Thin Co Films Research Articles

Co–B nanoparticles supported on carbon film synthesized by pulsed laser deposition for hydrolysis of ammonia borane

Thin films of Carbon-supported Co–B nanoparticles were synthesized by using Pulsed Laser Deposition (PLD) and used as catalysts in the hydrolysis of Ammonia Borane (AB) to produce molecular hydrogen. Amorphous Co–B-based catalyst powders, produced by chemical reduction of cobalt salts, were used as target material for nanoparticles-assembled Co–B film catalysts preparation through PLD. Various Ar pressures (10–50 Pa) were used during deposition of carbon films to obtain extremely irregular and porous carbon support with high surface area prior to Co–B film deposition. Surface morphology of the catalyst films was studied using Scanning Electron Microscopy, while structural characterization was carried out using X-Ray diffraction. The hydrogen generation rate attained by carbon-supported Co–B catalyst film is significantly higher as compared to unsupported Co–B film and conventional Co–B powder. Almost complete conversion (95%) of AB was obtained at room temperature by using present film catalyst. Morphological analysis showed that the Co–B nanoparticles produced after the laser ablation process act as active catalytic centers for hydrolysis while the carbon support provides high initial surface area for the Co–B nanoparticles with better dispersion and tolerance against aggregation. The efficient nature of our carbon-supported Co–B film is well supported by the obtained very low activation energy (∼29 kJ (mol)−1) and exceptionally high H2 generation rate (13.5 L H2 min−1 (g of Co)−1) by the hydrolysis of AB.

Exchange bias in nanopatterned Co antidots prepared by self-assembling polystyrene nanospheres

The exchange bias properties of nanopatterned thin films of Co, on top of which a native Co-oxide layer develops spontaneously, are studied by means of magnetic and magneto-resistance measurements. Both continuous and patterned films are investigated, the latter in the form of antidot arrays prepared with the self-assembling polystyrene nanospheres technique. The obtained antidot arrays are in the hexagonal close-packed configuration and cover a surface area of several square millimetres. Nanopatterned samples turn out to have a very good repeatability of their magnetic and magneto-resistive properties. The presence of a native oxide is responsible for the development of an exchange bias effect at temperatures below ~150 K, which has been reported both on hysteresis loops and on magneto-resistance curves; these consist of a superposition of an anisotropic magneto resistance (AMR) effect and a giant magneto-resistance (GMR)-like effect. The determination of the bias field by means of the two different sets of data is consistent and gives a complete picture of the phenomenology in this kind of nanopatterned magnetic systems.

Effects of N-Doping on Magnetic Properties of ZnCoO Diluted Magnetic Semiconductor Thin Films

Radio-frequency magnetron sputtering technique was used in manufacturing thin films of Co doped and (Co, N) co-doped ZnO (Co:ZnO and Co:Zn(N, O)). The sputtering target was a commercial Zn-Co-O compound with 5% Co in weight. The sputtering gas of pure argon was used for Co:ZnO films; and two other different gases, a mixture of Ar and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (gas flow-rate ratio 15 : 5 seem) and a pure N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , was used for (Co, N):ZnO films. Without N-doping, the films show n-type electrical conduction. Upon N-doping, films become poorly conductive and even exhibit p-type conduction for pure N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sputtering-gas. Paramagnetism is observed in the film with no N-doping (Co:ZnO); but superparamagnetism and then a mixture of ferromagnetism plus paramagnetism were observed with increasing N-doping. Based on the magnetic phase diagram proposed by Coey, we explain in detail the distinct magnetic properties which are closely related with the microstructure and electrical properties of each sample.

Patterned Growth of Graphene over Epitaxial Catalyst

Rectangle- and triangle-shaped microscale graphene films are grown on epitaxial Co films deposited on single-crystal MgO substrates with (001) and (111) planes, respectively. A thin film of Co or Ni metal is epitaxially deposited on a MgO substrate by sputtering while heating the substrate. Thermal decomposition of polystyrene over this epitaxial metal film in vacuum gives rectangular or triangular pit structures whose orientation and shape are strongly dependent on the crystallographic orientation of the MgO substrate. Raman mapping measurements indicate preferential formation of few-layer graphene films inside these pits. The rectangular graphene films are transferred onto a SiO(2)/Si substrate while maintaining the original shape and field-effect transistors are fabricated using the transferred films. These findings on the formation of rectangular/triangular graphene give new insights on the formation mechanism of graphene and can be applied for more advanced/controlled graphene growth.

