Molecular movie of gold nanoparticle oscillations driven by displaced electrons

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 experimental observations of a so-called molecular movie that cannot be explained by established models in Nano Letters. The team including researchers from DESY provides a new theoretical model that explains the dynamics of excited gold nanoparticles observed in their experiments.

Plasmons are collective electron oscillations associated with highly localised 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 which 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 wanting to make use of these processes. 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, so far 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), the CFEL at DESY, and TU Berlin joined forces. Led by Holger Lange, Jochen Küpper, and 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 diffractive imaging (SPI), performed at DESY’s FLASH facility, and transient absorption spectroscopy (TA), the researchers determined both the structural size and the 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 previously assumed. This observation directly proved the need for an immediate excitation source other than the temperature rise and associated expansion of the particle.

Read more on the DESY website

Image: Optical excitation of gold nanoparticles directly sets the particle into an oscillatory motion in which the particle periodically expands and contracts.

Credit: Univ. Hamburg/H. Lange