Molecular Information Processing And Molecular Electronic Devices

Computer chips with molecular sized circuits made from proteins still belong to scientific fantasy. Yet reality has matched part of that dream as microprocessors have shrunk to the size of a nail and transistor have been made from semiconducting polymers. 

The fields of molecular electronics and nanoelectronics have become hot issues in the scientific and technological world in less then 10 years. Thanks to the pioneering work of H. Kuhn in the 60's and André Barraud from CEA, new materials made from LB films helped dozens of laboratories around the world to study the properties of molecular electronic devices. 

The first interdisciplinary research groups interested in molecular electronics met in Washington at the Naval Research Lab in 1978 during a workshop organized by the late Forest L. Carter. Since that time many other conferences, seminars and workshops in the United States, Europe, USSR and Japan have contributed to the fast progress of this field. 

One of the most promising areas of molecular information processing results from the convergence of microelectronics, solid state physics, and molecular biology. This hybrid technology is leading to the development of molecular electronic devices (MED). Technological breakthroughs in this area are opening the way to the development of "chemical computers." Molecular electronic devices could represent a significant breakthrough for computers in the next 30 years. From the tube era (40s to 60s), to the transistor era (60s to 80s), we are now entering into the molecular electron devices era (90s to 2000 and beyond). The challenge is enormous. We need to start from scratch and reinvent all the components of present day micro-electronics. 

Scientists will have to synthezise molecules sensitive to outside stimuli (like photons, magnetic or electric fields) and which present new properties when connected together For example as conductors or semiconductors, optoelectronic, piezo-electric or iono-electronic devices. Then the researchers will need to control the cooperative properties of supra-molecular assemblies made of such molecules through LB films, orientation of liquid cristals or growth of ordered films of molecular material. Eventually such supra-molecular assemblies will have to be integrated in electronic, opto-electronic, magnetic or catalytic components. 

These components will mimic the fundamental elements of present day micro-electronics. 

1- First, engineers have to work on a switch, able to shift information in one state or another. By interrogating this component. one should be able to find out in which state the switch is. 

2- The second major achievement will be the building of a true memory with different switches, that is, an array of molecules which could undergo reversible alterations, and can therefore be reused. 

3- The third major step is to construct molecular wires, able to transport information through distances, like conjugated chains of carbon atoms on which solitons move rapidly in order to connect switches and memories. 

4- The fourth step will be represented by the assembly of switches, memories and wires in three-dimensional structures or arrays, organized in assemblies of modules at several levels of communication and interconnections, able to perform coordinated functions. 

This is where one of the major breakthroughs of the new molecular engineering techniques will come into play : from the properties of atoms and molecules, the system will self-assemble in highly organized structures, putting atoms, molecules, and macromolecules into place, in order to perform dedicated functions. At this step, immunology techniques could be used to fit molecules at predetermined spots, or to deposit heavy metals which could be used as conductors at other levels of communications. 

5- Finally, the system should be reparable. Modules which do not perform correctly will have to be detected, corrections made, and sometimes a device replaced. These types of self-repairing automata already exist in biological systems. We begin to understand how they function at the molecular level, and how they self-assemble. Other supramolecular assemblies, like the quantosome, the photosome, the oxysome are small self-contained factories which perform important functions for the cell. These minute devices are packed with an enormous amount of molecules and information. We are slowly starting to understand their structures, and are able for the first time to copy them. But we still have to learn more from nature's design, understand how nature operates, and then translate what we have learnt into the new micromachines. 

What has been achieved by research labs around the world to meet such exciting challenges ? 

In Strasbourg, France, the group headed by Jean-Marie Lehn, Nobel Price in Chemistry has been able to build molecular wires and DNA-like double helical molecules. 

Last year Francis Garnier and his group of the Laboratoire des Matériaux Moléculaires du CNRS à Thiais produced what is recognized world-wide as the first "plastic transistor" made of polythiophène and with properties comparable to those of devices made from amorphous silicon. 

