Natural order

Researchers turn to nature for help in constructing nanoscale circuits

NANOTECHNOLOGY, which involves creating and using tools measured in billionths of a meter, holds great promise for applications such as medicine and quantum computing, but producing the devices in usable quantities in reasonable time remains a challenge.

Researchers at the University of Maryland's A. James Clark School of Engineering are working to enlist nature's help to produce nanocircuits economically.

'While we understand how to make working nanoscale devices, making things out of a countable number of atoms takes a long time,' said Ray Phaneuf, associate professor of materials science and engineering. 'Industry needs to be able to mass-produce them on a practical time scale.'

That's where nature comes in. 'Nature is very good at making many copies of an object' through self-assembly, Phaneuf said. But nature knows how to make only a limited range of patterns for these complex structures, such as shells or crystals. Phaneuf's work focuses on the use of templates to teach nature some new tricks.

'The idea of using templates is not new,' Phaneuf said. 'What is new is the idea of trying to convince nature, based on the topography of the template, that it should assemble objects in a particular place,' atom by atom.

One application for the process could be quantum computing. A host of schemes propose harnessing the quantum states of atomic particles to do complex calculations. One involves assembling pairs of quantum dots ' tiny semiconductors containing from one to 100 particles with elementary electric charges ' to create the qubits used in quantum calculations.

Assembling the billions of dots in the precise patterns needed for massively parallel computing may be possible, but, Phaneuf said, 'it may not be doable within the age of the universe' with current techniques.

'Nature already knows how to assemble quantum dots,' he said. 'We are working on the step before self-assembly, the self-organization of the substrate,' which will act as the template for the dots.

The silicon substrate is etched into steps using lithography, but it is difficult to reach the level of precision required at the atomic scale using lithography alone. Heat and cold can be used to add or subtract atoms on the surfaces and precisely shape the step patterns. The steps can also be shaped. The step patterns, which are stiff, tend to straighten out under heat but are limited by the surrounding patterns in how much they can straighten.

'We play this stiffness off with the repulsive interaction between steps' to create the sizes and shapes needed, Phaneuf said.

The result is a substrate that can be reused many times as a template for growing nanostructures with silicon and gallium arsenide for computer and cell phone components.

'It still is in the development stages,' he said.

'There is still quite a lot to do before we make practical devices out of it. I don't think we're quite ready to make transistors on the chips.'

And the market for the end products has not yet developed. You are not likely to find any deals on quantum computers from Dell or HP in the ads of your Sunday supplements this weekend. More-immediate applications for this technology are likely to be biochips used in biology and medicine.

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