as rectifying current or storing charge. This proposal is aimed at synthesising extended molecular wires from a class of molecules called porphyrins, whose synthesis and functionalisation is the focus of the Oxford group. These electrical properties of these well-characterised molecules will be investigated at Cardiff and Liverpool, particularly their efficiency as molecular wires, their contacts with metal electrodes and their potential for electrochemical control in devices. The porphyrin wires to be synthesised are conjugated, stiff and contain sites where charge can be localised. Other key attributes include lengths greater than 10 nm, remarkable stability, their ability to bind a wide range of metal ions and their capacity to be tuned with electrochemistry or photochemistry. It is expected that their attributes will allow them to conduct electrons over long distances. Their redox activity and ability to support pendent molecular groups will in turn provide avenues for current rectification, switching or charge storage. The investigations of the electrical properties of these porphyrin wires will require us to wire-up single molecules. This is clearly a big experimental challenge but the Liverpool group has recently developed new techniques using the scanning tunnelling microscope which makes this procedure more straightforward and reliable. These techniques will provide robust chemical contact of the single porphyrin molecules at both ends to metallic contacts. The role of the molecule/metal contact remains one of the most poorly understood and yet extremely important aspects of single molecule electronics. We will systematically investigate these contact effects through the use of several differing chemical groups for binding to the metal electrodes and complementary determination of the lineup of energy levels between the metal and the molecule. Although most of the electrical characterisation will be performed with two metal contacts at either end of the wire, in the later stages of the project a scanning probe contact will be introduced which can be scanned along the length of the wire probing electrical properties along the wire. The final goals of the project are to produce a series of novel porphyrin molecular wires and to have defined and understood electron transport across them down to the single molecule level and in different environments including UHV and electrolyte. Being able to probe the key variables (temperature, environment, oxidation state, metal atom, molecule stiffness, contact chemistry) for one molecular system will provide a systematic approach for formulating detailed mechanisms. In particular, the limiting roles of contact chemistries and non-ideal charge transport (inelastic scattering) processes will be defined. The ideal porphyrin molecular wire would support high currents and we will assess how closely this goal (quantum conductance limit) can be approached. We will also have evaluated the ability of these molecules to act as active molecular wires through the placement of redox addressable groups along their length.


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Description There is strong interest in developing electronics based on molecular systems such as polymers (plastics) or smaller molecules to augment or as low-cost alternatives to silicon electronics. Much current research and development is focused on thin molecular films. The focus of this grant was much longer term - on single molecules - the ultimate in miniaturisation. Specifically, we aimed to develop electronics based on single porphyrins, at the heart of photosynthesis and oxygen transport in mammals.

A central challenge for single molecule electronics is that the conductance of a single molecule diminishes rapidly with increasing molecular length, characterised by a beta-factor.
We demonstrated one of the lowest beta-factors achieved in molecular systems (0.04 Ang-1), an extremely promising result for molecular systems.

A second contribution is to the understanding of transition voltage spectroscopy, a recently developed approach. Using this, we demonstrated different transport characteristics related to distinct molecular con?gurations at the electrode.

A third area of discovery, likely to be the most significant in the long term, is the extension of the technique to studying conduction in single proteins containing porphyrins. An extremely surprising result was that the conductance of a porphyrin-bearing protein, cytochrome b562, was significantly higher than that of the porphyrin ring alone. This work has engendered work in other groups internationally. This work was described in a Perspective review as 'To this end,
the work by Macdonald and colleagues may enable a new type of tunneling spectroscopy,namely tunneling current noise spectroscopy for biomolecular systems the work by Macdonald and colleagues is truly enabling, in that it demonstrates the precision needed for these new types of experiments' (Inkpen and Albrecht, doi 10.1021/nn205016v)
This approach holds promise for novel biosensors.
Exploitation Route Our technical developments are already being implemented by others.
We are actively inverstigating possible new biosensors on graphene and carbon nanotubes based on our protein-based work.
Sectors Electronics,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

Description The findings have not been used commercially - single molecule electronics is yet to be developed. The potential impact of the work was identified in a Perspective article as This work has engendered work in other groups internationally. This work was described in a Perspective review: 'To this end, the work by Macdonald and colleagues may enable a new type of tunneling spectroscopy,namely tunneling current noise spectroscopy for biomolecular systems the work by Macdonald and colleagues is
Sector Education
Impact Types Cultural,Societal

Description Lecture on Nanoscience (University of Sarajevo) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Undergraduate students
Results and Impact On 16th April 2015, 120 undergraduates, postgraduates and general public attended a lecture on Nanotechnology at the University of Sarajevo. Local TV journalists attended and it led to an interview broadcast on Bosnian State Television.
It also led to discussion of possible research involvement in nanoscience in a university setting where little dedicated equipment exists.
Year(s) Of Engagement Activity 2014,2015