E. B. Gordon

Nanowire formation by gold nano-fragment coalescence on quantized vortices in He II

We demonstrate that laser ablation of a gold target immersed in superfluid and normal fluid helium leads to the formation of elongated gold nano-fragments. In the superfluid phase these nano-fragments aggregate into filaments with extremely large aspect ratios displaying metallic electric conductivity. We attribute this unusual structure to the coalescence of gold particles trapped on quantized vortices. Our observations suggest new ways to visualize the structure of quantized vortex bundles and a new approach for producing centimeter-long metal nanowires.

Structure and Properties of Platinum, Gold and Mercury Nanowires Grown in Superfluid Helium

Webs consisting of nanowires made of gold, platinum and mercury were produced by the technique based on laser ablation of metals inside superfluid helium. Their morphology and structure as well as their electrical conductivity have been studied. Diameters of gold and platinum nanowires are 4.5 and 3 nm, respectively. Fortunately, they are close to diameters of nanospheres made of these metals, which, as known from the literature, possess anomalous catalytic activity. Web resistivities for all metals up to room temperature are controlled by conductive electron scattering on a wire surface, thus they are almost independent of T. Nanowires in the webs are electrically interconnected, and therefore the web can be used as a catalyst without any support. Possible advantages of this type of nanocatalyst are outlined.

Embedding Impurities into Liquid Helium

We have introduced guest particles into superfluid helium using a directed helium jet containing traces of species under study. The distinguishing peculiarity of the method consists in that the whole system is sealed from the cryostat main helium bath. This allows: (i) on the account of the absence of evaporating helium upflow to realize a complete capture of the impurities from the jet into liquid helium; (ii) to eliminate the dependence of the process conditions on liquid He level position in the main bath as well as on the amount of liquid He condensed inside a cell; and (iii) this method can be used to introduce impurities into liquid 3He. Two modifications of the technique have been designed—one for an optical cryostat and another for a cryostat with narrow 1” tail typical for use in a very high magnetic field. Optical and X-ray diffraction studies have confirmed the possibility of embedding in superfluid helium samples consisting of submicron D2 particles with a rate of 10mmoles per hour. Such samples are necessary for the achievement of strong D2 nuclear spin polarization by the brute force method.

Experimental study of thermal stability of thin nanowires.

Thin (D < 10 nm) nanowires are in principle promising for their application as catalysts and as elements of nanocomputers and quantum devices. To perform these tasks, their structure and properties must be stable at least at standard conditions. Using our technique based on the capture of small particles to the core of quantized vortices in superfluid helium, we synthesized nanowires made of various metals and alloys and investigated their thermal stability. The indium nanowires (D = 8 nm) were shown to be stable when heated to 100 °C, i.e., almost to the melting point, whereas the silver nanowires (D = 5 nm) disintegrated into traces of individual nanoclusters at 300 K. The gold and platinum nanowires also decomposed at temperatures more than twice as low as the melting point. A model is proposed to explain the premature decay of thin nanowires by unfreezing of the surface-atom mobility in combination with the anomalous dependence of the surface tension on the nanowire radius. Methods for improving the stability limits of thin nanowires by saturation of their surface with immobilized atoms as well as by surface oxidation have been proposed and experimentally tested.

Catalysis of carbon monoxide oxidation with oxygen in the presence of palladium nanowires and nanoparticles

A new synthesis method based on the coagulation of metal nanoparticles, introduced by laser ablation into superfluid helium, inside of quantized vortices has been used for the fabrication of nanoweb consisting of interconnected palladium wires of a 4 nm diameter. It has been found that at temperatures above 523 K, the Pd nanoweb effectively catalyzes the oxidation of CO with molecular oxygen. Temperature cycling leads to a shift of Pd nanoweb activity to lower temperatures. The catalytic action of the Pd nanoweb has been compared to that of Pd nanoparticles with a diameter of about 2 nm prepared by laser electrodispersion.

