Extending the Debye scattering equation for diffraction from a cylindrically averaged group of atoms: detecting molecular orientation at an interface.

The Debye scattering equation is now over 100 years old and has been widely used to interpret diffraction patterns from randomly oriented groups of atoms. The present work develops and applies a related equation that calculates diffraction intensity from groups of atoms randomly oriented about a fixed axis, a scenario that occurs when molecules are oriented at an interface by the presentation of a binding motif as in antibody binding. Using an example biomolecule, the high level of sensitivity of the diffraction pattern to the orientation of the molecule and to the direction of the incident beam is shown. The use of the method is proposed not only for determining the orientation of molecules in biosensors and at membrane interfaces, but also for determining molecular conformation without the need for crystallization.

Sensory gating in bilayer amorphous carbon memristors.

Multi-state amorphous carbon-based memory devices have been developed that exhibit both bipolar and unipolar resistive switching behaviour. These modes of operation were implemented independently to access multiple resistance states, enabling higher memory density than conventional binary non-volatile memory technologies. The switching characteristics have been further utilised to study synaptic computational functions that could be implemented in artificial neural networks. Notably, paired-pulse inhibition (PPI) is observed at bio-realistic timescales (<100 ms). Devices displaying this rich synaptic behaviour could function as robust stand-alone synapse-inspired memory or be applied as filters for specialised neuromorphic circuits and sensors.

Graphitization of Glassy Carbon after Compression at Room Temperature.

Glassy carbon is a technologically important material with isotropic properties that is nongraphitizing up to ∼3000 °C and displays complete or "superelastic" recovery from large compression. The pressure limit of these properties is not yet known. Here we use experiments and modeling to show permanent densification, and preferred orientation occurs in glassy carbon loaded to 45 GPa and above, where 45 GPa represents the limit to the superelastic and nongraphitizing properties of the material. The changes are explained by a transformation from its sp^{2} rich starting structure to a sp^{3} rich phase that reverts to fully sp^{2} bonded oriented graphite during pressure release.

The near edge structure of cubic boron nitride.

We compare the near edge structure (NES) of cubic boron nitride (cBN) measured using both electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) with that calculated using three commonly used theoretical approaches. The boron and nitrogen K-edges collected using EELS and XAS from cBN powder were found to be nearly identical. These experimental edges were compared to calculations obtained using an all-electron density functional theory code (WIEN2k), a pseudopotential density functional theory code (CASTEP) and a multiple scattering code (FEFF). All three codes were found to reproduce the major features in the NES for both ionisation edges when a core-hole was included in the calculations. A partial core hole (1/2 of a 1s electron) was found to be essential for correctly reproducing features near the edge threshold in the nitrogen K-edge and to correctly obtain the positions of all main peaks. CASTEP and WIEN2k were found to give almost identical results. These codes were also found to produce NES which most closely matched experiment based on χ² calculations used to qualitatively compare theory and experiment. This work demonstrated that a combined experimental and theoretical approach to the study of NES is a powerful way of investigating bonding and electronic structure in boron nitride and related materials.

Enhancing the hardness of Al/W nanostructured coatings.

Two-component multilayer thin films frequently show hardness enhancements at specific repeat periods above that of the constituent layers. This study of hardness enhancements in W/Al nanostructured coatings provides strong new evidence that hardness enhancements in this system arise not only from the presence of a layered structure, but also from the presence of defects introduced by changing the deposition conditions. Samples with well defined layers of W and Al were produced by sputtering to cover a wide range of periods from 10 to 200 nm. No evidence of enhanced hardness in these films was found by nanoindentation. On the other hand, samples deposited from cathodic arc sources showed strong hardness enhancement above that of pure W. However, the samples of highest hardness did not contain Al layers for much of their thickness. The hardening mechanism therefore could not be attributed to the presence of a multilayer structure. Examination of the microstructure showed that the interruptions to the W deposition caused by operation of the Al source introduced defects which acted as pinning sites for dislocations. The nanoindentation hardness data were well described using a modified Hall-Petch relation.

The origin of preferred orientation during carbon film growth.

