Surface Review and Letters

A quantum mechanical approach is developed to calculate the surface-induced electrical conductivities of very thin metallic films with isotropic and anisotropic surfaces. Two particular cases are analyzed with this formalism: (1) films with isotropic surfaces and (2) films with anisotropic surfaces which have different morphological properties along two orthogonal directions. It is shown that, depending on the differences between these morphological properties, the surface-induced resistivities can be different along these directions. In order to investigate the validity of these predictions we have fabricated Pt films, with thickness in the 0.90 ≤ d ≤ 11.10 nm range, with very different morphological properties along two orthogonal directions. Measuring the electrical resistivities of these films, we have found different resistivities along these directions. We show that this anisotropic resistivity can be well explained by our theoretical approach.

Bi₂Te₃ has attracted attention due to its potential applications in the microfabrication of integrated thermoelectric devices. It is also interesting to study the metallization process of this compound. Metallic nanostructures were deposited by means of an electron gun evaporator in ultra high vacuum (UHV) conditions (10^-8 Pa) on the freshly cleaved 0001 surface of the crystal Bi₂Te₃. Measurements were conducted using the commercially available Omicron UHV scanning tunneling microscope (STM). Scanning tunneling spectroscopy (STS) measurements were performed using current imaging tunneling spectroscopy (CITS), and subsequent calculation of the dI/dV maps. Metallic characteristics were observed on nickel islands since early stages of the growth. CITS and dI/dV maps showed distinct contrast between the substrate and metallic islands. Similar contrast was not observed in the case of titanium, most probably due to an intercalation process. Occurring of such a process was confirmed by the appearance of the superlattice structure.

In this paper, the applications and qualifications of biomedical materials are introduced. In regard to the hard tissue implants, the biocompatibility can be improved by preparing various bio-ceramic and bio-glass coatings. In view of this, the principles, characteristics, and applications of surface modification (plasma spraying, electrostatic spray deposition, micro-arc oxidation, pulsed laser deposition, sol–gel deposition, and magnetron sputtering) in biomedical materials are reviewed. In addition, the research direction of improving biocompatibility by surface modification is presented.

Plasma-spraying parameters are of crucial importance to the fabrication of high-quality hydroxyapatite (HA) coating for biomedical use. In this paper, hydroxyapaptite was coated onto carbon fiber reinforced carbon composites (C/C composites) by a plasma spraying method. The effect of stand-off distances on the microstructure, phase composition and shear strength of hydroxyapatite coatings was studied. X-ray diffraction (XRD) was used to characterize the phase composition and scanning electron microscopy (SEM) examined the morphology of the sprayed surface and polished cross-sections. The shear strength of the HA coatings–C/C substrates was detected on a RGD-5 electric tension machine. The interface of the HA coating–C/C composites was also detected. Results showed that the extent of melting of particles increased with increasing stand-off distances, while the crystallinity of the HA decreased at a power level of 30 kW. Mechanical bonding plays a dominant role at the interface of the rough substrate and the deformed HA particles.

Tris(8-hydroxyquinolato) aluminum (Alq₃), a prototypical molecule for organic light-emitting devices, has been studied via synchrotron radiation photoemission to investigate (1) the surface electronic structure of the molecules at room temperature and at elevated temperatures, (2) adsorption onto the inorganic Si(001)-2×1 surface, and (3) doping with the alkaline metal Mg. For case (1), three chemical environments of carbon are resolved. Moreover, the shake-up satellite structures are detected in all the N 1s, C 1s, O 1s, and Al 2p core-level spectra, but with different magnitudes. Annealing allows for a charge redistribution within Alq₃ itself. As to case (2), the organic molecules not only passivate the dangling bonds, but also rupture the dimer bonds. The wave function of the surface dangling bonds and of the pyridyl side of an 8-quinolinol ligand overlaps greatly so that charge is polarized towards, the organic adlayer. However, the polarization diminishes at greater coverage. With regard to case (3), the N 1s core-level spectra appear as an Mg-induced charge-transfer component with a binding energy lower than the original component. This new component grows gradually in intensity with increasing concentration of the dopant. Moreover, Mg also affects the O 1s core, as manifested by a component lying at a +1.09 eV higher binding energy than the original component. The Mg 2p core-level spectra, although rather broad, exhibit a shift toward a lower binding energy with increasing Mg vapor. Upon examining all these experimental results, we propose that Mg in the surface Alq₃ molecules forms clusters. Each cluster attaches to a pyridyl ring, affecting not only the nitrogen atom at that ring, but also the oxygen atom in the adjacent phenoxide ring. The depleted charge in the affected oxygen then flows about its adherent ligand and resides on the pyridyl ring at that ligand, resulting in a high Alq₃ anion state.

