Jolting crystals into nanostructures.

Researchers have used the needle tips of scanning probe microscopes to manipulate atoms on a material's surface. They have positioned atoms to spell out nanoscopic messages and moved them about to create and characterize a variety of structures (SN: 10/9/93, p. 228; 9/14/96, p. 167). Now, chemist Charles M. Lieber and his coworkers at Harvard University have discovered that a voltage applied to the tip of a scanning tunneling microscope can alter the nearby surface crystal structure. An electric pulse creates a nanocrystalline island with an atomic arrangement different from that of the rest of the material. This technique represents a new approach to the fabrication of nanostructures, Lieber says. The scientists report their achievement in the Nov. 1 Science. Lieber and his team start with a single crystal of tantalum diselenide oriented to display a surface consisting of an array of selenium atoms closely packed into a hexagonal arrangement. They use a special scanning tunneling microscope, operated in an ultrahigh vacuum and at a frigid 4.8 kelvins (the temperature of liquid helium), to modify this surface. A pulse of electricity delivered through the microscope's tip induces surface selenium atoms to shift slightly away from the tip. These movements alter the bonding arrangement between the selenium atoms and the underlying tantalum atoms, creating a new local crystal structure. As long as they keep the voltage of the pulse above a certain threshold value, the researchers can vary the voltage to create crystal islands that range in width from 7 nanometers to more than 100 nm. Because Lieber can now, with "fair reproducibility," control the size of the nanocrystals created on a tantalum diselenide surface, he and his team can investigate the effect of confinement on the distribution of electrons in nanocrystals and on other physical characteristics important for determining potential applications. "We have enough control at this point to put two or three of these little nanoclusters very close together," Lieber says. "We can start to create arrays to study how these nanostructures might interact with one another at different separations."