Working out why quantum dots blink
A fresh understanding of quantum dot behaviour may lead to better televisions, writes Joel F. Hooper.
You may not have heard of quantum dots (QDs) yet, but you will be seeing them everywhere soon. These tiny semiconductor particles, only nanometres in size, emit specific wavelengths of light when excited by light or electricity, and are finding applications in quantum computing, solar cells and next-generation televisions.
But quantum dots show some strange behaviour: their light emissions turn on and off, seemingly at random. Understanding this “blinking” of QDs is key to developing future applications in nanotech.
Now, an international team of researchers, led by Paul Mulvaney at the University of Melbourne, Australia, has examined the two leading theories regarding this blinking, and shows that both proposed mechanisms in fact operate side-by-side. The results are presented in a paper on ChemRxiv, the chemistry pre-print service.
Quantum dots have properties that are quite different from those of larger particles: they behave more like single molecules than bulk materials. The electrons in a QD are restricted to defined energy levels, much like the orbitals of a single atom. When the dot absorbs energy, electrons are boosted to a higher energy level, and light is emitted when they relax again. The size, shape and composition of the dot can be tuned to control the wavelength of this light, which, along with the dots’ stability, makes them very attractive for display purposes.
However, when a quantum dot is in its “off” state, the excited electrons can relax in a way that produces heat rather than light. Several theories have been put forward to explain why they switch between these two modes, but two have emerged as frontrunners.
The “Auger-blinking” theory states that QDs only emit light when they are electrically neutral, and switch to an off state when they carry a positive or negative charge. However, this doesn’t quite explain all the blinking behaviour. A second theory, “BC-blinking”, suggests that uncharged dots blink because other materials stick to their surface.
Experiments by Mulvaney and his colleagues have shown that both Auger and BC-blinking occur in individual dots. This understanding may help scientists better control dots in future, giving us brighter and more efficient displays, along with other applications, like blinking labels that can be attached to cells and biomolecules.