V. I. Matyushenko

Structure of metallic nanowires and nanoclusters formed in superfluid helium

It is shown that metallic nanowires (5–8 nm in diameter) that form during laser ablation of Ni, Pb, In, and Sn targets embedded in HeII contain extended single-crystal segments, while spherical clusters (about 2 μm in diameter) that form under these conditions have a regular shape and an atomically smooth surface. Such structures are explained by melting of metal ablation products under their coalescence in HeII. The short-term action of a low-intensity beam of electrons with an energy of 200 keV initiates the explosion in metallic spheres preserved in the vacuum chamber of a transmission electron microscope, which is accompanied with the formation of thousands of clusters with a diameter of a few nanometers. This effect is due to metastability of internal mechanical stresses produced upon sharp cooling of molten spheres by liquid helium. A mechanism of condensation of atoms and nanoparticles in quantized vortices of superfluid helium is proposed.

Electric properties of metallic nanowires obtained in quantum vortices of superfluid helium

Laser ablation of metals in superfluid helium has been used to obtain nickel, indium, and lead nanowire bundles. Wires 5–8nm in diameter demonstrate metallic conductivity and are coupled with one another by point contacts. It is shown that the wire bundles attach to sharp tips introduced into the region of condensation and are up to 1cm long. The high intensity and low threshold of electron field emission are explained by the smallness of the radius of individual wires and the long length of a bundle. The superconducting transition temperature is shifted downwards by 2.9K in lead nanowires and upwards by more than 1K in indium 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.

Application of Au–Cu nanowires fabricated by laser ablation in superfluid helium as catalysts for CO oxidation

The copper-doped gold nanowires (4xa0nm in diameter) were produced by the novel technique based of laser ablation of Au–Cu alloy inside superfluid helium. The principle of the method is using the quantized vortices as the 1D template for the condensation of the ablation products into thin threads. The nanowires were applied as the catalyst in СО oxidation with oxygen. The activity of Au–Cu nanowires deposited on glass filters was compared with that for monometallic and bimetallic Au and Cu particles (3–8xa0nm in diameter) deposited on alumina by traditional deposition–precipitation and impregnation techniques. The apparent activation energies of the reaction (Еа) were 95 and 98, 150, and 147xa0kJ/mol for Au–Cu nanowires and Au–Cu, Au, and Cu particles, respectively. During running-in of the Au–Cu nanowire–based catalyst, Еа decreased to 20xa0kJ/mol and retained at this level in the subsequent cycles of lowering and raising the reactor temperature.

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.

Stability of micron-sized spheres formed by pulsed laser ablation of metals in superfluid helium and water

It has been shown that micrometer-sized balls, resulting from the condensation of products of laser ablation of fusible metals in both superfluid helium and water, are in the state of strong internal tension counterbalanced by external compression. By radiation-induced or chemical damage to the integrity of their surface, they break up, ejecting a plurality of nanoparticles. The empty shells of the microspheres, which nonetheless remain intact, are identical to the “hollow spheres” of unclear origin that have been observed previously under laser ablation in usual liquids. The metastability of the microparticles produced by ablation in a liquid should be taken into consideration in their use in engineering and medicine.

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.

Realization of mechanical rotation in superfluid helium

The possibility of using miniaturized low-power electric motors submerged in superfluid helium for organization of rotation inside a cryostat has been investigated. It has been revealed that many of commercial micromotors can operate in liquid helium consuming low power. Turret with 5 sample holders, assembled on the base of stepper motor, has been successfully tested in experiments on the nanowire production in quantized vortices of superfluid helium. Application of the stepper motor made it possible in a single experiment to study the effect of various experimental parameters on the yield and quality of the nanowires. The promises for continuous fast rotation of the bath filled by superfluid helium by using high-speed brushless micromotor were outlined and tested. Being realized, this approach will open new possibility to study the guest particles interaction with the array of parallel linear vortices in He II.

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.

Non-isothermal physical and chemical processes in superfluid helium

Metal atoms and small clusters introduced into superfluid helium (He II) concentrate there in quantized vortices to form (by further coagulation) the thin nanowires. The nanowires thickness and structure are well predicted by a double-staged mechanism. On the first stage the coagulation of cold particles in the vortex cores leads to melting of their fusion product, which acquires a spherical shape due to surface tension. Then (second stage) provided these particles reach a certain size they do not possess sufficient energy to melt and eventually coalesce into the nano-wires. Nevertheless the assumption of melting for such refractory metal as tungsten, especially in He II, which possesses an extremely high thermal conductivity, induces natural skepticism. That is why we decided to register directly the visible thermal emission accompanying metals coagulation in He II. The brightness temperatures of this radiation for the tungsten, molybdenum, and platinum coagulation were found to be noticeably higher tha...