Surface Preparation Conditions Research Articles

Orientation-dependent growth of Ni clusters on SrTiO3 (001), (110), and (111) surfaces

Surface conditions of oxide substrates greatly influence the physical properties of clusters and films grown on them. To investigate the effects of surface orientations and preparation conditions, Ni clusters were grown on SrTiO3 (001), (110), and (111) substrates, and studied using a combined ultrahigh-vacuum transmission electron microscopy/scanning tunnelling microscopy (UHV-TEM/STM) system. Before Ni deposition, SrTiO3 (111) surfaces showed a partial formation of the (5 × 5) superstructure on STM samples and the (3 × 3) structure on TEM samples. The (5 × 5) superstructure formation is attributed to oxygen depletion during electron beam annealing. The (3 × 3) superstructure formation is assumed to be due to amorphization and oxygen removal during ion milling followed by enhanced oxygen self-diffusion and re-oxidation. Some of the Ni clusters have equilibrium shapes, such as square, rectangle, and hexagon, when grown on the SrTiO3 (001), (110), and (111) substrates, respectively. These can be interpreted on the basis of Winterbottom construction and are clarified as truncated pyramids, huts, and hexagons, respectively. No particular epitaxial relationship was obtained for clusters on SrTiO3 (111).

Interplay between bulk atomic clusters and surface structure in complex intermetallic compounds: The case study of theAl5Co2(001)surface

The ${\mathrm{Al}}_{5}{\mathrm{Co}}_{2}$ crystal is a complex intermetallic compound, whose structure can be described by a stacking of chemically bonded atomic motifs. It is a potentially new catalytic material for heterogeneous hydrogenation. A single crystal of this phase has been grown by the Czochralski technique in order to study the influence of the three-dimensional bulk substructure on the two-dimensional surface using both experimental ultrahigh vacuum surface techniques and ab initio methods based on the density functional theory. Some bulk properties are first presented, focusing on chemical bond strengths, the determination of the Al and Co chemical potentials in ${\mathrm{Al}}_{5}{\mathrm{Co}}_{2}$, the vibrational properties, and the specific heat. Then, the combination of experimental and computational approaches allows the identification of the surface structure, which was found to depend on the surface preparation conditions. In all cases, the surface terminates at specific bulk layers (Al-rich puckered layers) where various fractions of specific sets of Al atoms are missing, identified as ${\mathrm{Al}}_{3}$ atoms left at the surface resulting from cluster truncation. Finally, electron density of states calculations and spectroscopic measurements were compared and indicate a strong $sp\text{\ensuremath{-}}d$ hybridization of the topmost pure Al layer with subsurface Co atoms. This could influence the surface reactivity and the catalytic performances of this material.

TiO2 anatase with a bandgap in the visible region.

TiO2 anatase plays a central role in energy and environmental research. A major bottleneck toward developing artificial photosynthesis with TiO2 is that it only absorbs ultraviolet light, owing to its large bandgap of 3.2 eV. If one could reduce the bandgap of anatase to the visible region, TiO2-based photocatalysis could become a competitive clean energy source. Here, using scanning tunneling microscopy and spectroscopy in conjunction with density functional theory calculations, we report the discovery of a highly reactive titanium-terminated anatase surface with a reduced bandgap of less than 2 eV, stretching into the red portion of the solar spectrum. By tuning the surface preparation conditions, we can reversibly switch between the standard anatase surface and the newly discovered low bandgap surface phase. The identification of a TiO2 anatase surface phase with a bandgap in the visible and high chemical reactivity has important implications for solar energy conversion, photocatalysis, and artificial photosynthesis.

Optimized Pre-Treatment Process for MOS-GaN Devices Passivation

In this letter, we present an effective GaN surface passivation process, which was developed by optimizing the surface chemical pretreatment prior to the PECVD- SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> deposition. It is demonstrated that the electronic properties of the GaN/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> interface are drastically influenced by the surface preparation conditions. Among the used chemicals, we found that KOH/HCl leads to the best GaN/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> interface quality. MOS capacitors fabricated using this pretreatment have shown a near ideal capacitance-voltage characteristics, with a good surface potential modulation, small flatband voltage shift, low hysteresis, and no significant frequency dispersion. Using this optimized passivation process, AlGaN/GaN-based MOS-high electron mobility transistors (HEMTs) were fabricated. Electrical characterizations have shown up to four orders of magnitude lower gate leakage current and three orders of magnitude lower off-state current compared with the reference Schottky gate HEMT.

