Molecular Electronics has two principal strands; firstly, the use of the novel combinations of
macroscopic properties offered by organic materials in electronic and optoelectronics devices; and secondly, the study of systems at the molecular scale in order to determine their potential as components of molecular scale devices. The use of liquid crystals in displays and semiconducting polymers in the xerographic process are the prime examples of the impact that molecular materials can make on technology. Conjugated, conducting polymers offer good prospects for mid-term application but still require basic studies to underpin the development of materials suitable for use in practical devices. An example of how rapidly this field has developed is illustrated by the fact that Philips have now developed an electrolytic capacitor using a conducting polymer counter electrode. True molecular scale electronics (< 10 nm) is a much longer term target which, at present, lacks a strong fundamental science base. However, atomic scale resolution microscopies, and other emerging techniques offer the tools with which to develop the science essential for the exploration of this topic.
The EPSRC in the UK has in recent years supported a UK-wide initiative in Molecular Electronics to extend the use of molecular materials in electronics. In generation research has focused on the investigation of electronic properties at the molecular scale that might lead to new devices. Many different types of molecules have been proposed for synthesis or further investigation. A practical approach to the problems of molecular electronics has been taken in Glasgow by concentrating current efforts on developing reliable techniques for substrate attachment and patterning of molecules for new devices or materials. The techniques of photolithography and electron beam lithography have been successfully applied to patterning techniques at nanoelectronic scales and there is a strong interest in exploiting the electronic properties of biological molecules imparted by either their structure and/or their functionality (see Bioelectronics).
COOPER,J.M. , SHEN, J.,YOUNG,F M.,CONNOLLY ,P, BARKER, J.R., AND MOORES, G.
The Imaging of Streptavidin and Avidin using Scanning Tunnelling Microscopy
J. Materials Science: Materials in Electronics 5 (1994)
BARKER,J R, CONNOLLY, P C AND MOORES, G
Interfacing to biological and molecular structures
in Physics of granular electonic systems, edited by D K Ferry, J R Barker, C Jacoboni, Plenum,425-440 (1991).
BARKER, J R
Building molecular electronic systems
Chemistry in Britain 27 728-731 (1991)
J.R. Barker
This theoretical work is aimed at establishing suitable architectures for the very high density electronic circuits which are implied by molecular electronics. Many formidable problems have to be overcome before a clear route to molecular information processing is possible, including; the functional specification of a macromolecular computing structure; the specification of a logic architecture which is manageable at the very high levels of complexity implied by molecular switches; the formulation of a practical fabrication scheme; the production of an interface between the molecular structure and the outside world, namely optical and solid state electronic interfaces. Possible practical routes to the synthesis of a demonstrator macromolecular computational structure are currently being studied at Glasgow. Considerations of functionality, simplicity of logic structure, logic structural stability and the problem of adequate input/output interfacing suggest that block-structured cellular automata are a natural architecture for future molecular electronic systems. There is considerable interest in quasi-mechanical computational schemes including the artificial microtubule model and the molecular abacus schemes.
BARKER J R, Molecular electronic logic and architectures in Introduction to Molecular Electronics edited by M Petty, D. Bloor and M. Bryce; Edward Arnold: London , Chapter 16, 345-376 (1995).
J R Barker and F Young
As part of the ESPRIT programme TOPFIT we are developing tools for the visualisation and analysis of molecular images obtained locally and with collaborators in Mainz from scanning tunelling microscopy. At the simplest level we are developing software for producing fully rendered "landscape" images in both direct and stereo pair forms. Simulation tools are being developed which allow the experimenter to effectively fly over and around the surface terrain of a target molecule viwed by the STM and to then produce fully rendered computer movies of the scene. Morphing and compression techniques are being used to make this procedure cmputationally efficient.
It has been found that electrochemical STM images of arrays of molecules covering a suitable substrate such as graphite (HOPG) have a number of surprising features which makes interpretation of the images very difficult or ambiguous. For example in recent data from our partners in Mainz it has been found that a monolayer of didodecylbenzene on graphite presents a multiple image rather than a single surface image and the result is very sensitive to the tip-substrate bias potential. For example at low bias the image reveals high contrast cones associated with the benzene group but the alkyl chains are virtually invisible but there expected location is filled with a direct image of the graphite substrate which appears to shine through the monolayer. At higher bias levels the graphite image fades and is replaced with the characteristic images of the alky chains. To assist the interpretation of such complex "scenes" we are developing adaptive pattern recognition programs which are trained to recognise features in an STM image and to classify a scene into a superposition of known features and a residual "novelty" element. The technique is based on a synergetic non-linear dynamical systems algorithm which uses known images to construct attracting basins in a generalised potential function to filter out known components of a target scheme. The method is complemented by a new algorithm for determining the position, scale and rotation of recognised objects based on affine transformations. By using test images which include noisy substrates, STM artefacts and defects as well as known or predicted molecular images it is possible to give "expert" assistance to the task of image interpretation.
COOPER,J.M. , SHEN, J.,YOUNG,F M.,CONNOLLY ,P, BARKER, J.R., AND MOORES, G.
The Imaging of Streptavidin and Avidin using Scanning Tunnelling Microscopy
J. Materials Science: Materials in Electronics 5 (1994)
J. Barker and J. Cooper in collaboration with Philips Electronics (Eindhoven), and the Universities of Mainz, Mons and Durham.
