This reaction creates unique excitonic complex particles, multiple electrons, and holes strongly bound together. These particles possess a new quantum degree of freedom, called "valley spin." The "valley spin" is similar to the spin of electrons, which has been extensively used in information storage such as hard drives and is also a promising candidate for quantum computing.
Assist Prof. Shi's research focuses on low dimensional quantum materials and their quantum effects, with a particular interest in materials with strong light-matter interactions. These materials include graphene, transitional metal dichacogenides (TMDs), such as tungsten diselenide (WSe2), and topological insulators.
TMDs represent a new class of atomically thin semiconductors with superior optical and optoelectronic properties. Optical excitation on the 2D single-layer TMDs will generate a strongly bound electron-hole pair called an exciton, instead of freely moving electrons and holes as in traditional bulk semiconductors. This is due to the giant binding energy in monolayer TMDs, which is orders of magnitude larger than that of conventional semiconductors. As a result, the exciton can survive at room temperature and can thus be used for application of excitonic devices.
As the density of the exciton increases, more electrons and holes pair together, forming four-particle and even five-particle excitonic complexes. An understanding of the many-particle excitonic complexes not only gives rise to a fundamental understanding of the light-matter interaction in 2D, it also leads to novel applications, since the many-particle excitonic complexes maintain the "valley spin" properties better than the exciton.
However, despite recent developments in the understanding of excitons and trions in TMDs, said Assit Prof. Shi, an unambiguous measure of the biexciton-binding energy has remained elusive.
"Now, for the first time, we have revealed the true biexciton state, a unique four-particle complex responding to light," he continued. "We also revealed the nature of the charged biexciton, a five-particle complex."
At Rensselaer, the team says it developed a way to build an extremely clean sample to reveal this unique light-matter interaction. The device was built by stacking multiple atomically thin materials together, including graphene, boron nitride (BN), and WSe2, through van der Waals (vdW) interaction, representing the state-of-the-art fabrication technique of 2D materials.
