Nanowire photodetectors
According to researchers at the University of California at San Diego (UCSD), semiconducting nanowires can be used to build perfect light detectors with single-photon sensitivity. The zinc oxide (ZnO) nanowires they've used are ideally suited to develop "new photodetector architectures for sensing, imaging, memory storage, intrachip optical communications and other nanoscale applications." So far, the engineers have demonstrated in their labs that nanowires are extremely sensitive photodetectors because of their specific geometry which combines large surface areas and small volumes. Now we have to wait for real applications.
On the left, you can see a scanning electron microscope (SEM) image of the zinc oxide (ZnO) nanowires grown in the Deli Wang lab at UCSD, where this research is been done (Credit: UCSD). Deli Wang is an electrical and computer engineering (ECE) professor from the UCSD Jacobs School of Engineering, who said that their "results are encouraging and suggest a bright future for nanowire photodetectors, including single-photon detectors, built from nanowire structures."
Here is why a nanowire can be used as a photodetector. "For a nanowire to serve as a photodetector, photons of light with sufficient energy must hit the nanowire in such a way that electrons are split from their positively charged holes. Electrons must remain free from their holes long enough to zip along the nanowire and generate electric current under an applied electric field -- a sure sign that light has been detected."
And here is why the geometry of nanowires -- with so much surface area compared to volume -- is so efficient for trapping light. "Dangling bonds on vast nanowire surfaces trap holes -- and when holes are trapped, the time it takes electrons and holes to recombine increases. Delaying the reunion of an electron and its hole increases the number of times that electron travels down the nanowire, which in turn triggers an increase in current and results in 'internal photoconductive gain.'"
It should even be possible to design nanowire photodetectors for specific colors. Cesare Soci, one of two primary authors on the Nano Letters paper and a postdoctoral researcher in the Deli Wang lab at the Jacobs School. Soci is standing next to equipment used to grow nanowires. "Different kinds of nanowires detect different wavelengths of light. You could make a red-green-blue photodetector on the nanoscale by combining the right three kinds of nanowires," said Cesare Soci, a postdoctoral researcher in the Deli Wang lab.
For those of you who want more information, this research work has been published by Nano Letters under the name "ZnO Nanowire UV Photodetectors with High Internal Gain" (Volume 7, Issue 4, Pages 1003-1009, April 2007). Here are two links to the abstract and to the full paper (PDF format, 7 pages, 315 KB).
Sources: University of California at San Diego, via EurekAlert!, April 25, 2007; and various websites
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