Although this is a basic research action, we are pursuing our targets utilising conducting polymers for electronics in collaboration with industry. Central to these studies is the provision of precisely defined oligomers and polymers designed both to provide macroscopic conductivity and the potential for molecular scale manipulation and study. The synthesis of these materials will be targeted to give highly defined stereochemistry and molecular weight. The chemical structures that have been chosen give conjugated backbones with small intrinsic gaps between valence and conduction bands. Experimental and theoretical studies have been directed to providing an understanding of the conduction processes in both intrinsically semiconducting and 'doped' metallic samples. In particular the measurement and understanding of the evolution of properties as a function of chain length from short length oligomers to high molecular weight polymers will be a major goal for these studies. The mid- and long-term application of such materials to both passive and active components, e.g. conductive screens, capacitors, biosensors, FETs, etc., will be explored.
Following guide-lines established by scanning tunnelling microscopy observations we believe that molecular structures favouring the formation of highly ordered monolayers can readily be obtained by the same routes used to produce the conductive materials. Thus the programme of synthesis will also provide the basis for studies of properties at the molecular scale. Methods for the reproducible preparation of ordered monolayers on solid surface have been explored. The structure, physical and chemical properties of such layers has been studied using atomic resolution microscopy allied to appropriate spectroscopic techniques. The aim will be to obtain monolayers that can be manipulated to provide elementary data storage and processing functions. Both chemical and physical processes occurring within the monolayers will be studied as means to achieve this aim.
Such studies cannot be conducted in isolation from developments in nanometre scale lithography and computational science since both will be important in determining likely device configurations. Thus parallel work will be initiated in these areas in order to provide a broad, well balanced basis for future developments beyond the lifetime of this project. The research is currently funded by ESPRIT as a basic research programme.
J.M. Cooper in collaboration with C. Wharton and N. Berovic (University of Birmingham)
Luciferase is the enzyme that enables the firefly Photinus pyralis to glow in the dark. In this research we will isolate the enzyme from the worm and investigate its application in Biomolecular Electronics. Previously, work at Birmingham has shown that the quantum yield of luciferase and its spectral emission and kinetics are both dependent on pH of the environment immediately surrounding the enzyme. It is now proposed to immobilise luciferase onto arrays of micro-electrodes using novel techniques that have been developed this year in Glasgow, and to modulate the amount of light emitted using the electrochemical surface of the micro-electrodes to control the local pH environment. We also have a second handle on the enzyme activity in the form of a 'messenger' molecule (c-ATP) which can be switched ON/OFF at different wavelengths.
Prototype devices, developed prior to this funding, have suggested that these systems may have applications in pattern recognition and image, data storage and light amplification. Novel devices will be fabricated in the photolithographic facility within the Department of Electronics and Electrical Engineering, and will be used to perform some basic tasks including. The research is funded by the Molecular Electronics Committee of the EPSRC.
(i) Pattern recognition, based upon a parallel processing pattern recognition system with a minimum processing time that is equal to the rise time for the luciferase enzyme system. Work carried out has suggested that this is approximately 200 ms. We recognise the fact that this is considerably slower than comparable electronic systems and work is currently in progress to further enhance this.
(ii) Image and data storage, using read/write functions involving the photo-activation/inactivation of the messenger molecule and enzyme switching by modulating the local pH.
(iii) Light amplification. Unfortunately, nature has intended that the light amplification should be slow, although optimisation of both the cycle time and the gain of the amplification is currently being investigated. The system at present seems limited to applications with response times of ~ 1kHz.
J. Cooper, J. Weaver and J. Williamson in collaboration with B. Moore (University of Strathclyde)
The properties of nano-scale semiconductor devices are beginning to approach molecular dimensions. In these systems, electron transport has been studied in one dimensional or zero dimensional regimes and, as a result, many new effects due to electron waveguide modes, quantum interference, and single electron charging have been observed. Exploitation of these effects at room temperature requires a reduction in size and better device regularity, which, as yet, can not be achieved by lithographic techniques alone. Further device miniaturisation therefore requires the use of a hybrid technology which interfaces molecular materials directly with state of the art nanofabricated methods.
In order to achieve this, we are fabricating solid-state devices consisting of a nanometre-scale gap between two metal electrodes, using a technology unique to the Department of Electronics at the University of Glasgow. We are exploring methods for organising or promoting self-assembly of molecules within this gap. In the first instance, model molecular materials such as conducting polymers and charge transfer salts are being positioned electrochemically within the gap. Although conducting polymers offer a low degree of molecular organisation, they provide an example of an easily-assembled molecular materials which will enable conductivity measurements to be carried out at an early stage in the project. In addition, the physico-chemical and theoretical properties of these materials are also well understood. For example, it is known that the high conductivity of these polymeric materials is restricted by electron hopping between individual chains (typically 40 monomer units or 20 nm in length) and by defects within the chains. Given that at the tip of a nanoelectrode there may be only two or three initiation sites for polymer growth, it should be possible to grow either single chains or small bundles of polymer chains. In the future, more sophisticated molecular systems, such as biological materials will be investigated using these systems. This research is funded by the Molecular Electronics Committee of the EPSRC.