'Blinking' Stopped

The scientists at JILA have induced quantum dots to emit photons faster and more consistently. By bathing the dots in a watery solution of an antioxidant chemical used as a food additive, the 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, Fellow David Nesbitt says. The 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 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 apparently attaches to these imperfections, blocking the electron from being trapped and thereby preventing the dot from blinking off.

The advance could make quantum dots more sensitive as fluorescent tags in biomedical tests and single-molecule studies and steadier sources of single photons for “unbreakable” quantum encryption.

COMPAMED.de; Source: National Institute of Standards and Technology (NIST)