Exciton Dynamics in Proximitized Chiral Heterostructures

Nontechnical Summary Chirality is a geometric property of objects and materials that lack mirror image symmetry. Your hands are mirror images of each other but cannot be superimposed on each other. Chirality is seen widely in nature at the molecular level. For example, amino acids in proteins are left-handed, while sugars in DNA and RNA are right-handed. Such handedness also determines how many semiconductors emit or absorb polarized light or transport electrons. While materials chirality has applications from drug design to nanoelectronics, the transfer of chirality to neighboring non-chiral materials remains poorly understood. Chirality transfer also appears in a large class of non-chiral inorganic semiconductors, known as perovskites, when they incorporate chiral organic molecules. The focus of this project is on chirality transfer from hybrid chiral perovskites to two-dimensional (2D) semiconductors with excellent optical properties. Chirality transfer in heterostructures of chiral perovskites and non-chiral 2D semiconductors will discriminate emission and absorption of polarized light as well electrons of spin up and down. The project will reveal new insights into how spin-based effects can be shared between materials without the need for magnets. This could unlock new possibilities for future quantum or optical technologies. This project includes a closely integrated educational and outreach components, benefitting students in the Buffalo area. Through summer workshops led by the investigators, students have the opportunity to explore cutting-edge topics in materials physics. Technical Summary With proximity effects, where a given material acquires properties of its neighbors, it is possible to complement the conventional materials design by doping and functionalization, as well as to overcome their limitations. This research investigates dynamics of interlayer excitons between chiral perovskites and 2D transition metal dichalcogenides (TMDs), focusing on how proximity effects influence chirality transfer across the interface. Chiral perovskites and TMDs are extensively studied and recognized for their strongly bound excitons which are expected to expand modern applications in photovoltaics, optical communications, and lasers. However, their heterostructures are less understood and are expected to be transformed by chiral proximity effects, including removing of the valley degeneracy of TMD to unlock their spin-selective properties. This implies that excitons, demonstrated to be transported in TMDs over a macroscopic distance, through chiral and spin-orbit coupling effects could acquire a nontrivial spin structure and contribute to spin-dependent transport. The project focuses on elucidating the dominant mechanism for the transfer of chirality and spin angular momentum across the interface in chiral perovskite/TMD heterostructures. Experimentally, three key mechanisms are investigated: the spin-polarized charge transfer, spin-orbital coupling proximity effect, and the effect of chiral rotation orientation. Following the growth and characterization of the heterostructures, the optical spectroscopy is employed to visualize the polarization states by mapping the photoluminescence in space and time. The experimental results are supported systematically by many-body first-principles studies of electronic structure and excitons and symmetry analysis of the effective Hamiltonian to also guide materials selection. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.