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Cure In Sight For 'Pink Noise' In Quantum Dots

By R. Colin Johnson
EE Times

The troublesome flickering that plagues quantum-dot development today may be closer to a cure, thanks to a team of researchers at the University of Chicago.

By analyzing the power spectrum of quantum-dot blinks, the group found that, unlike semiconductors, individual quantum dots exhibit exactly the same qualities as their aggregations, thereby simplifying the measuring apparatus needed to fix the problem.

"This blinking phenomenon happens everywhere in nature at the nanoscale," said Matthew Pelton, a research associate at the university's James Franck Institute. In MOSFETs, for example, "current blinks when electrons get stuck in these nanoscale traps," each of which holds the electrons for a particular time. "One trap might hold electrons for a short amount of time and another holds them for a long time," Pelton said, but together they exhibit so-called 1/f noise, or "pink noise."

If quantum dots behaved like MOSFETs, which until now was the common wisdom, then curing a flickering problem would mean studying the individual quantum dots, since each would have a characteristic blinking frequency. However, Pelton and his colleagues Philippe Guyot-Sionnest and David Grier (who has since moved to New York University) found that each quantum dot exhibited the same pink-noise profile all by itself.

Therefore, researchers can study the problem at the macroscale with less-expensive equipment and still get the same-quality results.

"We analyzed the power spectrum, a 200-year-old technique that for some reason no one had yet thought to apply to quantum-dot blinking," Pelton said. "What we found was that unlike MOSFETs, where each trap has a single blinking frequency, each quantum dot blinks at all the different frequencies."

Thus, when quantum dots are observed at the macroscale, the same phenomenon is seen as when the output of a single quantum dot is observed.

With MOSFETs, by contrast, studying flickering at the microscale only provides an average of what is happening at the nanoscale.

Pink noise, also called "flicker noise," exhibits a power spectrum within a band where all the frequencies have equal energy per octave (ideal white noise has the same intensity at all frequencies). Now that quantum dots are known to each exhibit 1/f noise, Pelton's group suggests that instead of $100,000 microscopes and CCD imagers, researchers should use existing laboratory lasers and a $1,000 silicon photodiode hooked to an analog-to-digital converter.

"We think our discovery will enable a quicker solution to this blinking problem in quantum dots, and we have to solve this problem in order to engineer new properties into semiconductor materials on the nanoscale," Pelton said. "Can you imagine trying to design an on-chip communications laser if it flickered?"

The research at the University of Chicago was supported by the Materials Science and Engineering Research Center, the National Science Foundation and the American Chemical Society.