I. I. Khodos

Superconductivity in Ropes of Single-Walled Carbon Nanotubes

We report measurements on ropes of single-walled carbon nanotubes (SWNT) in low-resistance contact to nonsuperconducting (normal) metallic pads, at low voltage and at temperatures down to 70 mK. In one sample, we find a 2 orders of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic field of the order of 1 T, or by a dc current greater than 2.5 microA. These features strongly suggest the existence of superconductivity in ropes of SWNT.

Quantum transport through carbon nanotubes: Proximity-induced and intrinsic superconductivity

We report low-temperature transport measurements on suspended single-walled carbon nanotubes ~both individual tubes and ropes!. The technique we have developed, where tubes are soldered on low-resistive metallic contacts across a slit, enables a good characterization of the samples by transmission electron microscopy. It is possible to obtain individual tubes with a room-temperature resistance smaller than 40 k V, which remain metallic down to very low temperatures. When the contact pads are superconducting, nanotubes exhibit proximity-induced superconductivity with surprisingly large values of supercurrent. We have also recently observed intrinsic superconductivity in ropes of single-walled carbon nanotubes connected to normal contacts, when the distance between the normal electrodes is large enough, since otherwise superconductivity is destroyed by ~inverse! proximity effect. These experiments indicate the presence of attractive interactions in carbon nanotubes which overcome Coulomb repulsive interactions at low temperature, and enable investigation of superconductivity in a one-dimensional limit never explored before.

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.

The electrical conductivity of bundles of superconducting nanowires produced by laser ablation of metals in superfluid helium

The temperature dependencies of the electrical resistance are reported for bundles of permalloy, indium, tin, and lead nanowires having a similar diameter of about 8 nm which were grown by catalytic coagulation of laser ablation products in quantized vortices of superfluid helium. In all metals, the constant residual resistance found at low temperatures changes to a superlinear growth at higher temperatures. For all superconducting nanowires, the transition to superconductivity is broadened with a width of δT = 0.6 K. The size shift of the transition center ΔT is largest for lead, ΔT = −2 K, whereas for indium and tin ΔT is 0.2 K and <0.1 K, respectively.

Luminescent Properties of ZnO Films Doped with Group-IB Acceptors

It is shown by experiment that the doping of ZnO films with Group-IB acceptors—Cu, Ag, and Au—influences the photoluminescence spectra of the films as well as their electrical properties. Specifically, this reduces the emission in the UV region and intensifies it in the visible region. The respective distances of the Cu, Ag, and Au energy levels from the valence band are found to be 0.38, 0.20, and 0.45 eV.

Morphology of SiC nanowires grown on the surface of carbon fibers

SiC nanowires (NWs) with diameters of 20–200 nm and lengths from tens to hundreds of micrometers have been synthesized by the carbothermal reduction of colloidal silica. The morphology and microstructure of NWs have been studied in detail by electron microscopy techniques. SiC NWs have been found to be hexagonal prisms, “bamboo-like” nanorods and nanobelts. The NWs with a [111] growth axis are hexagonal prism nanorods, while the nanobelts have growth directions varying from [110] to [113]. It has been found that NW growth proceeds in two stages. Initially, SiC crystallites grow on the carbon fiber surface. These crystallites serve as seeds, on which the SiC NWs nucleate and grow. The crystallites containing microtwins and stacking faults (SFs) with a preferential [111] growth direction give rise to the growth of nanorods, while the nanobelts start growing on the (111) facets of relatively perfect crystallites. Wires with core (SiC)–shell (SiO2) structure have been obtained under special temperature treatment in air. The core–shell structure has been confirmed by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) mapping techniques.

Mechanochemical synthesis of nonstoichiometric nanocrystals La1 − yCayF3 − y with a tysonite structure and nanoceramic materials from CaF2 and LaF3 crystals

The nonstoichiometric phases La1 − yCayF3 − y (y = 0.15, 0.20) with a tysonite (LaF3) structure have been prepared for the first time by the mechanochemical synthesis from CaF2 and LaF3 crystals. The average size of coherent scattering regions is approximately equal to 10–30 nm. It has been shown that the compositions of the phases prepared by the mechanochemical synthesis are inconsistent with the phase diagram of the CaF2-LaF3 system. The “mechanohydrolysis” of the La1 − yCayF3 − y phase has been observed for the first time. Under these conditions, the La1 − yCayF3 − y phase partially transforms into lanthanum calcium oxyfluoride for a milling time of 180 min with intermediate sampling. The La1 − yCayF3 − y nanoceramic materials have been prepared from a powder of the mechanochemical synthesis product by pressing under a pressure of (2–6) × 108 Pa at room temperature. The electrical conductivity of the synthesized materials at a temperature of 200°C is equal to 4.9(6) × 10−4 S/cm, and the activation energy of electrical conduction is 0.46(2) eV. These data for the nanoceramic materials coincide with those obtained for migration of fluorine vacancies in single-crystal tysonite fluoride materials.

The effect of self-ions bombardment on the structure and properties of thin metal films

Metal films were deposited by the partially ionized beam technique, using a source with high ionization efficiency. Some characteristics of the source were investigated. Metals with low, medium and high melting temperatures were used for deposition. It was found that ionization efficiency of low-melting metals is higher than that of metals with medium and high-melting temperatures. The effect of the amount of self-ions in the beam of a deposited material as well as the effect of their energy on the structure and properties of the resulting films were studied.