Surface microstructure of the oxide protective layers grown on vanadium-free Ti alloys for use in biomedical applications

In this work, we present a surface study by SFM (scanning force microscopy) of three new Ti alloys of composition (in wt%) Ti-7Nb-6Al, Ti-13Nb-13Zr and Ti-15Zr-4Nb, developed for biomedical applications. V was not included in these alloys since this element has been reported to be cytotoxic. The surface of these materials has been modified by a thermal treatment in air at 750 °C for different times. As a consequence of this treatment an oxide layer develops on the surface, resulting in both an improvement of the corrosion resistance and an increase of the roughness, which enhances the adhesion of the tissue cells to the implant. SFM has been used to characterize the surface structure and topography of the oxide layers grown on the three alloys. The surface roughness analysis obtained by SFM points to a correlation between the mean square roughness, the thickness of the oxide layer, and the α-phase/β-phase ratio in the base material.     

In situ surface analytical investigation of the thermal oxidation of Ti–Al intermetallics up to 1000 °C

The low pressure high temperature oxidation behavior of Ti–Al intermetallics are of interest to power technology aiming to synthesize this material by sintering of powders. This paper presents in situ surface analytical studies of the composition of a two-phase TiAl/Ti₃Al bulk microcrystalline system after oxidizing the same (sputtered) reference surface for 30 min at various oxygen partial pressures and temperatures varying between room temperature (RT) and 1000 °C. The results show that oxidation already begins at very low (<5×10⁻¹⁰ mbar) oxygen pressure, producing Al₂O₃ and the lower oxidation states of Ti. As the oxygen pressure and oxidation temperature increases, TiO₂ becomes dominant up to 900 °C. No phase transition of Al₂O₃ has been observed in this range. No sign of a blocking behavior of the oxide layer is seen. At 1000 °C a new oxide phase, Al₂TiO₅ appears, changing the composition and behavior of the surface drastically. The observed results can be explained by qualitative thermodynamic arguments. The thickness and composition of the oxide overlayer is, however, primarily determined by the oxygen supply.     

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator layer

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator (SOI) layer has been studied. A square-shaped 12 nm thick SOI layer was patterned by lithography and by selective etching with a KOH solution. The structural change by ultrahigh vacuum annealing in a temperature range of 900–1100 °C was observed by atomic force microscopy. The agglomeration takes place preferentially from the pattern edges at a lower annealing temperature than that for the unpatterned layer, indicating enhanced diffusion of Si atoms at the edges. Additionally, the patterning causes formation of smaller islands than those for the unpatterned layer, reflecting that the patterning limits the amount of Si atoms supplied for the island formation.     

Growth mechanism of the Pd(100)-p(2×2)-p4g-Al surface alloy

We have studied the growth mechanism of a Pd(100)-p(2×2)-p4g-Al surface alloy by scanning tunneling microscopy (STM). The surface alloy has a bilayer structure and is formed by annealing at 450–700 K (depending on the initial aluminum coverage) after the deposition of aluminum on Pd(100) at room temperature. The ratio of the surface-alloy coverage to the initial aluminum coverage is found to be constant (0.44) irrespective of the initial aluminum coverage from 0.5 monolayers (ML) up to 2 ML. The growth mechanism of the surface alloy is proposed on the basis of the STM measurements at various annealing temperatures. Upon annealing at 450 K, some of the surface aluminum atoms migrate into the bulk and, instead, palladium atoms come out to the surface. These palladium atoms react with aluminum atoms remaining on the surface to form a surface alloy. When the initial aluminum coverage is less than 1 ML, bilayer-high islands of the surface alloy with an average area of 100 nm² are formed at 450–500 K, which diffuse on the terrace at 500–700 K and coalesce to form larger islands. A possible role of the percolation transition of aluminum islands in the formation of the surface alloy is discussed.     

