2D Material Combines Long Lifetime with Freedom of Movement

Researchers at Rensselaer Polytechnic Institute (RPI) have developed a method to isolate thin layers of tungsten diselenide (WSe₂) — a 2D material — from crystals, to allow layers of WSe₂ to be stacked on top of other atomically thin materials such as boron nitride and graphene. The discovery could further the use of WSe₂ for building smaller, more efficient computing devices.

When the WSe₂ layer is sandwiched between two boron nitride flakes and interacts with light, an exciton is formed. The exciton in the WSe₂ contains a property known as “valley spin”— an expanded degree of freedom of movement that could be useful in the building of quantum computing devices. However, excitons usually do not have a long lifetime.

A special “dark” exciton that typically cannot be seen but that has a longer lifetime than a traditional exciton was demonstrated in previous work by RPI professor Sufei Shi and his team. However, this “dark” exciton did not have the valley-spin property that gives the exciton in the WSe₂ a quantum degree of freedom.

An RPI team led by Sufei Shi, assistant professor of chemical and biological engineering, figured out how to brighten the “dark” exciton, that is, to make the “dark” exciton interact with another quasiparticle (a phonon) to create a completely new quasiparticle that has both properties researchers want: long lifetime and valley spin. Courtesy of Rensselaer Polytechnic Institute.

Through new research, Shi and his team discovered a way to brighten the “dark” exciton. When the “dark” exciton interacts with a phonon, a new quasiparticle is created that has both of the desired properties. First-principles calculations showed that the exciton-phonon interaction permitted the simultaneous emission of a chiral phonon and a circularly polarized photon. The team’s discovery and understanding of the phonon replica could open a new route to manipulating valley-spin.

“We found a new quasiparticle that has a quantum degree of freedom and also a long lifetime,” Shi said. “We have the quantum property of the ‘bright’ exciton, but also have the long lifetime of the ‘dark’ exciton.”

Atomically thin, 2D semiconductor materials, less than 1 nm thick, are expected to play a significant role in the development of the next generation of devices. The ability to manipulate WSe₂ could further advance the potential of this 2D material.

The research was published in Nature Communications (https://doi.org/10.1038/s41467-019-10477-6).