Influence of oxygen partial pressure on structural, transport and magnetic properties of Co doped [formula omitted] films

Pulse laser deposited (PLD) thin films of Co doped TiO 2 on silicon and quartz substrates are investigated. A mixture (1:1) of argon and oxygen with various total pressures (6.6 mPa to 53 Pa) is used to vary the oxygen content in the samples. The crystal structure and transport/ magnetic properties of Co x Ti 1− x O 2−δ ( x = 0.01 , 0.03, 0.06) thin films are found to have strong dependence on oxygen stoichiometry. X-ray diffraction (XRD) data reveal mixed phase material containing both anatase and rutile. However, the stability of each phase depends on the amount of oxygen present in the chamber during the growth of the films. X-ray Photoelectron Spectroscopy (XPS) shows the incorporation of Co in TiO 2 and is in the 2+ oxidation state. There occurs an enhancement in electrical conductivity and magnetization due to the off stoichiometric oxygen. The resistivity data follow a simple thermal activation model, giving carriers’ activation energies in the range of 20 to 140 meV. A bound magnetic polaron model is adopted to explain the observed magnetic behavior of the films.

MeV Au ion induced modifications at Co/Si interface

We report on room temperature MeV Au ion induced modifications at the Co/Si interfaces. Nanometers size thin film of Co and Si were grown by ultra high vacuum (UHV) electron beam evaporation technique on Si(1 1 1) surface and were irradiated by 1.5 MeV Au 2+ ions at a fluence of 5 × 10 14 ions cm −2. High-resolution transmission electron microscopy (HRTEM) along with energy filter imaging technique has been employed to study the formation of Co–Si alloy at the interface. Formation of such surface alloy has been discussed in the light of ion-matter interaction in nanometer scale regime.

Nano-sized Co(II)-8-hydroxyquinolate complex thin film via surface layer-by-layer chemical deposition method: Optimized factors and optical properties

An extensive study was performed and reported for evaluation and optimization of the factors affecting thin film formation of nano-sized Co(II)-8-hydroxyquinolate complex by surface layer-by-layer chemical deposition method. The formation of uniform thin films of nano-crystalline metal complex is heavily dependent on several important factors. Variation in metal and ligand concentrations (1:1–1:3) was found to show insignificant contribution to the molar stoichiometric ratio of the synthesized thin film of nano-sized Co(II)-8-hydroxyquinolate. The number of dipping cycles (2–50) was characterized by strong influence on the thin film thickness. The dependence of the immersion time (2–50 s) was proved to influence the crystal growth and homogeneity of the thin film. The role of pH of metal and ligand solutions was identified by strong contribution in development and formation of deposited Co(II)-8-hydroxyquinolate complex thin film. Finally, the role of solvent on the thin film formation was also studied and evaluated. Metal analysis, SEM, EI-MS, FT-IR and TGA were applied as monitoring techniques of these factors. The optical properties of Co(II)-8-hydroxyquinolate complex were also studied and the complex thin films were characterized by the highest optical transition from π– π∗ or n, π∗ states with energy gap in the UV-range at 3.13 eV. The lowest optical transition resulted from d–d transition or metal centered transition with energy 1.5 eV while, the optical transition at 2.35 eV is the contribution of metal ligand or ligand metal transition. In the light of the optical measurement, Co(II)-8-hydroxyquinolate complex can be considered as an organic semiconductor with the potential applications in the design of organic light-emitting diodes (OLEMs).

Magnetism in strained pseudomorphic ultrathin films of fcc 3d-transition metals (Cr, Mn, Fe, Co and Ni) with lateral lattice parameters of bulk fcc- [formula omitted

Pseudomorphic ultrathin (1–4 ML) free standing films, overlayers, and sandwiches of Cr, Mn, Fe, Co and Ni on fcc-Cu ( 0 0 1 ) are investigated using spin-polarized ab-initio calculations. All free standing thin films have quite high magnetic moment, with the exception of the 2 ML Mn film which has zero magnetic moment. Sandwiches show the same magnetic behavior as overlayers, but with reduced magnetic moments. The 3 ML overlayer of Fe has a different structure than any other overlayer structure. Sandwiches built of monolayers of Ni and Cr in Cu have zero magnetic moment. The ultrathin free standing films, overlayers and sandwiches of Cr, Mn and Fe exhibit an oscillating magnetic moment depending upon the number of layers. We attribute this to alternating ferromagnetic and antiferromagnetic coupling. On the contrary, thin films of Co and Ni always show a ferromagnetic coupling, with a magnetic moment per atom converging towards a constant value. The magnetic moment induced in Cu atoms at the interface is significant for Mn, Fe, and Co (0.02– 0.07 μ B ), while it is zero for the Ni/Cu interface.

