Nickel Monolayers Research Articles

Voltammetric Behavior of Self-assembled Monolayers of Novel Nickel (II/III) Pentaazamacrocyclic Complex on Gold Electrode and Electrocatalytic Oxidation of Hydrogen Peroxide

Voltammetric Behavior of Self-assembled Monolayers of Novel Nickel (II/III) Pentaazamacrocyclic Complex on Gold Electrode and Electrocatalytic Oxidation of Hydrogen Peroxide

Voltammetric response and electrocatalysis of glassy carbon electrode modified with monolayer nickel hydroxide

The glassy carbon (GC) electrode modified with a monolayer nickel hydroxide (GC/Ni(OH)2) was prepared by immersion of GC substrate in 1.0×10−3 mol/L NiSO4 solution, and then cyclic voltammetric scanning in 0.20 mol/L KOH. Similarly, GC/Co (OH)2 electrode was prepared too. The experiments showed that the voltammetric behavior of GC/Ni (OH)2 electrode in 0.20 mol/L KOH is more stable than that of GC/Co(OH)2. It was found that the GC/Ni(OH)2 electrode acts as an effective electrocatalysis for the oxidation of hydrazine.

The magnetism of nickel monolayers

Nickel allows to study the largest variety of phenomena in the magnetism of UHV ultrathin films. The low critical temperature of 630 K for the bulk favors experiments from 0 K to above T c and from one monolayer to infinite thick films. The thickness dependence of T c (d) for the (001) and the (111) orientation is compared. Susceptibility measurements in UHV are presented, and from χ max the film geometry can be deduced. Ferromagnetic resonance measures the second- and fourth-order anisotropy constants. These give a clear understanding of when and how the reorientation transition from the inplane to the perpendicular orientation occurs and its nature. Magnetic resonance and circular X-ray dichroism measure the spin and orbital parts of the magnetic moment μ, its anisotropy Δμ, and the 3d and 4sp contributions. Finally, we show how a 4 Monolayer (ML) Ni (001) film can be transformed into NiO by controlled oxygen dosage and thermal treatment.

The magnetism of nickel monolayers

The magnetism of nickel monolayers

Metal-Insulator Transitions and Possible Quantum Effects in Supported Nickel Monolayers

Metal-Insulator Transitions and Possible Quantum Effects in Supported Nickel Monolayers

Energy of spin wave excitations in nickel monolayers

The spin wave stiffness constant D (q) of unsupported FCC (100), (110), and (111) nickel monolayers are calculated for different directions, q, of spin wave propagation. It is shown that D is isotropic for the (100) and (111) orientations, and highly anisotropic for the (110) orientation. Results indicate that the strength of the exchange coupling between nearest neighbour magnetic moments is quite sensitive to the effective space dimensionality as well as the crystalline structure of the system.

Influence of interfacial hydrogen and oxygen on the Schottky barrier height of nickel on (111) and (100) diamond surfaces.

In this paper we report on Schottky barrier height measurements of nickel on both diamond (111) and (100) surfaces, as a function of surface preparation. The Schottky barriers of thin (5 \AA{}) nickel films on natural type-IIb (p-type semiconducting) diamond (111) and (100) surfaces were determined with ultraviolet photoemission spectroscopy. Exposing the diamond (111) surfaces to an argon plasma while heated to 350 \ifmmode^\circ\else\textdegree\fi{}C resulted in a change from a negative-electron-affinity surface to a positive-electron-affinity surface. This effect was used as an indication that a hydrogen-free surface had been obtained. Deposition of a monolayer of nickel on the hydrogen-free diamond (111) surface resulted in a Schottky barrier height of 0.5 eV. The nickel caused the surface to exhibit a negative-electron-affinity surface. Nickel deposited on a diamond (111) surface with a negative electron affinity, indicative of a monohydride-terminated surface, resulted in a \ensuremath{\sim}1.0-eV Schottky barrier height. Diamond (100) surfaces were prepared by vacuum annealing to temperatures ranging from 500 \ifmmode^\circ\else\textdegree\fi{}C to 1070 \ifmmode^\circ\else\textdegree\fi{}C. The various anneals resulted in a lowering of the electron affinity by up to \ensuremath{\sim}1 eV, which resulted in a negative electron affinity after the surface had been annealed to \ensuremath{\sim}1000 \ifmmode^\circ\else\textdegree\fi{}C. Oxygen was initially present on the surface but could not be observed after the 1000 \ifmmode^\circ\else\textdegree\fi{}C anneal. The removal of oxygen and the appearance of a negative electron affinity coincided with the appearance of a 2\ifmmode\times\else\texttimes\fi{}1 surface reconstruction. Nickel was deposited after the various anneals, and Schottky barrier heights were found, ranging from 1.5 eV for the 545 \ifmmode^\circ\else\textdegree\fi{}C-annealed surface to 0.7 eV for the 1070 \ifmmode^\circ\else\textdegree\fi{}C-annealed surface. These measurements suggest that for both the (111) and the (100) diamond surfaces the presence of chemisorbed species, such as hydrogen and oxygen, results in an increase in the Schottky barrier height.

