at the University of Chicago have discovered a better
way to measure a confounding property of microscopic
high-tech particles called quantum dots.
Quantum dots, also called nanocrystals, emit light
in a rainbow of colors and are used in lasers, biological
studies and other applications, but their tendency
to blink hinders their technological value. Imagine
the annoyance caused by a randomly flickering light
"A quantum dot might blink for just a millionth
of a second or it might blink for 15 minutes,"
said Matthew Pelton, a Research Associate at the University
of Chicago's James Franck Institute. "This is
one of the problems we have to solve if we want to
engineer the properties of materials, particularly
semiconductor materials, on the nanoscale."
Pelton has found a way to measure the blinking that
is simpler and faster than the conventional method.
He will describe the measurements in the Aug. 2 issue
of Applied Physics Letters with co-authors David Grier,
now of New York University, and Philippe Guyot-Sionnest
of the University of Chicago.
Grier compares the light output or "noise"
of a blinking group of quantum dots to the babble
of a cocktail party conversation. "Even if everyone's
talking about the same thing you probably wouldn't
be able to figure out what they're saying because
they're all starting their conversations at random
times and there are different variations on their
conversations," he said.
"Matt has discovered that for these blinking
quantum dots, all the conversations are the same in
a very special way, and that allows you to figure
out an awful lot about what's being said by listening
to the whole crowd."
In previous studies, various research groups combined
powerful microscopes with video cameras to record
the blinking behavior of one quantum dot at a time,
but that method is expensive, time-consuming and difficult
to perform. It also required that the dots be placed
on a microscope slide. Pelton's method enables scientists
to study the blinking patterns of large quantities
of dots. And it can be done in just a few minutes
with standard laboratory equipment under a variety
of environmental conditions.
"Matt's approach is applicable to situations
where previous measurements could not be made,"
The four components of Pelton's system are a light
source, a photodetector (a device that measures the
intensity of light), an amplifier to boost the photodetector's
output, and an analogue-to-digital converter that
translates the amplified output into a string of numbers
for digital processing.
The system has already revealed new insights into
the behavior of quantum dots. Pelton's results contradict
the conventional wisdom about the blinking dots, which
states that environmental factors influence the behavior.
Pelton made his finding by applying a mathematical
tool commonly used by electrical engineers to the
problem of blinking quantum dots. "The mathematical
tool is almost 200 years old. No one had thought to
apply it to this problem before," Grier said.
Studying quantum dots one at a time with microscopes
and video cameras was limited by the capabilities
of the camera. For example, a camera that takes 40
frames a second would miss any blinks that occur more
rapidly. But Pelton's system includes a tool called
a power spectrum to trace blinking behavior. This
tool has established numerical recipes for handling
the time resolution problem.
The research team cannot say how long it might take
to crack the mystery of the blinking quantum dots.
What is certain is that quantum dots will continue
to generate interest in high-tech circles.
"Many scientists are trying to start up companies
to make nanocrystals and to find a new use for them,"
Quantum dot research at the University of Chicago
is supported by the Materials Science and Engineering
Research Center, the National Science Foundation and
the American Chemical Society.
University of Chicago