The adsorption of acetonitrile (CH₃CN) on Cu(001) has been studied using elastic Helium atom scattering (HAS) and has been analyzed using a novel method of direct HAS diffraction image inversion. At low temperatures (138 K) acetonitrile permanently adsorbs forming a c(4 × 2) structure at saturation. Annealing the saturated surface to temperatures above 200 K causes the desorption of all surface moieties leaving clean Cu(001). At the adsorption temperatures in the range 153–163 K acetonitrile does not adsorb permanently and demonstrates adsorption–desorption equilibrium. These dynamical studies suggest that the acetonitrile molecule does not dissociate during adsorption. Analyses of the c(4 × 2) HAS diffraction image have revealed several possible surface structures where the C–C–N axis of the adsorbed molecule lies approximately parallel to the Cu(001) surface.

Coagulation of colloidal alumina in aqueous solutions in the absence or presence of alkyl sulfates has been studied by means of measurements of electrokinetics, adsorption, and coagulate size in this work. The experimental results showed that the coagulation of colloidal alumina in aqueous alkyl sulfate solutions was much stronger than that in aqueous electrolytic solutions. It closely correlated with particle hydrophobicity rendered by the adsorption of alkyl sulfate anions on alumina/water interfaces, indicating hydrophobic coagulation. Also, it has been found that the hydrocarbon chain length of alkyl sulfate strongly influences the hydrophobic coagulation. The longer the chain, the stronger the coagulation and the lower the alkyl sulfate concentration needed for achieving the maximum coagulation degree.

The electronic structure of AgCd₂GaS₄ crystal has been studied with X-ray photoelectron spectroscopy (XPS). Chemical bonding effects have been observed by comparative analysis of binding energies of element core levels and crystal structure of AgCd₂GaS₄ and several ternary sulfides. It has been shown for Ga-bearing sulfides that the increase of mean chemical bond length between gallium and sulfur ions is directly related to the decrease of chemical shift of cation core level binding energy.

The surface concentrations and concentration depth profiles to the (110) surface of an Au₇₅Pd₂₅ alloy is studied by modified analytical embedded atom method (MAEAM) with the Monte Carlo simulations. The results indicate that Au enriched in the two topmost layers, but depleted in the third layer. The Au concentration in the non-reconstructed surface is less than that in the reconstructed surface. Au concentration in third layer of reconstructed surface, which is more agreement with experimental data in present simulations, is about 63% 61% and 55%, at 800K, 600K and 400K respectively. Thus the present simulations are helpful for a better understanding of surface segregation of AuPd alloys.

In the middle of the previous century, G. K. Boreskov, and J. Horiuti and S. Enomoto independently showed that for reversible reactions, running via a one-route mechanism with a rate-limiting step, there exist general relationships between the reaction rates in the forward and backward directions and also between the corresponding apparent activation energies and reaction heat. Their treatments are formally applicable to gas- and liquid-phase reactions and also to heterogeneous catalytic reactions (HCR) occurring in an ideal overlayer adsorbed on a uniform surface. In reality, HCR often run on heterogeneous surfaces, and the HCR kinetics are complicated by adsorbate–adsorbate lateral interactions. I explicitly demonstrate that in such situations the Boreskov–Horiuti–Enomoto rules are applicable as well.

ZnO thin films were prepared on silicon (001) and corning glass substrates using Pulsed laser deposition (PLD) technique with different oxygen pressures. The microstructure, crystallinity, and resistivity of the films depend on the oxygen pressure used. The effects of the films grown at room temperature and at 500°C with different oxygen pressures have been investigated by analyzing the optical and electrical properties of the film. The XRD analysis showed that the high intensity of c-axis orientation of ZnO thin films was obtained under high oxygen pressure and this leads to greater electrical and optical properties. By applying high pressure oxygen, the resistivity value was decreased and optical transmittance became higher in the visible region. The surface morphology of the films showed that the smooth surface was observed without any cracks.

Surface morphology of the {101} freshly cleaved planes of L-arginine trifluoroacetate crystals grown from aqueous solutions have been studied by atomic force microscopy. Various step patterns are described and discussed. Moreover, the dislocations are not easily found and all of them are devoid of hollow cores on the emergence points. The reason may be that the stress field is too weak to generate hollow cores.

SrBi₄Ti₄O₁₅ (SBTi) thin films were prepared on (100)- and (110)-oriented LaNiO₃(LNO) electrodes by a metalorgranic decomposition (MOD) technique at an annealing temperature of 650°C. c-axis-oriented SBTi thin film with volume fraction of 0.89 can be formed on a (100)-oriented LNO film due to the epitaxial relationship between c-axis-oriented SBTi and LNO(100). In contrast, SBTi film deposited on LNO(110) shows random orientations with strong (119) and (200) peaks. The remanent polarization (P_r) and coercive field (E_c) of the random oriented SBTi film were 18.1 μC/cm² and 70 kV/cm, respectively. This suggests that (110)-oriented LNO electrode is a better choice for obtaining SBTi films with higher volume fraction of a(b)-axis-orientated grains.

