"This is the first time that a hollow cage made of metal has been experimentally proved," said Lai-Sheng Wang of Pacific Northwest National Laboratory and Washington State University.

Scientists have been trying to create carbon-free hollow cages since the discovery of the C60 molecule, or buckyball. While they have succeeded in creating inorganic cages, elemental metal cages had proved elusive until now. For example, the only cage-like gold structures had been icosahedral Au12 cages but these were stabilized by a central metal atom.

Since Wang and colleagues knew that clusters of 20 gold atoms are pyramidal in shape and that clusters of 13 atoms are flat, they focused their attention on clusters with 14 to 19 atoms. Photoelectron spectroscopy and theoretical calculations helped them to analyse the molecules.

The study revealed that clusters of 16, 17, or 18 atoms were hollow, while those with 15 or fewer atoms remained flat and clusters with 19 atoms were near-pyramidal.

The free-standing hollow cages were stable at room temperature. "When deposited on a surface, the cluster may interact with the surface and the structure may change," said Wang.

The researchers believe they will be able to dope the cages with a number of different atoms. "These doped cages may very well survive on surfaces, depending on the dopants," said Wang.

The researchers reported their work in PNAS.