Ultrathin Nickel Films Research Articles

Ultrathin Transparent Nickel Electrodes for Thermoelectric Applications

AbstractIn this work, ultrathin nickel films are developed for application as transparent electrodes in thermoelectric devices. The quality of the films is determined systematically by electrical, optical, and morphological characterization in a series of samples with different thickness. The thermal properties of the films show a dramatic dependence of the Seebeck coefficient on the film thickness. This dependence, with values ranging from −16 to +5 𝜇V K−1 for thicknesses from 10 to 2 nm, includes a change in the behavior of the thermoelectric response from n‐ to p‐type. It has also been demonstrated that the accurate estimation of the thermal conductivity in thin films is challenging due to substrate effects. In this situation, a differential measurement method based on scanning thermal microscopy is proposed, as in these conditions the measurements are less sensitive to the substrate effects. In further works, the dependence of the thermal properties of ultrathin nickel films can be exploited as a tuning parameter for the design of thermoelectric devices.

Voltage-controlled magnetic anisotropy in an ultrathin nickel film studied by operando x-ray magnetic circular dichroism spectroscopy

Voltage-controlled magnetic anisotropy (VCMA) in an atomically thin Ni film placed at an epitaxial Fe-MgO interface has been studied. We characterize the VCMA by employing Fe/Ni/MgO-based magnetic tunnel junction. Moreover, we conduct x-ray magnetic circular dichroism (XMCD) spectroscopy around Ni absorption edges under an external voltage application to the tunnel junction device. This operando XMCD spectroscopy demonstrates voltage-induced changes of orbital and effective spin magnetic moments of Ni. As compared with the previous study, where operando XMCD spectroscopy was performed on an atomically thin Co film, we find that the magnitude of the VCMA effect in Ni and Co is proportional to the voltage-induced change of the orbital magnetic moment. This study would lead to a comprehensive understanding of how an electric field affects interfacial magnetism in ferromagnetic metals.

Quest for magnons in ultrathin nickel films

High-momentum spin waves (magnons) in ultrathin films of cobalt and iron have been explored thoroughly using inelastic scattering of low-energy electrons. The search for magnons in ultrathin nickel films failed, however, although high-energy magnons do exist in bulk nickel. The failure might be due to the weak coupling of nickel magnons to scattering electrons. In order to increase the coupling we deposited layers of cobalt onto Ni films and successfully studied the magnons of such films. The acoustic modes show the same dispersion as pure Co films, which is consistent with the nearly identical stiffness of bulk magnons in nickel and cobalt. Standing magnons in Ni films covered with Co are strongly damped at the Ni/Cu(100) interface when their total wave vector exceeds about $3\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{\ensuremath{-}1}$.

Negative differential resistance in nickel octabutoxy phthalocyanine and nickel octabutoxy phthalocyanine/graphene oxide ultrathin films

We find negative differential resistance (NDR) at room temperature in ultrathin films of nickel (II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine [NiPc(OBu)8] deposited on highly ordered pyrolytic graphite (HOPG) substrate [NiPc(OBu)8/HOPG] and NiPc(OBu)8 on graphene oxide (GO) deposited on HOPG [NiPc(OBu)8/GO/HOPG]. For the NiPc(OBu)8/HOPG system, NiPc(OBu)8 was transferred four times onto HOPG by the Langmuir-Blodgett (LB) technique. We have prepared a stable Langmuir monolayer of amphiphilic GO at the air-water interface and transferred it onto HOPG by the LB technique. Further, the monolayer of NiPc(OBu)8 was transferred four times for good coverage on GO to obtain the NiPc(OBu)8/GO/HOPG system. The current-voltage characteristics were carried out using a current sensing atomic force microscope (CSAFM) with a platinum (Pt) tip that forms Pt/NiPc(OBu)8/HOPG and Pt/NiPc(OBu)8/GO/HOPG junctions. The CSAFM, UV-visible spectroscopy, and cyclic voltammetry studies show that the NDR effect occurs due to molecular resonant tunneling. In the Pt/NiPc(OBu)8/GO/HOPG junction, we find that due to the presence of GO, the features of NDR become more prominent. Also, GO causes a shift in NDR voltage towards a lower value in the negative bias direction. We attribute this behavior to the role of GO in injecting holes into the NiPc(OBu)8 film.

