"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.