Cations perform a quick change

Researchers with the US Department of Energy's Lawrence Berkeley National Laboratory (Berkely Lab) and the University of California at Berkeley have shown that for nanocrystals, the doping process in which one type of positively charged atom, or cation, is exchanged for another, takes place at a much faster rate than for crystals of extended size and is fully reversible, something that is virtually forbidden in micro-sized crystals under the same environmental conditions.

"Our findings show that the cation exchange reaction offers a versatile route for expanding the range of nanoscale materials with diverse compositions, structures and shapes without having to develop new synthetic methods to produce each individual nanostructure," says chemist Paul Alivisatos, the principal author of a paper reporting this research.

Another important result from this study is that ionic nanocrystals apparently can be transformed into other materials with different physical and chemical properties but without altering their original shape, simply through an exchange of cations.

Doping a crystal to transform it into another type of material with different physical and chemical properties is a long-established practice in extended solids. This practice is being carried over into the transformation of nanocrystals grown from inorganic materials, including metals and semiconductors.

However, because nanocrystals have a high surface-to-volume ratio (meaning they are virtually all surface and no interior), their reactions to the various forms of doping can be quite different from the reactions of extended solids.

For example, in extended solids, chemical reactions run very slowly because of the high activation energies required to diffuse atoms and ions. These transformations are also not reversible.

Researchers worked with nanocrystals of the semiconductor cadmium-selenide (CdSe). They mixed a solution of CdSe nanocrystals together with a small quantity of silver nitrate at room temperatures.

In less than a second, the silver cations reacted with the CdSe spheres to produce spheres of silver-selenide (Ag_2 Se). When these Ag_2 Se spheres were mixed with a solution containing excessive cadmium cations, the reaction was reversed.

Though the reverse reaction took about a minute to complete, the final product was CdSe spheres.