Surface Review and Letters

The binding energy of a hydrogen-like impurity in an ellipsoidal parabolic quantum dot under the magnetic and electric fields have been discussed by using the effective mass approximation and the variational method. We have calculated the effects of the magnetic and electric fields on the binding energy of donor impurities in the quantum dots with different confinement potentials. We conclude that the structural confinement is very effective, and especially in the weak confinement potential case the magnetic field dependence of the donor binding energy is more pronounced.

The DC conduction mechanisms in metal–insulator–metal sandwich structure based on amorphous thin films of SnO₂ have been studied in the thickness range 100–400 nm, in the substrate temperature range 293–543 K, and in the annealing temperature range 473–773 K, and the results are discussed in terms of current theory. It is observed that at low field and low temperature the conduction mechanism is found to obey the hopping model, at higher temperature the conduction takes place by transport in the extended states but at high field the main barrier lowering effect is associated with localized centers. The increase in electrical conductivity with film thickness is caused by the oxygen vacancies and defects which generate carriers in the films. The increase in electrical conductivity due to an increase in substrate temperature is ascribed to the increasing concentration of ionized donors and hoping of electrons between metal ions in two different valence states. The formation of tin species of lower valence states and doubly ionized oxygen vacancies are thought to be responsible for the increase in electrical conductivity at higher annealing temperature.

Monte Carlo simulations of the kinetics of migration and coalescence of nm-sized metal particles on a uniform surface and a surface with traps in the physically reasonable case when the diffusion coefficient of particles varies inversely as the fourth power of the particle size are presented. The Monte Carlo kinetics are demonstrated to be in good agreement with the mean-field results. The size distribution of particles is found to be nearly symmetrical with respect to the average value.

The Fe–C–H interaction near defects in iron structures was studied using qualitative structure calculations in the framework of the atom superposition and electron delocalization molecular orbital. Calculations were performed using three Fe clusters to simulate an edge dislocation, a divacancy; both in bcc iron and a stacking fault in an fcc iron structure. In all cases, the most stable location for C atom inside the clusters was determined. Therefore, H atom was approximated to a minimum energy region where the C atom resides. The total energy of the cluster decreases when the C atom is located near the defects zone. In addition, the presence of C in the defects zone makes no favorable H accumulation. The C acts as an expeller of H in a way that reduces the hydrogen Fe–Fe bonds weakening.

In this work, the influence of laser pulse trains on thin mild steel sheet blades is investigated. The method is based on varying the duty cycle (DC) of a pulsed RF slab CO₂ laser during the process of laser percussion drilling. Here, an experimental methodology to correlate laser parameters and their effects on the characteristics of the melting material ejected was carried out. In the experiments, the interaction time such that of 5%, 10%, and 15% DC were considered for the pulse trains. Results showed a significant effect on the characteristics of melt material ejection during laser percussion. We demonstrate that it was not evident the influence and molten material for pulse width of less than 500 μs and larger than 4500 μs, respectively, when having 5% DC; spatter formation (droplets) for 10% and 15% DC with pulse periods > 2000 μs; debris formation (re-solidified material) at pulse periods from 200 to 1250 μs at 15% DC.

This paper describes the oxygen adsorption properties on magnesium oxide surface. The results are compared with theoretical adsorption kinetics. Temperature and time dependences of adsorption mechanisms and chemiluminescence are discussed.

Synthesis of the precursors, using research grade materials and locally available facilities, is optimized in order to obtain a suitable sol for BaTiO₃. The optimally synthesized BaTiO₃ sol is then spun onto Cu substrates to obtain films of thickness ~0.5 μm. The deposited BaTiO₃ films are heated up to a temperature of 300°C. Surface and structural changes, with heating, are observed by optical microscope, scanning electron microscope, atomic force microscope, and X-ray diffraction. The surface cracked in some cases but is generally uniform, as seen with SEM and AFM, in case of optimized conditions. The SEM images reveal large crystallites, which are correlated to the XRD observation of tetragonal perovskite structure of these sol–gel films. The AFM scan, however, shows an rms roughness of 35.6 nm for an area of 3 μm × 3 μm. Tetragonal perovskite BaTiO₃ is obtained at a temperature of as low as 300°C. The dielectric constant and loss angle, in the 30 Hz–3 MHz range, are calculated at room temperature and up to 200°C. At high frequencies the dielectric constant (~290) and loss angle (~2 × 10^-6) values indicate the usefulness of such films in electronic applications. The leakage current is also measured at room temperature and a conductivity of 2.0 × 10^-15S/cm is obtained.

