David R. McKenzie

EELS analysis of vacuum arc-deposited diamond-like films

Abstract Electron energy-loss spectroscopy measurements have been made on amorphous diamond-like carbon films produced by condensing the plasma stream from a filtered vacuum arc. The results are compared with spectra from diamond, graphitized carbon and amorphous carbon. Although the energy-loss spectra reveal the presence of a small quantity of sp2-bonded material in the diamond-like film, the high plasmon frequency and shape of the K-edge show that the material is essentially an amorphous form of diamond. The fraction of sp2- bonded carbon was quantified and found to be of the order of 15%. It was not possible to determine if the sp2 carbon was on the surface or throughout the bulk.

Compressive stress induced formation of cubic boron nitride

Abstract Compressive biaxial stress fields in thin film boron nitride are shown to provide a means of accessing the region of the phase diagram in which cubic BN (c-BN) is the stable phase. This process differs from high pressure, high temperature synthesis in that c-BN is synthesised from an incoming flux of ions or atoms rather than converted from the hexagonal phase. Experimental evidence is presented that the compressive stress mechanism for c-BN formation is operating during the reactive ion plating deposition process. A well defined threshold stress value is found for the production of c-BN under a wide range of preparation conditions. The microstructure of the films as studied by electron optical methods and the behaviour of the infrared absorption spectrum after stress relief provides further confirmation of the compressive stress mechanism.

Properties of tetrahedral amorphous carbon prepared by vacuum arc deposition

Abstract The structural, optical, electrical and physical properties of amorphous carbon deposited from the filtered plasma stream of a vacuum arc were investigated. The structure was determined by electron diffraction, neutron diffraction and energy loss spectroscopy and the tetrahedral coordination of the material was confirmed. The measurements gave a nearest neighbour distance of 1.53 A, a bond angle of 110 and a coordination number of four. A model is proposed in which the compressive stress generated in the film by energetic ion impact produces pressure and temperature conditions lying well inside the region of the carbon phase diagram within which diamond is stable. The model is confirmed by measurements of stress and plasmon energy as a function of ion energy. The model also predicts the formation of sp2-rich materials on the surface owing to stress relaxation and this is confirmed by a study of the surface plasmon energy. Some nuclear magnetic resonance, infrared and optical properties are reported and the behaviour of diodes using tetrahedral amorphous carbon is discussed.

Aphrodite's iridescence

The most intense colours displayed in nature result from either multilayer reflectors or linear diffraction gratings1,2,3. Here we investigate the spectacular iridescence of a spine (notoseta) from the sea mouse Aphrodita sp. (Polychaeta: Aphroditidae). The spine normally appears to be deep red in colour, but when light is incident perpendicular to the axis of the spine, different colours are seen as stripes running parallel to the axis of the spine; over a range of smaller incident angles, the complete visible spectrum is reflected with a reflectivity of 100% to the human eye. The simple structure responsible for this effect is a remarkable example of photonic engineering by a living organism.

The conductivity of lattices of spheres II. The body centred and face centred cubic lattices

The conductivities of body-centred (b. c. c.) and face-centred (f. c. c.) cubic lattices of spheres of a conducting material in a conducting matrix are calculated by using a method originally devised by Lord Rayleigh. Measurements of the conductivity of the b. c. c. lattice of perfectly conducting spheres are presented. Good agreement between theory and experiment is obtained. Our results are shown to be in agreement with asymptotic equations derived by other authors. A formula is given for the case of a disordered array.

Residual stress, microstructure, and structure of tungsten thin films deposited by magnetron sputtering

The residual stress and structural properties of tungsten thin films prepared by magnetron sputtering as a function of sputtering-gas pressure are reported. The films were analyzed in situ by a cantilever beam technique, and ex situ by x-ray diffraction, cross-sectional transmission electron microscopy (TEM), x-ray photoelectron spectroscopy, electron energy-loss spectrometry, and energy-filtered electron diffraction. It is found that the residual stress, microstructure, and surface morphology are clearly correlated. The film stresses, determined in real time during the film formation, depend strongly on the argon pressure and change from highly compressive to highly tensile in a relatively narrow pressure range of 12–26 mTorr. For pressures exceeding ∼60 mTorr, the stress in the film is nearly zero. It is also found that the nonequilibrium A15 W structure is responsible for the observed tensile stress, whereas the stable bcc W or a mixture of bcc W and A15 W are in compression. Cross-sectional TEM eviden...

