Displaced electrons drive nanoparticle oscillations

Photocatalysis, sensors, solar cells: Plasmons promise a variety of applications if the processes triggered by optical excitation in the nanoparticles can be controlled. A research team from Hamburg and Berlin reports in Nano Letters experimental observations that cannot be explained by established models and describes a new theoretical model that explains the dynamics of excited gold nanoparticles observed in experiments.

Plasmons are collective electron oscillations associated with highly localized fields. The decay of these oscillations after optical excitation is currently the subject of intense debate. Researchers assume that very energetic "hot" electrons are generated in the process, that lose their energy by electron-electron scattering into a "warm" electron gas. The gas heats up the particle, which eventually releases the excess energy into the environment. The efficiency of the energy transfer between the "hot electron", "warm electron" and "warm particle" stages is important for applications that want to make use of each process. In particular, the energy transfer from the warm electron gas to the nanoparticle appears to be so efficient that the particle is heated extremely quickly. In the process, it expands explosively, causing it to oscillate collectively, like a breathing sphere. However, direct experimental studies resolving the breathing-oscillation have been missing.

For their study, researchers from the Departments of Physics and Chemistry at Universität Hamburg, the Max Planck Institute for the Structure and Dynamics of Matter (MPSD), DESY and TU Berlin joined forces. Led by Holger Lange, Jochen Küpper und Kartik Ayyer, who all conduct research in the Cluster of Excellence "CUI: Advanced Imaging of Matter", and Andreas Knorr from Berlin, the team combined theory and experiment for an accurate description of the dynamics of excited gold nanoparticles. Using single-particle X-ray imaging (SPI) and transient absorption spectroscopy (TA), the researchers determined the size and electron temperature of the nanoparticles after optical excitation as a function of time.  They observed that the particles already expanded with the optical excitation pulse, much faster than expected. This observation directly proved the need for an immediate excitation source other than the temperature rise and associated expansion of the particle.

To explain their findings, the researchers set up a new, fundamental model. The theoretical calculations confirmed the experimental results, giving a consistent picture of all observed aspects of the excitation. Two source terms for the "particle breathing" also emerge from the calculations: the "classical" thermal expansion and a new effect, optically induced distributions of the electrons, which directly drive the oscillation. The new excitation term shows that the plasma dynamics processes are a lot more intertwined than assumed and that existing models on warm and hot electrons must be questioned, with implications for photocatalysis and other energy transformation pathways.

The new theoretical model also provides a general approach to the plasmon particle interaction and is already being used in first other CUI projects.


Text courtesy of CUI: Advanced Imaging of Matter

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