Electric double layer capacitors (EDLCs) with carbon nanotubes (CNTs) film electrodes and gel polymer/polyacid electrolytes have been demonstrated. The low-cost CNTs film is directly grown on Cu–Ni current collector by low pressure and low temperature thermal chemical vapor deposition. The electrolytes consist of gel polymer poly(vinyl alcohol), polyacid phosphomolybdic acid (PMA) with different concentrations from 10 to 40 wt.% and KCl. The electrochemical measurement of the EDLCs by cyclic voltammetry and chronopotentiometry shows that gel polymer/polyacid electrolytes can work stable at a wide potential range of -1.5 to 1.5 V and EDLCs with electrolytes containing 30 wt.% PMA exhibit optimum capacitive behavior with 8.09 F/g specific capacitance.

The addition of hydrogen to the carbon–carbon double bond of 2-butenes adsorbed on Pd(111) was studied within the density functional theory (DFT) and using a periodic slab model. For that purpose, the Horiuti–Polanyi mechanisms for both complete hydrogenation and isomerization were considered. The hydrogenation of cis and trans-2-butene to produce butane proceeds via the formation of eclipsed and staggered-2-butyl intermediates, respectively. In both cases, a relatively high energy barrier to produce the half-hydrogenated intermediate makes the first hydrogen addition the slowest step of the reaction. The competitive production of trans-2-butene from cis-2-butene requires the conversion from the eclipsed-2-butyl to the staggered-2-butyl isomer. As the corresponding energy barrier is relatively small and because the first of these isomers is less stable than the second, an easy conversion is predicted.

In this work, a porous layer on an n⁺ emitter by a chemical route was realized. The MEB observation shows a nanoporous shape of the surface. The subsequent contact depositions needs a heat treatment that has an influence on hydrogen and oxygen distributions in the porous layer. After heat treatments, the porous silicon layer is analyzed by secondary ion mass spectroscopy. The concentration profile of light elements like H, O, C, F and N are measured and compared with untreated porous layer. The results show that oxygen is present at high level at ambient temperature and then decreases from 25°C to 775°C. This means that oxygen desorption is observed on the surface. At 800°C the oxygen content increases again showing an oxidation of porous layer surface. For hydrogen, the concentration decreases from the ambient temperature until 750°C was noted. Then the hydrogen is restored at its first concentration. The FTIR spectra correlate this hydrogen distribution. The absorption spectra show the appearance of Si–H_x bonds (with x = 1, 2, 3) at 2089, 2115, and 2140 cm^-1, respectively. Both elements O and H are present deeply in the porous silicon layer as shown by the sputter time.

A simply modified bath deposition technique has been successfully used to deposit uniform CdS thin films using cadmium chloride or cadmium acetate as the cadmium ion source, and thiourea as the sulfur source on glass substrates. Both the traditional magnetic agitation and the frequent substrate vibration by hand were done simultaneously during the deposition. Various properties of the deposited films such as surface morphology, crystallinity, and optical properties were investigated. The CdS films deposited from cadmium acetate show a good crystallinity and exhibit a preferential orientation along the hexagonal (002) direction. Their surface morphologies are more homogeneous with smaller grains than those from cadmium chloride. The CdS films prepared from both cadmium ion sources present a high optical transmission (more than 80%) in the visible range with the band gap value of about 2.4 eV. The substrate vibration reduces the formation and residence of gas bubbles on the glass substrate during growth and resulted in deposition of CdS thin films with high quality. XRD, SEM, and UV–Vis measurements have provided the supported data. The fundamental CBD growth mechanisms involving different cadmium salts are discussed.

La₂O₃ films were deposited on Si(100) substrates by ion beam assistant electron beam evaporation and were annealed at 450–900°C. Crystalline structure was found to change from amorphous structure to cubic and hexagonal structure with the rise in annealing temperature. The near infrared transmittance of the film annealed at 600°C was relatively high and could be further increased using assistant ion beam in deposition. The maximum leakage current density increased dramatically from 10^-7 to 10^-4 A/cm² with the rise in annealing temperature. The film annealed at 600°C with cubic structure had a relatively high dielectric constant of ~18.

The corrosion inhibition of mild steel in 1 M H₂SO₄ in the presence of polyvinylpyrollidone (PVP) and polyacrylamide (PA) as inhibitors at 30–60°C was studied using gravimetric and gasometric techniques. The inhibition efficiency (1%) increased with increase in concentration of the inhibitors. Increase in temperature increased the corrosion rate in the absence and presence of inhibitors but decreased the inhibition efficiency. Both PVP and PA were found to obey Temkin adsorption isotherm and Kinetic-Thermodynamic Model of El-Awady at all the concentrations and temperatures studied. Phenomenon of physical adsorption is proposed from the activation parameters obtained. Thermodynamic parameters reveal that the adsorption process is spontaneous. PVP was found to be a better inhibitor than PA.