Corrosion Behavior of Surface Modified NdFeB Permanent Magnet in Dilute Chloride Environments

Nanoporous oxide layer was formed on the NdFeB permanent magnets by a simple anodization process in fluoride containing ethylene glycol at 50V for 15minutes. The electrochemical polarization behavior of the commercial permanent magnetic material was evaluated in 0.1M NaH2PO4 solution (pH: 4.75) containing 0–0.017M of chloride at room temperature. The material was evaluated in magnetized and demagnetized condition with different surface preparation conditions such as: Ni-Cu-Ni coated, uncoated (bare), and anodized. The nickel coated magnetic sample showed reasonable pitting corrosion resistance up to 0.0017M chloride addition. The uncoated samples showed passivity in chloride free phosphate solution. Chloride additions caused pitting and the pitting protection potential was equal to the corrosion potential with 0.0017M chloride addition. Demagnetization did not alter the electrochemical polarization. However, the passive film formed on the magnetic sample showed higher charge carrier density than the film formed on the demagnetized sample.

High quality Ge epitaxy on GaAs (100) grown by metal-organic chemical vapor deposition

High quality Ge epitaxy on a GaAs (100) substrate was grown by metal-organic chemical vapor deposition using germane. The effects of growth temperature and deposition rate on the quality of the Ge epitaxy are investigated. Significant improvement in surface root-mean-square roughness is observed with increasing Ge growth temperature or deposition rate, while keeping all other deposition parameters unchanged. Investigation of the Ge material quality grown after different GaAs surface preparation conditions shows that Ga rich surfaces are beneficial for smooth Ge surfaces, which is attributed to the formation of Ge–Ga bonds at the initiation of the Ge deposition. The good crystalline quality of grown Ge film was further confirmed by high-resolution X-ray diffraction and secondary ion mass spectrometry characterization.

Taguchi analysis of bonded composite single-lap joints using a combined interface–adhesive damage model

The mechanical behaviour of bonded composite joints depends on several factors, such as the strength of the composite–adhesive interface, the strength of the adhesive and the strength of the composite itself. In this regard, a finite element model was developed using a combined interface–adhesive damage approach. A cohesive zone model is used to represent the composite–adhesive interface and a continuum damage model for the adhesive bondline. The influence of the composite–adhesive interfacial adhesion and the strength of the adhesive on the performance of a bonded composite single-lap joint was investigated numerically. A Taguchi analysis was conducted to rank the influence of material parameters on the static behaviour of the joint. It was found that the composite–adhesive interfacial fracture energy and the mechanical properties of the adhesive predominantly govern the static performance of the joints. A parametric study was performed by varying the most important material parameters, and a response surface equation is proposed to predict the joint strength. It is shown that the influence of experimental parameter variations, e.g. variation in adhesive curing and surface preparation conditions, can be numerically accommodated to investigate the static behaviour of bonded composite joints by combining finite element and statistical techniques. The methods presented could be used by practicing engineers to describe the failure envelope of adhesively bonded composite joints.

The structure of epitaxial V2O3 films and their surfaces: A medium energy ion scattering study

Medium energy ion scattering, using 100keVH+ incident ions, has been used to investigate the growth of epitaxial films, up to thicknesses of ~200Å, of V2O3 on both Pd(111) and Au(111). Scattered-ion energy spectra provide a measure of the average film thickness and the variations in this thickness, and show that, with suitable annealing, the crystalline quality is good. Plots of the scattering yield as a function of scattering angle, so-called blocking curves, have been measured for two different incidence directions and have been used to determine the surface structure. Specifically, scattering simulations for a range of different model structures show poor agreement with experiment for half-metal (….V′O3V) and vanadyl (….V′O3V=O) terminations, with and without surface interlayer relaxations. However, good agreement with experiment is found for the modified oxygen-termination structure, first proposed by Kresse et al., in which a subsurface V half-metal layer is moved up into the outermost V buckled metal layer to produce a VO2 overlayer on the underlying V2O3, with an associated layer structure of ….O3VV′′V ′O3. This result is consistent with the predictions of thermodynamic equilibrium at the surface under the surface preparation conditions, but is at variance with the conclusions of earlier studies of this system that have favoured the vanadyl termination. The results of these previous studies are re-evaluated in the light of the new result.

