Vanadium Layers Research Articles

Growth of Vanadium and Vanadium Oxide on a Rh(110) Surface

We have studied the epitaxial growth of thin layers of vanadium and vanadium oxide on a Rh(110) surface with low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). Up to six monolayers of V/VOx were deposited with temperatures varying between 370 and 770 K. For V deposition, (1 × n) and (n × 1) overlayers with n = 2 and 4 are observed. For the deposition of 3 MLE of VOx, a (12 × 1) structure is obtained as a stable structure. XPS of V 2p3/2 reveals the simultaneous presence of an oxidic and a metallic or strongly reduced (V2+) V component.

Strain in Vanadium Thin Film due Nanosecond Laser Treatment

Strain in Vanadium Thin Film due Nanosecond Laser Treatment A thin coating of vanadium was grown on a glass substrate using vacuum coating technique. Piezoelectric thickness monitor was used to measure the thickness of the deposited coatings in-situ. The vanadium layer was then treated with a nanosecond pulsed fiber laser source in ambient conditions, and this resulted in the surface cracking due to heat dissipation from the laser. The density of cracks was observed to increase in a direct proportion to the laser fluence. The increase in the density of the cracks was accompanied by the surface modification of the of the vanadium layer as observed in the high-resolution scanning electron microscope. At higher laser fluence there are pinholes forming on the surface of the laser, and this is due to the laser punching and taking place at those experimental conditions.

Mathematical simulation of atomic hydrogen diffusion transfer through a multilayer metal membrane at finite pressures

Diffusion transfer of atomic hydrogen through multilayer metal membranes has been studied within the lattice model of an ideal gas, with the transfer being described by a set of nonlinear algebraic equations. It has been shown that, for multilayer membranes composed of less than four layers, an analytical expression describing a diffusion flux can be derived. Atomic hydrogen transfer through a membrane consisting of a vanadium layer, the surfaces of which are coated with palladium films, has been analyzed in detail. It has been found that the value of the flux may depend on the transfer direction. The effect of diffusion asymmetry arises at finite pressures of hydrogen on the outer membrane surfaces, when its dissolution in metals is described by nonlinear sorption isotherms. The degree of the diffusion asymmetry increases with a rise in hydrogen pressure and depends on the arrangement of the layers composing a membrane.

Perpendicular Anisotropy in Heusler Alloy Layers Induced by a V Seed Layer

We have studied the effects of a seed layer of vanadium on the anisotropy of the full-Heusler alloy Co2FeSi (CFS) for application as a perpendicularly anisotropic ferromagnetic layer for a current-perpendicular-to-plane giant magnetoresistance device. Perpendicular magnetic anisotropy (PMA) was estimated from the susceptibility in the linear magnetizing region of hysteresis loops for various layer thicknesses of CFS. Introduction of a second vanadium interface with the CFS was found to increase the observed PMA to a value of $K = 3.4\times 10^{4}$ erg/cm3. Basic spin-valve structures were fabricated to investigate the effect of spacer layers on two layers of CFS.

Enhancement of Photovoltaic Performance by Utilizing Readily Accessible Hole Transporting Layer of Vanadium(V) Oxide Hydrate in a Polymer-Fullerene Blend Solar Cell.

Herein, a successful application of V2O5·nH2O film as hole transporting layer (HTL) instead of PSS in polymer solar cells is demonstrated. The V2O5·nH2O layer was spin-coated from V2O5·nH2O sol made from melting-quenching sol-gel method by directly using vanadium oxide powder, which is readily accessible and cost-effective. V2O5·nH2O (n ≈ 1) HTL is found to have comparable work function and smooth surface to that of PSS. For the solar cell containing V2O5·nH2O HTL and the active layer of the blend of a novel polymer donor (PBDSe-DT2PyT) and the acceptor of PC71BM, the PCE was significantly improved to 5.87% with a 30% increase over 4.55% attained with PSS HTL. Incorporation of V2O5·nH2O as HTL in the polymer solar cell was found to enhance the crystallinity of the active layer, electron-blocking at the anode and the light-harvest in the wavelength range of 400-550 nm in the cell. V2O5·nH2O HTL improves the charge generation and collection and suppress the charge recombination within the PBDSe-DT2PyT:PC71BM solar cell, leading to a simultaneous enhancement in Voc, Jsc, and FF. The V2O5·nH2O HTL proposed in this work is envisioned to be of great potential to fabricate highly efficient PSCs with low-cost and massive production.

