Credit: © 2008 NPG

Materials made from nanoscale crystals can withstand remarkably high stress, but tend to break under extreme strain. Now Andrew Minor at Lawrence Berkeley National Laboratory and the University of California, Berkeley and co-workers1 have observed large deformation in hollow spheres made from nanocrystals of cadmium sulphide, a material usually considered to be very brittle. The work shows that the shape of nanocrystalline materials has a strong influence on their structural properties.

The researchers tested hollow shells of between 200 and 450 nanometres in diameter by squashing them between two substrates, while monitoring their deformation with a transmission electron microscope. The experiment showed that the shells can withstand stresses close to the ideal value of cadmium sulphide, and can be deformed by up to 20% of the sphere's diameter without breaking-up. In general, larger spheres were capable of withstanding greater forces before fracturing.

These spheres are a good example of 'structural hierarchy', because their small crystal size provides strength, whereas the overall hollow shell shape allows large deformation. Work is still needed to determine exactly how the shells deform without breaking.