Polarity-dependent photoemission spectra of wurtzite-type zinc oxide

The polar surfaces of wurtzite-type zinc oxide (ZnO) were characterized by x-ray photoemission spectroscopy to identify the origin of the polarity dependence of the valence band spectra. A characteristic sub-peak always appeared in the valence band spectra of the (0001) face regardless of the surface preparation conditions. It also appeared in the valence band spectra of the (101¯2) face, but only when the photoelectron take-off angle was parallel to the c-axis of ZnO. Our analysis demonstrates that this take-off angle dependency originates not from the surface state, photoelectron diffraction, or the presence of surfactants but from the crystal polarity.

Microstructure and Transport Properties in Alloyed Ohmic Contacts to P-Type SiC and GaN for Power Devices Applications

The formation of good Ohmic contacts to p-type silicon carbide (SiC) and gallium nitride (GaN) is an important physical and technological concern, because of the difficulty to find metals with low Schottky barriers to p-type wide band gap materials, and due to the high ionization energies of p-type dopant impurities. Typically, to overcome these issues, alloyed metallic compounds are used. In this work, the electrical properties of alloyed Ohmic contacts to p-type (Al-implanted) 4H-SiC and p-type (Mg-doped epilayers) GaN are presented and correlated with their microstructure. The impact of the surface preparation and annealing conditions are discussed, reporting the cases of Al/Ti contacts to p-SiC and Au/Ni contacts to p-GaN. The electrical characterization as a function of temperature allowed to define the dominant transport mechanism and to determine the barrier heights.

The effects of various irrigating solutions on intra-radicular dentinal surface: An SEM analysis

Aim:The action of irrigant solutions on intra- radicular dentinal surface were evaluated in an in vitro setting using a scanning electron microscope (SEM) and it was observed that sodium hypochlorite and MTAD produced the cleanest surface and that none of the irrigants were able to produce an ideal preparation of the dentinal surface when used individually. The primary objective of endodontic therapy is to achieve a clean, optimal environment in root canals to avoid unsuccessful treatment outcomes. The complexities of the root canal system necessitate the use of irrigating solutions which act on radicular dentin surface, modifying it. The action of irrigants can be beneficial, and yet at the same time, as they modify the surface structure of dentin, they can have an adverse impact on the properties of dentin. The present study was undertaken to assess the effect of various irrigants on the dentinal surface using an SEM.Materials and Methods:Forty-five roots were randomly divided into nine groups (n=5) and prepared by sectioning at the level of cemento-enamel junction (CEJ) and 10 mm from the CEJ and split longitudinally. The dentin surface was prepared and the cemental surfaces were coated with double layer of varnish. The irrigants tested were normal saline, de-ionized water, 17% ethylenediaminetetraacetic acid (EDTA), 5% sodium hypochlorite (NaOCl), 5% NaOCl with ultrasonic agitation, 3% hydrogen peroxide, 2% chlorhexidine (CHX), MTAD, and MTAD with ultrasonic agitation. The prepared samples were placed in the irrigant solution for 3 min, subsequently dehydrated, sputter coated, and observed under SEM. The images were subsequently analyzed for dentinal surface changes.Results:17% EDTA and MTAD produced the cleanest dentinal surface. Ultrasonic agitation enhanced the effect of irrigants. 5% NaOCl and 3% hydrogen peroxide were efficient at removal of organic debris, but were unable to remove the smear layer. De-ionized water, normal saline, and 2% chlorhexidine were not effective at removing the debris or the smear layer.Conclusion:None of the irrigants individually were able to achieve conditions of an ideal dentinal surface preparation.

A metastable Fe(A) termination at the Fe 3O 4(001) surface

A metastable Fe(A) terminated Fe 3O 4(001) surface was prepared by tailoring the surface preparation conditions. STM, LEIS and LEED are utilized to demonstrate that annealing the Ar + sputtered surface to 350 °C produces an Fe(A) terminated surface with a (√2 × √2)R45° superstructure. Within the superstructure both single Fe atoms and Fe dimer species are observed. The surface is reoxidized upon annealing to higher temperatures, eventually leading to the recovery of the energetically favorable Jahn–Teller distorted surface at 700 °C. The ability to reproducibly prepare the Fe(A) termination in this simple manner will allow investigations into the structure–function relationship for this important technological material.

