Catherine M. Cotell

Effect of ion energy on the mechanical properties of ion beam assisted deposition (IBAD) wear resistant coatings

Ion beam assisted physical vapor deposition offers a number of advantages over other surface modification methods with respect to the properties of wear resistant coatings. These properties include density, adhesion to the substrate, crystallographic orientation, durability and resistance to degradation by moisture. These improved film properties derive from the physical and chemical processes associated with the impingement of energetic ions onto the surface of a growing film. Such processes include sputtering and desorption of atoms from the substrate, implantation of atoms into the substrate, chemical reactions at the surface, cluster nucleation and dissociation, enhancement of surface diffusion, atomic displacements in the bulk, as well as energy and momentum transfer to the atoms in the growing film. A review of models for the influence of ion energy and momentum transfer to a growing film suggests that the effects of ion bombardment may be categorized according to the ion energy.

Pulsed laser deposition of thin film hydroxyapatite: applications for flexible catheters

Skin exit site infections are a major source of morbidity in patients with indwelling percutaneous catheters. Ceramic materials, such as hydroxyapatite (HA) and alumina, have demonstrated excellent biocompatibility and low rates of infection in soft tissues. Previous attempts to design ceramic materials for use as percutaneous connectors have resulted in rigid discs or solid cylindrical tubes. In order to take advantage of the inherent properties of HA without reducing patient comfort or mobility, the feasibility of applying a thin film of HA directly onto a flexible polymeric catheter was studied. The coating was applied by pulsed laser deposition (PLD). The beam from a KrF excimer laser impinged upon a target of pressed and sintered HA, producing a plume of ablated material that was deposited onto the catheter tubing. By rotating the tubing, an even coating of HA was applied to the catheter at a thickness of approximately 0.50 microm. The coating did not compromise the flexibility of the catheter tubing. Hence, PLD of a thin film of HA at the exit site of percutaneous catheters may be a means of incorporating the bioactive and biocompatible properties of HA with the mobility and patient comfort that characterize polymeric catheters.

Ion beam assisted deposition of metal nanoclusters in silica thin films

Abstract Gold and silver nanoclusters in silica were deposited by coevaporation of gold and silicon or silver and silicon under simultaneous oxygen bombardment. The noble metal concentration, the linear absorption coefficient and the cluster size can be controlled by variation of the process parameters. The position of the absorption peak was shifted from 400 to 700 nm by the choice of the cluster metal and the dielectric matrix.

ION-BEAM-ASSISTED DEPOSITION OF AU NANOCLUSTER/NB2O5 THIN FILMS WITH NONLINEAR OPTICAL PROPERTIES

Abstract Gold nanocluster thin films (∼ 200 nm thickness) consisting of metal clusters ∼ 5 nm in size embedded in a matrix of Nb2O5 were deposited by ion beam-assisted deposition (IBAD) by coevaporation of Au and Nb with O2+ ion bombardment. The microstructure and optical characteristics of these films were examined as-deposited and after annealing at 600°C. Annealing crystallized the amorphous oxide matrix and ripened the nanoclusters. A strong linear absorption at the wavelength of the surface plasmon resonance for Au developed as a result of annealing. The linear optical behavior was modeled using Mie scattering theory. Good agreement was found between the nanocluster sizes predicted by the theory and the particle sizes observed experimentally using transmission electron microscopy (TEM). The nonlinear optical (NLO) properties of the nanocluster films were probed experimentally using degenerate four wave mixing and nonlinear transmission. The wavelength was near the peak of the surface plasmon resonance as measured by VIS/UV spectroscopy. Values of |χxxxx(3)| were 7.3 × 10−8 and 3.0 × 10−10 esu for annealed and unannealed samples, respe The dominant mechanism for the nonlinear response was change in dielectric constant due to the generation of a distribution of hot, photoexcited electrons.

Secondary ion mass spectrometry and scanning transmission and transmission electron microscopy studies of the effects of reactive elements on nickel oxidation

Abstract Implantation of reactive elements (REs), such as cerium and yttrium, at doses of 0.5−5 × 10 16 ions cm −2 has been shown to reduce the NiO scale growth rate substantially during the oxidation of nickel in oxygen at 900 °C, at least during 5 h exposures. Understanding the mechanism involved necessitates establishing the location and form of the RE within the scale. These questions have now been addressed, primarily by secondary ion mass spectrometry (SIMS) and by scanning transmission and transmission electron microscopy (TEM/STEM) examination, on NiO scales formed on nickel implanted with either cerium or yttrium ions to 5 × 10 16 cm -2 during oxidation in oxygen at 900 °C for periods between 0.25 and 150 h. SIMS demonstrated that initially the peak RE concentration within the scale was located near the gas interface but with increasing time it became positioned at increasing depth. NiO growth on unimplanted nickel is controlled primarily by outward cation transport. Assuming the position of the RE peak concentration is indicative of the original nickel surface then, at least initially, RE ion implantation changed the scale growth such that inward oxidant movement was dominant. However, with time, growth reverted increasingly to outward cation transport control. The resurgence of cation transport occured earlier during the oxidation of cerium than yttrium implanted nickel. TEM/STEM has established that cerium was present as small oxide particles within both the NiO grains and grain boundaries and also that cerium and yttrium segregated along the scale grain boundaries. The segregant concentrations diminished with oxidation time, indicating diffusion along the boundaries, which was consistent with the SIMS observations. There would seem, therefore, to be a plausible correlation between the disposition of the REs within the NiO scale and their effect on the oxidation behaviour of nickel.