Growth and thermal properties of ultrathin cerium oxide layers on Rh(1 1 1)

Ultrathin layers of cerium oxide have been deposited on a Rh(1 1 1) surface and their growth morphology, structure, and thermal stability have been investigated by LEED, STM, XPS, and valence band resonant photoemission. STM and LEED indicate that the ceria grows epitaxially in form of ordered CeO₂ islands at elevated substrate temperature (250–300 °C), with (1 1 1) faces parallel and orientationally aligned to the main azimuthal directions of the substrate. The ultrathin ceria films contain significant amounts of reduced Ce³⁺ species, which appear to be located predominantly at the ceria–Rh interface. For thicker films (>6 equivalent monolayers) stoichiometric CeO₂ is detected in XPS. Vacuum annealing produces morphologically well-defined hexagonal islands, accompanied by partial reduction and the formation of oxygen vacancies at the ceria surface. The thermal stability and the degree of reduction is a function of the oxide layer thickness, with thinner layers being thermally less stable. At temperatures >800 °C, the ceria decomposes and Ce–Rh alloy phases are identified.     

Formation of cementite in tempered Fe–Co–C alloys

In Fe–Co–C alloys, undesirable grain coarsening results from the specific austenite orientation variants that form after the γ→→γ transformations. Tempering of martensite before reheating prevents austenite returning to its original orientation and also limits grain coarsening. However, the reasons for this are unclear. It may be assumed that some differences between cementite formed in tempered and rapidly heated alloys may cause the variation in the final austenite structure. In the present work the orientation relationships between cementite and martensite in two tempered Fe–Co–C alloys have been studied using microbeam electron diffraction in a transmission electron microscope. In both alloys after short-term (rapid heating at 100°C s⁻¹ followed by quench) and long-term (1 and 3 h) tempering treatments the orientation relationships were shown to obey the Isaichev orientation relationships:

However, after rapid tempering, only one carbide variant was found in each crystal, while after long-term tempering, up to three variants were present. This might account for the observed crystallographic reversibility in rapidly heated alloys, contrary to the multiplication of γ variants formed from the long-term tempered martensite.     

STM and LEED observations of erbium silicide nanostructures grown on Si(1 0 0) surface: atomic-scale understandings

In this work, erbium silicide is grown on the Si(1 0 0) surface by depositing Er onto the substrate and annealing at 600–700 °C. Many nanowires of Er silicide are formed with lengths in the range 10–100 nm. The formation and evolution of this nanostructure are investigated at atomic scale directly with scanning tunneling microscopy and low-energy electron diffraction. The direction of these nanowires is found perpendicular to that of Si dimer rows. It is shown that Er coverage and annealing temperature have an effect on the formation of nanowires. On the surface between nanowires, new (5×2) and c(5×4) reconstructions are observed, giving an implication to understand the growth behaviors of Er silicide on Si(1 0 0) surface.     

Intermixing at the interface of the system Nb/Fe(1 1 0) investigated by STM

The growth of niobium on the Fe(1 1 0) surface at a deposition temperature between room temperature (RT) and 680 K was studied using in situ STM and LEED. At RT we observe no indication of intermixing. Although a final roughness of only 1.7 Å is reached, the crystalline quality is low. At elevated growth temperatures the development of a surface alloy was observed, whose formation is ascribed to an exchange mechanism through which Nb adatoms are incorporated into the Fe surface. These Nb atoms arrange themselves in chains along the [0 0 1] direction. The expelled Fe atoms form islands on the Nb/Fe-alloy substrate. At higher coverage additionally a Nb wetting layer and intermixed 3D islands evolve.     

Subsurface miscibility of metal overlayers with V, Nb and Ta substrates

The high solubility and diffusivity of oxygen in Nb, Ta and V are responsible for the difficulty in the preparation of a clean, well-ordered (1 0 0) surfaces of these early transition metals. The deposition and subsequent annealing of a metal overlayer on Nb(1 0 0) are a convenient route for the preparation of flat surfaces with the Nb lattice constant and a metallic character. Such well-ordered, stable, inert and easily reproducible Nb(1 0 0)-like surfaces can be produced due to the suppression of oxygen surface segregation by a suitable layer blocking sub-surface oxygen diffusion. For example, a Nb(1 0 0)-like surface can be obtained by annealing thin Au or Pd films deposited on Nb(1 0 0), since this results in a Au–Nb or Pd–Nb alloy situated just below the surface which preserves the structure of the Nb-lattice and effectively suppresses the oxygen segregation toward the surface. In contrast, Ag and Cu layers do not show this property as these metals do not form a bulk alloy with Nb. The presence or absence of bulk alloying and its relation to surface oxygen contamination is a general phenomena observed for many metallic layers deposited on V, Nb and Ta substrates. A discussion of results reported in the literature is given for these adsorbate systems.     

