Friday, July 23, 2004

"QUANTUM: Cure in sight for 'pink noise' in quantum dots"
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. 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.