Nanostructured p-type cobalt layered double hydroxide/n-type polymer bulk heterojunction yields an inexpensive photovoltaic cell

A low-cost, environmentally benign method was used to prepare nanostructured thin films of Co 5(OH) 8(NO 3) 2·2H 2O, a layered double hydroxide p-type semiconductor. When infilled with poly(3-butylthiophene) (P3BT), an n-type semiconducting polymer, the resulting hybrid bulk heterojunction yields a photovoltaic device. The indium-doped tin oxide/Co 5(OH) 8(NO 3) 2·2H 2O/P3BT/Al cell described here is an unprecedented example of an optoelectronic device fabricated by a low-cost biologically inspired pathway independent of organic structure-directing agents. Under illumination, this proof-of-principle device yields an open circuit voltage of 1.38 V, a short circuit current of 9 μA/cm 2, a fill factor of 26% and a power efficiency of 3.2·10 − 3 %. While the open circuit voltage of this prototype cell is close to its theoretical maximum, potential sources of the observed low efficiency are identified, and a suggested path for improvement is discussed.

The rotation of ripple pattern and the shape of the collision cascade in ion sputtered thin metal films

The sputter ripple formation in polycrystalline metal thin films of Al, Co, Cu, and Ag has been studied by 16.7 keV Ar+ and O2+ ion bombardment as a function of angle of ion incidence. The experimental results show the existence of a critical angle of ion incidence (θc) beyond which the ripples of wave vectors perpendicular to the projected ion beam direction appear. Monte Carlo simulation (SRIM) is carried out to calculate the depth, longitudinal and lateral straggling widths of energy deposition as these values are crucial in determining the critical angle θc. It is found that the radial energy distribution of the damage cascade has the maximum slightly away from the ion path in contradiction to the Gaussian distribution and the distribution is better characterized by an exponential function. The lower values of lateral straggling widths as those extracted from the measured critical angles using the Bradley and Harper theory indicate a highly anisotropic deposited-energy distribution.

Comparison of SERS Performances of Co and Ni Ultrathin Films over Silver to Electrochemically Activated Co and Ni Electrodes

In this work, the surface-enhanced Raman scattering (SERS) spectra of pyridine (py) on thin films of Co and Ni electrodeposited on an Ag electrode activated by oxidation−reduction cycles (ORC) are presented. The SERS spectra from the thin films were compared to those of py on activated bare transition metal electrodes. It was verified that the SERS spectra of py on 3 monolayers (ML)-thick films of Ni and Co presented only bands assignable to the py adsorbed on transition metal surfaces. It was also observed that even for 50 ML-thick transition metal films, the py SERS intensity was ca. 40% of the intensity from the 3 ML-thick films. The relative intensities of the SERS bands depended on the thickness of the films, and for films thicker than 7 ML for Co and 9 ML for Ni they were very similar to those of the bare transition metal electrodes. The transition metal thin films over Ag activated electrodes presented SERS intensities 3 orders of magnitude higher than the ones from bare transition metal electrodes. These films are more suitable to study the adsorption of low Raman cross-section molecules than are ORC-activated transition metal electrodes.

Electrocatalysts for bifunctional oxygen/air electrodes

Oxygen reduction and evolution have been studied with respect to the development of bifunctional air/oxygen electrode (BFE). Three groups of catalysts have been prepared: (i) Cu x Co 3− x O 4 by thermal decomposition of mixed nitrate and carbonate precursors; (ii) thin films of Co–Ni–Te–O and Co–Te–O were deposited by vacuum co-evaporation of Co, Ni and TeO 2 and (iii) Co x O v /ZrO 2 films were obtained by electrochemical deposition. The electrochemical behavior of the chemically synthesized catalysts was studied on classical bilayered gas diffusion electrodes (GDEs), where the catalyst is in form of powder. The GDE catalyzed with vacuum deposited catalysts was prepared by direct deposition of the catalyst on gas-supplying layer, thus creating a ready-to-use gas diffusion electrode. Catalysts prepared electrochemically were first deposited on Ni foam and than pressed onto the gas-supplying layer. Different catalysts deposited on classical and originally designed GDEs were compared by their electrochemical performances. Cu 0.3Co 2.7O 4 deposited on a classical bilayered GDE with loading of 50 mg cm −2 exhibits stable current–voltage characteristics after 200 charge–discharge cycles in a real metal hydride-air battery. The electrochemically and vacuum deposited Co x O v /ZrO 2, Co–Ni–Te–O and Co–Te–O films exhibit much higher mass activity compared to Cu 0.2Co 2.8O 4 for both oxygen reduction and evolution reactions. The difference is that the loading of electrochemically and vacuum deposited films is 0.06 mg cm −2 only, which is a substantial advantage from a practical viewpoint.

