Direct preparation of one-dimensional nanostructures in superfluid helium droplets Shengfu Yang,* Daniel Spence, Elspeth Latimer, Cheng Feng, Adrian Boatwright, Andrew M. Ellis Department of Chemistry, University of Leicester, LE1 7RH, United Kingdom
Helium droplets for the formation of nanowires P0 = 15 atm
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T0 = 4.5 – 6 K
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Figure 3 – Ultra-thin nanowires of
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Ni.
Figure 1 – Schematic of apparatus for the production of nanowires in superfluid helium droplets1
Controlled growth of nanowires
Helium gas held at 15 atm is pre-cooled to below 6K and expands
By controlling the doping rate of the metal atoms, we can control the average length of the wires produced (fig. 4). The diameter of the wires reaches a maximum of a few nm and is likely limited by the potential well of the vortex, or surface energy minimization. Wires have been produced using Au, Ni, Cr and Si (fig. 3), whilst silver preferentially forms chains of spherical particles (fig. 2).
into high vacuum through a pinhole. The expansion produces a beam of very large droplets of the order of microns in diameter, each at a temperature of 0.38 K, circulating around a vortex core spanning the length of the droplet. The droplets pass through sequential evaporator doping regions where they pick up foreign species. The dopants aggregate inside the droplet and become pinned to the vortex core by a confining velocity-gradient potential.
Figure 2 – A chain of Ag nanospheres formed by vortexinduced growth in a helium nanodroplet. Particle spacing is regular (~26nm) and Si can be insterted in the gaps prior to deposition (in droplet)
Vortices in superfluid helium In superfluid helium, circulation of the fluid is quantized, and only possible in the presence of a hollow core – the vortex core. These vortices were studied theoretically by Feynman2 and their traces observed experimentally in helium droplets recently by Vilesov et al. 3. We have recently obtained firm evidence of vortex-induced growth of nanowires and chains in helium droplets4. Fig. 2 shows a chain of completely spherical particles, ruling out spontaneous dipolar aligning forces, and providing firm evidence of vortex-induced growth inside the droplet.
Figure 4 – Controlled growth of fixed-diameter (5nm) Au nanowires
Applications of ultrathin nanowires This universal technique provides the opportunity to create nanowires of almost any material, for use in nanoelectronic circuits, state-of-the-art detector technology and biological sensing. Semiconductor nanowires may features in the next generation of solar energy technology.
1. A Boatwright et al., Faraday Discuss., 162 (2013) 113-124.
2. R P Feynman, Prog. Low Temp. Phys., 1, North-Holland (1955) pp. 17-53
3. L Gomez et al., Phys. Rev. Lett. 108 (2012) 155302.
4. D Spence et al., Phys. Chem. Chem. Phys., 16 (2014) 6903-‐6906 .