Correlation between interlayer exchange coupling and hydrogen absorption in V-Fe layered structures

Abstract 4 nm – Fe/d V -V/4 nm – Fe trilayers were prepared at room temperature using UHV (5 × 10 − 10 mbar) magnetron sputtering. A capping layer of 5 nm Pd was used to allow a fast uptake and release of hydrogen and to avoid oxidation of the top Fe sublayers. As a substrate we have used Si(100) wafers with an oxidised surface. The cleanness and planar growth of all layers was checked in-situ , immediately after deposition by X-ray Photoelectron Spectroscopy. Planar growth was also revealed in ex-situ studies by Atomic Force Microscopy and Scanning Tunnelling Microscopy. In the case of the trilayers the bottom and the top Fe sublayers were deposited in different deposition conditions to obtain relatively high difference in their coercive fields. Interlayer exchange coupling across vanadium was studied for “as prepared”, hydrogenated and dehydrogenated samples using minor hysteresis loop measurements. We have observed the strongest antiferromagnetic (AFM) interlayer exchange coupling energy of about 0.023 mJ/m 2 for hydrogenated sample. Results from the minor hysteresis loop measurements as a function of vanadium layer thickness for “as prepared” or dehydrogenated samples show three AFM peaks near d V ~ 1.95, 2.85, and 3.9 nm without transition to pure ferromagnetic coupling. Hydrogenation of the trilayers changes reversibly the interlayer exchange coupling strength and position of the 2nd and 3rd AFM peak.

Enhancement of Photovoltaic Performance by Utilizing Readily Accessible Hole Transporting Layer of Vanadium(V) Oxide Hydrate in a Polymer–Fullerene Blend Solar Cell

Enhancement of Photovoltaic Performance by Utilizing Readily Accessible Hole Transporting Layer of Vanadium(V) Oxide Hydrate in a Polymer–Fullerene Blend Solar Cell

Electron affinity of cubic boron nitride terminated with vanadium oxide

A thermally stable negative electron affinity (NEA) for a cubic boron nitride (c-BN) surface with vanadium-oxide-termination is achieved, and its electronic structure was analyzed with in-situ photoelectron spectroscopy. The c-BN films were prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition employing BF3 and N2 as precursors. Vanadium layers of ∼0.1 and 0.5 nm thickness were deposited on the c-BN surface in an electron beam deposition system. Oxidation of the metal layer was achieved by an oxygen plasma treatment. After 650 °C thermal annealing, the vanadium oxide on the c-BN surface was determined to be VO2, and the surfaces were found to be thermally stable, exhibiting an NEA. In comparison, the oxygen-terminated c-BN surface, where B2O3 was detected, showed a positive electron affinity of ∼1.2 eV. The B2O3 evidently acts as a negatively charged layer introducing a surface dipole directed into the c-BN. Through the interaction of VO2 with the B2O3 layer, a B-O-V layer structure would contribute a dipole between the O and V layers with the positive side facing vacuum. The lower enthalpy of formation for B2O3 is favorable for the formation of the B-O-V layer structure, which provides a thermally stable surface dipole and an NEA surface.

Computational screening of alloying elements for the development of sustainable V-based hydrogen separation membranes

Abstracts First-principles calculations are performed to derive promising alloying elements which inhibit inter-diffusion between vanadium and palladium-coating layers in vanadium-based hydrogen separation membranes. Paying attention to the inhibition of inter-diffusion by the grain boundary segregation of impurities, several physical features of various solute elements, grain boundary segregation, grain boundary embrittlement and solute–solute binding tendencies, are evaluated to construct a large database. Several alloying elements and their combinations similar to or even better than a previously reported yttrium are suggested for future experimental works by comprehensively analyzing these physical features.

Investigation of V-Ti/Al/Ni/Au Ohmic contact to AlGaN/GaN heterostructures with a thin GaN cap layer

In this paper, we report on a novel V-Ti/Al/Ni/Au metal stack as an Ohmic contact to AlGaN/GaN heterostructures with a thin GaN cap layer. The thin vanadium layer (10 nm) is employed as the first layer under the conventional Ti/Al/Ni/Au contact. This layer is shown to play a crucial role in achieving low specific contact resistance (ρc) and smooth surface morphology. The V-Ti/Al/Ni/Au contact exhibits significantly improved surface roughness when compared with Ti/Al/Ni/Au contacts with a root mean square value of 11 nm. The specific contact resistivity is measured to be 2.3 × 10−6 Ω cm2. According to cross-sectional transmission electron microscopy (TEM) analysis, very limited reaction is observed between the V-Ti/Al/Ni/Au contact and the AlGaN surface. It is demonstrated that the thin vanadium layer prevents excessive formation of TiN protrusions, which is considered to be the key to improving surface morphology with low contact resistance.

