Silver nanocrystals may store data densely

Researchers at the Georgia Institute of Technology (Georgia Tech; Atlanta, GA) have taken a process similar to photography down to a submicron scale and come up with a potential method for highly dense optical storage. Photoactivation of silver halide crystals has been the basis for photographic film exposure for more than 100 years. The Georgia researchers, however, have demonstrated binary optical storage by optically writing and reading simple images on thin films made up of silver oxide nanoclusters.¹ Each nanocluster contains between two and eight silver atoms.

"These nanomaterials have a remarkable new property: when you shine blue light with a wavelength of less than 520 nm onto them, you switch on their ability to fluoresce," said Robert M. Dickson, assistant professor of chemistry and biochemistry at Georgia Tech. "You can then read the fluorescence nondestructively by illuminating the clusters with longer-wavelength light."

The researchers are not yet sure exactly why the process works, although they suspect that it is related to quantum properties of silver. They observed that the blue light chemically reduced particles near the surface of the film into clusters of silver atoms. When subsequently exposed to green light, the photoactivated silver clusters fluoresced to emit red light intense enough to be seen without the aid of a microscope. When studied under a microscope, however, the nanoclusters also displayed a potential for improving upon the data density available with current optical storage media.

"If you look at an individual particle through the microscope, you see green emission, then red emission, then yellow emission all from the same particle," Dickson said. "Not only are you generating fluorescence, but you presumably are also changing the size and/or geometry of the cluster, which causes it to emit different wavelengths."

Such multicolor emissions could allow storage of more than one bit of information in each data point, Dickson said. And if the particles could be distributed in a three-dimensional matrix, they could provide a very dense storage medium that could be written and read in parallel.

Additional performance factors, such as the length of time that the data persist in the storage medium, whether data can be erased and rewritten, and of course the actual fluorescence mechanism are currently topics for further study. Dickson believes, however, that his group is the first to report fluorescence in silver clusters at room temperature.

REFERENCE

• L. A. Peyser, A. E. Vinson, A. P. Bartko, and R. M. Dickson, Science, 29, 103 (Jan. 5, 2001).