Single crystals of nonlinear optical material, L-arginine trifluoroacetate (abbreviated as LATF), were grown from an aqueous solution by the low temperature solution growth method. The cell parameters of the grown crystal were determined by the X-ray powder diffraction analysis. Atomic force microscopy (AFM) was used to investigate the surface morphology of {101} cleavage faces of LATF. Straight steps and two-dimensional nuclei were observed. Liquid inclusions and impurities as defects for revealing macrosteps were demonstrated. The molecular structure and crystal structure correlative with surface morphology were discussed.

Zirconium diboride is widely applied to high-temperature materials, but it is easily oxidized at high temperature. To increase the oxidation resistance of zirconium diboride at high temperature, the A1(OH)₃–Y(OH)₃ is coated on the ZrB₂ surface to prepare A1(OH)₃–Y(OH)₃/ZrB₂ composite particles. In this paper, the effect of coating content on the properties of A1(OH)₃–Y(OH)₃/ZrB₂ composite particles is investigated. It is analyzed that the particle size and particle size distribution of A1(OH)₃–Y(OH)₃/ZrB₂ composite particles is increased with the coating content. The dispersion of ZrB₂ particles is largely increased with the coating content of 0%–20%; the dispersion of ZrB₂ particles is similar when the coating content is from 20% to 30%. The oxidation resistance ratio of the ZrB₂ particles with 30% coating content is the best than that of other conditions—it is about three times more than that of the original ZrB₂ particles.

Diamond films were deposited on the WC/Co alloy substrates by a hot-filament chemical vapor deposition (HF CVD) reactor after the substrate surfaces were chemically pretreated with the two-step etching method. Some characteristics as morphology, texture, adhesion, and chemical quality of the diamond coatings on WC/Co alloy substrates were investigated by means of X-ray diffractometer (XRD), scanning electron microscope (SEM), hardness tester, and Raman spectrum. The results indicate that increasing the Co content from 0.12% to 3.22% within the etching depth of 5–10 μm caused a morphology transformation from prism diamond to spherical diamond, and a texture one from a {111} orientation to a {110} orientation. The Raman spectrum shows that the spherical diamond film contains more non-diamond phases (graphite, amorphous carbon, diamond-like carbon, etc.) and has lower chemical quality of diamond films. The diamond coated grain sizes became about four to five times smaller when the deposition temperatures on the substrate surface were reduced from 900°C to 800°C. Compared with the spherical diamond films, the prism diamond films exhibit better adhesion on the WC–Co substrates. It is also observed that the microcrystalline orientation diamond thin films with grain sizes of 1–3 μm on WC–Co substrate were formed under the circumstances of lower deposition temperature and higher gas pressure, and the microcrystalline growth mechanism of diamond thin films with a preferential orientation on WC/Co alloy is discussed.

Although particle size and its measurement are intuitively familiar to particle technologists, the concept of point of zero charge (pzc) is less widely understood and applied. This is unfortunate since it is at least as fundamentally important as particle size in determining the behavior of particulate materials, especially those with sizes in the colloidal range below a micrometer. pzc is related to the charge on the surface of the particle and strongly depends on the pH of the material; so it influences a wide range of properties of colloidal materials, such as their stability, interaction with electrolytes, suspension rheology, and ion exchange capacity.

The pH dependence of surface charges was quantified for four different adsorbent–aqueous solution interfaces. The points of zero charge were determined for activated charcoal, granite sand, lakhra coal, and ground corn cob materials using three methods: (1) the pH drift method, measuring pH where the adsorbent behaves as a neutral specie; (2) potentiometric titration, measuring the adsorption of H⁺ and OH^- on surfaces in solutions of varying ionic strengths; (3) direct assessment of the surface charge via nonspecific ion adsorption as a function of pH. The intrinsic acidity constants for acid and base equilibria, and , were also calculated. Lakhra coal was found to have the lowest pzc value among all other adsorbents studied owing to the presence of a large amount of humus material. The results were used to explain general connections among points of zero charges, cation exchange capacity, and base saturation % of adsorbents.