Scanning tunneling microscopy investigation of monolayer of metal complexes adsorbed on highly oriented pyrolytic graphite

Monolayer of nickel(II) complexes coordinated with N,N′-disalicylideneethylenediamine (salen) substituted by dodecyl side chains have been prepared at a water–air interface using the Langmuir–Blodgett method. The monolayer of the complex thus formed is scooped onto the cleaved surface of highly oriented pyrolitic graphite, and imaged by scanning tunneling microscopy.

Carbon, nitrogen, and sulfur on Ni(111): formation of complex structures and consequences for molecular decomposition

The decomposition of simple amines on Ni(111) produces a c(5√3 × 9)rect LEED pattern similar to the one reported earlier after reaction of H 2S, C 2H 4, CO, NO and CH 3CN on a Ni(111) surface. We have measured LEED IV curves of the structure obtained after deposition of carbon and nitrogen on a Ni(111) surface. The similarity between these IV curves shows that carbon and nitrogen have similar local geometries. The most intense spots of the c(5√3 × 9)rect LEED pattern are explained by the formation of a (100) orientation nickel monolayer, with the carbon or nitrogen atoms in the 4-fold hollow sites. This is supported by the qualitative similarity of the IV curves with those of the Ni(100)-(2 × 2)-2C structure. We have also produced a (2 × 6) nitrogen induced structure on Ni(111) and compared it to the very similar (5√3 × 2)rect structure obtained by adsorption of sulfur on Ni(111) that has also been interpreted as due to a (100) reconstruction of the Ni(111) crystal surface.

Exchange stiffness constant of two-dimensional Ni (100), (110) and (111) orientations

The exchange stiffness constant D of unsupported fcc (100), (110) and (111) nickel monolayers are calculated. It is shown that D is highly anisotropic for the (110) orientation. The relation between the strength of the magnetic coupling in the ground-state and the electronic structure of the monolayers is pointed out.

Structural transitions in ultra-thin nickel films on Rh{111}

For the first three monolayers, ultra-thin nickel films on Rh{111} grow in a close to layer-by-layer fashion. A nickel monolayer film is “pseudomorphic” with the underlying substrate. An automated tensor LEED I( V) analysis indicates the clean Rh{111} surface to be laterally unreconstructed with a small amplitude damped oscillatory relaxation of the outermost two atomic layer spacings of Δd z 12 = −2.7 ± 1.4% and Δd z 23 = + 0.4 ± 1.4%, resulting in a Pendry R-factor of 0.16. For the nickel adsorption site is found to be the fcc threefold hollow, with a nickel to rhodium interlayer spacing of 2.06 Å, slightly shorter than the sum of metallic hard sphere radii, with the outermost rhodium interlayer spacing relaxed to Δd z 12 = −1.4 ± 3.2%. The minimum Pendry R-factor of 0.28 was obtained. Spot profile analysis indicated significant beam broadening upon initiation of second layer growth with asymmetry at high parallel momentum transfer consistent with formation of small domains of incommensurate nickel with a nearest-neighbour separation intermediate between rhodium and nickel, finally yielding a complex pattern due to multiple scattering between the pseudomorphic p(1 × 1) slab and the incommensurate film for thicknesses beyond 1 ML. Model pair potential calculations indicate a layer dependent gradual relaxation of the nickelickel interlayer spacing towards that of bulk nickel in the outermost three atomic layers. Alloying occurs on an experimental time scale at temperatures above 600 K leading to formation of a substitutionally disordered RhNi alloy of variable composition.

Exchange stiffness constant of two-dimensional Ni and Fe

The exchange stiffness constant D 2d of unsupported (100) nickel monolayer is calculated in the tight-binding and random phase approximations. The tight-binding parameters are determined from a fit to ground-state spin density functional band structure calculations. The relative stability of the bulk and monolayer systems is discussed by comparing the results for D 2d and bulk D 3d. Preliminary results for (100) iron monolayer are also reported.

Growth of epitaxial NiSi2 on Si(111) at room temperature

Epitaxial type B NiSi2 thin layers have been grown at room temperature on Si(111). Deposition of a few monolayers of nickel followed by codeposition of NiSi2 has led to the growth of high quality single-crystal layers, with ion channeling χmin <2%. No disorder was found at the interfaces of these layers. The topography of the original substrate has a predominant effect on the structure of line defects at the NiSi2 interface. Codeposition at room temperature on annealed silicide thin layers also led to the overgrowth of high quality NiSi2. These results are suggestive of type B NiSi2 formation on Si(111) upon deposition of a few monolayers of nickel at room temperature.

