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Doctoral Studies
It's a really, really, really, really small world after all

>>Graduating from microstructures to nanostructures allows materials scientists to take technology to the next level.

Raghu Parthasarathy used to consider the big picture, but now he attends to the small details. As an astrophysics major at the University of California at Berkeley he became more interested in the tiny electronics of the telescope's inner space than the swirling clouds of outer space. Switching his focus from radio astronomy to semiconductors, the Ph.D. candidate in physics has moved to the other end of the scale-he measures in nanometers rather than parsecs.

Parthasarathy-who will graduate this summer after five years at Chicago-works with nanostructures, objects roughly 1 billionth of a meter in diameter, or several hundred times smaller than a human blood cell. Current technology operates on the microscale, measuring items about 1 millionth of a meter across. By examining the behavior of the much smaller nanostructures, materials scientists like Parthasarathy hope to pave the way for smaller and faster electronics.

Building devices on the nanoscale, however, presents problems of quantum mechanics not encountered on the microscale. "On the nanoscale you can only fit one extra electron on a device, as opposed to billions on a piece of wire," explains Parthasarathy. "Two electrons a micrometer away from each other will be fine, but put them a nanometer away and they will strongly repel each other. Only one will be able to occupy the space." Nanometric particles also create new physical phenomena-such as emitting differently colored lights-when squeezed into areas smaller than their wavelength. "It's not just new technology," he says. "It's new science."

Parthasarathy's dissertation work involves placing organically encased, six-nanometer diameter gold spheres into a slippery liquid on a chip (a process developed by lab partner Xiao-Min Lin, a postdoctoral researcher in physics). The slick surface helps the spheres-or nanocrystals-self-assemble into a hexagonal pack. When an electric charge is sent across the pack its electrons repel each other because of their close proximity, inhibiting the charge's progression across the array.

"If you send a current through a block of metal there is just a river of charge flowing through the material," Parthasarathy explains. "But because these crystals are so small, only one electron fits on each. So you get a flow of current that is more like a traffic jam, with spurts of movement here and there." Instead of a free-flowing river, the charge acts like water being forced through a sponge, following different paths to get to the other side. Parthasarathy and Lin have been able to characterize what these pokey flows look like, with organized as well as with messy arrays, to tell whether an array's orderliness affects how the paths branch.

Like all materials science, nanoscience advances in baby steps-some of Parthasarathy's experiments have no clear applications beyond forcing nanocrystals into situations just to see how they will react. While years away from the wonders of Star Trek, Parthasarathy hopes that it won't be long before physicists can manipulate nanostructures to create "artificial atoms." By assembling these crystals in different patterns to create new solids-the way nature assembles carbon atoms one way to make diamonds and another to make graphite-materials could be designed with specific electronic properties like magnetism or superconductivity. But before that can happen, physicists need to see how nanostructures can be manipulated under the microscope, all the while keeping sight of the big picture.
- C.S.



  APRIL 2002

  > > Volume 94, Number 4


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