Understanding the Influence of Low-Frequency Vibrations on Energy Relaxation Through Layered Nanomaterials

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Quantum-confinement occurs when an object's dimensions are small compared to the size of electron motion. Many modern electronic devices, which range from medical diagnostics and therapeutics instruments, to high-resolution optical displays and flexible electronics, rely on quantum-confined nanostructures. However, realization of their full potential requires a fundamental understanding of their interactions with light. With support from the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Kenneth Knappenberger from Pennsylvania State University (PSU) is developing advanced magneto-optical measurement capabilities to probe quantum-confined structures. Professor Knappenberger and his students are using the unique capabilities of this instrument to understand the behavior of electrons in transition metal dicholcogenide (TMD) materials, and how they interact with vibrations in the crystal lattice. The insights gained from the work could impact existing optoelectronic and photonic technologies, as well as emerging quantum information and sensing applications. The project is also providing training opportunities for future scientists in advanced experimental techniques. As part of this project, Professor Knappenberger has established a local student section of the Optical Society of America at Penn State. This activity facilitates a weekly brown-bag optics club lunch, hosted in the Millennium Science Complex at PSU. Students are able to present their research progress in an informal setting that includes faculty, postdoctoral, graduate, and undergraduate students. The program also includes a component for a high school volunteer student. This student acquires skills in data analysis and processing. The research team is developing and using novel variable-temperature, variable-magnetic field (VTVH) spectroscopy methods to characterize the electronic structure of a series of two-dimensionally confined quantum materials, including multi-component structures featuring combinations of 0-D/1-D/2-D systems. The overall objective is to understand how electronic carrier coupling to low-frequency phonon modes affects the relaxation dynamics of these systems. In order to achieve this fundamental research goal, the team is developing VTVH ultrafast two-dimensional spectroscopy (VTVH-2DES) capabilities, and using them to characterize key parameters of transient exciton states, such as Lande g factors, zero-field splitting energies, and electron-phonon coupling strengths. Monitoring transient signal amplitudes from these excitons in the time- and energy-domains provides insight into phonon-mediated energy relaxation and transfer in quantum materials. The research activities include 1) understanding the optical properties and electronic structure for single and multi-layer 2-D confined TMDs, 2) development of VTVH-2DES infrastructure of characterizing TMD transient states, 3) quantitatively describing quantum-state-specific electron-phonon coupling and carrier dynamics for TMDs; and 4) understanding fundamental mechanisms of phonon-mediated energy transfer in multi-component quantum structures.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.