Helium Droplets – A Very Clean and Controlled Way to Produce Nanoparticles
Philipp Thaler
11:00 - 11:40 Friday 22 May 2015 HS P3 PH02112

Clusters and nanoparticles are an interesting class of materials because their properties can differ from the bulk material as well as from the pure atomic species. Their optical, magnetic and catalytic characteristics can strongly depend on their size and shape as well as on their surroundings (oxide layers, ligands, solvents, substrates). The utilization of helium droplets as reactors to grow nanoparticles allows a good control over the size of the produced particles and provides an ultra-pure environment, excluding any unwanted reactions. Recently, the existence of quantized vortices inside helium droplets has been demonstrated, opening the possibility to create one-dimensional shapes on the nanoscale. Furthermore, sequential addition of two or more species to the helium droplet allows material combinations which would not be possible using other techniques.
Helium droplet mediated cluster aggregation is still in its infancy, but by now we are able to produce samples with a desired size distribution and material composition with good reliability. In the past two years we characterized the growth behavior of large metastable five-fold symmetric multiply twinned particles. We explored the vortex-regime with the objective of producing nanowires and tested the limitations of the technique caused by the Rayleigh breakup effect. We employed sequential doping of the droplets with different materials and were able to produce Au-core Ag-shell as well as Ag-core Au-shell nanowires. Additionally, we investigated chemical reactions and diffusion processes in single-phase and multi-phase nanoparticles and nanowires.
In this talk I will give an overview of the experimental setup, and present the results of the experiments mentioned above.