Synthesis of Single‐Layer Graphene on Nickel Using a Droplet CVD Process

A liquid metal “droplet” chemical vapor deposition (CVD) process is developed for the synthesis of single‐layer graphene flakes utilizing molten nickel droplets (nominal diameter from 0.5 to 1 µm) as the catalysts. Ultrathin nickel films melt and form nickel droplets at around 1000 °C, providing liquid‐phase, single‐grain catalytic sites for the nucleation of graphene flakes via carbon precipitation process. Experimentally, both single and double‐layer graphene flakes with low defects have been synthesized by using nickel thin films of either less or more than 100 nm (up to 130 nm) in thickness, respectively. Furthermore, the synthesized graphene flakes are hybridized by the nickel orbitals as suggested by a density function analysis and experimentally observed blue shifts in Raman spectra tests. When the original nickel thin film is 75 nm in thickness, the resulting nickel droplets are physically and electrically isolated, while the graphene flakes are found to be electrically connected probably due to the outgrowth of graphene from the separated nickel droplets. These electrically connected graphene sheets could be readily available for device applications without the need of transfer processes. This work demonstrates the direct generation of photocurrents (up to 0.53 µA mm−2 W−1) due to the photothermal effect based on as‐fabricated graphene sheets under a visible light source.

Dissolution-and-reduction CVD synthesis of few-layer graphene on ultra-thin nickel film lifted off for mode-locking fiber lasers.

The in-situ dissolution-and-reduction CVD synthesized few-layer graphene on ultra-thin nickel catalyst film is demonstrated at temperature as low as 550 °C, which can be employed to form transmission-type or reflection-type saturable absorber (SA) for mode-locking the erbium-doped fiber lasers (EDFLs). With transmission-type graphene SA, the EDFL shortens its pulsewidth from 483 to 441 fs and broadens its spectral linewidth from 4.2 to 6.1 nm with enlarging the pumping current from 200 to 900 mA. In contrast, the reflection-type SA only compresses the pulsewidth from 875 to 796 fs with corresponding spectral linewidth broadened from 2.2 to 3.3 nm. The reflection-type graphene mode-locker increases twice of its equivalent layer number to cause more insertion loss than the transmission-type one. Nevertheless, the reflection-type based saturable absorber system can generate stabilized soliton-like pulse easier than that of transmission-type system, because the nonlinearity induced self-amplitude modulation depth is simultaneously enlarged when passing through the graphene twice under the retro-reflector design.

Ultrathin and stable Nickel films as transparent conductive electrodes

Ultrathin stable transparent conductive nickel films were deposited on quartz substrates by radio frequency sputtering at room temperature. Such films showed visible transmittance up to 80% and conductivity up to 1.8×104S/cm, further increased to 2,3×105S/cm by incorporation of a micrometric silver grid. Atomic force microscopy and scanning electron microscopy revealed quite compact, smooth and low surface roughness films. Excellent film stability, ease, fast and low cost process fabrication make these films highly competitive compared to indium tin oxide alternative transparent conductors. Films were characterized regarding their morphological, optical and electrical properties.

ChemInform Abstract: High‐Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation.

AbstractDeposition of a thin nickel film (approximately 2 nm thick) on n‐type silicon with its native oxide yields a high‐performance metal‐insulator‐semiconductor photoanode for photoelectrochemical water oxidation in both aqueous KOH (pH 14) and aqueous borate buffer (pH 9.5) solutions.

