2, Johns Hopkins University, Baltimore, Maryland, United States
Plasmonic nanostructures have been employed as sensitizers in photocatalysis to achieve higher catalytic efficiency. Although the most widely used materials are gold and silver, aluminum has attracted interest in recent years as a new plasmonic material due to its natural high abundance, low toxicity and relatively high free carrier density. Moreover, it exhibits plasmon resonances which can be tuned from the ultraviolet through the visible via adjustment of size, shape, composition and the surrounding dielectric environment. Hot carriers generated in a plasmonic material can be injected into a catalytic semiconductor in hundreds of femtoseconds and enhance the photocatalytic activity. Understanding hot carrier relaxation processes is therefore crucial for applications.
In this work, we synthesized large aluminum nanocrystals in the solution phase (~98±12 nm in diameter) and report the first photophysical characterization of energy dynamics in these types of particles. Our nanoparticles displayed dipolar and quadrupolar localized surface plasmon resonance peaks at 392 nm and 269 nm, respectively. High resolution transmission electron microscopy shows the formation of a 3.7 nm thick native aluminum oxide layer. We used ultrafast transient spectroscopy to study the electron relaxation dynamics of these particles. We showed that the particles exhibit a decrease in light transmission across the broad visible and near-infrared regions on a 2 ps timescale associated with electron-lattice relaxation processes. The spectral response was qualitatively different near the interband transition region with a persistent bleach signal that provides a window into electron-electron thermalization dynamics. We found that these large particles exhibited fast thermal energy transfer (~250 ps timescale) which is comparable to that predicted for much smaller particles with diameters of ~10 nm. Using an extended two interface model, we demonstrated that the thin native oxide layer on the particles plays an important role in mediating fast thermal energy transfer, with minimal dependence on the shell thickness. We propose that using controlled surface modification strategies is an effective approach for engineering heat transfer rates in large nanoparticles. This is a beneficial strategy for photocatalytic and sensing applications where heat management is critical.