A tale of helium droplets: from nanoparticles to quantized vortices

This project focused mainly on the fabrication and characterisation of atomic clusters, molecular clusters and nanostructures made with the aid of helium droplets. Nanoparticles with different sizes have been fabricated by controlling the droplet source expansion conditions. Large silver nanoparticles with both spherical and non-spherical shapes were produced successfully using Cs+ ion as a seeding agent in helium droplets. Self-assembled nanostructures constructed with well-defined distances has also been achieved using organic molecules as structural templates. In addition to particles, nanowires were also investigated. The growth of nanowires is thought to derive from quantized vortices inside large helium droplets. Quantized vortices can act as an aligning tool for one-dimensional growth. TEM images from particle deposition have provided the first experimental evidence that multiply quantized vortices might exist in large helium droplets formed at the edge of helium droplet expansion due to the faster rotation in the helium droplets formed at that position. Finally, in this project we have fabricated, for the first time, binary metal/semiconductor nanoparticles and nanowires by the sequential addition of metal and then CdS to a helium droplet. The formation of Au/CdS core-shell nanowires was confirmed through the crystalline structures of the core and shell regions using High Resolution Transmission Electron Microscopy (HRTEM) images.