Ultrapowerful computers might one day work by manipulating the magnetic orientation, or spin, of electrons. Ideal building blocks for these "spintronic" devices are quantum dots, nanometer-sized specks of semiconductors and metals, which retain magnetic information. But spin doctors have had a hard time shuttling spin-based information from dot to dot, a basic requirement for quantum dot-based computing. Now researchers have hurdled that obstacle and may be poised to make additional leaps.
As reported online in Science this week, chemist Min Ouyang and physicist David Awschalom of the University of California, Santa Barbara (UCSB), report a simple scheme in which they coated a surface with successive layers of quantum dots, each wired to others above and below with molecular bridges containing benzene rings. Atoms in benzene rings keep only a loose grip on their electrons, letting them roam freely in the vicinity of neighboring molecules; the UCSB pair suspected that this free-ranging motion would help electrons travel from one quantum dot to another. Using a pair of ultrashort pulse lasers, the researchers watched electron spins hop between dots much as electrical charges move through a wire.
The big surprise, however, was that the spin-hopping picked up considerably as the researchers increased the temperature. Practical spintronic devices have been stalled for years because even modest amounts of heat tend to scramble electron spins, wiping out any information they contain. The UCSB team's coupled quantum dots, however, buck the trend: They transfer spins at about 12% efficiency near absolute zero but at 20% efficiency at room temperature.
"This is very significant work," says Jeremy Levy, a physicist and spintronics expert at the University of Pittsburgh in Pennsylvania. Levy says the simple, flexible method for wiring dots together should open the door for other groups to create and test a kaleidoscopic range of new quantum dot assemblies. That ability, Levy and others say, could lead to breakthroughs in solar cells, molecular electronics, sensors, light-emitting displays, and other technologies. "It's clear there will be other people using this approach, modifying it, and taking it in new directions," Levy says.