Steady State Photothermalization and Hot Electron Dynamics in Noble Metals
Abstract
There has been significant recent interest in using the resonant absorption in plasmonic nanostructures to produce transiently excited populations of photo-excited non-equilibrium hot electrons that can then be utilized in a variety of applications including photocatlysis and optoelectronic energy conversion. Traditional studies of these hot electrons are done using high power, pulsed excitation, though there is particular interest in being able to expand the current understanding to conditions of continuous wave excitation that may be more directly relevant to conditions used in emerging applications, such as solar fuel production.
I show that it is possible to systematically understand and control photothermalization in plasmonic materials by quantifying the connection between absorption, relaxation, and thermal emission. Using fundamental principles, the phononic temperature of a material can be predicted based on its absorption and emission properties, which then allows for optimization of the nanostructure geometry to maximize temperature for thermal applications. Increases in phononic temperature can in turn be used to inform the electronic temperature. By fitting to anti-Stokes Raman spectra, I develop a model that describes electrons in equilibrium with both the phononic and electronic temperature as well as giving a quantitative measure of the number of hot electrons. Additional analysis allows further insight into the hot electron dynamics inside the material including electron-phonon coupling and hot electron lifetime. Longer lifetimes increase the chance that hot electrons can be used in power cycles or chemical reactions.
Using this spectroscopic technique, I explore how the hot electron population in nanostructures is dependent on shape and material composition. Increased hot electron reactivity with surface species in copper is promising for photocatalytis. In contrast, gold nanostructures are less reactive and reached higher temperatures which has increased utility for solar-thermal applications. Further, variations in nanostructure geometry modified access of the hot electrons to the environment which changes not only the coupling constant but also the electronic temperature. This information opens opportunities for optimization of not only phononic temperature, but additionally rational design of nanostructures to have hot electrons with longer lifetimes or higher energies, properties which are extremely useful in developing hot electron technologies.
Citation
Hogan, Nicki Lynne (2020). Steady State Photothermalization and Hot Electron Dynamics in Noble Metals. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192318.