High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation

Silicon's sensitivity to corrosion has hindered its use in photoanode applications. We found that deposition of a ~2-nanometer nickel film on n-type silicon (n-Si) with its native oxide affords a high-performance metal-insulator-semiconductor photoanode for photoelectrochemical (PEC) water oxidation in both aqueous potassium hydroxide (KOH, pH = 14) and aqueous borate buffer (pH = 9.5) solutions. The Ni film acted as a surface protection layer against corrosion and as a nonprecious metal electrocatalyst for oxygen evolution. In 1 M aqueous KOH, the Ni/n-Si photoanodes exhibited high PEC activity with a low onset potential (~1.07 volts versus reversible hydrogen electrode), high photocurrent density, and durability. The electrode showed no sign of decay after ~80 hours of continuous PEC water oxidation in a mixed lithium borate-potassium borate electrolyte. The high photovoltage was attributed to a high built-in potential in a metal-insulator-semiconductor-like device with an ultrathin, incomplete screening Ni/NiO(x) layer from the electrolyte.

Fabrication of Ni electrode films by sintering Ni nanoparticle pastes: Compositional dependence of specific resistance and optimal composition

Ultra-thin nickel films are essential for the internal electrodes of the high density multi-layered ceramic capacitors (MLCCs) used in a variety of electronic devices. In this study, Ni electrode films were fabricated by sintering Ni nanoparticle pastes prepared by mechanically stirring mixtures of Ni nanoparticles (d = 40–150 nm), dispersant, binder, and solvent. Paste compositions were varied by using three solvents, two binders, one dispersant, and different amounts of nickel nanoparticles. In total 36 pastes and electrode films were prepared. The electrode film defects such as voids in films and exfoliation (film loss due to lack of adhesion to substrate) were noted, and film thicknesses and specific resistances (ρ) were measured. Voids were observed in all samples whereas exfoliations were observed only for several samples. Ni electrode film thicknesses and ρ values ranged from 2 to 7 μm and from 10−5 to 10−4 Ω cm, respectively, depending on paste composition. Based on considerations of ρ values, an optimal composition was determined. Of the three solvents and two binders used, terpineol and EC produced better quality films with lower ρ values and without exfoliation, suggesting that they are probably the most suitable for fabricating internal electrodes for high density MLCCs.

Enhanced charge storage characteristics of nickel nanocrystals embedded flash memory structures

Nickel nanocrystals (NCs) embedded flash memory structure with SiO2 tunnelling and pulsed laser deposition grown Al2O3 blocking oxide is reported here. The post-deposition thermal annealing of an ultrathin (∼5 nm) nickel film evaporated on thermally grown SiO2 tunnelling barrier has been performed to form the nickel NCs. The formation of tiny nickel NCs is confirmed from the high-resolution transmission electron microscope micrographs. The electrical capacitance–voltage (C–V) characteristics of the optimised 1000°C, 5 min annealed sample shows a large memory window of 20 V at ±20 V sweeping voltage. The charge storage properties are found to be significantly improved as compared to control samples. The frequency dependent C–V measurements indicate the dominant charge trapping in Ni NCs, making the memory structure attractive for use in future nanoscale high-performance applications.

Hydrogen-free PECVD growth of few-layer graphene on an ultra-thin nickel film at the threshold dissolution temperature

The synthesis of few-layer graphene sheets on an ultra-thin nickel film-coated SiO2/Si substrate using hydrogen-free plasma-enhanced chemical vapor deposition (PECVD) with in situ low-temperature carbon dissolution is preliminarily demonstrated. The deposited carbon atoms are initially dissolved into the nickel matrix and subsequently precipitate out onto the nickel film surface. The threshold carbon dissolution temperature for synthesizing few-layer graphene is observed to be as low as 475 °C, and the critical thickness of the host nickel film is at least 30 nm. Due to the ultra-low solubility of the carbon atoms in the nickel film at a threshold temperature of 475 °C, the layer number in few-layer graphene can be precisely controlled. Raman scattering analysis indicates almost identical D and 2D peak intensities for nickel films with different thicknesses, whereas the G peak is enhanced with an increasing layer number of graphene which precipitates from thicker nickel films. Saturation of the G peak for the 50 nm thick nickel film is observed, due to the finite carbon dissolution within a limited deposition time, and results in a stabilized, high quality precipitated few-layer graphene. The linear transmittance of few-layer graphene at 550 nm increases from 83 to 93% when the deposition time is shortened from 600 to 100 s, which corresponds to a decrease of the graphene layer number from 8 to 3 layers. The Raman scattering peak ratio of ID/IG decreases from 1.8 to 0.2 and the G-band linewidth shrinks from 67 to 37.2 cm−1, providing strong evidence for the improved quality of few-layer graphene synthesized by hydrogen-free PECVD at the threshold temperature on an ultra-thin nickel host.

