For Immediate Release
Using photon emissions from individual molecules of silver, researchers
at the Georgia Institute of Technology have created what may be the world's
smallest electroluminescent light source.
Believed to be the first demonstration of electroluminescence from individual
molecules, the work could lead to new types of nanometer-scale optical
interconnects, high-resolution optical microscopy, nanometer-scale lithography
and other applications that require very small light sources. And because
single molecules are known to emit one photon at a time, the technique
could ultimately be the basis for high-efficiency quantum information
processing and cryptography.
Though the effect was first reported in silver clusters composed of 2-8 atoms, the researchers also demonstrated electroluminescence in similarly prepared copper clusters, suggesting the effect may broadly apply to other metals. Details of the research were reported in the August 6 issue of the Proceedings of the National Academy of Sciences.
"This is the first time that anyone has seen electroluminescence
from individual molecules," said Robert
Dickson, assistant professor in Georgia Tech's School
of Chemistry and Biochemistry. "What we have observed involves
sub-nanometer scale sources to which an electric field is applied. These
molecules emit very strongly, and are very robust."
Dickson and collaborators Tae-Hee Lee and Jose Gonzalez began with thin
films of silver oxide that are not electroluminescent. By exposing the
film to electrical current of approximately one amp, they "activated"
some of the silver oxide molecules, which then appeared within "discolored"
regions in the film. When electrodes were attached to the film and an
alternating current applied, a thin line of silver clusters began to emit
light in colors that varied depending on the size of the clusters. The
system operated at room temperature.
"When you zoom in more closely, you can see the emissions coming
from single molecules," said Dickson. "They blink and have dipole
emission patterns. You see an incredibly thin line of emissive species
close to the middle of the sample."
Electroluminescence occurs when an electron recombines with a positively
charged molecule from which a single electron has been removed to create
an electron-hole pair. First, an electron is removed from a molecule,
creating a positive charge. Then, an electron is quickly injected into
a different state of the same molecule. Because of the charge differences,
the electron is attracted to the hole, and when they recombine, a photon
While normally stimulated by applying direct current (DC), the Georgia
Tech group observed a dramatically enhanced response from high frequency
alternating current (AC).
While DC voltage produced electroluminescence in the activated silver
clusters, Dickson and his colleagues found that high frequency AC voltage
-- above 150 megahertz -- produced a response as much as 10,000 times
greater. Dickson believes the AC voltage created rapid recombination within
single molecules in a very narrow section of a sample, producing the enhanced
response. Bulk materials normally cannot respond quickly enough to the
alternating current to enhance the electroluminescence to such a large
The AC current was more efficient than DC current at converting electrical
current to light because it injects the electron charge at just the right
time, minimizing the amount of energy lost to production of heat, Dickson
explained. From a practical standpoint, that increases the operating life
of the emitting clusters and reduces the amount of current required to
produce light, he noted.
"We know that the charge is recombining in the molecules because
you can simultaneously measure the electroluminescence and the current,
and the peaks are correlated," he said. "This is an extremely
interesting materials system, not only because of the single-molecule
electroluminescence, but also because of the resonance we see at relatively
Though the discovery may have important implications for optoelectronic
devices, Dickson's group is focusing first on understanding the basic
"We are concentrating on understanding the very fundamental aspects
of this: what the nature of the emission is, how the emission occurs,
the different time scales for electron injection, hole injection and recombination,"
he said. "We need to know how to better control this before we can
begin to use it in nanometer scale devices or as nanometer scale optoelectronic
components in circuitry. A lot of engineering will have to be done to
make any potential optoelectronic devices both useful and stable."
The electroluminescence research builds on earlier
work done by Dickson and colleagues Lynn Peyser and Amy Vinson that
demonstrated optical storage potential of thin-film silver oxide clusters.
In that work, reported in the journal Science in January 2001,
the researchers demonstrated binary optical storage by writing and reading
simple images recorded on films of silver oxide nanoparticles activated
by light of a specific frequency. That work is continuing, and advances
have been made toward potential optical storage systems.
Support for Dickson's research comes from the Sloan and Dreyfus Foundations,
as well as internal Georgia Tech research funds.
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WRITER: John Toon