Direct synthesis of six-coordinate nitro complexes of cobalt meso-tetraphenylporphyrinate with trans-O-donor ligands

The formation of a series of six-coordinate complexes (B)Co(TPP)(NO2) (B is ketone, aldehyde, ether or epoxide; TPP is meso-tetraphenylporphyrin dianion) on exposure of thin films of five-coordinate Co(TPP)(NO2) to vapors of the O-donor ligands was established by IR and UV/Vis spectroscopy (resorting to the data on isotope-substituted 15NO2). The complex formation is accompanied by shifts of the vibration frequencies of the trans-NO2 ligand and gives rise to new IR bands corresponding to the O-donors in the ν(C=O) or ν(C-O) region shifted with respect to these bands of free ligands. The complexes are rather stable in the solid phase but eliminate the O-donor ligand upon dissolution in an inert solvent (or upon maintenance in a vacuum for some complexes) being converted into the initial five-coordinate nitro complex. Using the IR spectra of CCl4 solutions of the complexes containing a large excess of O-donor molecules, the equilibrium constants and the formation enthalpies of some (B)Co(TPP)(NO2) complexes were determined, indicating weak coordination of these ligands.

Y3+ doping effect on the ferromagnetism of Ce0.97Co0.03O2−δ thin films on Al2O3 (0001) substrates

Abstract Co-doped CeO2 has been reported as a diluted magnetic oxide with high Curie temperature. In this paper, thin films of Co and Y co-doped CeO2 with the stoichiometry of Ce0.97−yYyCo0.03O2−δ (y=0, 0.01, 0.05, 0.1, respectively) were prepared on Al2O3 (0 0 0 1) substrates and the Y doping effect on its magnetic property was investigated. It indicates that Y doping introduces an additional amount of oxygen vacancies (VO) characterized by Raman spectra, which leads to enhanced magnetic moments for the as-deposited films. The experimental results are qualitatively consistent with the VO-mediated ferromagnetism proposed recently.

Comprehensive Investigation of Self-Assembled Monolayer Formation on Ferromagnetic Thin Film Surfaces

We report a simple, universal method for forming high surface coverage SAMs on ferromagnetic thin (< or =100 nm) films of Ni, Co, and Fe. Unlike previous reports, our technique is broadly applicable to different types of SAMs and surface types. Our data constitutes the first comprehensive examination of SAM formation on three different ferromagnetic surface types using two different surface-binding chemistries (thiol and isocyanide) under three different preparation conditions: (1) SAM formation on electroreduced films using a newly developed electroreduction approach, (2) SAM formation on freshly evaporated surfaces in the glovebox, and (3) SAM formation on films exposed to atmospheric conditions beforehand. The extent of SAM formation for all three conditions was probed by cyclic voltammetry for surfaces functionalized with either (11-thiolundecyl)ferrocene (Fc-(CH2) 11-SH) or (11-isocyanoundecyl)ferrocene (Fc-(CH2) 11-NC). SAM formation was also probed for straight-chain molecules, hexadecanethiol and hexadecaneisocyanide, with contact angle measurements, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy (RAIRS). The results show that high surface coverage SAMs with low surface-oxide content can be achieved for thin, evaporated Ni and Co films using our electroreduction process with thiols. The extent of SAM formation on electroreduced films is comparable to what has been observed for SAMs/Au and to what we observe for SAMs/Ni, Co, and Fe samples prepared in the glovebox.

