Mar 26, 2003
Don't mess with silicon nanospheres
Scientists from the University of Minnesota and Los Alamos National Laboratory, US, have shown that individual silicon nanospheres are extremely hard. The team found that defect-free nanospheres with a diameter of 40 nm were roughly four times harder than bulk silicon.
"This is the first time that a measurement of mechanical, rather than electromagnetic, properties of nanoparticles has been made," said William Gerberich of the University of Minnesota. "We can now compare it with the results of simulations - mechanical properties of materials at this scale are much more difficult to simulate than electromagnetic properties."
First, the researchers made defect-free silicon nanospheres by condensing silicon tetrachloride vapour onto a sapphire surface. Then they measured the hardness of the spheres by squeezing them between a diamond-tipped probe and the sapphire surface. The smaller the sphere, the harder it was: spheres with a diameter of 100 nm had a hardness of around 20 gigapascals (GPa), whereas 40 nm diameter spheres exhibited a hardness of 50 GPa. For comparison, bulk silicon has an average hardness of roughly 12 GPa, while sapphire has a hardness of about 40 GPa, and diamond 90 GPa.
"People have never had these perfect, defect-free spheres to test before," added Gerberich. "You can compare the silicon nanospheres with materials such as nitrides and carbides, which typically have hardness values in the range of 30-40 GPa."
The scientists also performed atom-by-atom simulations of the spheres' behaviour using a supercomputer. The hope is that their findings will enable them to design superhard materials and nanocomposites. "These measurements make it possible to pursue a bottom-up approach to materials design from a mechanical perspective," explained Gerberich.
Now the team plans to measure the hardness of silicon carbide nanospheres. The scientists will use two diamond surfaces for the measurements since a sapphire surface may well not be hard enough for the job.
The scientists reported their results in the Journal of the Mechanics and Physics of Solids.
About the author
Liz Kalaugher is editor of nanotechweb.org.