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
"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."