Niobium Films Research Articles

A Niobium and Nitrogen Co-Doped DLC Film Electrode and Its Electrochemical Properties

Niobium (Nb) and nitrogen (N) co-doped diamond-like carbon (DLC) film ((Nb:N)-DLC) was fabricated by reactive magnetron sputtering. Atomic force microscopy, Raman spectra and X-ray photoelectron spectroscopy were used to characterize the morphology, microstructure, and composition of the (Nb:N)-DLC film. The (Nb:N)-DLC film has typical DLC characteristics with enlarged and uniform specific surface area. Electrochemical tests show that the proposed co-doped DLC electrode not only presents fast apparent rate constant (1.92 × 10−2 cm s−1), low charge transfer resistance (13.86 Ω cm2), but also possesses excellent electrochemical reversibility and stability in K3Fe(CN)6 electrolyte. Furthermore, the (Nb:N)-DLC electrode was applied to the determination of dopamine (DA) and an apparent current response to a low concentration of 0.5 μM was obtained by differential pulse voltammetry.

Magnetic proximity effect and superconducting triplet correlations at the cuprate superconductor and oxide spin valve interface

A heterostructure consisting of a cuprate superconductor YBa2Cu3O7-δ and a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve was studied using SQUID magnetometry, ferromagnetic resonance, and neutron reflectometry. It is shown that because of the magnetic proximity effect a magnetic moment is excited in the superconducting portion of the heterostructure, whereas the magnetic moment in the spin valve becomes suppressed. The experimentally obtained value of a typical penetration depth of a magnetic moment into the superconductor is significantly greater than the coherence length of the cuprate superconductor, which indicates that the induced magnetic moment mechanism of Cu atoms is dominant. The mesastructure prepared by adding niobium film as a second superconducting electrode to the existing heterostructure, exhibited a superconducting current (dc Josephson effect) at interlayer thicknesses that are much greater than the coherence length of the ferromagnetic materials. The maximum of the critical current density dependence on the thickness of the spin valve material corresponds to the interlayer coherence length, which agrees with the theoretical predictions associated with spin-triplet pairing. The superconducting current is observed at magnetic fields that are two orders of magnitude greater than the field corresponding to the occurrence of one magnetic flux quantum in the mesastructure. The ratio of the second harmonic of the current-phase dependence of the mesastructure superconducting current to the first, determined according to the dependence of the Shapiro steps on the amplitude of microwave exposure, did not exceed 50%.

Performance analysis of superconducting rf cavities for the CERN rare isotope accelerator

The first cryomodule of the new HIE-ISOLDE rare isotope accelerator has recently been commissioned with beam at CERN, with the second cryomodule ready for installation. Each cryomodule contains five superconducting low-beta quarter wave cavities, produced with the technology of sputtering a thin niobium film onto the copper substrate ($\mathrm{Nb}/\mathrm{Cu}$). This technology has several benefits compared to the bulk niobium solution, but also drawbacks among which the most relevant is the increase of surface resistance with accelerating field. Recent work has established the possible connection of this phenomenon to local defects in the $\mathrm{Nb}/\mathrm{Cu}$ interface, which may lead to increased thermal impedance and thus local thermal runaway. We have analyzed the performance of the HIE-ISOLDE cavities series production, as well as of a few prototypes', in terms of this model, and found a strong correlation between the rf properties and one of the model characteristic quantities, namely the total surface having increased interface thermal impedance.

Diamond magnetometry of Meissner currents in a superconducting film

We study magnetic field penetration into a thin film made of a superconducting niobium. Imaging of magnetic field is performed by optically detecting magnetic resonances of negatively charged nitrogen-vacancy defects inside a single crystal diamond, which is attached to the niobium film under study. The experimental results are compared with theoretical predictions based on the critical state model, and good agreement is obtained.

Microstructure and nanomechanical properties of magnetron sputtered Ti − Nb films

Abstract Titanium niobium (Ti − Nb) films, covering a broad spectrum of compositions, are deposited on silicon substrates using magnetron co-sputtering. The morphological, crystallographic and mechanical characteristics of the films are studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and nanoindentation, respectively. The Ti − Nb films exhibit a columnar growth and contain porosity at volumetric percentages of 2–14 vol%. Furthermore, it is observed that the allotropic β − Ti phase can be stabilized at room temperature for Nb compositions beyond 20 at%. Phase composition and microstructural details have subsequent implications on the mechanical properties of the films which are studied experimentally and micromechanically. Among the tested compositions, Ti 85 Nb 15 exhibits the lowest elastic modulus. The obtained structure–property relations could serve as a tool for material optimization.

