Ni 2p Core-level Spectra Research Articles

Evolution of structural phase transition from hexagonal wurtzite ZnO to cubic rocksalt NiO in Ni doped ZnO thin films and their electronic structures

This study thoroughly examines the collective influence of compositional variation and annealing temperature on the electronic structure of sol–gel derived NixZn1−xO (x = 0 to 1) thin films annealed at different temperatures (700 ° C, 800 ° C, and 900 ° C) using x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy. A gradual structural phase transition from hexagonal wurtzite ZnO to cubic rocksalt NiO with increasing Ni concentration was revealed by x-ray diffraction (XRD). Grain growth was observed from scanning electron microscopy with increasing annealing temperature. Photoluminescence measurements indicate the presence of interstitial oxygen when Ni atoms are incorporated in the film. The Ni L 3,2 absorption edge shows an intensity enhancement in the white-line feature with increasing Ni concentration, evidencing the presence of higher oxidation states. Concurring results were observed by XPS where both Ni2+ and Ni3+ free ion multiplets are present in the Ni 2p core level spectrum for 20% and higher Ni concentration. O K and Zn L 3,2 XAS spectra demonstrated the e g-t 2g sub-band splitting at higher Ni concentration, triggered by band anti-crossing interaction and crystal field splitting. The extended x-ray absorption fine structure (EXAFS) simulation for the Zn K edge revealed a Zn–Zn/Ni bond length change for 60% Ni concentration. The thermal disorder factor increased up to 40% Ni concentration, and beyond that, it decreased due to stable NiO phase dominance in the alloy composite. Ni K edge EXAFS fitting indicated an insignificant change in the Ni-O and Ni–Ni/Zn bond lengths throughout the range of varying Ni concentrations. The thermal disorder factor increases with increasing annealing temperature, indicating a more disordered lattice. Such investigation is essential where the electronic properties of nanometer-sized materials determine the performance of functional devices. The present work critically elucidates the combined impact of compositional variations and annealing temperatures on electronic structures.

Engineered Mott ground state in artificial LaNiO3/(NiO)n/LaNiO3 heterostructure

Understanding and controlling the metal–insulator transition (MIT) can provide great opportunities for electronic devices. Here, artificial LaNiO3/(NiO)n/LaNiO3 has been synthesized. MIT temperatures have been tuned by changing the thickness of the artificial NiO insert layer. The Ni 2p core-level spectra and O K-edge have been investigated. The linear relationship between the hybridization T (or bandwidth W) and the MIT temperature has been clearly demonstrated. In this work, we realized the Mott ground state by modulating the parameters of T and the covalency W, which might be significant for the development of multifunctional materials.

The influence of Gd/Ce substitution on the magnetic properties, electronic structure and the valence of cerium in the solid solution CexGd1−xNi3

The polycrystalline compounds CexGd1−xNi3 with a rhombohedral PuNi3 (x ≤ 0.2) and hexagonal CeNi3 (x ≥ 0.8) – type of crystal structures have been obtained. The change of the lattice parameters may suggest an unstable valence of cerium in the studied system. The effect of partial substitution of Gd by Ce is reflected in a change of the cell volume V(x), the Curie temperature TC(x) and in the temperature dependence of the magnetic susceptibility and electrical resistivity. Thus, TC(x) decreases from 115 K (x = 0) to ∼6.8 K (x = 0.8). The XPS spectra have been measured at room temperature. The valence band spectra (VB) as well as the core level lines have been analysed as influence of the Gd/Ce substitution on the electronic structure. The VB near the Fermi level (EF) is dominated by Ni3d states. In the VB region some slight effects of hybridization and changes of intensity of states on EF have been noticed. The analysis of Ce core level lines reveals the occurrence of possible intermediate valence. The values of the Ce4f - state occupation parameter (nf) and the hybridization energy (Δ) have been estimated. The gradual filling of the Ni3d band is revealed by a reduction of the 6 eV satellite intensities in the Ni2p core level spectrum. All these effects can modify the magnetic properties of the investigated compounds.

Nano-size protein at the surface of a Ti–Zr–Ni quasi-crystalline alloy: Fibronectin adsorption on metallic nano-composites

Surface characterisation of alloys before and after their interaction with proteins allows a more rational design of biomaterials. In this work, fibronectin adsorption on a Ti-based quasicrystals-forming alloy was studied by X-ray photoelectron spectroscopy (XPS) and compared to those obtained on pure titanium substrates (cp Ti). Different fibronectin (Fn) solution concentrations were tested in interaction with the quasi-crystalline/amorphous Ti45Zr38Ni17 composite. N 1s, C 1s, Ti 2p, Zr 3d and Ni 2p core level spectra were systematically recorded. The results indicate that fibronectin is chemically adsorbed on Ti45Zr38Ni17 as well as on cp Ti. The alloy surface composition and oxide thickness are not significantly changed in presence of the adsorbed protein. Data on the elemental distribution in the alloy surface obtained by XPS were completed by depth profiling with Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Surface saturation is clearly reached for Fn solution concentrations of 100–150μgmL−1 for both surfaces. On the plateau, the equivalent protein thickness is 6nm for Ti45Zr38Ni17 and 8nm for cp Ti, both corresponding to one fibronectin monolayer. These results are in accordance with previous observations on osteoblasts adhesion, confirming the biocompatibility of Ti45Zr38Ni17. However, quasicrystals do not seem to influence the alloy surface reactivity.

