Wednesday, August 13, 2008

First All-Nanowire Sensor

Original Article
Researchers integrate nanowire sensors and electronics on a chip.

Researchers at the University of California, Berkeley, have created the first integrated circuit that uses nanowires as both sensors and electronic components. With a simple printing technique, the group was able to fabricate large arrays of uniform circuits, which could serve as image sensors. "Our goal is to develop all-nanowire sensors" that could be used in a variety of applications, says Ali Javey, an electrical-engineering professor at UC Berkeley, who led the research.

Nanowires make good sensors because their small dimensions enhance their sensitivity. Nanowire-based light sensors, for example, can detect just a few photons. But to be useful in practical devices, the sensors have to be integrated with electronics that can amplify and process such small signals. This has been a problem, because the materials used for sensing and electronics cannot easily be assembled on the same surface. What's more, a reliable way of aligning the tiny nanowires that could be practical on a large scale has been hard to come by.

A printing method developed by the Berkeley group could solve both problems. First, the researchers deposit a polymer on a silicon substrate and use lithography to etch out patterns where the optical sensing nanowires should be. They then print a single layer of cadmium selenide nanowires over the pattern; removing the polymer leaves only the nanowires in the desired location for the circuit. They repeat the process with the second type of nanowires, which have germanium cores and silicon shells and form the basis of the transistors. Finally, they deposit electrodes to complete the circuits.

The printed nanowires are first grown on separate substrates, which the researchers press onto and slide across the silicon. "This type of nanowire transfer is good for aligning the wires," says Deli Wang, a professor of electrical and computer engineering at the University of California, Santa Barbara, who was not involved in the research. Good alignment is necessary for the device to work properly,since the optical signal depends on the polarization of light, which in turn is dependent on the orientation of the nanowires. Similarly, transistors require a high degree of alignment to switch on and off well.

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