Preparation of atomically clean and flat Si(1 0 0) surfaces by low-energy ion sputtering and low-temperature annealing

Si(1 0 0) surfaces were prepared by wet-chemical etching followed by 0.3–1.5 keV Ar ion sputtering, either at elevated or room temperature (RT). After a brief anneal under ultrahigh vacuum (UHV) conditions, the resulting surfaces were examined by scanning tunneling microscopy. We find that wet-chemical etching alone cannot produce a clean and flat Si(1 0 0) surface. However, subsequent 300 eV Ar ion sputtering at room temperature followed by a 700 °C anneal yields atomically clean and flat Si(1 0 0) surfaces suitable for nanoscale device fabrication.     

Preparation of tantalum oxide thin films by photo-assisted atomic layer deposition

Tantalum oxide thin films were prepared by photo-assisted atomic layer deposition (Photo-ALD) in the substrate temperature range of 170–400 °C using Ta(OC₂H₅)₅ and H₂O as precursors. The constant growth rates of 0.42 and 0.47 Å per cycle were achieved for the films grown by normal ALD and Photo-ALD, respectively. The increased growth rate in Photo-ALD is probably due to the reactive surface by photon energy and faster surface reaction. In Photo-ALD, however, the constant growth rate started at lower temperature of 30 °C and one cycle time shortened up to 5.7 s than that of normal ALD. The films grown by normal ALD and Photo-ALD were amorphous and very smooth (0.21–0.35 nm) as examined by X-ray diffractometer and atomic force microscopy, respectively. Also, the refractive index was found to be 2.12–2.16 at the substrate temperature of 190–300 °C, similar to that of the film grown by normal ALD. However, the remarkably low leakage current density of 0.6×10⁻⁶ A/cm² to 1×10⁻⁶ A/cm² at applied field of 1 MV/cm is several order of magnitude smaller than that of normal ALD, probably due to the presence of reactive atom species.     

Dynamic observations of the formation of thin Cu layers on clean and hydrogen-terminated Si(1 1 1) surfaces

The growth of Cu on the clean and hydrogen-terminated Si(1 1 1) surfaces is studied in situ by low-energy electron microscopy (LEEM). The dependence of the growth of the “5×5” layer on the clean Si(1 1 1) 7×7 surface upon the deposition temperature is investigated by combining LEEM with LEED. After completion of the “5×5” layer not only the regular-shaped three-dimensional islands reported before are observed but also irregular shaped more two-dimensional islands. On the hydrogen-terminated Si(1 1 1) surface the formation of the “5×5” structure is suppressed and nano-scale islands form preferentially at the step edges and domain boundaries. This is attributed to the enhancement of the surface migration of Cu atoms by the elimination of the surface dangling bonds.     

Regular step formation on concave-shaped surfaces on 6H–SiC(0 0 0 1)

A concave-shaped surface has been prepared in a 6H–SiC(0 0 0 1) substrate by mechanical grinding. As a consequence, the different crystallographic planes building up the 6H–SiC polytype are cut under continuously changing polar angles in all azimuthal directions. Through hydrogen etching, this curved surface breaks up into a whole set of surfaces vicinal to the initial 6H(0 0 0 1) orientation. The local structural reorganisation after hydrogen etching has been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Two types of local bond environments are present at the step edges leading to a strong anisotropy in the surface etching with hydrogen. As a result, the distribution of the terrace width and the step heights varies with the azimuthal angle and reflects the sixfold symmetry of the bulk crystal. For most azimuthal directions, an alternation of large and small terraces, separated by steps of 0.75 nm heights (height of half the 6H polytype, three bilayers) is observed and only for well defined azimuthal directions, equally spaced terraces separated by steps of 1.5 nm height (one unit cell of 6H–SiC, six bilayers) are found. In addition, the polar variations have been studied by taking various line-scans along the concave-shaped surface with AFM. It seems that for polar angles above 3°, step bunching of several SiC steps occurs whereas below 3° the bimodal terrace width distribution is observed.     