Metastable materials formation by ion beam assisted deposition: application to metal clusters in ceramic matrices

Abstract The collision cascade, the fundamental event in ion-solid interactions, is responsible for the beneficial effects on thin films deposited by low energy ion beam assisted deposition (IBAD) or by energetic ion assisted deposition processes in general. However, the fundamental implications of the marriage of collision cascades and film growth processes have yet to be fully realized. The first section of this paper reviews the effects of ion bombardment on film growth and reaches the conclusion that IBAD processing drives material synthesis parameter phase space into new regimes. Therefore, IBAD processing leads to microstructures far from equilibrium that are unattainable by other materials synthesis methods. The second part of this paper reviews an application of IBAD which tends to support the hypothesis that it is a distinct form of ion-solid interaction. It describes the production of unique metastable materials, demonstrated through deposition of metal nanoclusters in a dielectric matrix by means of beam assisted phase separation (BAPS), a term coined here to describe deposition of phase-separated multicomponent materials.

Pulsed Laser Deposition of Biocompatible Thin Films: Calcium Hydroxylapatrte and Other Calcium Phosphates

Calcium phosphate-based ceramics were deposited on Ti, Ti-6Al-4V and Si substrates by pulsed laser deposition. The calcium phosphate phase (amorphous, α- or β-tricalciumn phosphate, tetracalcium phosphate, or hydroxylapatite), Ca/P ratio and surface morphology of the films were related to deposition conditions. At substrate temperatures 2 , deposition of α-tricalcium phosphate was favored between 400 and 700°C and β-tricalcium phosphate was favored above 700°C. Films deposited in low pressures of O 2 ( 2 in the chamber during deposition increased. In water vapor-enriched inert gas environments, deposition of hydroxylapatite was observed at temperatures between 400 and 700°C and tetracalcium phosphate was observed at temperatures above 700°C. The surface morphology of hydroxylapatite films depended on the temperature of deposition. Surfaces deposited at higher substrate temperatures showed more recrystallization than those deposited at lower substrate temperatures.

Copper-Carbon Alloy Films by Methane Beam Assisted Epitaxial Copper Deposition

Copper-carbon alloy films containing between 7 and 40 at.% carbon, as measured with 6.2 MeV He ++ scattering, were deposited by ion beam assisted deposition (IBAD) on pure epitaxial Cu(100) films on Si(100) substrates. The IBAD process involved simultaneous deposition of evaporated copper (0.3 to 2.0 nm/s) and bombardment with 400 eV ions (340 μA/cm 2 on target) from a Kaufman ion source fed with methane. Transmission electron microscopy (TEM) showed that an IBAD layer with 15 at.%C was epitaxial with the underlying pure Cu(100) layer. X-ray diffraction (XRD) indicated that the IBAD films were epitaxial at low carbon concentration but polycrystalline at high concentration. The small range of carbon ions in copper and the evidence of sputtering suggest that this loss of epitaxy resulted from increased surface carbon interfering with epitaxy. Although carbon and copper are immiscible, TEM and XRD have revealed no evidence of carbon precipitation, indicating that the carbon may be in solution and that such IBAD layers may be suitable for diamond formation by laser annealing.

Ion beam processing of nanocluster-containing thin films

We show that ion beam assisted deposition (IBAD) can result in films of metal or semiconductor nanoclusters in dielectric matrices with nonlinear optical (NLO) or photoluminescence (PL) properties. Gold nanocluster thin films consisting of metal clusters 5-30 nm in size embedded in a Nb2O5 or a SiO2 oxide matrix were deposited by ion beam assisted deposition by co-evaporation of Au and Nb or Si with O2+ ion bombardment. Semiconductor nanocluster films have been prepared by IBAD as well. Silicon-rich silica films were deposited by coevaporation of silica and silicon with and without simultaneous ion bombardment. PL attributable to defects in SiO2 was observed at about 550 nm. After annealing, the PL peak shifts to 750 nm and increases in intensity, indicating the formation of Si nanoclusters. Transmission electron microscopy (TEM) images of annealed IBAD films show a dense distribution of mostly spherically shaped crystalline Si nanoclusters, about 1-4 nm in diameter, in a featureless amorphous SiO2 matrix. TEM images of films prepared without ion assist showed many less crystalline Si clusters, that were more irregularly shaped, within 0.1 micrometers amorphous silica grains. Passivation of the films with hydrogen removes the defect PL peak and enhances the peak due to nanoclusters. We have also prepared Ge nanoclusters in silica films with and without IBAD.

Metastable Materials Formation by ion Beam Assisted Deposition: Application to M Clusters in Ceramic Matrices

The collision cascade, the fundamental event in ion-solid interactions, is responsible for the beneficial effects on thin films deposited by low energy ion beam assisted deposition (IBAD) or by energetic ion assisted deposition processes in general. However, the fundamental implications of the marriage of collision cascades and film growth processes have yet to be fully realized. The first half of this paper reviews the effects of ion bombardment on film growth and reaches some new conclusions. We propose that IBAD represents a different ion-solid interaction in a fundamental sense, and that as such, it should lead to new microstructures unattainable by other materials synthesis methods. The second part of this paper discusses the deposition of metal nanoclusters in a dielectric matrix by means of beam assisted phase separation (BAPS), a term coined here to describe deposition of phase-separated multicomponent materials. Examples discussed are gold nanoparticles in both niobium oxide and silica matrices.