Jeffrey J. Weimer

An investigation of the activity of coprecipitated gold catalysts for methane oxidation

Coprecipitated Au on transition metal oxide catalysts have been tested for their activity toward methane oxidation. Catalyst activities fall in the order Au/Co3O4>Au/NiO> Au/MnOx> Au/Fe2O3 > Au/CeO. The Au/Co3O4 catalyst is active just below about 250°C. The catalysts are proposed to have more than one type of reactive site since the supports are also active at higher temperatures. Analysis of spent catalysts with X-ray photoelectron spectroscopy indicates that Au exists in at least two oxidation states on some of them, a reduced state and an oxidized state. The activity for methane oxidation increases with increasing oxidation of Auin the oxidized state.

Atomic Force Microscopy Imaging of DNA Covalently Immobilized on a Functionalized Mica Substrate

A procedure for covalent binding of DNA to a functionalized mica substrate is described. The approach is based on photochemical cross-linking of DNA to immobilized psoralen derivatives. A tetrafluorphenyl (TFP) ester of trimethyl psoralen (trioxalen) was synthesized, and the procedure to immobilize it onto a functionalized aminopropyl mica surface (AP-mica) was developed. DNA molecules were cross-linked to trioxalen moieties by UV irradiation of complexes. The steps of the sample preparation procedure were analyzed with x-ray photoelectron spectroscopy (XPS). Results from XPS show that an AP-mica surface can be formed by vapor phase deposition of silane and that this surface can be derivatized with trioxalen. The derivatized surface is capable of binding of DNA molecules such that, after UV cross-linking, they withstand a thorough rinsing with SDS. Observations with atomic force microscopy showed that derivatized surfaces remain smooth, so DNA molecules are easily visualized. Linear and circular DNA molecules were photochemically immobilized on the surface. The molecules are distributed over the surface uniformly, indicating rather even modification of AP-mica with trioxalen. Generally, the shapes of supercoiled molecules electrostatically immobilized on AP-mica and those photocross-linked on trioxalen-functionalized surfaces remain quite similar. This suggests that UV cross-linking does not induce formation of a noticeable number of single-stranded breaks in DNA molecules.

Mesenchymal stem cell interaction with ultra smooth nanostructured diamond for wear resistant orthopaedic implants

Ultra-smooth nanostructured diamond (USND) can be applied to greatly increase the wear resistance of orthopaedic implants over conventional designs. Herein we describe surface modification techniques and cytocompatibility studies performed on this new material. We report that hydrogen (H)-terminated USND surfaces supported robust mesenchymal stem cell (MSC) adhesion and survival, while oxygen- (O) and fluorine (F)-terminated surfaces resisted cell adhesion, indicating that USND can be modified to either promote or prevent cell/biomaterial interactions. Given the favorable cell response to H-terminated USND, this material was further compared with two commonly used biocompatible metals, titanium alloy (Ti-6Al-4V) and cobalt chrome (CoCrMo). MSC adhesion and proliferation were significantly improved on USND compared with CoCrMo, although cell adhesion was greatest on Ti-6Al-4V. Comparable amounts of the pro-adhesive protein, fibronectin, were deposited from serum on the three substrates. Finally, MSCs were induced to undergo osteoblastic differentiation on the three materials, and deposition of a mineralized matrix was quantified. Similar amounts of mineral were deposited onto USND and CoCrMo, whereas mineral deposition was slightly higher on Ti-6Al-4V. When coupled with recently published wear studies, these in vitro results suggest that USND has the potential to reduce debris particle release from orthopaedic implants without compromising osseointegration.

Surface characterization of ion-beam sputter-deposited Ca-P coatings after in vitro immersion

Abstract The surface properties of ion-beam sputter-deposited calcium phosphate (Ca-P) coatings on titanium (Ti) substrates were investigated using atomic force microscopy (AFM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The Ca-P coatings showed columnar growth with AFM. Although FTIR analyses revealed structural changes as a result of the coating process, negligible changes in the structural integrity of the coatings were observed after immersion in an alpha-modification of Eagles Medium (α-MEM). This finding was confirmed by XPS and AES analyses, indicating constant binding and kinetic energies for P 2p and O 1s spectra, and O KLL and P LVV Auger lines, respectively, for all controls and samples immersed in α-MEM solutions. Although XPS analyses indicated no significant change in the P concentration, a reduction in the Ca concentration was observed, thereby resulting in a decrease in the Ca/P ratio from 2.3 to 1.5 after 12 days of immersion.

Surface topography, corrosion and microhardness of nitrogen-diffusion-hardened titanium alloy

Mechanical-electrochemical interactions accelerate corrosion in mixed-metal modular hip prostheses. These interactions can be reduced by improving the modular component machining tolerances or by improving the resistance of the components to scratch or fretting damage. Wrought cobalt-alloy (CoCrMo) is known to have better tribological properties compared to the titanium alloy (Ti64). Thus, improving the tribological properties of this mixed-metal interface should center around improving the tribological properties of the Ti64 alloy. This study used scanning probe microscopy (contact, tapping and phase contrast mode), scanning electron microscopy, corrosion testing, and microhardness testing to determine the effect of a nitrogen-diffusion hardening process on the surface morphology, electrochemistry and surface hardness of the Ti64 alloy. The nitrogen-diffusion-hardened titanium alloy samples (N-Ti64) had a more pronounced grain structure, more nodular surface, and significantly (P<0.01) higher mean roughness values than the control-Ti64 samples. The N-Ti64 samples also exhibited at least equivalent corrosion behavior and a definite increase in surface hardness compared to the control Ti64 samples. The equivalent corrosion behavior and improved surface hardness indicate the potential for N-Ti64 samples to resist similar and mixed-metal scratch and fretting damage. The use of N-Ti64 as opposed to control-Ti64 may therefore reduce the occurrence of mechanical-electrochemical degradation in mixed-metal modular total hip prostheses.

