Nanoclusters Of 2-8 Silver Atoms
May Be Basis For New Optical
Nanoclusters composed of 2-8 silver atoms could be the basis for a
new type of optical data storage. Fluorescent emissions from the
clusters could potentially also be used in biological labels and
Writing in the journal Science, researchers at the Georgia Institute of
Technology report that they have successfully demonstrated binary
optical storage with the new system, writing and reading simple
images recorded on thin films made up of silver oxide (Ag2O)
"These nanomaterials have a remarkable new property: when you
shine blue light with a wavelength of less than 520 nanometers 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 begin by producing extremely thin films (less than 20
nanometers thick) of silver oxide nanoparticles on a glass slide. They
then selectively expose portions of the film to light in the blue
spectrum. The light chemically reduces particles near the surface of
the film, partially converting them to clusters of silver atoms. When
researchers then expose these photoactivated silver clusters to longer
wavelength (greater than 520 nanometers) green light, they fluoresce
strongly, emitting red light easily visible to the naked eye. Silver
oxide particles not photoactivated by exposure to the blue light do
Dickson's research group, including Lynn A. Peyser, Amy E. Vinson
and Andrew P. Bartko, have used the technique to store images of
simple geometric shapes and the letter "L."
When studied under a microscope, the individual silver particles
display an additional property that may ultimately prove useful for
increasing the density of optical data storage.
"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
By using the correct distribution of particle sizes, these multi-color
emissions could allow storage of more than one bit of information in
each data point. 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.
"We have already demonstrated binary optical storage because we can
write fluorescent patterns in which an individual particle is either on
or off," Dickson noted. "But we can imagine being able to write and
read more than binary storage. These silver clusters could potentially
be very useful optical storage materials because of the potential for
writing and reading in parallel, and/or storing more than one bit of
information per data point."
When exposed to laser-generated blue light at a wavelength of 515
nanometers, the nanoclusters produce a seemingly random blinking
pattern of yellow, red and green light. Exposure to blue light,
however, photoactivates additional silver oxide particles, destroying
the original image.
Images stored on the silver oxide film can be read nondestructively by
green light for at least two days, the longest period of time the
researchers left them on the stage of their microscope. How long the
effect will persist is a topic for further study.
Though they have demonstrated an ability to optically write and read
information with the new system, the researchers do not yet know if
the information can be optically erased and the film re-written.
Fluorescence has previously been reported in silver clusters at low
temperatures and in rare gas environments, but Dickson believes this
is the first time the phenomenon has been reported at room
Having demonstrated a potentially valuable new technique, the
researchers are now working to understand the fundamental issues
governing the properties of the nanoclusters.
"We really want to understand the underlying physics and chemistry
of this material," Dickson said. "While we have an eye toward
developing applications, the issue now is understanding what gives
rise to the fluorescence, understanding the size and geometry of these
clusters, how to control the composition and what factors are
important for generating the fluorescence."
A physical chemist with a background in optically-active organic
dyes, Dickson expected to see fluorescence in the silver clusters, but
he was surprised at the strength of the emissions produced. "We were
also amazed at the beauty of the fluorescence from the sample," he
Photoactivation of silver halide crystals has been the basis for
commercial photographic processes used for more than 100 years.
The new technique is similar, though photographic materials use
larger crystals of silver salts as the photoactivable material.
While the researchers do not yet understand why the particles
fluoresce, Dickson believes the phenomenon's cause relates to the
quantum mechanical properties of atomic silver. "Interesting things
happen when materials that behave in one way as bulk materials are
reduced to the small scale," he added.
Students involved in this research were supported in part by the
Georgia Tech Molecular Design Institute and the National Science
Foundation's Research Experiences for Undergraduates program.
Provisional patent protection has been applied for to protect the
The paper appeared in the January 5 issue of Science.
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