Study of the formation processes of gold droplet arrays on Si substrates by high temperature anneals

In this study, the peculiarities of the transformations of gold films deposited on the Si wafer surfaces as a result of high temperature anneals are investigated experimentally depending on the conditions of wafer surface preparation and the annealing regimes. The morphology and the distribution functions of the crystallites of gold films as well as the gold droplets formed as a result of anneals are studied as functions of annealing temperature, type of annealing (rapid thermal or rapid furnace annealing), and the state of the surface of Si wafers. The results obtained can be used for the controlled preparation of the arrays of catalytic gold droplets for subsequent growth of Si wire-like crystals.

Structure of clean and hydrated α-Al2O3 (11̄02) surfaces: implication on surface charge

Periodic DFT calculations coupled to a first-principle thermodynamic approach have allowed us to establish a surface phase diagram for the different terminations of the α-Al(2)O(3) (1102) surface in various temperature and water pressure conditions. Theoretical results are compared with previous experimental data from the literature. Under a wide range of temperature and water pressure (including ambient conditions) the most stable surface (denoted C2_1H(2)O in this work) is terminated with singly coordinated hydroxyls on four-fold coordinated aluminium (Al(4C)-μ(1)-OH) while most existing surface models are only considering six-fold coordinated surface Al atoms as in the bulk structure of alumina. The presence of more acidic Al(4C)-μ(1)-OH sites helps explain the low Point of Zero Charge (PZC) (between 5 and 6) determined from the onset of Mo oxoanions adsorption on (1102) single crystal wafers. It is also postulated that another termination (corresponding to the hydration of the non-polar, stoichiometric surface, stable in dehydrated conditions) may be observed in aqueous solution depending on the surface preparation conditions.

Aggregate and disaggregate statistical evaluation of the performance-based effectiveness of long-term pavement performance specific pavement study-5 (LTPP SPS-5) flexible pavement rehabilitation treatments

Engineers continually seek effective techniques for preserving highway infrastructure. Using data from the specific pavement study#5 of the long-term pavement performance (LTPP) programme's western region, this article evaluated the performance of eight flexible pavement rehabilitation treatments. Aggregate and disaggregate post-treatment performance models were developed for each treatment. Effectiveness was measured in the short term (roughness reduction) and long term (estimated treatment service life and area bounded by the performance curve (ABP)). The results showed that compared to 2-inch treatments, 5-inch treatments were on average more effective in terms of the following measures: 47% (estimated service life) and 35% (ABP) depending on the level of surface preparation, mix type and initial pavement condition. Also, relative to minimal surface preparation, intensive surface preparation generally yielded greater effectiveness: 25% (estimated service life) and 49% (ABP) depending on added thickness, mix type and initial pavement condition. Compared to recycled mix treatments, virgin mix was marginally more effective. Finally, compared to pavements treated in poor condition, those treated in good condition were significantly more effective. Treatment effectiveness models were developed for predicting the expected effectiveness of future treatments on the basis of attributes such as treatment type, added layer thickness, level of surface preparation and mix type.

Fermi level unpinning of GaSb (100) using plasma enhanced atomic layer deposition of Al2O3

N-type and p-type GaSb metal-oxide-semiconductor capacitors (MOSCAPs) with atomic-layer-deposited (ALD) and plasma-enhanced-ALD (PEALD) Al2O3 dielectrics are studied to identify the optimum surface preparation and oxide deposition conditions for a high quality oxide-semiconductor interface. The ALD Al2O3/GaSb MOSCAPs exhibit strongly pinned C-V characteristics with high interface state density (Dit) whereas the PEALD Al2O3/GaSb MOSCAPs show unpinned C-V characteristics (low Dit). The reduction in Sb2O3 to metallic Sb is suppressed for the PEALD samples due to lower process temperature, identified by x-ray photoelectron spectroscopy analysis. Absence of elemental Sb is attributed to unpinning of Fermi level at the PEALD Al2O3/GaSb interface.

