Abstract
The formation of Ag nanoparticles by the addition of Ag atoms to helium droplets has been investigated. The resulting nanoparticles were then imaged by transmission electron microscopy after being deposited on a thin solid surface. In large helium droplets chains of Ag nanorods were observed similar to recently reported track-like deposits [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302]. However, by adjusting the experimental conditions chains of spherical nanoparticles could also be seen with a nearly uniform inter-particle spacing. Given that spherical Ag nanoparticles have no intrinsic anisotropy, the only viable explanation is that these particles must be guided into position by interaction with a quantized vortex spanning the diameter of the helium droplet. Furthermore, addition of Si to the droplets immediately after Ag resulted in Si inserting between the Ag nanoparticles to form continuous nanowires. This eliminates the possibility that the segmented Ag nanostructures are the result of nanowire fragmentation when the helium droplets collide with the deposition substrate. Thus segmented Ag chains are shown to be an intrinsic feature of Ag aggregation in helium droplets in the presence of a quantized vortex.
Highlights
Liquid helium is a superfluid at temperatures below 2.17 K
Given that spherical Ag nanoparticles have no intrinsic anisotropy, the only viable explanation is that these particles must be guided into position by interaction with a quantized vortex spanning the diameter of the helium droplet
The first real indication that quantized vortices may be present in helium droplets came from experiments by Gomez et al.,[11] in which multiple Ag atoms were added to relatively large helium droplets and aggregated into nanoparticles
Summary
Liquid helium is a superfluid at temperatures below 2.17 K. The superfluid phase transition temperature these particles become pinned to quantized vortices and light scattering allows images of the vortices to be made.[3,4,5,6] In other recent experiments, laser. Liquid helium droplets are the nanoscale counterparts of bulk superfluid helium. While well known in the bulk liquid, evidence for quantized vortices in its nanoscale counterpart, superfluid helium droplets, is sparse. The first real indication that quantized vortices may be present in helium droplets came from experiments by Gomez et al.,[11] in which multiple Ag atoms were added to relatively large helium droplets (with diameters 4 300 nm) and aggregated into nanoparticles. The explanation offered by Gomez et al for these 1D nanostructures was that Ag atoms become pinned to quantized vortices spanning the helium droplets, which directs the formation of elongated structures. The observation of aligned nanorods is potentially problematic, since the rods themselves introduce an anisotropy
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