Hot-carrier Dynamics in Photoexcited Gold Nanostructures: Analysis of Plasmon Excitation, Interband Transitions and Ballistic Transport

Plasmon-enhanced photo-excitation and collection of ‘hot’ charge carriers from metallic nanostructures has the potential to enable new light harvesting mechanisms for optoelectronic devices. While it is now established... [ view full abstract ]

Plasmon-enhanced photo-excitation and collection of ‘hot’ charge carriers from metallic nanostructures has the potential to enable new light harvesting mechanisms for optoelectronic devices. While it is now established that plasmon excitation improves device performances, such as responsivity of photo-detectors or incident-photon-to-charge-carrier efficiency of photocatalytic cells, detailed understanding of the role of plasmon in either light absorption or hot-carriers generation, transport and collection (i.e. internal quantum efficiency, IQE) is yet to be achieved. Theoretical ab-initio studies have clarified individual aspects of these processes, establishing the major mechanisms for hot-carriers generation upon plasmon non-radiative decay (inter- and intra-band transitions) as well as the resulting energy distribution, hot-carrier mean free path and momentum. However, the complex convolutions of these aspects is less understood and experimental results are still largely interpreted in terms of the semi-classical Fowler theory.

In this work we combine experimental and theoretical methods to gain a detailed description of hot-carrier transport in nanostructures. We indeed experimentally determine the IQE spectra of several Au-on-GaN photo-electrodes supporting different plasmon resonances and show that plasmon excitation is primarily responsible for absorption enhancement. Furthermore, using a recently developed theoretical framework, we are able to determine the energy distribution of the carriers which reach the Au/GaN interface as well as track the number of scattering events which occurred before extraction. This calculation, whose results agree well to experimental data, predicts ballistic carrier transport and collection for wavelengths greater than 620 nm. Finally, by assessing the relative contribution of intra- and inter-band transitions, we are able to relate the characteristic feature of the IQE spectra to the competing role of these two mechanisms, restrospectively supporting previous experimental results.

Overall the discussed approach coupling experiments with theoretical calculations enables a new level of physical detail in the study of plasmon-enhanced hot-carrier devices.