Surface Review and Letters

Effect of pH on the one-step electrodeposition of Cu(In, Al)Se₂ chalcopyrite layer in the presence of citric acid has been investigated by applying different electrochemical and characterization techniques. It has been observed that at pH of 1.5, nanocrystalline phase of chalcopyrite and small amount of binary phase of Cu₂Se with overall composition of Cu_0.91In_0.32Al_0.39Se₂ have been deposited. On the other hand, at pH of 4, the film composition changed to Cu_1.9In_0.05Al_0.21Se₂ and an additional binary phase of copper selenide (CuSe) has also been formed. Morphological investigation illustrated that smooth and compact layer with fine spherical particles having the size of ~20 nm has been obtained at pH of 1.5 whereas mixture of planar and spherical particles with size of 450–550 nm have been formed at pH of 4. In alkaline environment (pH~9), the deposition current has been noticeably decreased and no deposition occurred due to the formation of a stable complex of citric acid with metal ions. The mechanism of citric acid interaction with metal ions at different pH has also been studied by cyclic voltammetry measurement.

Formation of nickel–boron–molybdenum (Ni–B–Mo) coating on steel by electroless plating and evaluation of their morphology, hardness and tribological properties post heat treatment at different temperatures for 1 h is investigated. The 25 μm thick coating is uniform and adhesion between the substrate and coating is good. Ni–B–Mo coating was amorphous-like structure in their as-plated condition and by 400°C heat-treated coating, nickel fully crystallized and nickel borides and molybdenum carbide were formed. All coatings exhibited higher hardness than the substrate steel. Hardness values of all coatings up to 400°C did not change distinctively but decreased partly beyond 400°C. Friction coefficient reached lowest value post heat treatment at 300°C but later increased with increasing tempering temperature. Wear resistance was lowest in as-plated coating; however it reached the highest value at 300°C. Worn surface of the coatings showed the abrasive wear as the dominant wear mechanism. An additional adhesive wear mechanism was detected in coating tempered at 550°C. Moreover, our results confirmed that the molybdenum addition improved the thermal stability of the resulting coating. Therefore, Ni–B–Mo coating has potential for application in precision mould, optical parts mould or bipolar plates, where thermal stability is essential.

It has been shown that on both clean and (1 × 1) Be-covered Mo (112) surfaces, oxygen will readily dissociate without any noticeable barrier, and the presence of Be in the furrows facilitates rather than hinders the dissociation. The O₂ molecules can also be adsorbed on Be rows in several metastable configurations, which can be considered precursor states for the dissociation. The O atoms will occupy bridge-on-row sites, which remain available for oxygen in presence of the (1 × 1) Be monolayer, since Be atoms occupy adsorption sites in furrows. This structure of Be–O monolayer is metastable, such that the O atoms, being slightly shifted from symmetrical positions in bridge-on-row sites, tend to move towards Be atoms and spontaneously form Be₂O or, at higher oxygen exposures, BeO, in agreement with recent AES studies of this system. The estimated gain in energy due to BeO formation on the Mo (112) surface is 1.17 eV. At lower oxygen coverages, the formation of Be₂O is more favorable (by 0.35 eV) than the configuration of BeO with some nonoxidized Be adatoms on the surface.

Single-molecule magnets (SMMs) have unique magnetic properties such as quantum tunneling of magnetization and quantum coherent oscillation, which have potential applications in quantum computation and information storage. In this paper, using the tip-deposition method, we have grafted individual Fe₄ molecules onto the semi-metallic Bi(111) surface. Low temperature scanning tunneling microscope (LT-STM) was used to characterize the molecular morphology and electronic structures. It was found that individual Fe₄ molecules reveal a triangle shape, which is consistent with the molecular structure of Fe₄. Scanning tunneling spectroscopy (STS) analysis indicated that the HOMO–LUMO gap is 0.49 eV. These studies provide direct information about the adsorption of individual SMMs on semi-metal surfaces.