Nanostructured Anodic Oxide Films of Niobium: Features of Electrochemical Formation, Functional Properties and Applications (Review)

The review summarizes data on the anodic behavior of niobium in aqueous solutions. Features of electrochemical formation nanostructured oxide coatings on niobium by anodic oxidation are systematized. The article deals with theoretical aspects formation of the porous anodic oxide layers. The influence of process parameters and the characteristics of the electrolyte on layer of niobium oxide were analyzed. The functional properties of porous coating on niobium are considered and promising areas of practical application identified.

Structural and nanomechanical characterization of niobium films deposited by DC magnetron sputtering

Nb thin films were deposited onto Si wafers by direct current (DC) magnetron sputtering at different deposition pressures. The microstructure and nanomechanical properties of Nb films were investigated by scanning electron microscope, X-ray diffractometer, transmission electron microscope, atomic force microscope and nanoindenter. The results revealed that the grain size, thickness, surface roughness, the reduced elastic modulus (Er) and hardness (H) values of Nb thin films increased at the pressure range of 0.61–0.68 Pa. Meanwhile, the porosity of Nb films decreased with the increase in deposition pressure. The lattice deformation of Nb thin films changed from negative to positive with the increase in deposition pressure. It is concluded that deposition pressure influences the microstructure and nanomechanical properties of Nb films.

Characterization of Superconducting Thin Films and nanoSQUIDs for Nanoparticle Investigation at High Magnetic Field

We present an experimental investigation aimed to fabricate a nanoSQUID for high-magnetic-field applications. In particular, we produced niobium films having different thicknesses (5, 10, and 15 nm) with passivation layers to investigate the transport properties of very thin niobium film in high magnetic field parallel to the film plane. The results indicate that niobium thin films are superconducting at high field; however, the passivation layers are not sufficient to avoid a degradation of superconducting properties when niobium films are cycled between room and cryogenic temperatures. We also fabricated and measured a nanosensor based on niobium nitride. Niobium nitride films have much higher magnetic critical field, and they are more stable compared with niobium ones. The characterization of the nanodevice in the temperature range from 2 to 7 K includes measurements of current–voltage and critical current versus magnetic flux characteristic. Albeit the coherence length of niobium nitride is very short compared with the niobium one, and much smaller than the Dayem bridge length, the nanoSQUIDs present modulations of critical current larger than 5% at 2 K. The modulation can be observed up to 8 T, making NbN nanoSQUIDs good candidates to measure magnetic properties of magnetic nanoparticles and nanostructures.

Spectral identification scheme for epitaxially grown single-phase niobium dioxide

This publication describes how to distinguish under-oxidized and over-oxidized phases of the metal-insulator transition material NbO2 by employing a combination of the Nb 3d and O 1s core-level and valence-band spectra. Niobium oxides (NbOx) are grown under various conditions by molecular beam epitaxy on several different substrates, mostly (111)-oriented strontium titanate (SrTiO3), in order to obtain phase-pure NbO2. The films are characterized by in situ reflection high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy to determine crystallinity, lattice constants, and epitaxial relationships. Niobium metal oxidation states and film stoichiometry are determined with in situ X-ray photoelectron spectroscopy. Asymmetric Nb 3d core-level spectral components are observed in phase-pure NbO2, with a binding energy for Nb 3d5/2 of 206 eV. The effect of substrate type, growth temperature, and oxygen pressure on the film composition is also described.