Structural changes during the reaction of Ni thin films with (1 0 0) silicon substrates

Ultrathin films of nickel deposited onto (100) Si substrates were found to form kinetically constrained multilayered interface structures characterized by structural and compositional gradients. The presence of a native SiO2 on the substrate surface in tandem with thickness-dependent intrinsic stress of the metal film limits the solid-state reaction between Ni and Si. A roughly 6.5nm thick Ni film on top of the native oxide was observed regardless of the initial nominal film thickness of either 5 or 15nm. The thickness of the silicide layer that formed by Ni diffusion into the Si substrate, however, scales with the nominal film thickness. Cross-sectional in situ annealing experiments in the transmission electron microscope elucidate the kinetics of interface transformation towards thermodynamic equilibrium. Two competing mechanisms are active during thermal annealing: thermally activated diffusion of Ni through the native oxide layer and subsequent transformation of the observed compositional gradient into a thick reaction layer of NiSi2 with an epitaxial orientation relationship to the Si substrate; and, secondly, metal film dispersion and subsequent formation of faceted Ni islands on top of the native oxide layer.

Structural, magnetic and magneto-optical properties of ultrathin films of nickel on iridium (001)

A detailed theoretical study of the structural, magnetic and magneto-optical behaviours of ultrathin films of nickel grown by pseudomorphic epitaxy on semi-infinite Ir(001) is given. The crystalline structure is found to be body centered tetragonal. The total energies are calculated by Spin-Polarized Relativistic Linear Muffin-Tin Orbitals with Atomic Sphere Approximation method. The calculation of the magnetic properties shows a ferromagnetic inter-layer coupling. The polar magneto-optical Kerr effect spectra are calculated over a photon energy range extended to 9 eV. The microscopic origin of the most interesting features is explained by the interband transitions between the localized spin projected states. These transitions are characterized by a spin-flip.

Preparation of a Nickel Ion Containing Langmuir–Blodgett Multilayer and an Ultra-Thin Nickel Film Deposited on an Interpenetrating Polymer Network Substrate

The nickel ion containing Langmuir–Blodgett (LB) multilayer was prepared by transferring nickel acetate, spread on the surface of a sub-phase of ultra-pure water and stearic acid–chloroform mixture, onto an interpenetrating polymer network (IPN) substrate. The substrate was prepared by dip-pulling a hydrophilic silicon wafer or a glass plate into precursors, followed by solidification at room temperature, in order to create an ultra-thin and uniform surface for metal deposition. The multilayer was then converted into an ultra-thin nickel film after chemical reduction by 0.01 mol/l sodium borohydride. The surface pressure of the monolayer and dipping speed of substrate were determined by measuring the transfer coefficient. Fourier transform infrared spectroscopy (FT-IR) was used to investigate the interactions of the nickel ions with the IPN during the multilayer deposition on substrate, and the metal transformations of nickel ion in the ultra-thin film. The shifted peak location for –C=O verified the interactions between the IPN and the nickel ion and the transformation of the nickel ion by reduction. Further reduction caused the organic phase to dissolve, resulting in most of it being removed from the multilayer. The surface morphologies of the LB multilayer were detected by atomic force microscopy (AFM). Compared with the IPN, which formed a uniform and flat film, within a range of nanometers, the Ni/IPN multilayer and Ni ultra-thin film both showed some surface fluctuations, although these were still within nanometer range. The roughness of the nickel ion containing multilayer and the ultra-thin nickel film was 1.123 nm and 0.773 nm with a maximum height of 12.451 nm and 14.933 nm, respectively, which were larger than that of pure IPN substrate of 0.593 nm with the max height of 6.795 nm.

