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Scientists prevent quantum dots from "blinking"

27 Feb 2008

Quantum dots can be made to emit single photons more efficiently and reliably by using a new method to suppress quantum dot "blinking".

A US research team has found a way to reduce quantum dot (QD) "blinking" by a factor of 100, and in doing so has also increased the photon emission rate by four to five times. This could make QDs more sensitive as fluorescent tags in biomedical tests and single-molecule studies, and could also yield steadier sources of single photons for quantum encryption (Nano Letters 8 287).

"QD blinking is an ubiquitous phenomenon, which has been known about for 10 years, but is still not well understood. We are trying to understand and ultimately control this phenomenon," David Nesbitt, a researcher from JILA, a joint venture between the National Institute of Standards and Technology and the University of Colorado, told optics.org. "We have reduced the average time-delay between the excitation of a QD and the photon emission from 21 ns to 4 ns while also reducing the probability of blinking by a factor of 100."

QDs normally produce single photons via the electron-hole recombination process. Blinking occurs when the electron fails to return to its hole and is instead ejected to imperfections on the dot's surface, leaving a charged hole inside the dot. "The resulting electric field - which can be 108 V/cm easily, is thought to reduce the quantum yield for emission of another electron-hole pair formation in the QD," explained Nesbitt.

While it's not necessary to suppress QD blinking in all applications, it can become a critical problem in single molecule assays, where fluorescence is used to denote if something is present or not. This means that blinking of a QD could mimic something disappearing when it really hasn't.

Nesbitt's team found that almost complete suppression of blinking could be achieved by controlling the solution environment around their QDs. The QDs, measuring 4 nm wide, were composed of cadmium selenide and coated with zinc sulphide. "We added propyl gallate solution in a microfluidic device to the QDs," explained Nesbitt. "The presumed mechanism is that the propyl gallate additive binds to trap states on the surface of the QD, preventing electrons from leaving the centre of the QD and residing there."

Chemical suppression of blinking has been seen previously, but Nesbitt's group found that in this case the suppression was accompanied by a dramatic increase in photon emission rate. "Suppression of blinking is not new - it is the demonstration that the solution environment can decrease the radiative lifetime of the QD, resulting in an increase in photon emission rate," he said. "This represents a novel way in which fluctuations in the quantum yield can be controlled."

The future goal for the team is to fully understand the mechanism by which suppression is achieved. "We are trying to isolate the real ingredient responsible for filling the surface trap states," concluded Nesbitt. "We are also investigating other chemical analogues for blinking suppression as a function of pH."

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