Passivation of CdZnTe surfaces by oxidation in low energy atomic oxygen

A method of surface passivation of Cd1−xZnxTe (CZT) x-ray and gamma ray detectors has been established by using microwave-assisted atomic oxygen bombardment. Detector performance is significantly enhanced due to the reduction of surface leakage current. CZT samples were exposed to an atomic oxygen environment at the University of Alabama in Huntsville’s Thermal Atomic Oxygen Facility. This system generates neutral atomic oxygen species with kinetic energies of 0.1–0.2 eV. The surface chemical composition and its morphology modification due to atomic oxygen exposure were studied by x-ray photoelectron spectroscopy and atomic force microscopy and the results were correlated with current-voltage measurements and with room temperature spectral responses to 133Ba and 241Am radiation. A reduction of leakage current by about a factor of 2 is reported, together with significant improvement in the gamma-ray line resolution.

Effect of passivation and dry heat-sterilization on surface energy and topography of unalloyed titanium implants

Abstract The effects of nitric acid passivation (P) and dry heat-sterilization (S) on the surface topography and energy of unalloyed titanium were examined. Surfaces were treated using the following sequences: cleaned in phosphoric acid (C), C and passivated (CP), CP and dry heat-sterilized (CPS), CPS and resterilized (CPSS), and C and dry heat-sterilized (CS). Surface roughnesses were examined with atomic force microscopy (AFM) and mechanical profilometry. Critical surface tensions γ c and solid surface tensions γ t were determined from contact angles of distilled water and diiodomethane. Analysis of micro-roughness using AFM revealed no statistically significant differences among most roughness parameters for the various treatment conditions. However, the CS samples had a lower ratio of real to projected surface area. Macro-roughness analysis using mechanical profilometry showed that CPSS had lower peak count (PC, i.e. number of peaks above the mean line) values than CP and CS; PC values for C surfaces were also lower than CS. Also, CPSS had higher macro- R q values than C and CPS. Values of γ c correlated positively with the non-polar/dispersive components of γ t ; values were highest with the CS specimens. Higher γ t values were found for the CPSS and CPS treated surfaces, reflecting greater polar contributions on these surfaces. Thus, P and S together resulted in increased surface energy, which is thought to be a desirable surface property for bone implants.

Depth profiling analysis of aluminum oxidation during film deposition in a conventional high vacuum system

The oxidation of aluminum thin films deposited in a conventional high vacuum chamber has been investigated using x‐ray photoelectron spectroscopy (XPS) and depth profiling. The state of the Al layer was preserved by coating it with a protective MgF2 layer in the deposition chamber. Oxygen concentrations in the film layers were determined as a function of sputter time (depth into the film). The results show that an oxidized layer is formed at the start of Al deposition and that a less extensively oxidized Al layer is deposited if the deposition rate is fast. The top surface of the Al layer oxidizes very quickly. This top oxidized layer may be thicker than has been previously reported by optical methods. Maximum oxygen concentrations measured by XPS at each Al interface are related to pressure to rate ratios determined during the Al layer deposition.

Achieving zero stress in iridium, chromium, and nickel thin films

We examine a method for achieving zero intrinsic stress in thin films of iridium, chromium, and nickel deposited by magnetron sputter deposition. The examination of the stress in these materials is motivated by efforts to advance the optical performance of light-weight x-ray space telescopes into the regime of sub-arc second resolution. A characteristic feature of the intrinsic stress behavior in chromium and nickel is their sensitivity to the magnitude and sign of the intrinsic stress with argon gas pressure, including the existence of a critical pressure that results in zero film stress. This critical pressure scales linearly with the film’s density. While the effect of stress reversal with argon pressure has been previously reported by Hoffman and others for nickel and chromium, we have discovered a similar behavior for the intrinsic stress in iridium films. Additionally, we have identified zero stress in iridium shortly after island coalescence in the high adatom mobility growth regime. This feature of film growth is used for achieving a total internal stress of -2.89 MPa for a 15.8 nm thick iridium film with a surface roughness of 5.0 ± 0.5Å based on x-ray reflectivity (XRR) measurement at CuKα. The surface topography was also examined using atomic force microscopy (AFM). The examination of the stress in these films has been performed with a novel in-situ measurement device. The methodology and sensitivity of the in-situ instrument is also described herein.

Composition and morphology of a MgF2/Al multilayer thin film reflective coating

Multilayer Al thin films with interleaving and protective MgF2 dielectric layers for use as far and extreme ultraviolet reflectors have been characterized using x‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The devices were prepared in a conventional high vacuum system. Sputter depth profiles were obtained using XPS in a separate ultrahigh vacuum system. The MgF2 layers may contain Mg vacancies, and F appeared to be segregated to the MgF2/Al boundaries where it may form AlF3. Oxide regions on the Al surfaces are thicker than previously reported from optical measurements and are not pure Al2O3 throughout. The AFM images showed columnar growth of the Al and MgF2 layers. Root‐mean‐square roughnesses determined across these surfaces were on the order of at least 1–2 nm. A more comprehensive picture of the composition and morphology of each layer in a multilayer device was obtained from this investigation than typically provided from measurements with optical techniques.