2D Semiconductors Preserve Excitons, Explored for Integrated Nanophotonics

Combining two different ultrathin semiconductors — each just one layer of atoms thick — creates a 2D heterostructure with potential uses in clean energy and optically active electronics.

University of Washington scientists have created a system to study the special properties of these atomically thin layers and their potential to answer basic questions about physics, as well as to advance electronic and photonic technologies.

Courtesy of Kyle Seyler, Pasqual Rivera.

Led by professor Xiaodong Xu, the team synthesized and investigated the optical properties of the semiconductor sandwich.

"What we're seeing here is distinct from heterostructures made of 3D semiconductors," Xu said.

When semiconductors absorb light, pairs of positive and negative charges can form and bind together to create so-called excitons. When they are squeezed down to the 2D limit in these atomically thin materials, surprising interactions can occur.

While traditional semiconductors manipulate the flow of electron charge, this device allows excitons to be preserved in "valleys," a concept from quantum mechanics similar to the spin of electrons. This is a critical step in the development of new nanoscale technologies that integrate light with electronics.

"Many groups have studied the optical properties of single 2D sheets," said doctoral student Kyle Seyler. "What we do here is carefully stack one material on top of another and then study the new properties that arise at the interface."

The team obtained two types of semiconducting crystals, tungsten diselenide (WSe₂) and molybdenum diselenide (MoSe₂), from collaborators at Oak Ridge National Laboratory. They used facilities developed in-house to precisely arrange two layers, one derived from each crystal in a process that took a few years to develop.

The properties of electrons in each layer posed a unique challenge in getting these devices to emit light, the researchers said.

"Once you have these two sheets of material, an essential question is how to position the two layers together," said Seyler. The electrons in each layer have unique spin and valley properties, and "how you position them — their twist angle — affects how they interact with light."

By aligning the crystal lattices, the authors excited the heterostructure with a laser to create optically active excitons between the two layers.

"These excitons at the interface can store valley information for orders of magnitude longer than either of the layers on their own," said doctoral student Pasqual Rivera. "This long lifetime allows for fascinating effects which may lead to further optical and electronic applications with valley functionality."

The team would next like to explore other physical properties, such as the variation of exciton behavior as they angles between layers, the quantum properties of excitons between layers and electrically driven light emission.

"There's a whole industry that wants to use these 2D semiconductors to make new electronic and photonic devices," said Xu. "So we're trying to study the fundamental properties of these new heterostructures for things like efficient laser technology, LEDs and light-harvesting devices. These will hopefully be useful for clean energy and information technology applications.”

The work was published in Science (doi: 10.1126/science.aac7820).