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QD ‘Blinking’ Mystery Unravels

Photonics.com
Nov 2011
LOS ALAMOS, N.M., Nov. 21, 2011 — A significantly better understanding of why quantum dots blink, and how this blinking can be controlled and even completely suppressed, should lead to many sought-after applications, such as single-particle tracking, novel LEDs and single-photon sources, and more energy-efficient solar cells.

Quantum dots, measuring 1 to 10 nm in diameter, have been on the research scene for decades, can be precisely tuned from the infrared to the ultraviolet, and are cheap and easy to make. They can be fabricated using wet-chemistry techniques, and their quantum properties make them attractive for many applications.

However, set against their many advantages for research is a drawback: Their optical properties can vary randomly over time. Perhaps the most dramatic manifestation of this variation is quantum dot "blinking."


Artist’s concept of how solving the problem of quantum blinking may lead to applications in areas such as solid-state lighting. (Image: LANL)

If energized by electrical current or light, quantum dots are characterized by an effect known as Auger recombination, which both competes with light emission in LEDs and reduces current output in solar cells. Both blinking and Auger recombination reduce the efficiency of quantum dots, and controlling these phenomena has been the focus of intense research.

To probe the mechanism responsible for blinking, scientists at Los Alamos National Laboratory (LANL) developed a spectro-electrochemical experiment that allowed them to controllably charge and discharge a single quantum dot while monitoring its blinking behavior.

”Our work is an important step in the development of nanostructures with stable, blinking-free properties for applications from LEDs and single-photon sources to solar cells,” said Victor Klimov, LANL scientist and director for the Center for Advanced Solar Photophysics (CASP).

The main result of the experiment is the discovery of two distinct blinking mechanisms.

The first is consistent with the traditional concept of quantum dot blinking — the random electrical charging and discharging of the core of the dot. In this model, a charged state is “dark” due to highly efficient nonradiative Auger recombination.

The second mechanism was a surprise: The majority of quantum dots blink due to the filling and emptying of a surface defect “trap” on the dot. If not occupied, this trap intercepts a “hot” electron that otherwise would produce photon emission, thus causing a blink.

With further research into the photophysical properties of quantum dots, the scientists hope to provide a comprehensive theoretical model of this phenomenon.

”The new single nanocrystal spectro-electrochemistry technique developed here could readily be extended to study the effect of charging in a wide array of nanostructures, including carbon nanotubes and nanowires,” said researcher Han Htoon.

Experiments were conducted at the Center for Integrated Nanotechnologies (CINT), a Department of Energy Office of Science User Facility and Nanoscale Science Research Center. CASP is an Energy Frontier Research Center funded by the DoE Office of Science, Office of Basic Energy Sciences.

The findings were published in Nature on Nov. 9, 2011.

For more information, visit: casp.lanl.gov  


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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