Size does matter in computer future

November 3, 2000

Liverpool scientists unveil nanoscale electronic components that build themselves.

The electronics industry underwent a fundamental change with the invention of the transistor, paving the way for the microelectronics revolution. Devices are now constructed in complex arrays using lithographic methods derived from classical photography. However, as the size of the circuit shrinks, the resolution of the features that can be fabricated by this so-called "top-down" methodology reaches a limit - currently about 100 nanometres.

If computers are to become even smaller - and hence faster - the size of the circuit must be reduced towards the "nanoscale" domain. One prospect is the so-called "bottom-up" approach in which smaller electronic components are constructed from even smaller building blocks, preferably by a process of chemical self-assembly.

Our report, published in Nature yesterday, demonstrates for the first time that this is feasible. An electronic switch was built from a single 6-nanometre diameter cluster of gold atoms, attached to a gold contact by a layer of organic molecules that acts as a spacer, or chemical glue. The largest dimension of the device itself was some 8 nanometres.

An important issue concerning the continued miniaturisation of electronics is whether the ease of electron transfer across structures of nanometre dimensions can be controlled by injection of a small number of electrons into the spacer molecules.

The strategy we followed was based on the self-organising properties of compounds that carry sulphur-containing thiol groups that allow molecules to fix onto gold surfaces. By using molecules with a thiol group at each end, it becomes possible to chemically fix gold nanoparticles in a predetermined position with respect to the base gold contact.

Each spacer molecule included a central chemical group, related to the herbicide Paraquat. The group can easily accept one or two electrons, thereby changing its electronic state. This chemistry allowed a group of spacer molecules to behave as controllable molecular wires, connecting the gold nanoparticle to the base contact.

The materials used in this work were synthesised from readily available chemicals and used simple procedures. The nanostructure showed low electrical conductivity between the base contact and the gold cluster. Changing the electronic state of the spacer molecules could be achieved by established electrochemical techniques that allow a rigorous control of the number of electrons injected into each molecule.

The results we obtained demonstrate that the introduction of one electron into each spacer molecule caused the collapse of the barrier to electronic conduction between the nanoparticles and the base contact. Surprisingly, the introduction of a second electron lowered the electrical conductivity.

The number of electrons per nanoparticle required to achieve each of these effects is probably less than 30. Our report shows that it is possible to control electrical conduction through these structures by the electronic state of the spacer molecules.

These research efforts represent a step towards decreasing the size of electronic components using chemical means. The challenges for further development are twofold. The design and creation of chemical structures that are able to organise themselves will permit the construction of more elaborate nanoscopic devices by self-assembly. There is also a need for the development of methods by which these elements may be connected together and interfaced to the macroscopic world using the engineering nano-fabrication techniques that are becoming available.

The future will probably involve the conjunction of the two approaches to nanotechnology: bottom-up, which has its roots in the synthetic and physical chemistry of self-assembled nanostructures, and top-down, representing the engineering approach to the construction of nanoscopic objects.

Donald Bethell is professor of physical organic chemistry, David Gittins is a research scientist, Richard Nichols is reader in physical chemistry and David Schiffrin is professor of physical chemistry and director of the Centre for Nanoscale Science, Liverpool University.

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