Carbon films were prepared using a filtered cathodic vacuum arc deposition system operated with a substrate bias varying linearly with time during growth. Ion energies were in the range between 95 and 620 eV. Alternating dark, high density (sp(3) rich) bands and light, low density (sp(2) rich) bands were observed using cross-sectional transmission electron microscopy, corresponding to abrupt transitions between materials with densities of approximately 3.1 and 2.6 g cm(-3). No intermediate densities were observed in the samples. The low density bands show strong preferred orientation with graphitic sheets aligned normal to the film. After annealing, the low density bands became more oriented and the thinner high density layers were converted to low density material. In molecular dynamics modelling of film growth, temperature activated structural rearrangements occurring over long timescales ([Formula: see text] ps) caused the transition from sp(3) rich to oriented sp(2) rich structure. Once this oriented growth was initiated, the sputtering yield decreased and channelling was observed. However, we conclude that sputtering and channelling events, while they occur, are not the cause of the transition to the oriented structure.

Abrupt stress induced transformation in amorphous carbon films with a highly conductive transition phase.

We demonstrate that when, and only when, the biaxial stress is increased above a critical value of 6+/-1 GPa during the growth of a carbon film at room temperature, tetrahedral amorphous carbon is formed. This confirms that the stress present during the formation of an amorphous carbon film determines its sp;{3} bonding fraction. In the vicinity of the critical stress, a highly oriented graphitelike material is formed which exhibits low electrical resistance and provides Ohmic contacts to silicon. Atomistic simulations reveal that the structural transitions are thermodynamically driven and not the result of dynamical effects.

Nanostructured SnO(2) films prepared from evaporated Sn and their application as gas sensors.

This paper describes the morphology, stoichiometry, microstructure and gas sensing properties of nanoclustered SnO(x) thin films prepared by Sn evaporation followed by a rheotaxial growth and thermal oxidation process. Electron microscopy was used to investigate, in detail, the evolution of the films as the oxidation temperature was increased. The results showed that the contact angle, perpendicular height, volume and microstructure of the clusters all changed significantly as a result of the thermal oxidation processes. Electron diffraction and x-ray photoelectron spectroscopy measurements revealed that after oxidation at a temperature of 600 °C, the Sn clusters were fully transformed into porous three-dimensional polycrystalline SnO(2) clusters. On the basis of these results, a prototype SnO(2) sensor was fabricated and sensing measurements were performed with H(2) and NO(2) gases. At operating temperatures of 150-200 °C the film produced measurable responses to concentrations of H(2) as low as 600 ppm and NO(2) as low as 500 ppb.

Modeling of structure and porosity in amorphous silicon systems using Monte Carlo methods.

Porous solids are very important from a scientific point of view as they provide a medium in which to study the behavior of confined fluids. Although some porous solids have a well defined pore geometry such as zeolites, many porous solids lack crystalline order and are usually described as amorphous. The description of the pore geometry in such structures is very difficult. The authors develop a modeling approach using a Monte Carlo algorithm to simulate porosity within amorphous systems based on constraints for the internal volume and surface area. To illustrate this approach, a model of microporous amorphous silicon is presented. Structural aspects of the porous model are then compared against hybrid reverse Monte Carlo simulations of nonporous amorphous silicon and published results from the literature. It is found that coordination defects are predominately located at the pore surface walls.

Refinements in the collection of energy filtered diffraction patterns from disordered materials.

In this paper a method for collecting electron diffraction patterns using a Gatan imaging filter is presented. The method enables high-quality diffraction data to be measured at scattering angles comparable to those that can be obtained using X-ray and neutron diffraction. In addition, the method offers the capability for examining small regions of sample in, for example, thin films and nano-structures. Using X-ray, neutron and electron diffraction data collected from the same sample, we demonstrate quantitative agreement between all three. We also present a novel method for obtaining the single scattering contribution to the total diffracted intensity by collecting data at various electron wavelengths. This approach allows pair distribution functions to be determined from electron diffraction in cases where there exists significant multiple scattering.

A study of Technegas employing X-ray photoelectron spectroscopy, scanning transmission electron microscopy and wet-chemical methods.

Scanning transmission electron microscopy (STEM), coupled with energy dispersive X-ray analysis (EDS), X-ray photoelectron spectroscopy (XPS) or radionuclear chemical methods, indicates that the active agent in Technegas is either polymeric TcO2[i.e. (TcO2)n] or (TcO2)n bound to a carbon nanoparticle. The particle size observed using STEM is in good agreement with other published results. XPS has also been used to investigate technetium residues remaining on spent crucibles. The chemical form of technetium in this residue is quite different to the form detected in the aerosol particles. We conclude that the small fraction that migrates into the crucible framework upon resistive heating is reduced to either metallic technetium or carbidic forms, with the remaining nuclide evaporating as (TcO2)n with or without carbon before complete reduction can occur.