Direct observation of surface plasmon-driven hot carrier generation with photoconductive AFM

Abstract

Understanding the intrinsic relation between surface plasmon and hot carriers in plasmonic nanostructures is an important approach to achieve expanded applications of hot carrier-based photovoltaic devices. Since these carriers, called hot electron/or hot hole, dissipate their energies within the femtosecond resolution, it is hard to directly observe the fast-delivering information during photocatalytic reaction. Thus, research on identifying surface plasmon-driven hot carrier dynamics at the nanoscale is of great importance. In this dissertation, Chapter 1 and 2 introduce the research background and experimental setup, respectively. In Chapter 3, we measured photo-induced hot electrons of a triangular Au nanoprism on n-type TiO2 under incident light with photoconductive atomic force microscopy (pc-AFM). Under incident light with a wavelength of 640 nm near the absorption peak of the Au nanoprism, localized surface plasmon resonance resulted in generation of higher photo-induced hot electron flow of Au nanoprism/TiO2 than that of a wavelength of 532 nm. Due to the geometry of triangular shaped Au, measured photocurrent mapping revealed that field confinement at the boundary of an Au nanoprism which leads to an enhancement of hot electron flow at the boundary than inner area of the Au nanoprism. Furthermore, we show that the application of reverse bias results in the higher photocurrent of an Au nanoprism/TiO2 that is associated with image force barrier lowering. In Chapter 4, we also demonstrate the direct observation of surface plasmon-driven hot holes created in a Au nanoprism/p-GaN platform using pc-AFM. Significant enhancement of photocurrent in the plasmonic platforms under light irradiation was revealed, providing direct evidence of plasmonic hot hole generation. Experimental and numerical analysis verified that a confined |E|-field surrounding a single Au nanoprism spurred resonant coupling between localized surface plasmon resonance (LSPR) and surface charges, thus boosting hot hole generation. Furthermore, geometrical and size dependence on the extraction of LSPR-driven hot holes suggests an optimized pathway for their efficient utilization. The direct visualization of hot carriers at the nanoscale provides significant opportunities for harnessing the underlying nature and potential of plasmonic hot carriers.