The interaction of copper and nickel : nickel monolayers on a Cu{100} single crystal: M A Morris et al, Thin Solid Films, 156, 1988, 65–78

The interaction of copper and nickel : nickel monolayers on a Cu{100} single crystal: M A Morris et al, Thin Solid Films, 156, 1988, 65–78

Nickel monolayers on copper surfaces: CO adsorption and ni diffusion

Nickel monolayers on the Cu(111) surface have been studied in an attempt to understand metal-metal interface mixing. High resolution energy loss (HREEL) spectra of carbon monoxide is used to monitor the surface by distinguishing between CO adsorption on top and bridge sites of the nickel overlayer. Results are discussed for nickel deposition at temperatures in the range of 300–475 K. Room temperature nickel deposition leads to nickel island formation; at slightly elevated temperatures nickel and copper mix in the first layer ; and by 475 K, nickel becomes significantly covered by copper, the thermodynamically favored configuration.

The interaction of copper and nickel:nickel monolayers on a Cu{100} single crystal

Thin films of nickel were evaporated onto a Cu{100} surface and the coverage regime of 0–5 monolayers was investigated. It is shown that at room temperature and at about 0.4 of a monolayer, a 2d alloy is formed which dissolves at greater coverages. Higher coverages result in the formation of pseudomorphic layers of nickel above the Cu{100} surface which interact weakly with the copper substrate. At low annealing temperatures (of about 500 K) an alloy is formed by interdiffusion. At higher temperatures, copper very rapidly segregates to the surface forming a bulk-like structure. The use of evaporated materials to measure surface core level shifts is discussed.

Growth of ultrathin single-crystal NiSi2 layers on Si(111)

Reaction of monolayers of nickel with Si(111) toward the formation of single-crystal epitaxial NiSi2 layers is examined in detail. New results are presented to illustrate the dependence on the processing parameters. A model is shown to successfully account for the experimental results.

High activity catalyst from exfoliated MoS 2

By using exfoliated MoS 2 suspended as single layers in solution we have been able to prepare what appear to be unusually high activity catalysts, where hydrogenation of CO, methanation, was used as the model reaction. The process depends on three features in the preparation; first, the preparation of the single molecular layer MoS 2; second, the deposition of these as single layers onto high-surface-area Al 2O 3 (or rather deposition of Al 2O 3 onto single layers of MoS 2); and third, the deposition of monolayers of nickel onto the single layers of MoS 2. With this structure, suitably calcined and reduced to improve the interaction between the Mo, the Ni, and the Al 2O 3, we have a very high density of active sites per unit area (and per unit mass). The most active sites appear to be a form of oxysulfide. The active sites when nickel is present behave differently from Ni supported on Al 2O 3. The Ni/Mo/Al 2O 3 composition yields CO 2 as the by-product of methanation, instead of the H 2O observed with the Ni Al 2O 3 composition, and has a different activation energy. A specific formulation designed to give the maximum density of accessible Ni/Mo/Al 2O 3 sites shows an activity substantially higher than Ni Al 2O 3 . For formulation without Ni the activity of catalysts prepared by exfoliation shows substantially higher activity than those prepared by precipitation from ammonium heptamolybdate.

Initial growth of nickel on copper (100) studied by ion-scattering

Medium-energy helium ion-scattering in combination with channeling and blocking is used to study the structure of a clean and a nickel covered copper (100) surface. For clean copper (100) a surface relaxation of − 2 ± 2% is found together with an one-dimensional thermal vibration amplitude of 0.12 Å for atoms in the first layer and 0.10 Å for atoms in the second layer. Moreover under channeling conditions a very high electronic stopping power and straggling is observed when He + ions are backscattered from the surface layers. The high stopping power enables us to differentiate between ions backscattered from the first and ions backscattered from the second surface layer. The growth mechanism of nickel on copper is analyzed by covering the copper (100) surface with one third of a monolayer nickel. From our experiments it is concluded that nickel atoms are incorporated in the crystal structure of copper. The data suggest that the first surface layer contains 23% nickel, the second surface layer 9% and the third and deeper layers less than 4%. These results indicate either a Volmer-Weber type island-growth mechanism or an indiffusion of nickel into the copper substrate.

Electronic properties of rho (1 x 1) Ni films on Cu(100).

Angle-resolved photoelectron spectroscopy is used to determine the electronic structure and magnetic exchange splitting of an ordered nickel monolayer on Cu(100).