Yttria-stabilized zirconia (YSZ) coatings have been frequently used as a thermal protective layer on the metal or alloy component surfaces. In the present study, ZrO₂-7%Y₂O₃ thermal barrier coatings (TBCs) were successfully deposited by DC (direct current) plasma spray process under very low pressure conditions (less than 1 mbar) using low-energy plasma guns F4-VB and F100. The experiments were performed to evaluate the thermal shock resistance of different TBC specimens which were heated to 1373 K at a high-temperature cycling furnace and held for 0.5 h, followed by air cooling at room temperature for 0.2 h. For comparison, a corresponding atmospheric plasma spray (APS) counterpart was also elaborated to carry out the similar experiments. The results indicated that the very low pressure plasma spray (VLPPS) coatings displayed better thermal shock resistance. Moreover, the failure mechanism of the coatings was elucidated.

In this work, pure indium and aluminum targets were co-sputtered in a reactive argon–nitrogen environment at 200°C to deposit InAlN film on the GaAs substrate in the presence of a ZnO buffer layer. The as-grown film was annealed at 750°C for 1 h in a high temperature furnace under nitrogen ambient. XRD pattern of the as-grown film did not display any diffraction peak relating to the InAlN due to its poor structural crystallinity, however, the annealed film exhibited InAlN diffraction peaks corresponding to (002), (101) and (102) planes. A significant increase in the grain size and the surface roughness was observed after the films' annealing. Raman spectroscopy revealed A₁ (LO) and E₂ (high) phonon modes whereas the PL analysis showed a luminescence peak at 2 eV in the annealed film. The Hall measurements indicated an increase in the carrier concentration and electron mobility after the annealing which was accompanied by a decrease in electrical resistivity of the film. The dark current–voltage (I–V) characteristics of the as-grown and the annealed films were also recorded to investigate the barrier height and the ideality factor.

TiNi composite thin films were fabricated using a closed-field unbalanced magnetron sputtering system equipped with optical emission spectroscopy monitor (OEM). The thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation. Results show that the TiNi films are amorphous, and their composition varies approximately linearly with the OEM value. Thus, the film composition could be controlled by in situ real-time OEM. The structure of the single B2 parent phase was observed in the annealed TiNi film. The hardness and elastic modulus of the films increased because of the precipitation of the Ti₃Ni₄ phase in the single B2 parent phase.

Antimony doped tin oxide (ATO) nanoparticles were prepared by wet chemical coprecipitation method with different contents of polyvinyl alcohol (PVA) dispersant. The prepared ATO nanoparticles have been characterized by means of XRD, SEM, HRTEM, SAED, EDS, bulk density and electrical resistivity measurement. Results indicated that the approach functionalized by PVA dispersant enables a synchronous improvement of two important properties namely the dispersibility and electrical conductivity due to the mechanism of avoiding the formation of agglomeration of nanoparticles, which could be regarded as primary factors for the enhanced electron transfer of powders: The surface area over which are crucial for the interfacial arrangement and electron charge scattering/transfer processes. The bulk density and electrical resistivity decreased to a minimum of 0.90 g/cm³ and 1.44 Ω⋅cm at PVA dispersant content of 5%, and increased rapidly at higher PVA contents. The prepared ATO nanoparticles can serve as a kind of effective conductive filler in insulating species such as plastics, textile and rubber.

The aim of this work is to synthesize polypyrrole (PPy) films on nonconducting cellulosic substrate and modified by copper oxide particles for use in the nitrate electroreduction process. Firstly, the chemical polymerization of polypyrrole onto cellulosic substrate is conducted by using FeCl₃ as an oxidant and pyrrole as monomer. The thickness and topography of the different PPy films obtained were estimated using a profilometer apparatus. The electrochemical reactivity of the obtained electrodes was tested by voltamperometry technique and electrochemical impedance spectroscopy. Secondly, the modification of the PPy film surface by incorporation of copper oxide particles is conducted by applying a galvanostatic procedure from a CuCl₂ solution. The SEM, EDX and XRD analysis showed the presence of CuO particles in the polymer films with dimensions less than 50 nm. From cyclic voltamperometry experiments, the composite activity for the nitrate electroreduction reaction was evaluated and the peak of nitrate reduction is found to vary linearly with initial nitrate concentration.