Investigation of surface and non-local screening effects in the Ni 2p core level photoemission spectra of NiO

We studied the Ni 2p core-level spectra of NiO using cluster model calculations and X-ray photoemission spectra. The calculations were performed for a NiO6, a NiO5, and a NiO6:NiO5 cluster, which allows studying the bulk, the surface, and the non-local effects in the spectra. The photoemission spectra were taken with soft (or hard) X-rays to enhance the surface (or bulk) contribution. The calculation shows that both surface and non-local-effects contribute to the shoulder in the experimental data. The photoemission spectra show that the relative intensity of the surface contribution decreases for Hard X-rays.

Magnetic properties, crystal and electronic structure of GdNi 5− xCu x series

The effect of Ni/Cu substitution on the magnetic properties, crystal and electronic structure of the polycrystalline GdNi 5− x Cu x series has been studied. All compounds crystallize in the hexagonal CaCu5 type of crystal structure (space group P6/ mmm). The temperature dependence of magnetic phase transition ( T mag) estimated from χ AC( T) susceptibility as well as magnetization M( T) below room temperature indicates the maximum for x = 1.0 copper concentration. In the paramagnetic range (above 300 K) the magnetic susceptibilities follow a Curie–Weiss-type dependence. The effective magnetic moments are higher than theoretical value for free Gd 3+. From X-ray photoelectron spectroscopy (XPS) data the valence band as well as the core level spectra have been analyzed. The filling of Ni3d band in the GdNi 5− x Cu x system by charge transfer of Gd conduction electrons is revealed by a reduction of the satellite intensities in the Ni2p core level spectrum. The obtained results exhibit that the valence bands at the Fermi level are dominated by hybridized Ni3d and Gd5d states, when Cu3d states are rather localized about 3 eV below the Fermi level. Quite good relation between the magnetic properties and electronic structure has been found.

Excitonic Insulator State inTa2NiSe5Probed by Photoemission Spectroscopy

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Revisiting the Valence-Band and Core-Level Photoemission Spectra of NiO

We have reexamined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopies. The spectral weight of the lowest energy state was found to be enhanced in the bulk sensitive Ni 2p core-level spectrum. A configuration-interaction model including a bound state screening has shown agreement with the core-level spectrum and off- and on-resonance VB spectra. These results identify the lowest energy states in the core-level and VB spectra as the Zhang-Rice (ZR) doublet bound states, consistent with the spin-fermion model and recent ab initio calculations within dynamical mean-field theory. The results indicate that the ZR character first ionization (the lowest hole-addition) states are responsible for transport properties in NiO and doped NiO.

Unusual Superexchange Pathways in anNiS2Triangular Lattice with Negative Charge-Transfer Energy

We have studied the electronic structure of the Ni triangular lattice in NiGa(2)S(4) using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2p core-level spectrum shows that the S 3p to Ni 3d charge-transfer energy is approximately -1 eV and the ground state is dominated by the d(9)L configuration (L is a S 3p hole). Cell perturbation analysis for the NiS(2) triangular lattice indicates that the strong S 3p hole character of the ground state provides the enhanced superexchange interaction between the third-nearest-neighbor sites.

Surface composition and electronic structure of Ni 2+ xMn 1− xGa studied by X-ray photoelectron spectroscopy

The surface of Ni 2+ x Mn 1− x Ga have been studied using X-ray photoelectron spectroscopy under different conditions of surface preparation. We find that the surface becomes Ni rich by argon ion sputtering. However, the change in surface composition due to sputtering can be reverted by annealing at appropriate temperature so that the bulk composition is restored at the surface. The origin of a satellite feature observed both in the valence band and the Ni 2p core-level spectra is discussed. The magnetic microstructures at the surface have been observed by using magnetic force microscopy.

Electrochromic properties of NiO xH y thin films

NiO x H y films were prepared by DC magnetron sputtering in H 2/O 2 atmosphere. NiO x H y coatings with transparency and high electrochromic efficiency were obtained by changing H 2 content. A 60 nm thick NiO x H y film with transmittance of 0.57 (as-deposited state), 0.78 (bleached state) and 0.24 (coloured state) at wavelength of 550 nm was deposited in an atmosphere of H 2(60%)+O 2(40%). Analysis of infrared spectra (6000∼2400 cm −1) showed that the absorption peaks for bleached and colored states are associated with free ‘OH’ and OH stretching vibrations, respectively. XPS Ni2p core level spectra of colored NiO x H y film exhibited a peak at 856.2±0.2 eV which is attributed to Ni 3+. Ni2p core level spectra of the bleached and as-deposited films exhibited two peaks at 856.4±0.2 and 854.6±0.2 eV which are attributed to Ni 3+ and Ni 2+.

The interaction of oxygen with ultrathin Ni deposits on yttria-stabilized ZrO2(1 0 0)

The interaction of oxygen with vapor-deposited Ni on yttria-stabilized ZrO2(100) and the thermal stability of the oxygenated deposit were studied by XPS, UPS and CO TPD experiments, for two different Ni deposits. For 0.4 ML of Ni deposited at 300 K, the Ni2p core level spectra show complete oxidation above 110 L of O2 at 300 K, whereas for 5 ML of Ni deposited at 570 K, the Ni2p and the UP spectra show that oxidation slowly approaches completion above 4500 L of O2 at 420 K. Oxidation extends up to the Ni2+ state and is accompanied by a drastic decrease of CO desorption temperature and desorbed amount. Upon annealing at increasing temperature, oxide decomposition is in both cases already complete at 870 K. At 700 K, the low-coverage Ni deposit has been reduced by more than 70%, whereas the high-coverage deposit by less than 40%. The results are discussed in comparison with the well studied interaction of O2 with Ni monocrystals and a few available reports on the oxidation of Ni clusters on other substrates.