Driving chemical reactions optically represents a paradigm shift in our ability to synthesize materials. Plasmonic catalysis – using photoexcited hot carriers generated in metal nanoparticles to drive reactions – represents an exciting approach for chemical conversion. Importantly, the shape and size of the nanoparticle has been demonstrated to modulate the selectivity and efficiency of the reaction, allowing for novel reaction pathways to be found [1]. To realise the full potential of plamonic catalysis we need to understand the fundamental processes behind it. To this end I will present two complementary approaches, based on spectroscopy and scanning probe microscopy, that help unravel the microscopic dynamics of charge carriers in plasmonic metals.

Starting from gold monocrystalline flakes, we fabricated nanodisks to drive chemical reactions, with plasmonic resonances in the 600-800 nm wavelength range.  Using a combination of Scanning Electrochemical Microscopy and microscale optical characterization we were able to go beyond external quantum efficiencies and extract the internal quantum efficiency of the system, that is, the probability of a chemical reactions per absorbed photons. State-of-the-art density-functional-theory (DFT) parameterized calculations of the same nanostructures allowed us to calculate the fraction of photoexcited charge carriers that reach the exposed gold surfaces. Together these results allow us to find the probability of hot carrier charge transfer from a nanoparticle to a molecule as a function of electron and hole energy, a key and difficult to access parameter. For the example reaction of oxidation of ferrocyanide we show that the chemistry is dominated by high energy hot hole transfer [2].

Subsequently, we explored the potential of luminescence from gold as a non-invasive probe of hot carrier dynamics, and hence chemical reactions. We studied the luminescence from unpatterned monocrystalline gold flakes and modelled our results in DFT, enabling, for the first time, a complete understanding of this process. Our measurements show that pre-scattered hot carriers are responsible of a large fraction of the signal close to the excitation energy. Furthermore, for gold less than 40 nm thick quantum mechanical effects play a significant role in the luminescence spectrum, higher thicknesses than previously assumed. Overall, this work reveals that gold luminescence can be employed as a probe for nanoscale chemistry [3].

In summary, these projects realise new methodologies for probing plasmonic catalysis at the micro- and nanoscale, with implications for a wide range of chemical processes and materials.