Gold atomic aggregates are fabricated by vapor-depositing Au atoms onto a silicone oil surface and the microstructure evolution is investigated by atomic force microscopy (AFM) observation. It is found that the Au aggregates are composed of Au circular nanoparticles with diameter around 45 nm, which is independent with the nominal film thickness d. As d increases from 1 nm to 15 nm, the height of the nanoparticles increases from 15 nm to 25 nm, indicating the geometric shape of the Au nanoparticles evolves from plateau to spherical. Furthermore, the roughness analysis shows that the mean surface roughness increases linearly with d in the range of 1 nm–15 nm, which is quite different from the findings in Ag system. The anomalous microstructure evolution of Au aggregates suggests that the growth of Au aggregates may be dominated by the shadowing effect.

In this paper, the thermal diffusion behavior of small two-dimensional Ag-islands on Ag(1 1 1) surface has been explored using molecular dynamics (MD) simulations. The approach is based on semi-empirical potentials. The key microscopic processes responsible for the diffusion of Ag₁₋₅ adislands on Ag(1 1 1) surface are identified. The hopping and zigzag concerted motion along with rotation are observed for Ag one-atom to three-atom islands while single-atom and multi-atom processes are revealed for Ag four-atom and five-atom islands, during the diffusion on Ag(1 1 1) surface. The same increasing/decreasing trend in the diffusion coefficient and effective energy barrier is observed in both the self learning kinetic Monte Carlo (SLKMC) and MD calculations, for the temperature range of 300–700 K. An increase in the value of effective energy barrier is noticed with corresponding increase in the number of atoms in Ag-adislands. A reasonable linear fit is observed for the diffusion coefficient for studied temperatures (300, 500 and 700 K). For the observed diffusion mechanisms, our findings are in good agreement with ab initio density-functional theory (DFT) calculations for Al/Al(1 1 1) while the energy barrier values are in same range as the experimental values for Cu/Ag(1 1 1) and the theoretical values using ab initio DFT supplemented with embedded-atom method for Ag/Ag(1 1 1).

Nanostructured CoFe thin films were prepared on the glass substrates by spray pyrolysis deposition (SPD) method. The as-deposited films were separately annealed at 500°C in ambient air and hydrogen atmospheres to investigate the effect of annealing atmosphere on different properties of the deposited films. Structural, morphological, magnetic and electrical properties of annealed thin films were investigated by the X-ray diffractometer (XRD), scanning electron microscope (SEM), vibrating sample magnetometer (VSM) and four-point probe measurements, respectively. XRD patterns show that by changing the annealing atmosphere from ambient air to hydrogen, the crystal structure transformed from cubic spinel phase (Fd-3m) to cubic CsCl-type phase (Pm-3m). SEM images demonstrated that the morphology, grain size and thickness of annealed thin films depend greatly on the atmosphere type. Thickness of the films annealed in ambient air and hydrogen atmosphere were 378 and 356 nm, respectively. The morphology of the films changes from small sized granular for the ambient air-annealed films to coral-like structures for the hydrogen atmosphere annealed films. The magnetic properties of the annealed thin films depend on the annealing atmosphere, the applied field directions and film microstructural properties. Hydrogen annealing induced a perpendicular magnetic anisotropy in CoFe thin films. Room temperature hysteresis measurements show soft magnetic behavior of the hydrogen atmosphere annealed CoFe films in the perpendicular direction suggested its application in perpendicular magnetic recording media.