Two-dimensional semiconductors, heralded as the subsequent generation in semiconductor technology, are characterised by their single atomic layer thickness. On account of their ultra-thin structure, two-dimensional semiconductors exhibit remarkable optical properties and supply flexibility, together with excellent integration capability with other materials, for a big selection of applications. Leveraging these attributes, they’re being applied in diverse fields comparable to advanced flexible devices, nanophotonic devices, and solar cells. A key aspect of the optical characteristics in two-dimensional semiconductors is the presence of excitons, that are electron-hole pairs. Harnessing the generation and recombination of those excitons opens avenues for the event of light-emitting devices and various optical and electronic applications. One other significant optical phenomenon involves the precise control of trions, that are charged excitons. The manipulation of trions offers quite a few functionalities for device applications.
Collaborative research involving Professor Kyoung-Duck Park and Mingu Kang, a PhD candidate, from the Department of Physics at Pohang University of Science and Technology (POSTECH), Professor Yung Doug Suh from the Department of Chemistry at Ulsan National Institute of Science and Technology (UNIST) who can be Deputy Director of the IBS Center for Multidimensional Carbon Materials, and Professor Hyun Seok Lee from the Department of Physics at Chungbuk National University has developed tip-enhanced cavity-spectroscopy system with a gold nanowire structure. This technique dynamically manipulates the interconversion of excitons and trions, allowing nanoscale control and investigation of their emission properties. The approach enabled the researchers to successfully discover the mechanism of trion generation.
The mixing of metals and semiconductors can result in the event of multifunctional optoelectronic devices possessing unique optical and electrical properties. The research team fabricated a hybrid structure of gold nanowires with a monolayer of two-dimensional semiconductor, molybdenum diselenide (MoSe2). In addition they built a tip-enhanced cavity-spectroscopy system, by integrating the hybrid structure with tip-enhanced nano-spectroscopy.
Illumination of laser to the well-designed gold nanowire structure induces the formation of standing waves of plasmons on the surface. Initially, it was intended to induce conversion of excitons to trions within the two-dimensional semiconductor, however the multipole mode was found to contribute to the conversion process. The tip-enhanced cavity-spectroscopy system facilitates the investigation of optical properties of excitons and trions on the nanoscale by achieving a spatial resolution of roughly 10 nm, which is surpassing optical diffraction limits. This technique can reveal the underlying principles of trion generation and enable the dynamic manipulation of exciton-trion interconversion in a reversible manner.
Furthermore, the dynamically controlled gold tip, which concentrates light inside a nanoscale region, generates high-energy hot electrons. These electrons will be injected into the two-dimensional semiconductor, affecting the generation of trions. This approach not only led to the event of high-precision measurement tools that enable the manipulation of materials with ultra-high resolution, but additionally prompted the researchers to propose a novel platform for the control of excitons and trions in semiconductors on the nanoscale.
Mingu Kang, the lead writer of the research, remarked, “We successfully display the nanoscale manipulation of excitons and trions, and reveal the principles governing the interaction between excitonic quasiparticles, plasmons, and hot electrons.” He added, “This breakthrough could open recent avenues for optoelectronic device applications using excitons and trions comparable to solar cells and photoelectric integrated circuits.”
Su Jin Kim from the Department of Physics at Chungbuk National University, and Huitae Joo, Yeonjeong Koo, and Hyeongwoo Lee, integrated PhD students on the POSTECH, were contributors to the research. The research, recently published within the international journal Nano Letters, was sponsored by the National Research Foundation of Korea, the Ministry of Science and ICT, the Electronics and Telecommunications Research Institute, the Samsung Future Technology Incubation Program, the Commercializations Promotion Agency for R&D Outcomes, the Korea Research Institute of Chemical Technology, the UNIST, and the Institute for Basic Science (IBS).