In the present communication, the monolayer characteristics of pyrene mixed with stearic acid (SA) at the air–water interface have been reported. The monolayer properties are investigated by recording and analyzing the surface pressure–area per molecule isotherm (π–A) of the pyrene–SA mixed films. It is observed that the pyrene and SA are miscible in the mixed monolayer. This miscibility/nonideality leads to phase separation between the constituent components (pyrene and SA). BAM image of the mixed monolayer confirms the miscibility or nonideal mixing at the mixed monolayer.

We report the electronic structure of the interface of the ultrafilm Fe film with the O-terminated ZnO(000-1) polar surface studied by synchrotron radiation photoemission. It was found that Fe grows in a Stranski–Krastanov mode. The turning point of the growth mode was at the Fe coverage of ~2 Å. At low coverage, about 1 ML (monolayer) of iron (ca. 1.5 Å) was oxidized by the terminative oxygen atoms on the ZnO(000-1) surface. Analysis of the valence band and the Fe3p photoemission spectra indicates that below 2 Å coverage, only Fe²⁺ species could be detected at the Fe/ZnO interface, after that the formation of a metallic iron overlayer was observed. A sharp Fermi edge appeared at 5.1 Å coverage, indicating the formation of the bulk metallic Fe film. Furthermore, work function of the sample decreased to a minimum of 4.5 eV at 0.2 Å Fe, and then to 4.9 eV with the formation of a dipole layer at the interface.

Transparent conducting indium tin oxide (ITO) films are deposited on Porous Silicon (PS) substrates by spray pyrolysis technique. In this process, the films are formed over the surface and also incorporated into the pores of PS and thereby making a protecting layer as well as a contacting terminal. Thus, the ITO/PS/Si heterojunction light-emitting devices are fabricated. The growth of ITO on PS is thoroughly investigated by SEM and X-ray diffraction techniques. The features of growth on other substrates like single-crystal p-type (100) silicon and glass are also taken into consideration. The influence on the PS interface is correlated with the electrical and luminescent behavior of the resulting heterojunction diode structure.

The most stable structures for the adsorption and dissociation of phosphine (PH₃) on SiGe(100) (2 × 1) surface have been investigated by relative total energy calculations based on density functional theory. According to the optimization calculations, PH₃ is adsorbed on the Si (down) and Ge (down) site of the Ge–Si and Ge–Ge dimers on SiGe surface, respectively. The PH₂ and H products have been found to be thermodynamically favored in the dissociation path of PH₃ on SiGe surface when the system is thermally activated. Although PH₃ is adsorbed on the Ge–Ge and Ge–Si dimers directly, it dissociates on the SiGe surface by passing through a transition state. The asymmetric Ge–Si and Ge–Ge dimers on SiGe surface are found to be approximately symmetric after the dissociation of PH₃ on the surface. The present work has showed that PH₂ prefers to be adsorbed on Ge site of the Ge–Si dimer. Therefore, the adsorption of PH₂ on Ge site of the Ge–Si dimer, while PH₃ being dissociated on the Si site, has indicated the migration of PH₂ on SiGe surface.

The interaction of hydrogen with a platinum (111) cluster using the atom superposition and electron delocalization–higher binding ASED-TB quantum calculation method was studied. The metal surface was represented by a Pt cluster of seven layers. The effect of hydrogen on this metal substrate was studied by the analysis of density of states and crystal orbital overlap populations curves. The energy surface plots allow us to find a possible diffusion path through the cluster from one side to the other. The Pt–Pt metal bond is weakened during H adsorption and diffusion. The main components in the Pt–H bond are the Pt 6s (31%), 6p (26%), and 5 d_xz (16%) orbitals.

In this work, a Ti-pillared montmorillonite with high thermal stability has been prepared by using a Na-montmorillonite as the host clay and polyhydroxy-titania ions as the pillaring precursor. The formation of Ti-pillared montmorillonite has been confirmed from the characterizations through X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric–differential scanning calorimeter, and specific surface area analyses. In the preparation of Ti-pillared montmorillonite several parameters, such as the type of solvent in which the synthesis is realized, the ratio of polyhydroxy-titania ions and montmorillonite, the intercalation time, the calcining temperature, and calcining time, were tested to understand their effects on the basal spacing. It was shown that this method could produce a Ti-pillared montmorillonite with the basal spacing of 3.74 nm, specific surface area of 409 m²/g, and mean pore size of 2.94 nm, as well as a high thermal stability up to 900°C.