Electron diffraction analysis of polycrystalline and amorphous thin films

A rapid analytical technique has been developed for obtaining the reduced density function, G(r), from polycrystalline and amorphous thin films, using post-specimen scanning and an energy loss spectrometer on a transmission electron microscope. The technique gives on-line analysis of nearest-neighbour distances to an accuracy of 0.02 A, together with coordination numbers. It has the advantage over X-ray and neutron techniques that the information can be obtained from small (≲ 1 μm diameter) chosen regions of the specimen. Results from neighbouring selected regions can be compared.

Free radical functionalization of surfaces to prevent adverse responses to biomedical devices

Immobilizing a protein, that is fully compatible with the patient, on the surface of a biomedical device should make it possible to avoid adverse responses such as inflammation, rejection, or excessive fibrosis. A surface that strongly binds and does not denature the compatible protein is required. Hydrophilic surfaces do not induce denaturation of immobilized protein but exhibit a low binding affinity for protein. Here, we describe an energetic ion-assisted plasma process that can make any surface hydrophilic and at the same time enable it to covalently immobilize functional biological molecules. We show that the modification creates free radicals that migrate to the surface from a reservoir beneath. When they reach the surface, the radicals form covalent bonds with biomolecules. The kinetics and number densities of protein molecules in solution and free radicals in the reservoir control the time required to form a full protein monolayer that is covalently bound. The shelf life of the covalent binding capability is governed by the initial density of free radicals and the depth of the reservoir. We show that the high reactivity of the radicals renders the binding universal across all biological macromolecules. Because the free radical reservoir can be created on any solid material, this approach can be used in medical applications ranging from cardiovascular stents to heart-lung machines.

Monte Carlo calculation of the thermalization of atoms sputtered from the cathode of a sputtering discharge

The Monte Carlo technique has been used to simulate the thermalization of sputtered atoms in the filling or background gas within a planar sputtering discharge. The model uses Thompson’s theoretical distribution of the energy of atoms sputtered from the cathode surface together with an approximation to the 6–12 Lennard–Jones potential to describe collisions between sputtered atoms and background gas atoms. The diffusion of the thermalized atoms is included explicitly in the Monte Carlo calculations. The velocity distribution of sputtered atoms between the cathode and substrate is calculated, from which their average kinetic energy is determined as a function of the product Pd, where P is the background gas pressure and d is the distance from the cathode. Because of the effect of the finite cathode‐substrate separation on back diffusion, it was found that PD, where D is the cathode‐to‐substrate separation, is an important parameter in describing thermalization and, consequently, the average energy of sputt...

Covalent immobilisation of tropoelastin on a plasma deposited interface for enhancement of endothelialisation on metal surfaces

Currently available endovascular metallic implants such as stents exhibit suboptimal biocompatibility in that they re-endothelialise poorly leaving them susceptible to thrombosis. To improve the interaction of these implants with endothelial cells we developed a surface coating technology, enabling the covalent attachment of biomolecules to previously inert metal surfaces. Using horseradish peroxidase as a probe, we demonstrate that the polymerised surface can retain the presentation and activity of an immobilised protein. We further demonstrated the attachment of tropoelastin, an extracellular matrix protein critical to the correct arrangement and function of vasculature. Not only it is structurally important, but it plays a major role in supporting endothelial cell growth, while modulating smooth muscle cell infiltration. Tropoelastin was shown to bind to the surface in a covalent monolayer, supplemented with additional physisorbed multilayers on extended incubation. The physisorbed tropoelastin layers can be washed away in buffer or SDS while the first layer of tropoelastin remains tightly bound. The plasma coated stainless steel surface with immobilised tropoelastin was subsequently found to have improved biocompatibility by promoting endothelial cell attachment and proliferation relative to uncoated stainless steel controls. Tropoelastin coatings applied to otherwise inert substrates using this technology could thus have broad applications to a range of non-polymeric vascular devices.