Improvements of performance on sputter-coated niobium films for superconducting cavities by adding a NbN interlayer

A new type of superconducting film is studied at Peking University in order to improve the properties of sputter-coated films for superconducting cavities. NbN film and NbN–Nb film have been prepared by DC diode sputtering technology at certain nitrogen content and temperature. NbN film is prepared between copper and Nb film as a barrier against copper diffusion into Nb. Micro-structure analyses show that the NbN–Nb films grow well on the copper substrate. The T_c of the Cu–NbN–Nb increases to 9.5 K compared to the 9.2 K transition temperature of bulk Nb.     

Influence of NH beam bombarding energy on structural characterization and cell attachment of CNx coating

The carbon nitride (CN_x) coating with its novel properties will be excellent candidate for biomedical applications. CN_x coatings were prepared on the surface of Ti–6Al–4V by ion-beam-assisted deposition (IBAD) with different NHⁿ⁺ beam bombarding energies at low substrate temperature. The coatings were characterized by Scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy. The result showed that the wear-resistance of CN_x coatings was better at higher beam bombarding energy. The cell attachment tests also gave interesting results that CN_x coatings exhibited low macrophage attachment and provide desirable surface for the normal cellular growth and morphology of the fibroblasts. Structural analysis showed that NHⁿ⁺ beam bombardment at the energies of 300–400 eV could result in more nitrogen concentration and fraction of sp³CN bonds in the structure of CN_x coatings, which may be responsible for the improvement in the wear-resistance and the cell attachment.     

Characterization by electrolyte electroreflectance and X-ray photoelectron spectroscopy of amorphous Ni59Nb40Pt1−xSnx alloys and their activation by HF solutions

The influence of chemical etching with HF on the nature of the surface of amorphous Ni₅₉Nb₄₀Pt_1−xSn_x alloys has been studied in situ by electrolyte electroreflectance (EER) and ex situ by X-ray photoelectron spectroscopy (XPS). The EER spectrum of the untreated alloy in 0.5 M H₂SO₄ shows a bipolar band, which disappears after the HF treatment yielding a structureless EER spectrum similar to that of Pt, but reappears after several hours in the 0.5 M H₂SO₄ electrolyte. This process of dissolution by HF of an oxide species and its reappearance after a few hours cannot be followed by XPS, since the time interval between sample withdrawal from the electrolyte and actual measurement is of a few hours as well. XPS spectra showed the presence of metallic Nb before and after the HF treatment, and that niobium pentoxide was the main species in the as-quenched alloy, but that after treatment with HF it became a minor component, the main one being NbO. The main effect of the HF treatment is to produce a platinum enrichment of the surface, as unequivocally determined by cyclic voltammetry, XPS and EER. After Ar sputtering for 9 min the XPS spectrum of the untreated alloy showed metallic Nb only, while in the HF-treated alloy the peaks of metallic Nb were swamped by those of NbO and some Nb₂O₅. We interpret this difference as being due to the formation by the HF attack of a porous Nb film which becomes oxidized in the electrolyte and/or during transfer to the spectrometer, and so thick that it is not eliminated by Ar sputtering for 9 min.     

Atomic force microscopy analysis of wool fibre surfaces in air and under water

Wool fibre surfaces have been treated by solvent cleaning which leaves the native covalently bound surface lipid layer intact, and by alcoholic alkali which removes the lipid layer. The resultant surfaces have been analysed by atomic force microscopy (AFM), with particular emphasis on force–distance (F–d) methods. Methodologies were developed for investigation in situ in water of both the surface topography and the characteristics of the lipid layer. Longitudinal surface texturing was resolved in images of wool fibre surfaces in air; the texturing remained prominent after exposure to water. High resolution F–d curves revealed features associated with the lipid layer. A simple formalism was used to show that the layer had a thickness of a few nm, and an effective stiffness of some 0.12±0.01 N/m. Strong adhesive interactions, equivalent to a pressure of 0.1 MPa, acted on the tip at the tip-to-substrate interface. The methodology and formalism are likely to be relevant in the broad field of thin-film analysis and for fibre technology.     