Measurements of Thermal Diffusivity of Impurity-Helium Solids

Thermal diffusivity of the Kr- and N2-impurity-helium solids (IHS) has been measured for the samples, immersed into liquid helium in the temperature range of 2.2–4.2 K. It has been found that the thermal diffusivity for both solids does not differ from that of liquid helium when the convection is absent even for the case when the samples were compact 10 times as compared to their initial volume. It should be emphasized that the presence of less than 0.5% mole fraction of a rare gas in the form of IHS allows us to suppress completely any convection in liquid helium (this effect was not observed when deuterium was used as an impurity). IHS can be used as a specific porous medium for investigations of critical phenomena of the He-I–He-II phase transition, as the He-aerogel systems.

Quasi-1D metals (Pd, Pt, Nb) as catalysts for oxidation of CO

Nanowebs of palladium, platinum, and niobium consisting of nanowires 3–4 nm in diameter with average length of about 200 nm were produced by laser ablation of metallic targets in superfluid helium. When the nanowebs were tested in the oxidation of CO by oxygen at 573 K, an appreciable yield of CO2was observed for all the catalysts. According to TEM, palladium and platinum nanowires at this temperature in air disintegrate into chains consisting of individual nanoparticles; the nanowires of niobium remain intact. According to XPS contact between niobium wires and oxygen leads to oxidation of the niobium to Nb2O5, the palladium nanowires are partially oxidized, while the platinum nanowires remain in the metallic state. Despite the substantial differences in the morphology and structure the activity of the investigated metal nanowires in the oxidation of CO is fairly close.

Processes in condensed inert gases involving excess electrons

Drift of an excess electron in dense and condensed inert gases in external electric field and excitation of atoms by electron impact in these systems are analyzed. The effective potential energy surface for an excess electron at a given electric field strength consists of wells and hills, and the actions of neighboring atoms are therefore separated by saddles of the potential energy. At such atomic densities that the difference of interaction potentials for an excess electron between neighboring wells and hills of the potential energy surface becomes small, the electron mobility is large. This is realized for heavy inert gases (Ar, Kr, Xe) with a negative scattering length of an electron on individual atoms. In these cases, the average potential energy of the electron interaction with atoms corresponds to attraction at low atomic densities and to repulsion at high densities. The transition from attraction to repulsion at moderate atomic densities leads to a maximum of the electron mobility. A gas model for electron drift in condensed inert gases is constructed on the basis of this character of interaction. Due to high electron mobility, condensed inert gases provide high efficiency of transformation of the electric field energy into the energy of emitting photons through drifting electrons. It is shown that, although the role of formation of autodetaching states in the course of electron drift is more important for condensed inert gases than for rare gases, this effect acts weakly on exciton production at optimal atomic densities. The parameters of a self-maintained electric discharge in condensed inert gases as a source of ultraviolet radiation are discussed from the standpoint of electron drift processes.

Electronic excitation of the matrix during drift of excess electrons through solid xenon

It is shown experimentally that the exciton luminescence λ=172 nm) quantum yield excited by excess electrons drifting through solid xenon at 77 K in fields of 10 kV/cm amounts to 20±5 per electron and that luminescence takes place during the entire drift process. A CW bulky discharge through solid xenon (with a current up to 20 A/cm2) is realized, and intense visible luminescence due to excitation of impurities by electron impacts is observed. The prospects for using solid rare gases as matrices for studying processes in low-temperature plasmas and for creating effective electric energy converters in the vacuum ultraviolet range are discussed.

The Tube Character of Electron Drift in Condensed Inert Gases

The behavior of an excess electron in condensed inert gases in an external electric field is considered at densities and temperatures at which the mobility of a slow electron is relatively high. On the basis of experimental data and a model of a pair electron interaction with atoms, an effective potential energy surface is constructed for an excess electron inside a dense inert gas. The region available for a slow electron consists of many intersecting channels that form a Delaunay network located between atoms. A drifting electron, as a quantum object, propagates along these channels (tubes), and electron transition between intersecting potential energy tubes of different directions provides an effective electron scattering. This mechanism of electron drift and scattering differs from that in gases and crystals. Peculiarities of electron drift inside dense inert gases are analyzed within the framework of this mechanism of electron scattering, leading to a moderate change of the electron mobility upon melting.