X-ray fluorescence evaluation of the surface density of vanadium nanolayers on various substrates

An auxiliary system in the form of thin vanadium layers on polymer film substrates has been used in surface density determinations. We have calculated correction coefficients that take into account the absorption of the primary radiation of an X-ray tube and that of analytical lines of the elements present in the substrate. A technique has been developed that allows one to determine the surface density of thin films when samples and references are located on substrates from different materials.

Strong and Weak Interlayer Exchange Coupling in V/Fe Multilayers

The dV-V/0.6 nm-Fe multilayers with constant thickness sublayers were prepared onto naturally oxidised Si(100) substrate using UHV (5× 10−10 mbar) DC/RF magnetron sputtering. Results showed that the saturation eld of the V/Fe multilayers oscillate with antiferromagnetic interlayer exchange coupling peaks near the V spacer thickness of about 1.3, 1.6, 2.05 nm. Furthermore, all the samples with vanadium layer thickness near local maxima of the antiferromagnetic coupling show zero remanence. The short period of the antiferromagnetic peak oscillations is due to indirect Ruderman Kittel Kasuya Yosida (RKKY)-type coupling of the Fe layers across vanadium spacer. The absence of the antiferromagnetic peaks in the very strongly coupled region (below dV ≈1 nm) could be explained by direct ferromagnetic exchange coupling of the Fe layers due to magnetic polarization of V atoms near V Fe interface.

Impacts of vanadium and coke deposits on the CO2 gasification of nickel catalysts supported on activated carbon from petroleum coke

One of the ways to dispose of spent carbon-based catalysts is gasification. In this study, the kinetics of CO2 gasification of Ni/activated carbon (AC) catalysts, produced from petroleum coke, were investigated at 700–900°C using a thermogravimetric analyzer (TGA). The catalysts were coked by exposure to ethylene at 400–600°C and/or impregnated with 3 or 8wt% vanadium, which is typically deposited during hydrodemetalization (HDM) of heavy oil. The gasification rates of all samples were 30–400 times greater than the rate of gasification of uncatalyzed (i.e., no metal) AC. Rate profiles as a function of conversion were different for all samples, which suggested that nickel was promoted/inhibited as a gasification catalyst in different ways depending on the composition of the sample. Transmission electron microscopy was used to analyze samples before and after gasification. The particle morphologies and chemical analysis (energy-dispersive X-ray) help to explain the different gasification behaviors. In particular, the nickel particles on the coked Ni/AC sample became encapsulated by graphite and sintered over time from 20nm initially to 150nm after 80% conversion (∼10min at 900°C). For the Ni/AC catalysts impregnated with vanadium, the AC support was decorated with ∼10nm nickel particles and wetted with a layer of vanadium after preparation at 500°C. After heating to 900°C, however, sintering of both metals was evident as well as the formation of new vanadium species including vanadium carbide in some areas. The interaction between nickel and vanadium lowered the initial gasification rate but also slowed the deactivation of the nickel during gasification.

Fabrication of Tungsten-Vanadium Hybrid Material with Sufficient Toughness for High-Temperature Applications by Diffusion Bonding

The design of fusion plasma-facing components is challenging, as their materials have to meet rigorous requirements in terms of low activation and high-temperature strength. At the same time, sufficient ductility is required even in the low-temperature range. Unfortunately, these properties are not found in conventional materials. To solve this problem, a hybrid material that combines the high strength of one material with the high ductility of the other material was developed. This paper presents the hybrid material, which consists of thin tungsten and vanadium layers. This hybrid material was produced by means of diffusion bonding at relatively low temperature in a vacuum chamber. Microstructural investigations and nanoindentation tests indicated no cracks, no delamination, and no brittle intermetallic phases along the bond interfaces. Investigations of the mechanical properties of the hybrid material by instrumented Charpy impact tests revealed a relatively low ductile-to-brittle transition temperature (DBTT) at 124°C (compared to the DBTT of polycrystalline tungsten of >441°C) with an absorbed Charpy impact energy of 4.53 J [kleinst (KLST)-specimen]. Additionally, the tested Charpy impact specimens were found to be not fractured thoroughly even at room temperature.

Proximity effects on H absorption in ultrathin V layers

We discuss proximity effects on the hydrogen uptake in 14 atomic layers of vanadium. The enthalpy and entropy of solution were measured and compared to ab initio density functional calculations. We show that there exists a large difference in the hydrogen uptake of V in Cr/V and Fe/V superlattices, in which the V is under close to identical strain states. The calculations show that neither local strain effects nor charge redistributions can be the cause for the observed effect. This leaves magnetic and long ranged elastic strain fields---neither captured by the current calculations---as possible mechanisms for the observed effects.

