US team put right the 'hidden dirty secret' of quantum dots

The researchers at JILA discovered that by bathing the quantum dots in a solution of an andioxidant chemical used as a food additive, the dot's photon emission rate increased four- to five-fold. JILA is a joint venture of the National Institute of Standards and Technology (NIST) and the University of Colorado. Therefore, JILA research David Nesbitt describes their work as "shocking"​ because the rate at which light radiates is generally considered an immutable property of the dot. Quantum dots are nanoscopic semiconductor crystals made up of as few as 10 atoms, small enough so that their electron energy levels are discrete, with gaps determined by the materials size and shape. When the atoms are excited by a brief laser pulse, electron are promoted to higher energy levels and when they decay back to a lower level, a single proton is sometimes released. This bandgap and the wavelength of light emitted is therefore directly related to the dot's size. The experiments done at JILA could enable scientists to make quantum dots more sensitive as fluorescent tags in biomedical tests and single-molecule studies and also steadier sources of single photons for "unbreakable" quantum encryption. For example, dots have been used to find specific sequences of DNA by making them light up beneath a microscope, and also to image cancer tumours. Scientists are also trying to develop a technique to attach a drug to the quantum dot so that if it should find diseased cell, they could treat it. 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 described the blinking as the "hidden dirty secret"​ of quantum dots. The dots used in this particular piece of research were made of cadmium-selenide cores just four nanometres wide coated with zinc sulphide. Every so often, an excited electron in a dot doesn't decay and is instead ejected to imperfections on the dot's surface. The chemical used at JILA apparently attaches to these imperfections and blocks the electron from being trapped. The dot is thereby prevented from blinking off. Nesbitt also admitted that the 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.