Niobium Films as Rewritable Imaging Media

Niobium Films as Rewritable Imaging Media

Temperature dependences of transport properties of films, bulk samples of nanocrystals, and single crystals of niobium triselenide

The temperature dependences of transport properties of bulk samples of nanocrystals of niobium triselenide (NbSe3) and films prepared from its colloid dispersions have been investigated. The results have been compared with the data for NbSe3 single crystals. Changes in positions of maxima of the resistivity ρ(T) at 145 and 59 K, which are associated with charge density waves (CDWs), by ∼1 K as compared to those observed for NbSe3 single crystals are found for bulk samples of nanocrystals. The main contribution to resistivity ρ(T) in film samples is introduced by resistance of contacts between nanocrystals, the temperature dependence of which is described by a model of the fluctuation-induced tunneling conduction. Contributions to σ(T) = 1/ρ(T) from CDWs at a level of 10% are observed in the vicinity of temperatures of 145 and 59 K. However, in contrast to bulk samples of nanocrystals, these peaks for films not only shift downward by temperature but also broaden approximately twofold. These variations are associated with a decrease in transverse size of nanocrystallites in the films to magnitudes smaller than the CDW phase coherence length (∼1 µm).

Field dependent surface resistance of niobium on copper cavities

The surface resistance Rs of superconducting cavities prepared by sputter coating a thin niobium film on a copper substrate increases significantly stronger with the applied RF field compared to cavities of bulk material. A possible cause is that due to the thermal boundary resistance between the copper substrate and the niobium film Rs is enhanced due to global heating of the inner cavity wall. Introducing helium gas in the cavity and measuring its pressure as a function of applied field allowed to conclude that the inner surface of the cavity is heated up by only 60+/-60 mK when Rs increases with Eacc by 100 nOhm. This is more than one order of magnitude less than what one would expect from global heating. Additionally the effect of cooldown speed and low temperature baking have been investigated in the framework of these experiments. It is shown that for current state of the art niobium on copper cavities there is only a detrimental effect of low temperature baking. A fast cooldown results in a lowered Rs.

Modulation of superconducting critical temperature in niobium film by using all-solid-state electric-double-layer transistor

An all-solid-state electric-double-layer transistor (EDLT) was fabricated for electrical modulation of the superconducting critical temperature (Tc) of Nb film epitaxially grown on α-Al2O3 (0001) single crystal. In an experiment, Tc was modulated from 8.33 to 8.39 K while the gate voltage (VG) was varied from 2.5 to −2.5 V. The specific difference of Tc for the applied VG was 12 mK/V, which is larger than that of an EDLT composed of ionic liquid. A Tc enhancement of 300 mK was found at the Li4SiO4/Nb film interface and is attributed to an increase in density of states near the Fermi level due to lattice constant modulation. This solid electrolyte gating method should enable development of practical superconducting devices highly compatible with other electronic devices.

Achieving reversibility of ultra-high mechanical stress by hydrogen loading of thin films

Nano-materials are commonly stabilized by supports to maintain their desired shape and size. When these nano-materials take up interstitial atoms, this attachment to the support induces mechanical stresses. These stresses can be high when the support is rigid. High stress in the nano-material is typically released by delamination from the support or by the generation of defects, e.g., dislocations. As high mechanical stress can be beneficial for tuning the nano-materials properties, it is of general interest to deduce how real high mechanical stress can be gained. Here, we show that below a threshold nano-material size, dislocation formation can be completely suppressed and, when delamination is inhibited, even the ultrahigh stress values of the linear elastic limit can be reached. Specifically, for hydrogen solved in epitaxial niobium films on sapphire substrate supports a threshold film thickness of 6 nm was found and mechanical stress of up to (−10 ± 1) GPa was reached. This finding is of basic interest for hydrogen energy applications, as the hydride stability in metals itself is affected by mechanical stress. Thus, tuning of the mechanical stress-state in nano-materials may lead to improved storage properties of nano-sized materials.

A Simple GHz Resonator for Superconducting Materials Characterization

This work examines the design and operation of a longitudinal resonant cavity, paired with monopole send and reciprocal patch receive antennae, that couples radio-frequency energy to a superconducting thin film carrying high current densities (~10 5 A/cm 2 ). The dielectric substrate supporting the film penetrates the waveguide, which operates in an evanescent mode below the design cutoff frequency of 18 GHz. Oscillatory vortex motion in the thin film is found to produce a small (~0.1 mV) dc voltage. When the niobium film is patterned to form an aperture that permits resonant conditions within the waveguide volume, the measured voltage increases by an order of magnitude. The increase is explained in the framework of the Larkin-Ovchinnikov model for quasiparticle behavior inside a moving normal vortex core. Operated near the superconducting transition, this device is useful for materials characterization, including the possibility to extract parameters including the pinning force. The authors suggest that the device could be used to characterize the pinning potential or to explore quasiparticle dynamics in superconducting thin films.

