By Josh Perry, Editor
[email protected]

Researchers at Chalmers University of Technology (Gothenburg, Sweden) and Regensburg University (Germany) detailed the ultrafast connections with special energy states, called interlayer excitons, that are formed when atomically-thin materials are layered to form 2-D heterostructures.

When atomically thin layers of two materials are stacked and twisted, a ‘heterostructure’ material emerges. (Brad Baxley/Chalmers University of Technology)

According to a report from Chalmers, the interlayer excitons exist in both material layers and the scientists can now explain how the energy states are formed and how they can be tuned by stacking and twisting the layers.

“They used two different lasers to follow the sequence of events,” the report indicated. “By twisting atomically thin materials towards each other, they have demonstrated that it is possible to control how quickly the exciton dynamics occurs.”

Scientists in Sweden conducted simulations and devised the theoretical framework that was confirmed by experiments in Germany.

The research was recently published in Nature Materials. The abstract read:

“Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic, Mott insulating or superconducting phases. In transition metal dichalcogenide heterostructures, electrons and holes residing in different monolayers can bind into spatially indirect excitons with a strong potential for optoelectronics, valleytronics, Bose condensation, superfluidity and moiré-induced nanodot lattices.

“Yet these ideas require a microscopic understanding of the formation, dissociation and thermalization dynamics of correlations including ultrafast phase transitions.

“Here we introduce a direct ultrafast access to Coulomb correlations between monolayers, where phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe₂/WS₂hetero-bilayers by revealing a novel 1s–2p resonance, explained by a fully quantum mechanical model.

“Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe₂ layer directly into interlayer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s–2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.”