Effects of exposure wavelength and surface preparation conditions on the generation of blister defects in organic insulator layer

We report a new factor for blister formation on organic photoresist (PR) and an improvement of this process. We have studied blister formation from more different standpoints, such as processes and instrumentation, than did previous reports. Unexpectedly, we observed radical blister formation in an experiment that involved exposure without any ultraviolet (UV) filters. After a series of experiments, the data showed that the organic PR blister problem was most likely caused by the specific wavelength of the UV light field on exposure. Surface preparations using wet and dry treatments prior to coating a thin film of organic PR on silicon nitride (SiNx) glass wafer were studied. By comparing exposure to different spectra, both with and without a UV filter, we confirmed the key point of blister formation in case of the organic PR. Additionally, various treatments of SiNx prior to the coating of organic PR were primarily performed to improve organic PR adhesion to SiNx glass substrate.

Effects of Deformation-Induced Heating on Bond Strength of Rolled Metal Multilayer

The bond strength of various metal multilayers produced by cold rolling of metal foils with different thermal conductivity was investigated. Results indicated that under the same conditions of deformation and surface preparation, the metallic multilayer system with low thermal conductivity exhibited relative high bond strength while high thermal conductivity metal system may fail to be roll-bonded together. The relationship between the deformation-induced localized heating and the bond strength were discussed. The deformation-induced localized heating in the low thermal conductivity metal multilayer systems may provide opportunities for achieving a successful accumulative roll bonding or a “cold roll/heat treatment/cold roll” process to synthesize metallic multilayer materials.

Density functional theory study of clean, hydrated, and defective alumina(11¯02)surfaces

We report an ab initio thermodynamic analysis of the $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}\text{ }(1\overline{1}02)$ surface aimed at understanding the experimentally observed terminations over a range of surface preparation conditions as well as a stoichiometric model for the $(2\ifmmode\times\else\texttimes\fi{}1)$ surface reconstruction observed after high-temperature annealing. As temperature is increased under both ultrahigh vacuum and ambient hydrated conditions, the predicted minimum-energy structural model goes through the same series of changes: from the hydroxylated ``missing-Al'' surface model (or half-layer model in which the topmost Al site of the stoichiometric surface has zero occupancy), to the hydroxylated stoichiometric model, to another hydroxylated missing-Al surface model with tetrahedral coordinated surface Al, and finally to the clean $(1\ifmmode\times\else\texttimes\fi{}1)$ stoichiometric model. These results are in agreement with observations of both missing-Al and bulklike stoichiometries under wet conditions and in agreement with similar trends reported for isostructural hematite. However, we observe that the models with excess oxygen have a relatively higher surface-free energy and distinct surface relaxations in the case of alumina as compared to hematite. At very high temperatures where oxygen defects are generated, we find that a stoichiometric, charge-neutral $(2\ifmmode\times\else\texttimes\fi{}1)$ structure becomes the most thermodynamically stable. This is consistent with the observation of a $(2\ifmmode\times\else\texttimes\fi{}1)$ electron diffraction pattern when the surface is annealed at 2000 K while a $(1\ifmmode\times\else\texttimes\fi{}1)$ pattern persists at lower annealing temperatures. A general rule that emerges from our modeling results is that while the full phase space of hydrated and defective surfaces is expansive, model stoichiometries that can be made charge neutral through either hydration or defects offer the greatest thermodynamic stability. However, the unique trends in structure and relative energies of alumina surface stoichiometries as compared to hematite can be understood based on the difference in electronic structure of the substrate.

Splice Resistance Measurements on 2G YBCO Coated Conductors

The Oak Ridge National Laboratory has been investigating the electrical splice resistance of second-generation (2G) YBCO coated conductor. The purpose of the experimental investigation is to study the splice resistance of 2G YBCO coated conductor as a function of: a) operating temperature, b) magnetic field strength (B-field), and c) magnetic field orientation (thetas). Understanding the splice resistance with its corresponding variation as a function of surface preparation and operating conditions is essential to the practical implementation of electric utility devices; e.g., motors, generators, transformers, cables, and fault-current limiters, etc. Preliminary test results indicate that the 2G YBCO splice resistance shows a weak temperature dependence and a significantly stronger dependence upon magnetic field strength and magnetic field orientation. Surface preparation conditions are also briefly discussed.