Ultrathin nickel films on Cu(001): Loss of strong ferromagnetism

The unoccupied electronic states in ultrathin Ni films on Cu(001) have been studied by spin- and angle-resolved inverse photoemission. Although the $d$ holes in a strong ferromagnet such as Ni are expected to have exclusively minority character, our inverse-photoemission results for a 6 ML film of Ni on Cu(001) provide evidence of significant majority contributions from empty $d$ bands. We examined carefully that the majority intensity is not caused by a reduced magnetization due to a lower Curie temperature as compared with the bulk case or by a modified magnetic structure of the film. Hence, it is attributed to a band structure different from the bulk situation, i.e., the majority $d$ bands are not entirely occupied. As a consequence, ultrathin Ni films on Cu(001) do not represent a strong ferromagnet.

Ligand Effect in Hydrogen Adsorption on Epitaxial Nickel Films

AbstractThe adsorption structure of hydrogen on ultrathin epitaxial nickel films in pseudomorphic arrangement on the unreconstructed Ir(100) surface is investigated by quantitative low-energy electron diffraction (LEED). This is highly interesting in the light of the fact that on Ni(100) adsorption is in hollow sites whilst on unreconstructed Ir(100) it is exclusively in bridge sites. On Ni films thicker than 1 monolayer (ML) the adsorption is in 4-fold coordinated hollow sites as it is on Ni(100). This is in spite of the considerable tensile strain of the Ni film (+8.9%) and is interpreted by the dominance of ensemble effects in the hydrogen bonding. In contrast, on the 1 ML Ni film on Ir(100), with iridium atoms being nearest neighbours to nickel atoms, there is occupation of both bridge and hollow sites of the nickel layer – a clear indication for the presence of ligand effects.

Facile preparation of an ultrathin nickel film coated nanoporous gold electrode with the unique catalytic activity to oxidation of glucose

A facile approach to prepare a Ni coated nanoporous gold (Ni@NPG) electrode with the combination of a UPD monolayer and galvanic displacement is described, which not only conserves the 3-D nanostructure of the NPG electrode but presents unique catalytic activity to the electrode surface to significantly enhance the performance of glucose detection.

Dynamic force microscopy study of the microstructural evolution of pulsed laser deposited ultrathin Ni and Ag films

Ultrathin films (6–10 nm) of silver and nickel were deposited by pulsed laser deposition (PLD) in high vacuum (1 × 10−6 mbar). Microstructural evolution of these films as function of incident laser energy, substrate temperature, substrate material [borosilicate glass, fused silica, MgO(100) and Si (311)] and target–substrate distance was studied in detail using dynamic force microscopy. It is shown that with increase in laser energy incident on the target, there is a substantial increase in particle size in the film. The effect of increased laser energy on microstructure is much more drastic than that for the increase of substrate temperature. In general, denser packing of nanoparticles and increased clustering have been observed at elevated substrate temperature. Increase in laser energy gives rise to higher average grain size, packing density, and clustering in comparison to substrate temperature. It is observed that the aspect ratio of grains is dependent on incident laser fluence and substrate temperature, but more drastically on the substrate material. Substrate coverage and aspect ratio of the grains are particularly dependent on the nature of crystallinity of the substrates. It is demonstrated that PLD provides a greater degree of microstructural manipulation than other physical vapor deposition techniques.

Temperature-dependent crossover of dimensionality in ultrathin nickel films

We report experimental results for nickel films which examine finite-size nanoscaling of the magnetic-order exponent β with temperature as a function of thickness. The data confirm the trends predicted by Monte Carlo calculations of an Ising spin lattice for thicknesses L much less than in-plane dimensions. A crossover in dimensionality D is seen in the magnetization M(T) at a rescaled reduced temperature txo, which delineates a change in the order parameter from three dimensional to two dimensional. This crossover temperature is a feature of ultrathin films over a wide range of thicknesses L.