Magnetization and Mössbauer study of partially oxidized iron cluster films deposited on HOPG

Iron clusters produced in a laser vaporization source were deposited to form cluster-assembled thin films with different thicknesses on highly oriented pyrolytic graphite substrates. The development of oxidation of the clusters with time, up to three years, was investigated by magnetic measurements using an alternating gradient magnetometer. Furthermore, to receive information about the oxidation states, clusters of 57Fe were studied using Mössbauer spectroscopy. The magnetic analysis shows a time evolution of the saturation magnetization, remanence, and coercivity, determined from the hysteresis curves characteristic of a progressing oxidation. The different thicknesses of the iron cluster films as well as a protective layer of vanadium influence the magnetic properties when the samples are subjected to oxidation with time. While the saturation magnetization and remanence decrease and reach half the initial values for almost all the samples after three years, the coercivity increases for all samples and is more substantial for the thickest sample with a vanadium protective layer. This value is three folded after three years. Furthermore, based on a core–shell model and using the saturation magnetization values we have been able to quantitatively calculate the amount of the increase of Fe-oxide as a function of time. The Mössbauer spectroscopy shows peaks corresponding to iron metal and maghemite.

Electrical properties and microstructure of vanadium-based contacts on ICP plasma etched n-type AlGaN:Si and GaN:Si surfaces

Light-emitting diodes emitting in the UV-B spectral range usually contain an n-type AlxGa1–xN contact layer with x of about 0.4 and require a low specific contact resistivity in the range of 10−6 Ω cm2 to operate at their optimal capacities. We compare the Ti/Al/Mo/Au metal system commonly used on GaN with the V/Al//V/Au metal system on n-Al0.4Ga0.6N:Si and GaN:Si surfaces. On Al0.4Ga0.6N, the lowest specific contact resistivity, (2.3 ± 0.2) ×10−6 Ω cm2, was reached with V/Al/V/Au whereas for Ti/Al/Mo/Au the lowest resistivity was (1.7 ± 0.7) ×10−3 Ω cm2. Independently of the metal system, lower contact resistivities are obtained on GaN where Ti/Al/Mo/Au is still performing at least one order of magnitude better than V/Al/V/Au. The contact resistivity of V/Al/V/Au on Al0.4Ga0.6N was found to be sensitive to surface treatments (wet chemical etching or sputtering) applied prior to metal deposition, to the thickness of the first vanadium layer and to that of the aluminum layer above. Structural and compositional investigations showed that a low contact resistivity is accompanied by the formation of an interfacial layer between AlGaN and V/Al/V/Au on the nm-scale, which contains aluminum, in the form of aluminum nitride, and separately vanadium as metal or metal nitride.

Controlling Exchange Coupling Strength in Ni x Cu1−x Thin Films

The thickness (d F) and concentration (x) dependence of the Curie temperature of Ni x Cu1−x (d F) ferromagnetic alloy layers (x=0.55, 0.65, d F=[4.5 nm÷12 nm]) in contact with a vanadium layer was studied. The Curie temperature of the ferromagnetic layers depends on the thickness when it is comparable with the transition layer between the F and the vanadium layers, which is attributed to the proximity coupling of the interface region with the rest of the F layer. The present study provides valuable information for fabrication of samples with controlled exchange coupling strength for studies of superconductor/ferromagnet proximity effects.

Investigation of vanadium and nitride alloys thin layers deposited by PVD

In this work we present the technique of magnetron vapor deposition and the effect of several deposition parameters on the structural and morphological properties of prepared thin films. It was noted that the deposition time has an effect on the crystallinity, mechanical properties such as residual stress, roughness surface and the layer composition from target products. Studies were carried out on layers of vanadium (V) and the nitride vanadium (VN).

Corrosion protection properties of vanadium films formed on zinc surfaces

Vanadium films were prepared on zinc surfaces by using a solution containing vanadate. Corrosion protection properties of vanadium-treated (V-treated), chromium-treated (Cr-treated), and untreated zinc surfaces in contact with a 3.5 wt.% NaCl solution were studied using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and neutral salt spray (NSS) tests. According to these results, the V-treated layer significantly improved the corrosion resistance of zinc surfaces. In comparison with the Cr-treated layer, the V-treated layer exhibited a better corrosion resistance. The composition of the V-treated layer was studied using X-ray photoelectron spectroscopy (XPS). XPS measurements indicated that the vanadium layer formed on zinc surfaces and the vanadium-rich coating was a hydrated oxide with a composition of V2O5, VO2, and its hydrates such as V2O5·nH2O and VO(OH)2.