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Unravelling the blinking process in silicon quantum dots

Posted: 17 Jun 2015     Print Version  Bookmark and Share

Keywords:University of Chicago  silicon  quantum dot  fluorescence intermittency 

University of Chicago researchers are at the forefront of identifying the causes behind the mysterious blinking phenomenon in silicon quantum dots. They were able to derive remarkable findings using computer simulations at the Department of Energy's National Energy Research Scientific Computing Centre (NERSC).

The results may bring scientists a step closer to understanding, and possibly remediating, the problem known as 'fluorescence intermittency.' Lasers and logic gates will not work well with variable light sources. Quantum dots can absorb specific colours of light, too, but using them to harvest sunlight in photovoltaics is not yet efficient, due in part to the mechanisms behind blinking.

Quantum dots possess some beneficial properties that their bulk counterparts lack.

Excite a quantum dot and it glows brightly in a specific colour of light. Vary the width by a few atoms and you can tune it to glow different colours: The smaller the dot, the bluer the light. The larger the dot, the redder. Quantum dots can likewise be tuned to absorb specific wavelengths of light, a useful property for solar cells.

Silicon quantum dot blinking

In comparison, the molecular structure of bulk semiconductors determines (and limits) the colours of light (or energies) emitted and absorbed. So, a light-emitting diode (LED) made of one material may glow green while another glows red. To get different colours, you must use different materials. Solar cells, likewise, use layers of different materials to capture various wavelengths of light.

So, why does a nanocrystal of semiconductor behave so differently than a larger lattice of the same material? In a word: size. Artificially fabricated to contain just a handful of atoms, quantum dots are so small that they exist in the twilight zone between Newtonian and quantum physics, sometimes obeying one set of rules, sometimes the other, often to surprising effect.

While the crystals of bulk semiconductors can lose and regain electrons (that's how they conduct a charge) the electrons of a quantum dot are confined within the dot. This state is called quantum confinement. When the electrons of a quantum dot interact with light, they can undergo a transition and 'jump' (quantum-mechanically) to a state that under normal conditions is unoccupied. The energy associated with the smallest jump is called the gap. The gap is thus the excess energy that electrons can shed, ideally as light (or in the case of photovoltaics, carriers) when snapping down to a lower energy state. As a result, the radius of the material defines the energy these dots can absorb and emit.

Quantum dots, however, tend to blink on and off. The blinking is not random (it obeys a 'power law'), but it also is not predictable which means that individual particles might go dark only for nanoseconds or remain dark for minutes at a time or some interval in between.

Scientists have some ideas about what causes the blinking, but still do not understand exactly how it works, said Marton Voros, a University of Chicago postdoctoral researcher who co-authored the study.

"There's been this idea that surface defects, for example a dangling bond on the surface of a nanocrystal, can trap electrons and cause this switching between bright and dark states," said Voros who performed the calculations at NERSC. "There are quite a few microscopic models already put forward by other groups that rely on defects but a complete understanding is still missing."

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