Adding structural diversity to roughness gradients formed from Sn

Nanoscale roughness gradients formed from gradients in the mass per unit area of sputtered Sn were previously reported in the literature. In the accompanying report, we identify three approaches that enhance or complement these gradients by allowing more diverse Sn structures to be generated. The first approach is to alter the substrate chemistry, which was explored by sputtering Sn onto the thin-film library Cu 1 − x Cr x (0 < x < 1) and onto thin films of Co, Ti, V and W. Sn structure was highly sensitive to substrate chemistry, which should provide a means of incorporating morphology as a variable alongside roughness in gradients of nanotopographies. In the second approach, a small amount of Cr is added to a film of Sn to reduce the size of Sn crystallites. By spatially varying the Cr composition, a gradient in crystallite size can be produced. This is another way of making a roughness gradient, which we have demonstrated by preparing the thin-film library Sn 1 − x Cr x (0 < x < 0.05). In the final approach, gradients in the mass per unit area of Sn were deposited at substrate temperatures of 14, 112 and 191 °C. While a minor increase in crystallite size was noted for increasing substrate temperature to 112 °C, significantly larger crystallites could be produced at 191 °C. This increase in size at 191 °C corresponded to a near doubling in roughness along the gradient, which suggests that elevating substrate temperature could be a viable means of extending the range in roughness attainable with Sn gradients.

Magnetic energy distribution in polycrystalline sputtered CoCr magnetic thin films

Magnetic thin films of Co 80 Cr 16 Ta 4 were sputtered onto identical CrMo seed-layers at –200 V bias and 3 different substrate temperatures (150, 200, and 250 °C). Energy-filtered transmission electron microscopy (EFTEM) was performed to analyze Cr levels at the grain boundaries as well as inside the grains. These quantitative Cr measurements were used to estimate the local values of magnetocrystalline anisotropy ( K u ) and, together with grain size distributions, calculate the product of K u and the grain volume ( K uV ), a quantity which is a measure of thermal stability. The results show that the coercivity as well as the fraction of stable grains increased with increasing substrate temperature. The increase in the fraction of stable grains is produced by the enhancement in the K u value due to Cr depletion of the grain interiors and the magnetic decoupling between the grains from Cr grain-boundary segregation.

Domain dynamics and magneto-electronics in magnetic thin films and nanowires

Recent theoretical results are highlighted that illustrate some of the interesting phenomena associated with magnetic domain boundary walls. Two problems will be discussed: dynamics associated with domain wall propagation, and effects related to spin transport through domain walls. For the first problem, an example of wall interaction and motion through a random potential will be discussed with reference to the general problem of roughening transitions. Images of domain dynamics in thin films of ion irradiated Co reveal a de-roughening transition associated with long range magnetostatic interactions between pairs of domain walls. A scaling theory of this transition is described in which a curious type of dynamic hysteresis can occur. For the second problem, results from calculations of ballistic charge and spin transport through domain boundary walls are discussed in terms of an effective circuit model.

Microstructure of Co∕X (X=Cu,Ag,Au) epitaxial thin films grown on Al2O3(0001) substrates

Epitaxial thin films of Co∕X (X=Cu,Ag,Au) were prepared on Al2O3(0001) substrates at substrate temperatures of 100 and 300°C by UHV molecular beam epitaxy. A complicated microstructure was realized for the epitaxial thin films. In-situ reflection high-energy electron diffraction observation has shown that X atoms of the buffer layer segregated to the surface during Co layer deposition, and it yielded a unique epitaxial granular structure. The structure consists of small Co grains buried in the X buffer layer, where both the magnetic small Co grains and the nonmagnetic X layer are epitaxially grown on the single crystal substrate. The structure varied depending on the X element and the substrate temperature. The crystal structure of Co grains is influenced by the buffer layer material and determined to be hcp and fcc structures for the buffer layer materials of Au and Cu, respectively.

Thin films of Co–B prepared by pulsed laser deposition as efficient catalysts in hydrogen producing reactions

Abstract Hydrogen generation by catalytic hydrolysis of sodium borohydride (NaBH 4 ) is studied by using Co–B-based thin film catalyst synthesized by pulsed laser deposition (PLD) technique. Co–B nanoparticles, produced in the catalyst film after the laser ablation process, act as active centers producing significantly higher H 2 generation rate than Co–B bulk powder. Surface morphology was studied by using scanning electron microscopy and compositional analysis was established by using X-photoelectron and infrared spectroscopies. Films were deposited at different PLD set-up parameters in order to understand the possible role of size and density of the nanoparticles in the catalytic process. Cobalt has been found here to act as an efficient catalyst only when alloyed with boron which partially prevents cobalt oxidation. In particular we report that when used as suitable thin films, Co–B produces H 2 with a maximum generation rate of about 3300 ml/min per gram of catalyst.