Effect of oxide thickness on the capacitance and conductance characteristics of MOS structures

In this work, the investigation of the interface states density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics in Au/SnO₂/n-Si (MOS) structures prepared at various SnO₂ layer thicknesses by spray deposition technique have been reported. It is fabricated five samples depending on deposition time. The thicknesses of SnO₂ films obtained from the measurement of the oxide capacitance in the strong accumulation region for MOS Schottky diodes are 37, 79, 274, 401, and 446 Å, for D1, D2, D3, D4, and D5 samples, respectively. The C–V and G–V measurements of Au/SnO₂/n-Si MOS structures are performed in the voltage range from −6 to +10 V and the frequency range from 500 Hz to 10 MHz at room temperature. It is observed that peaks in the forward C–V characteristics appeared because of the series resistance. It has been seen that the value of the series resistance R_s of samples D1 (47 Ω), D2 (64 Ω), D3 (98 Ω), D4 (151 Ω), and D5 (163 Ω) increases with increasing the oxide layer thickness. The interface state density D_it ranges from 2.40×10¹³ cm⁻² eV⁻¹ for D1 sample to 2.73×10¹² cm⁻² eV⁻¹ for D5 sample and increases with increasing the oxide layer thickness.     

Valence variations in the monolayer regime of Sm on the Nb(110) surface

Sm overlayers in the monolayer regime, deposited on a Nb(110) single-crystal surface at room temperature, have been studied by means of high-resolution photoelectron spectroscopy of the Nb 3d_5/2 core level and the Sm valence band. In the submonolayer regime, the Sm valence varies from mainly divalent for very low coverages to essentially trivalent close to a complete monolayer. Above 1 ML, a new divalent component appears in the Sm 4f spectra, corresonding to divalent Sm in the second layer. The mixed valence in this overlayer system is concluded to be heterogeneous (all Sm atoms have integer but site-dependent valence). Sm forms ordered overlayers on Nb(110) and the Sm growth is consistent with a layer-by-layer growth mode (Frank–van der Merwe growth).     

An examination of the initial oxidation of a uranium-base alloy (U–14.1 at.% Nb) by O2 and D2O using surface-sensitive techniques

The corrosion resistance of uranium is greatly enhanced by alloying with niobium. In this study the initial stages of corrosion of a specific uranium-base alloy (U–14.1 at.% Nb) by O₂ or D₂O have been examined using the surface specific techniques of X-ray photoelectron spectroscopy (XPS), thermal programmed desorption (TPD), static secondary-ion mass spectroscopy (SSIMS), and sputtered neutrals mass spectroscopy (SNMS). XPS studies of the U–14.1 at.% Nb surface following oxidation using O₂ at 300 K indicate production of a thin oxide overlayer of stoichiometric UO_2.0 intermixed with Nb₂O₅. The same stoichiometry is exhibited for uranium when the oxide is prepared at 500 K with O₂; although, niobium is much less oxidized exhibiting a mixture of NbO and Nb. Contrary to previous XPS literature, SNMS depth profiling studies reveal that oxidation by O₂ is much greater (as judged by oxide layer thickness) than that exhibited by D₂O. An oxide layer thickness of less than 20 Å was created using D₂O as an oxidant at 300 K with exposures >3500 L (oxide layers created from O₂ are significantly greater at much smaller exposures). Formation of a critical density of Nb₂O₅ is suggested to be responsible for the enhanced corrosion resistance by preventing diffusion of O⁻ (O²⁻) or OD⁻/OH⁻ into the oxide/metal interface region. The domains of stability of hydroxyl formation have also been followed using TPD, SSIMS and XPS. Maximal surface hydroxyl concentrations (Θ_rel=0.30) are obtained at a surface temperature of 175 K for these experimental conditions.