Dissertations, Theses, and Capstone Projects
Date of Degree
9-2024
Document Type
Dissertation
Degree Name
Ph.D.
Program
Physics
Advisor
Matthew Y. Sfeir
Committee Members
Andrea Alu
Maria Tamargo
Mingzhao Liu
Vinod Menon
Subject Categories
Condensed Matter Physics | Optics | Other Physics
Keywords
Semiconductors, Metasurfaces, Spectroscopy, Photoelectrocatalysis, cyclic voltammetry, Renewable energy
Abstract
Heterogeneous photocatalysis has attracted research interest in sustainable energy technologies. However, achieving high photocatalytic efficiency from visible light-absorbing semiconductors is challenging due to their weak absorption, transport losses, and low activity. While multilayer approaches have been developed to enhance the surface reactivity and stability independent of electronic factors, a comprehensive framework for optimizing light-matter interactions has not yet been established. Here, we introduce a photonic strategy using semiconductor metasurfaces, which is highly effective in enhancing the photocatalytic activity of GaAs, a high-performance semiconductor with a near-infrared bandgap. Our engineered pillar arrays with heights of ~150 nm exhibit Mie resonances near 700 nm that result in near-unity absorption and exhibit a field profile that maximizes charge carrier generation near the solid-liquid interface, enabling short transport distances. Our hybrid metasurface photoanodes facilitate oxygen evolution and exhibit enhanced incident photon-to-current efficiencies of approximately 22 times larger significant corresponding thin films for resonant excitation and 3 times larger for white light illumination.
Key to these improvements is the preferential generation of photogenerated carriers near the semiconductor interface, facilitated by the field enhancement profile of magnetic dipolar-type modes. We conducted time-resolved spectroscopy to investigate further carrier dynamics on n-GaAs, revealing defect-assisted rapid decay and extended intermediate decay components indicative of various defect states within the material. These defect states are detrimental to PEC cells. In contrast, MBE-grown GaAs demonstrated prolonged carrier presence, suggesting potential for enhanced photocatalytic activity. To explore the ultrafast response of the hybrid system, we performed transient pump-probe reflectivity measurements on 50nm Au film, which serves as a model for understanding the carrier dynamics in our Au/GaAs metasurface. The data showed carrier-carrier scattering happens in a few hundred femtoseconds, leading to the injection of hot electrons in the interfacial semiconductor within 1 ps time. This rapid response enhances the electric field distribution and overall photocatalytic activity.
Furthermore, we extended our investigation to study the interfacial dynamics in heterojunction materials useful for PEC applications using high-sensitivity transient optical measurements. The engineered Fe-O-Cu interfaces, characterized by strong covalent bonding, provided ultrafast interfacial charge transfer pathways on picosecond timescales, followed by long-lived charge-separated states.
Recommended Citation
Paudel, Yamuna, "Design, Fabrication, and Spectroscopy of Photocatalytic Materials in Visible Spectrum" (2024). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/5962