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It Takes Two to Tango

Published: 04-12-2009

When two electron/hole pairs, known as biexcitons, are formed inside a quantum dot, one pair can recombine, releasing enough energy to kick the remaining electron onto the surface of the dot. The quantum dot can’t blink on until the electron trapped on the surface is able to tunnel back into the dot and recombine with its hole. Credit: Greg Kuebler

Quantum dots are tiny structures made of semiconductor materials. With diameters of 1–5 nm, they are small enough to constrain their constituents in all three dimensions. This constraint means that when a photon of light knocks an electron into the conduction band and creates an electron/hole pair, the pair can’t get out of the dot. In terms of quantum mechanics, this confinement means that the wavelengths of the wave functions of both the electron and the hole are forced to be significantly smaller and more on top of each other inside a quantum dot than they would be in ordinary semiconductor material.

When electron/hole pairs recombine, they can release energy as light. When continuously illuminated by a laser, quantum dots blink on and off randomly. These on-and-off periods can last from a microsecond to several minutes, i.e., on time scales as short or as long as an experimentalist decides to observe. In 2001, Fellow David Nesbitt and his co-workers caught the attention of others in this field when they reported that the probabilities of the on and off times follow a power law. Power law behavior typically indicates that something fairly complicated is going on.

Recently, Nesbitt and former research associate Jeff Peterson (now on the faculty at the State University of New York at Geneseo) figured out what first turns blinking off and then back on again. In this work, the researchers used quantum dots made of cadmium selenide (CdSe) surrounded by a coating of zinc sulfide (ZnS). The CdSe dots are similar to those that are responsible for the beautiful orange colors in 12th century stained glass. The researchers wanted to understand why their modern orange quantum dots blink on and off like Christmas tree lights.

The key to solving the mystery was a paradox: The amount of time it takes an "on" quantum dot to blink off seems to depend on when it was observed and for how long. The longer a dot has been on, the more likely it is to decay exponentially, rather than follow power law behavior. In addition, the use of higher-power lasers dramatically increases the rate at which the dots turn off.

To account for these results, Nesbitt and Peterson came up with a mechanism that explains how blinking gets turned off in quantum dots: When a laser illuminates a quantum dot, there is a very small chance that two photons will interact with the dot, creating two electron/hole pairs, known as biexcitons. The chance of making a biexciton is only about one in a million in a microsecond. However, by the time a laser has shined for a million times longer than a microsecond (i.e., 1 second), the chances of forming a biexciton get a whole lot better. And, it’s the biexcitons that turn off emission!

Here’s how they do it: First, one of the electron/hole pairs recombines. Second, the energy released in this merger kicks the second electron out of the quantum dot onto the dot’s surface where it is trapped. The trapped electron creates a huge electric field across the dot. This field prevents the dot from emitting photons until the trapped electron can finally tunnel back into the dot and recombine with its hole.   - Julie Phillips


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