Novel materials bring excitons in from the cold

A team of physicists from the University of California, San Diego (UCSD), and the University of Manchester in the UK is creating novel tailor-made materials for cutting-edge research and perhaps a new generation of optoelectronic devices. These novel materials are making it easier for the researchers to manipulate excitons, which comprise an electron and an electron hole bound to each other by an electrostatic force.

Excitons are created when a laser is shone onto a semiconductor device; they can transport energy without transporting net electric charge, making them attractive for use in a range of new technologies. Inside the device, the excitons interact with each other and their surroundings, and then transform back into light that can be detected by extremely sensitive charge-coupled device (CCD) cameras.

Most of the team's previous work involved structures based on gallium arsenide (GaAs), which is a material commonly used by the semiconductor industry. Unfortunately, the excitonic devices they've developed so far come with a fundamental limitation: they require cryogenic temperatures (below 100K) – ruling out any commercial applications.

So the team made a radical material change to help bring their excitonic devices up to room temperature. They report their results in Applied Physics Letters.

"Our previous structures were built from thin layers of GaAs deposited on top of a substrate with a particular layer thickness and sequence to ensure the specific properties we wanted," explained Erica Calman, lead author and a graduate student in the Department of Physics at UCSD.

To make the new excitonic devices, the physicists turned to structures built from a specially-designed set of two-dimensional (2D) materials – molybdenum disulfide (MoS₂) and hexagonal boron nitride (hBN) – each just a single atom thick. These structures are produced via the famous ‘Scotch tape’ or mechanical exfoliation method developed by a group at the University of Manchester led by Andre Geim, a physicist awarded a Nobel Prize in physics in 2010 for his ground-breaking work on the 2D material graphene.

"Our specially designed structures help keep excitons bound more tightly together so that they can survive at room temperature – where GaAs excitons are torn apart," explains Calman.

Impressively, excitons can form a special quantum state known as a Bose-Einstein condensate; this state occurs within superfluids and allows currents of particles to flow without losses. The team discovered a similar exciton phenomenon at cold temperatures with GaAs materials.

"The results of our work suggest that we may be able to make new structures work all the way up to room temperature," said Calman. "We set out to prove that we could control the emission of neutral and charged excitations by voltage, temperature, and laser power ... and demonstrated just that."

This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.