Light-Harvesting Semiconductor Metasurfaces for the Enhancement of Photocatalytic Activity

Utilization of the enormous and constant amount of available solar energy as a clean alternative to fossil fuels is a task of utmost importance. However, a fundamental challenge in the field of heterogeneous photocatalysis is the design and optimization of visible and NIR absorbing semiconductors to maximize the light harvesting efficiency. We posit that a key limitation at longer optical wavelengths results from a mismatch of the characteristic length scales for charge transport, as determined by the depletion width, and absorption. To address this issue, we present a general framework for tailoring light absorption in nanopatterned arrays of conventional semiconductors. This approach maximizes the generation of carriers in the depletion region and overcomes limitations imposed by relatively poor material quality and weak absorption by nanoscale materials in the visible and NIR spectral region. Key to our design is the optimization of magnetic dipolar Mie resonances with near-unity absorption that preferentially generate charge carriers near the liquid-solid interface, enabling short transport distances and high conversion efficiencies. We demonstrate this approach using moderately doped (~ 10¹⁸ cm^-3) Gallium Arsenide (GaAs) thin films that are patterned to create plasmonic/dielectric hybrid metasurface of 2-D periodically surfaces of nanostructured resonators. These nanostructures are resonant scatterers that enhance absorption to near unity through a broad, geometrically determined Mie resonance that exploits the interference of the 1^st(magnetic dipole (MD) and 2^nd electric dipole (ED) resonance modes as well as confines the electromagnetic field within the nanostructures. This allows for a simultaneous suppression of almost all reflectance and transmittance and enhancement of the electron-hole generation rate near the liquid-solid interface of each nanostructure. We will discuss the modeling and simulation of the metasurface, the fabrication process for nanostructures of ~200 nm height, and the analysis of the resonant mode on the incident photo-to-current efficiency in a photoelectrochemical cell. Our optical measurements on the GaAs metasurface showed a strong absorption resonance near ~ 700 nm, which is inferred through the optical reflectivity being below 10 %. The incident photon-to-current conversion is 22x higher in fabricated GaAs metasurfaces for resonant excitation as compared to flat film of GaAs, which is strong evidence that these metasurfaces exhibit a strong electric field enhancement profile and maximize the charge carrier generation near the solid-liquid interface. Furthermore, we extend this approach to thin films of amorphous silicon (a-Si) to produce metasurfaces with strong absorption resonance at ~600 nm. These structures facilitate the carrier generation on non-epitaxial heterostructures and the use of high-sensitivity ultrafast spectroscopy methods. Transient optical measurements are used to understand the effect of the Mie resonance on charge carrier generation and collection.