Beam "Photography"

Many of the beautiful colors found in stained glass windows come from metal or oxide nanoclusters dispersed in the material. In the 19th century Michael Faraday correctly deduced that the glass was a mixture of different substances and in 1907 Gustav Mie explained the colors by showing how light in a medium gets scattered by particles on the size scale of the wavelength ("Mie scattering"). However, the mechanism by which the nanoclusters formed was usually lost in the complexities of glass chemistry.

Researchers at the Universities of Orsay and Paris (Harry Bernas, bernas@csnsm.in2p3.fr, 011-33-1-6915-5222), collaborating with glass experts, have now shown that by shooting MeV-energy ions at room temperature into a glass containing a metal oxide, one can initiate the aggregation or "nucleation" of pure metal nanoclusters and even control the density of nanoclusters within the material. Moreover, the nanoclusters only grow in size upon heating of the sample, allowing control over their size. This is analogous to the photographic process, in which photons hit metal-containing salts in the emulsion, thus allowing them to free metal atoms (usually silver) which nucleate into clusters, forming an invisible or "latent" image.

Upon developing, these clusters grow to sizes that form visible pixels. Here, ions replace photons and heat plays the role of the "developer." Two crucial advantages of the ion-beam method are that the density of nucleation sites in the "latent image" can be predicted precisely; and standard lithography techniques employing stencil-like "masks" may be used to design spatial patterns of clusters. Both could lead to applications in optoelectronics. (Valentin et al., Phys. Rev. Lett, 1 Jan. 2001; text at Physics News Select.)