Stephanie Brock and colleagues at Wayne State University in Detroit made the aerogels from metal chalcogenides -- compounds that contain a metal and a group VI element such as sulphur or selenium (J Mohanan et al. 2005 Science 307 397). "We have opened up a new class of aerogels," says Brock. "Previously, aerogels were limited to metal oxides and carbon." The useful properties of aerogels are a result of their combination of high surface area, quantum confinement effects and photoluminescence.

To prepare the aerogels, Brock and colleagues first made nanoparticles of various metal chalcogenides capped with molecules known as thiolates, which they then oxidized, causing the nanoparticles to form a gel. Next, they dried the gel with supercritical carbon dioxide to maintain the pore architecture.

The resulting structures contained pores between 2 and 50 nanometres in diameter. This means that the materials have surface areas as high as 250 square metres per gram. Moreover, the cadmium sulphide aerogel had a bulk density of 0.07 grams per cubic centimetre, which is just 1.4% of the density of a single crystal of cadmium sulphide. The Wayne State team also used the technique to make aerogels with cadmium selenide, zinc sulphide and lead sulphide.

"Finding ways to assemble nanoparticles into actual functional devices -- without losing the characteristic nanoscale properties -- is one of the current challenges for nanotechnology," says Brock. "This 'sol-gel' route provides a simple, versatile method to do this, and should be amenable for the creation of more complex composite materials too."