A new technique developped by Jean-Claude Wittmann and Paul Smith of the University of Santa Barbara in California provides a versatile method to produce ordered molecular films from a wide range of materials. They use a thin single cristal-like film of PTFE ( Polytetrafluoroethylene) deposited mechanically on a smooth surface like glass. This film acts as a matrix to align different materials in highly oriented thin films. 

A key issue in nanoelectronics is the connexion of molecular wires to create circuits. James Tour and Jeffry Schumm of the University of South Carolina at Columbia have been able to cross-connect molecular wires made of short chains of polythiophene rings through a junction box using silicon tetrachloride. 

An other important topic is related to molecular memory networks. Last year a Japanese group of the University of Tokyo headed by Z. Liu has produced LB films of a photochromic memory molecule derived from azobenzene wich changes shapes when it is struck by a photon of ultraviolet light allowing it to store a binary digit. 

Molecular diodes is also a hot subjet since the pioneering work of A. Aviram of IBM since 1974. Junction between silicon and organic conducting polymers to produce diodes is an important step on that road. Michael Sailor and his team from CalTech recently produced diodes usig a low-temperature chemical procedure in which contact to the semiconductor is made by a layer of polyacetylene. 

Ultimatly it will become possible to manipulate single molecules or clusters of molecules on any type of supra-molecular assembly through the use of a scanning and tunelling electron microscope (STEM) as demonstrated by Phaedon Avouris and In-Whan Lyo from IBM on silicon surfaces. 

These are only a few examples of recent developments in the field of molecular electronics. Further progress relies on the use of integrated and multidisciplinary strategies. For instance, in order to make ultramicrocircuits, several laboratories are trying to synthesize proteins which do not exist in nature. Such syntheses are feasible using present day genetic engineering techniques, automatic gene and protein synthesis, and computer reproductions of the bi and tridimensional structures of amino acid sequences. It is likely that from this research a large variety of molecular electronic devices and ultramicroeircuits could be conceived and manufactured. They could be rendered biocompatible, thus allowing the production of implantable logic circuits, offering the prospect of direct interface between the central nervous system of animals or human beings and computers. Such biocompatible circuits are now being implanted and tested in the brains of rats. Other applications of these ultramicrocircuits could be the production of prostheses for the blind, different transducers, or solar energy converters on soft plastic sheets. 

The production and assembly of these molecular circuits can be considered from two different approaches: a "passive" one (successive depositions, etching, grafting, doping) using technologies close to those presently used in the manufacturing of microcircuits; or an "active" one resulting from the spontaneous "growth" of the molecular circuit. In fact, automatic machines used today for synthesizing and analyzing genes and proteins, offer new models which may inspire the automation of the successive operations of molecular circuit production: growth of polymers, successive washing, reactions with other active groups, blocking, and reactivation of chemical groups. 

However, at the level where the presence or absence of a single chemical link can affect the performance of an entire circuit, it is virtually impossible to construct and assemble circuits with traditional macroscopic control techniques. It becomes necessary to use auto-assembly properties of biological macromolecules, observed for instance in Langmuir-Blodgett films or during the auto-organization of viruses or predissociated cellular organelles. In other words, instead of introducing the information from the outside, as we do today with most of our machinery (drill press, lathe, or even car construction robots), we will use information from the biopolymers themselves. Such information stored in the primary sequences of amino acids, allows the three dimensional folding of a protein. It is thus possible to benefit from the properties of biological macromolecules to assemble three dimensional molecular ultramicrocircuits. 

Many questions still remain unanswered : will these ultramicrocircuits be repairable? Will we be able to selectively break chemical linkages or rearrange them? Is it still necessary to utilize boolean logic, presently used in all computers? Will we be inspired by the neuronal networks of the brain? These circuits presently work, in all probability, in a non-boolean fashion, using parallel processing. To build the logic of the future, the convergence between molecular neurobiology and micro-electronics is in the forefront and it holds great promises.

Joël de Rosnay 

Director of Strategy

Cité des Sciences et de l'Insdustrie – La Villette – Paris – France 

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