RIKEN led group discovers quantum light source operating at room temperature at mixed-dimensional heterostructure interfaces

A joint research group comprising Special Postdoctoral Researcher (at the time of the research, now a visiting researcher) and Chief Scientist Yuichiro Kato, (Team Leader, Quantum Optoelectronics Research Team, Center for Advanced Photonics Research) of the Kato Group of Nanoscale Quantum Photonics Laboratory at RIKEN, Professor Susumu Okada (Nanostructured Materials Laboratory, Institute of Pure and Applied Sciences, University of Tsukuba), Professor Kosuke Nagashio (Department of Materials Engineering, Graduate School of Engineering, University of Tokyo), and Assistant Professor Shun Fujii (Department of Physics, Faculty of Science and Technology, Keio University) announced that they have discovered the existence of a quantum light source operating at room temperature at the interface of nano semiconductors with different dimensionality, namely one- and two-dimensional. This discovery is expected to lead to applications in quantum technologies such as quantum communication and quantum computation. The result is published in the international academic journal Nature Communications on April 11.

Low-dimensional semiconductors, which are made up of extremely thin layers with a thickness of approximately one atom, are attracting attention as a means to miniaturize semiconductor devices. Such atomically fine structures can be expected to exhibit new properties due to quantum effects even at room temperature. Thus, research on low-dimensional semiconductors such as single-walled carbon nanotubes (single-phase CNTs), which are one-dimensional semiconductors, and transition metal dichalcogenides (a group of compounds comprising chalcogens such as sulfur, selenium, and tellurium and transition metals), which are two-dimensional semiconductors, is expected to lead not only to overcoming the limits of miniaturization but also to applications in the next-generation quantum technology.

Single-walled CNTs are made up of a single atomic layer of carbon atoms arranged in a hexagonal lattice (graphene), rounded into a cylindrical shape with a diameter of 1-3 nm. Tungsten selenide, a type of transition metal dichalcogenide, is a layered two-dimensional semiconductor comprising tungsten and selenium atoms, with each layer having a thickness of 0.7 nm, and the layers are bound together by van der Waals force. When these two low-dimensional semiconductors with different dimensionality are joined to form a heterostructure, new properties and functions are expected in ultrathin semiconductor structures with only a few atomic layers utilizing the large band energy modulation of CNTs.

In 2023, RIKEN combined nanomaterials with structures of different dimensionality to construct normal, defect-free heterostructures by placing and joining CNTs with identified atomic arrangements and tungsten selenide with a specific number of layers in precise positions. They discovered the phenomenon of enhanced exciton transfer via band energy resonance. Exciton transfer occurs only in Type I heterostructures, while Type II heterostructures, in which electrons and holes are easily separated, can be used to develop a new type of excitonic state. In Type I, electrons and holes take on lower energy states in a material, whereas in Type II, each takes on a lower energy state in separate materials. Therefore, Type I is used in light-emitting devices such as LEDs, where electrons and holes easily recombine, while Type II is used in organic solar cells, where electrons and holes are easily separated.

In this study, the group used the anthracene-assisted transfer technique they developed previously to fabricate a mixed-dimensional heterostructure that combined CNTs and tungsten selenide. Their analysis revealed interface excitons exhibiting bright quantum emission at room temperature. Interface excitons were observed only with CNTs that form Type II heterostructures with high bandgap energy, and these excitons were found to be long-lived. Another unexpected property revealed in this study is that the interfacial exciton is localized even at room temperature, producing a single photon.

Kato said, "Advances in nanoscience have made it possible to use nanomaterials with a precisely understood structure at the atomic level. The mixed-dimensional heterostructure that was created in this research used CNTs with a specific atomic arrangement and an atomic layer material (tungsten selenide) with a specific number of layers. In an interface formed by 1D and 2D materials, a 1D electronic state, which has a smaller dimension, should be generated, but unexpectedly, a localized state corresponding to a 0D dimension was generated in this case. Moreover, I was surprised that it functioned as a quantum light source generating a single photon even at room temperature. I believe that the obtained results are a step toward atomically precise technology that utilizes atomic-scale structures smaller than the nanoscale."

Journal Information
Publication: Nature Communications
Title: Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructures
DOI: 10.1038/s41467-024-47099-6