A theory is presented for the magnetostatic modes in ferromagnetic and antiferromagnetic nanotubes, which have a large length-to-radius aspect ratio and an external magnetic field parallel to the cylindrical axis. The surface and bulk magnetic excitations (or magnetostatic spin waves) are studied for cases where the dipole–dipole interactions are dominant in the spin dynamics. This situation can be realized at sufficiently small wavevectors by inelastic light scattering or magnetic resonance techniques. A macroscopic continuum theory is developed, using the magnetostatic form of Maxwell's equations and the electromagnetic boundary conditions, and the characteristic equations (or dispersion relations) are deduced for the magnetostatic modes. Numerical calculations are presented for ferromagnetic and antiferromagnetic nanostructures, taking Ni₈₀Fe₂₀ and MnF₂, respectively. The spatial variations of the mode amplitudes are also investigated.

In calculating band structure, the local density approximation and density functional theory are widely popular and do reproduce a lot of the basic physics. Regrettably, without some fine tuning, the local density approximation and density functional theory do not generally get the details of the experimental band structure correct, in particular the band gap in semiconductors and insulators is generally found to be too small when compared with experiment. For experimentalists using commercial packages to calculate the electronic structure of materials, some caution is indicated, as some long-standing problems exist with the local density approximation and density functional theory.

In this work, tribological investigations on the neat polyester (NP) and woven (600 g/m²)-glass fabric reinforced polyester (WGRP) composite were carried out. Friction and wear characteristics of the WGRP composite were measured in three principal orientations, i.e., sliding directions relative to the woven glass fabric (WGF) orientations in the composites. These are longitudinal (L), transverse (T), and parallel (P) orientations. The experiments were conducted using a pin-on-disc (POD) machine under dry sliding conditions against a smooth stainless steel counterface. Results of friction coefficient and wear resistance of the composites were presented as function of normal loads (30–100 N) and sliding distances (0.5–7 km) at different sliding velocities, 1.7, 2.8, and 3.9 m/s. Scanning electron microscopy (SEM) was used to study the mechanisms of worn surfaces. Experimental results revealed that woven glass fabric improved the tribological performance of neat polyester in all three tested orientations. In L-orientation, at a low velocity of 1.7 m/s, WGRP exhibited significant improvements to wear resistance of the polyester composite compared to other orientations. Meanwhile, at high velocities (2.8 and 3.9 m/s), T-orientation gave higher wear resistance. SEM microphotographs showed different damage features on the worn surfaces, i.e., deformation, cracks, debonding of fiber, and microcracks.

The micro-laminated (ZrO₂–Y₂O₃)/(Al₂O₃–Y₂O₃) composite coatings were prepared on the surface of Fe–25Cr alloy by an electrochemical process and a sintering process alternately. FE-SEM analyses show that the cross section of the micro-laminated coatings has alternate six-layer films of ZrO₂–Y₂O₃ and Al₂O₃–Y₂O₃, with the thickness of each layer in the range of nanometer or submicron. The surface of the micro-laminated coatings is composed of nanoparticles. SEM, XRD, and mass gain measurements were adopted to study the oxidation resistance of coatings on the Fe–25Cr alloy. It has been found that all the coatings are effective in protecting the substrate from oxidation, and micro-laminated coatings exhibit more excellent protectiveness performance, and the oxidation resistance is increased with increasing the layers of the micro-laminated coatings. These beneficial effects can be contributed to the mechanism, by which such a micro-laminated coating synthesizes all the beneficial effects and overcomes all the disadvantages of both the ZrO₂–Y₂O₃ coating and the Al₂O₃–Y₂O₃ coating during the oxidation of the alloy.

The most stable structures for the dissociation of phosphine and arsine on Ge(100)(2x1) surface have been investigated by relative total energy calculations based on Density Functional Theory. It has been found that the thermodynamically preferred structures in the dissociation path of phosphine and arsine are the same; PH₂ and AsH₂ products prefer to be on a single Ge dimer bond, but PH and AsH prefer to be between the adjacent Ge dimers. According to the optimization calculations, the dissociation path started with the adsorption of PH₃(AsH₃) on the electron deficient side of the Ge dimer bond is ended with the formation of P–P (As–As) dimers parallel to the dimers of Ge.

Mo and C ions extracted from a metal vapor vacuum arc ion source were implanted into the surface of die steel (H13) to compare the wear resistance mechanisms of the implanted samples, respectively. The concentration depth profiles of implanted ions were measured using Rutherford backscattering spectroscopy and calculated by a code called TRIDYN. The structures of the implanted steel were observed by X-ray photoelectron spectroscopy and grazing-angle X-ray diffraction, respectively. It was found that the conventional heat-treated H13 steel could not be further hardened by the subsequent implanted C ions, and the thickness of the implanted layer was not an important factor for the Mo and C ion implantation to improve the wear resistance of the H13 steel. Mo ion implantation could obviously improve the wear resistance of the steel at an extraction voltage of 48 kV and a dose of 5 × 10¹⁷cm^-2 due to formation of a modification layer of little oxidation with Mo₂C in the implanted surface.