Helium droplets: a new route to nanoparticles

Abstract

Helium droplets are large helium clusters that are capable of picking up individual atoms and molecules and show promise as nano-reactors for the synthesis of unique nanoparticles. In particular, the sequential addition of materials of different types offers opportunities for the fabrication of novel core-shell nanoparticles that cannot be synthesised by other methods. To exploit this potential, here we have carried out a mass spectrometry investigation on metal clusters in order to establish how to control the doping conditions for the fabrication of nanoparticles in superfluid helium droplets, and in particular to develop a recipe to control core and shell ratios in the case of core-shell nanoparticles. Several types of metal nanoparticles, including pure Ag, Au and Ni nanoparticles, and Ag/Au and Ni/Au core-shell systems, have been synthesised and then removed from the helium droplets by deposition on substrates for ex situ investigations using high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The TEM imaging has been used to estimate the sizes of nanoparticles, which show a bimodel distribution under the conditions employed. We also present the first evidence that crystalline metal nanoparticles are formed by self-assembly of metal atoms in helium droplets. The XPS investigation of Ni/Au core-shell nanoparticles shows an absence of any Au 4f core-level shift that would occur on alloying of Au and Ni, which provides the first direct evidence for the successful formation of core-shell nanoparticles using superfluid helium droplets.

Preparation of Ultrathin Nanowires Using Superfluid Helium Droplets

“…29,32,44 Briefly, the vacuum system consists of four interconnected ultrahigh vacuum (UHV) regions: a source chamber, a dopant pick-up region, a deposition station, and a mass spectrometer. The helium droplets are produced in the source region by passing high purity helium (99.9999%) under pressure through a 5 μm nozzle that can be cooled to temperatures as low as 4 K by a closed-cycle helium cryostat (Sumitomo RDK-415D).…”

“…The traveling helium droplets can sequentially acquire different types of materials when they pass through different pickup regions, 32 making it possible to form multilayer nanowires. We note that in recent experiments in our laboratory on the formation of 1D Ag nanostructures, where Si was added to helium droplets after Ag, Ag−Si nanowires were formed with Si covering the surface of Ag nanorods and in-filling the gaps between the Ag nanorods.…”

“…These factors offer a route to a large array of new and potentially very interesting types of nanoparticles. 29 Although still in its infancy, a number of metallic nanoparticles have been synthesized using this technique and have been removed from the helium droplets for transmission electron microscopy (TEM) investigations, including Ag/Au nanoparticles composed of ∼500 metal atoms, 30 pure Ag nanoparticles composed of over 6000 Ag atoms, 31 Ag/Au and Ni/Au core−shell nanoparticles, 32 and polycrystalline Ag nanoparticles. 33 To form 1D nanostructures in helium droplets, we will take advantage of another unique feature of superfluid helium, the presence of quantized vortices, which provide anisotropy that can guide the one-dimensional growth of particles.…”

Exchange Bias in Fe@Cr Core–Shell Nanoparticles

“…In particular recent developments have enabled the synthesis of alloy [4] and core-shell [5,6] nanoparticle building blocks in which there is independent control over core size, shell thickness and flexible choice of element in either. In the work reported here we have exploited this flexibility to study how the magnetic behaviour of Fe nanoparticles evolves as they are coated with one or two monolayers of Cr.…”

Formation of Au and tetrapyridyl porphyrin complexes in superfluid helium

“…In previous experiments no elongated nanoparticles have been observed in helium droplets composed of <10 6 helium atoms, [31][32][33] suggesting that quantized vortices, even if they exist, do not have significant influence on the clustering of H2TPyP and Au in this work. Therefore, the formation of Au-H2TPyP complexes is presumably driven by intermolecular forces.…”