New Measurements Show Silicon Nanospheres Rank Among Hardest Known Materials

University of Minnesota researchers have made the first-ever hardness measurements on individual silicon nanospheres and shown that the nanospheres’ hardness falls between the conventional hardness of sapphire and diamond, which are among the hardest known materials. Being able to measure such nanoparticle properties may eventually help scientists design low-cost superhard materials from these nanoscale building blocks.

Up to four times harder than typical silicon-a principal ingredient of computer chips, glass and sand-the nanospheres demonstrate that other materials at the nanoscale, including sapphire, may also have vastly improved mechanical properties. The researchers’ results were published online March 18 by the Journal of the Mechanics and Physics of Solids and will appear in June 2003 issue. The work is supported by the National Science Foundation (NSF), the independent federal agency that supports basic research in all fields of science and engineering.

“These results give us two reasons to be excited,” said William Gerberich, chemical engineering and materials science professor at Minnesota and lead author on the paper along with his graduate student William Mook. “We can now look at the properties of these building blocks, and from there, we can begin to design superhard materials. In addition, we’ve now achieved a way to conduct experiments on a nanoscale particle and perform atom-by atom supercomputer simulations on a similarly sized particle.”

Such nanospheres might find early applications in rugged components of micro-electromechanical systems (MEMS), according to Gerberich. 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 research team first devised a method for producing defect-free silicon nanospheres in which the silicon spheres condensed out of a stream of silicon tetrachloride vapor onto a sapphire surface. (Defects in the spheres reduce the hardness by acting as sites for flow or fracture.) The hardness was measured by squeezing individual particles between a diamond-tipped probe and the sapphire.

The smaller the sphere, the harder it was. The spheres tested ranged in size from 100 nanometers to 40 nanometers in diameter, and the corresponding hardness ranged from 20 gigapascals up to 50 gigapascals for the smallest nanospheres. For comparison, stainless steel has a hardness of 1 gigapascal, sapphire of about 40 gigapascals, and diamond of around 90 gigapascals. Bulk silicon averages about 12 gigapascals.

“People have never had these perfect, defect-free spheres to test before,” Gerberich said. “You can compare the silicon nanospheres to materials such as nitrides and carbides, which typically have hardness values in the range of 30 to 40 gigapascals.” The research team will study silicon carbide nanospheres next, but they’ll need two diamond surfaces for the experiments, since squeezing a silicon carbide nanosphere would likely drill a hole into sapphire.

“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,” Gerberich said. “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 examined by the Minnesota researchers were composed of 5 million to 600 million atoms. Because materials science algorithms can simulate this number of atoms on supercomputers, the Minnesota team worked with Michael Baskes of Los Alamos National Laboratory to conduct some preliminary simulations, which corresponded well with the experimental findings.

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