Nanospheres: A gem of discovery

Silicon nanospheres are among the hardest known materials, ranking between the conventional hardness of sapphire and diamond, according to a recent study led by chemical engineering and materials science professor William Gerberich.

Gerberich's research team made the first-ever mechanical measurements on individual silicon nanospheres, which proved to be up to four times harder than typical silicon. The study suggests that other materials at the nanoscale also may have vastly improved mechanical properties. The ability to measure these properties may eventually help scientists design low-cost superhard materials from nanoscale building blocks, says Gerberich.

Such nanospheres might find early applications in rugged components of microelectro-mechanical systems. To produce a small gear, for example, the shape could be etched into a silicon wafer and filled with a composite including silicon carbide or silicon nitride nanospheres. The surrounding silicon could then be selectively etched away.

To make the measurements, the team first devised a method for producing defect-free silicon nanospheres. (Defects in the spheres reduce the hardness by acting as sites for flow or fracture.) The researchers then measured hardness by squeezing individual particles between a diamond-tipped probe and the sapphire. The smaller the sphere, the harder it was.

"This is the first time that a measurement of mechanical, rather than electromagnetic, properties of nanoparticles has been made, which we can now compare to the results of simulations," says Gerberich. "Mechanical properties of materials at this scale are much more difficult to simulate than electromagnetic properties."

A silicon sphere with a 40-nanometer diameter has approximately 40 million atoms. The spheres measured in the study were composed of five million to 600 million atoms. Preliminary computer simulations conducted at Los Alamos National Laboratory seem to support the research.

"Better designs for these sorts of nanocomposites will be based on a better understanding of what goes into them," Gerberich says. "These measurements make it possible to pursue a bottom-up approach to materials design from a mechanical perspective."