Exciton Motion Observed For The First Time; Could Greatly Advance Research In Electronics

By Shweta Iyer on April 16, 2014 1:42 PM EDT

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Scientists from MIT and the City College of New York have observed exciton movements directly, thus expanding their potential uses. (Photo: Photo courtesy of Shutterstock)

The concept of excitons, a bound state of an excited electron and an associated hole was first proposed by Yakov Frenkel in 1931. Since then there have been several theories and studies based on excitons but they had never been actually observed. Now for the first time scientists from MIT and the City College of New York have observed exciton movements directly, thus expanding their potential uses.Their research appears in this week's Nature Communications journal, according to a press release Wednesday..

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Excitons are electrically neutral quasiparticles that can transport energy without transporting net electric charge and this makes them useful in devices like solar cells, LEDs, and semiconductor circuits. The MIT researchers who conducted the experiments are Gleb Akselrod and Parag Deotare, professors Vladimir Bulovic and Marc Baldo, and four others.

"Excitons are at the heart of devices that are relevant to modern technology," Akselrod explains. Since the type of particles determine how energy moves at nanoscale, he says, "The efficiency of devices such as photovoltaics and LEDs depends on how well excitons move within the material." 

An exciton, which travels through matter as though it were a particle, pairs a negatively charged electron with a hole, which is formed when there is a lack of electron at a position where one could exist. Though it is neutrally charged, it can carry energy. For example, in a solar cell, an incoming photon may strike an electron, kicking it to a higher energy level. That higher energy is propagated through the material as an exciton. The particles themselves don't move, but the boosted energy gets passed along from one to another.

The speed of movement of excitons between points was known earlier but the scientists did not know how they moved. This information is necessary when designing materials that use exciton applications since certain characteristics of the material like the degree of molecular order or disorder will determine the speed of exciton motion.

"People always assumed certain behavior of the excitons," Deotare says. Now, using this new technique - which combines optical microscopy with the use of particular organic compounds that make the energy of excitons visible - "we can directly say what kind of behavior the excitons were moving around with." This advance provided the researchers with the ability to observe which of two possible kinds of "hopping" motion was actually taking place.

"This allows us to see new things," Deotare says, making it possible to demonstrate that the nanoscale structure of a material determines how quickly excitons get trapped as they move through it.

According to the scientists, in some applications like LEDs the exciton trapping needs to be maximized while in other applications like solar cells the trapping needs to be minimized. Their new research will help the scientists to understand the factors that determine the increase or decrease of trapping.

"We showed how energy flow is impeded by disorder, which is the defining characteristic of most materials for low-cost solar cells and LEDs," Baldo says.

The researchers carried out their experiment with a material called tetracene a well-studied archetype of a molecular crystal - the researchers say that the method should be applicable to almost any crystalline or thin-film material. They expect it to widely used in technical and educational fields.

"It's a very simple technique, once people learn about it," Akselrod says, "and the equipment

required is not that expensive."

Exciton movement can also be observed in a natural occurring energy-transfer process-photosynthesis. Plants absorb energy from photons from sunlight, and this energy is transferred by excitons to areas where it can be stored in chemical form for later use in supporting the plant's metabolism. Understanding exciton motion will also shed new light on photosynthesis.

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