Manipulating the Growth of Nanomaterials in Superfluid Helium Droplets: Formation of Hybrid Nanostructures and Nanoparticle Assemblies

This project intends to develop a technique to fabricate nanomaterials by manipulating the growth process using superfluid helium droplets as nanoreactors, which provide a cold and confined environment where atomic or molecular dopants can aggregate into clusters and even nanoparticles. A mass spectrometric study has been used to study binary molecular clusters containing a noble metal atom and an organic compound formed in helium droplets. The co-addition of metal has been found to influence the fragmentation patterns of the organic molecules depending on the relative ionisation energies of the selected molecules and metals. Nanoparticles with different sizes and forms have been fabricated by controlling the growth conditions such as the droplet source expansion and the pickup conditions. Direct growth of nanoparticle assemblies has been demonstrated for the first time as a novel bottom-up technique for nanoparticle synthesis. The direct growth technique is based on a templated growth of nanoparticle assembly in superfluid helium droplets, in which the nanoparticle grows directly at the binding sites in a molecular template. By using this technique, we have demonstrated the feasibility to grow nanoparticle with a high degree of control on both size and the interparticle distances. Besides, semiconductor CdS and core-shell of metal/CdS in the form of nanoparticles and nanowires were also prepared by the sequential doping of metal and CdS to the helium droplets. The nanoparticles and nanowires were grown by using different nozzle positions and nozzle temperatures. The nanowires formation was due to the presence of quantised vortices inside large helium droplets. A further study on the quantised vortices shows different patterns observed from different parts of the expansion region, which reflects the different rotational behaviour of the helium droplets. Images collected from the offset nozzle position provide the first evidence for the presence of multiply quantised vortices (rotational quanta > 1) in helium droplets, where small particles are seen at the vicinity of the chains of large particles.