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JILA scientists have found a way to suppress the
blinking problem in quantum dots by bathing them in a chemical
solution. This animated image, which demonstrates the basic
blinking phenomenon, is made from a series of 40 images taken
about one minute apart. Quantum dots actually blink on time scales
ranging from millionths of a second to tens of seconds or longer.
Image by K Kuno/JILA
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By bathing the dots in a watery solution of an
antioxidant chemical used as a food additive, the JILA team increased
photon emission rate four- to fivefold, a “shocking” result because
the rate at which light radiates is generally considered an immutable
property of the dot, JILA/NIST Fellow David Nesbitt says. The JILA
scientists dramatically reduced the average time delay between
excitation of a quantum dot and resulting photon emission from 21
nanoseconds to 4 nanoseconds while reducing the probability of
blinking up to 100 fold. Nesbitt calls blinking the “hidden dirty
secret” of quantum dots. (Nesbitt notes that blinking is not always an
annoyance. For example, it can serve as a measurement probe of very
slow rates of electron flow through nanoscale materials).
The quantum dots used in the JILA experiments were made of
cadmium-selenide cores just 4 nanometers wide coated with zinc sulfide.
When a dot is excited by a brief laser pulse, one electron is
separated from the “hole” it normally occupies. A few nanoseconds
later, the electron typically falls back into the hole, sometimes
producing a single photon—always in a color that depends on dot size,
greenish-yellow in this case. But every so often the electron fails to
make it back to its hole and instead is ejected to imperfections on
the dot’s surface. The chemical added at JILA apparently attaches to
these imperfections, blocking the electron from being trapped and
thereby preventing the dot from blinking off.
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