Aging of Ultra-Thin Niobium Films

We characterize the evolution of the electrical properties of ultra-thin niobium films stored in ambient conditions over a period of approximately seven months. Patterned films with thicknesses between 8 and 16 nm were fabricated via electron-beam deposition on unheated silicon substrates using a lift-off process. The film quality is similar to previous results obtained with sputter deposition onto unheated silicon substrates. The increase of the resistance and the decrease of the superconducting critical temperature are well described by an exponential function with a time constant of approximately 37 days.

Special pinning phenomena in arrays of defects with gradient spatial distributions on niobium film

Flux pinning effect is related to the strong influence by the presence of defects that act as pinning centers. We present transport experiments that investigate the pinning properties in a spacing-graded array of pinning sites. We have found the asymmetric pinning by arrays of defects with modulated gradient spatial distributions on niobium film. One can clearly see the asymmetric pinning effect for the vortex motion. The dc voltage depends on the amplitude of the ac current and that there is a sharp maximum at matching fields. This curve, however, has an interesting sign reversal phenomenon below the first matching field, which will be investigated more in the future. The dc voltage as a function of dc and ac applied current gives evidence that the artificial arrays of gradient pinning site density induce ratchet effect. This effect can be used to control the vortex motion, which will be important for the application of vortex based on electronic devices.

Nanostructures of Tungsten Trioxide, Nickel Oxide and Niobium Oxide for Chemical Sensing Applications

Tungsten trioxide nanowires, nickel oxide nanowires and niobium oxide nanostructures weresynthesized by thermal oxidation, vapour-liquid-solid (VLS) technique and hydrothermal method,respectively. Alumina (2 · 2 mm2) substrates were used for growth of the different metal oxidesnanostructures. Thin films of tungsten or niobium are used as source material for WO3 and Nb2O5. Anultrathin film of gold was deposited on alumina as catalyst for NiO. All these materials were depositedby RF magnetron sputtering. The morphology was investigated by scanning electron microscope andthe functional properties of these structures were tested towards several gaseous species at differentworking temperatures. The results obtained show that these sensing materials have similarperformances considering the different n- and p-type semiconductors behaviour. Tungstentrioxide (WO3) nanowires, nickel oxide (NiO) nanowires and niobium oxide (Nb2O5) nanostructures were synthesized by thermal oxidation, vapor-liquid-solid (VLS) technique and hydrothermal method, respectively. The morphology was investigated by scanning electron microscope and the functional properties of these structures as gas sensors were tested for several gaseous species at different working temperatures. In this work a comparison between the sensitivity of these devices towards acetone is shown.

Niobium and Tungsten Oxide Nanowires for Chemical Sensor

Thin films of tungsten and niobium (WNb) were the starting point for the growth of metal oxide nanowires. RF Magnetron sputtering was used to deposit a layer of 100nm on 2 · 2 mm2 alumina substrates. Prepared samples were thermally oxidized to obtain the growth of nanostructures whose morphology was investigated by a scanning electron microscope (SEM). Structural properties of these nanostructures were investigated by RAMAN spectroscopy. Functional characterization was carried out using a homemade test chamber. Interdigitated contacts and heating elements were deposited by DC Magnetron sputtering, and samples were bonded to TO packages thanks to gold wires. The gas sensing properties of samples were analyzed in a wide range of working temperatures for different target gases in order to verify the best working condition.

Superconductive niobium films coating carbon nanotube fibers

Superconducting niobium (Nb) has been successfully obtained by sputter deposition on carbon nanotube fibers. The transport properties of the niobium coating the fibers are compared to those of niobium thin films deposited on oxidized Si substrates during the same deposition run. For niobium films with thicknesses above 300 nm, the niobium coating the fibers and the thin films show similar normal state and superconducting properties with critical current density, measured at T = 4.2 K, of the order of 105 A cm−2. Thinner niobium layers coating the fibers also show the onset of the superconducting transition in the resistivity versus temperature dependence, but zero resistance is not observed down to T = 1 K. We evidence by scanning electron microscopy (SEM) and current-voltage measurements that the granular structure of the samples is the main reason for the lack of true global superconductivity for thicknesses below 300 nm.