D.G. McCulloch

The anodized crystalline WO3 nanoporous network with enhanced electrochromic properties

We demonstrate that a three dimensional (3D) crystalline tungsten trioxide (WO(3)) nanoporous network, directly grown on a transparent conductive oxide (TCO) substrate, is a suitable working electrode material for high performance electrochromic devices. This nanostructure, with achievable thicknesses of up to 2 μm, is prepared at room temperature by the electrochemical anodization of a RF-sputtered tungsten film deposited on a fluoride doped tin oxide (FTO) conductive glass, under low applied anodic voltages and mild chemical dissolution conditions. For the crystalline nanoporous network with thicknesses ranging from 0.6 to 1 μm, impressive coloration efficiencies of up to 141.5 cm(2) C(-1) are achieved by applying a low coloration voltage of -0.25 V. It is also observed that there is no significant degradation of the electrochromic properties of the porous film after 2000 continuous coloration-bleaching cycles. The remarkable electrochromic characteristics of this crystalline and nanoporous WO(3) are mainly ascribed to the combination of a large surface area, facilitating increased intercalation of protons, as well as excellent continuous and directional paths for charge transfer and proton migration in the highly crystalline material.

Hybrid approach for generating realistic amorphous carbon structure using metropolis and reverse Monte Carlo

An improved method for the modelling of carbon structures based on a hybrid reverse Monte Carlo (HRMC) method is presented. This algorithm incorporates an accurate environment dependent interaction potential (EDIP) in conjunction with the commonly used constraints derived from experimental data. In this work, we compare this new method with other modelling results for a small system of 2.9 g/cc amorphous carbon. We find that the new approach greatly improves the structural description, alleviating the common problem in standard reverse Monte Carlo method (RMC) of generating structures with a high proportion of unphysical small rings. The advantage of our method is that larger systems can now be modelled, allowing the incorporation of mesoscopic scale features.

Plasma-based ion implantation utilising a cathodic arc plasma

Plasma-based ion implantation (PBII) is usually carried out with isotropic gaseous plasmas, such as a discharge in nitrogen. More recently, it has been applied using drifting plasmas, such as those produced by cathodic arcs, in order to allow efficient implantation of metallic species. The condensable nature of a cathodic arc plasma allows for the deposition of ion-stitched thin film coatings, as well as surface modification by ion implantation. In this paper the promising results for biomaterial fabrication are discussed in light of current limitations of the technique. The use of PBII to control preferred orientation in titanium nitride films is also discussed, together with implications for the physical mechanisms involved in the development of preferred orientations in thin films.

Controlled surface modification of boron nitride nanotubes

Controlled surface modification of boron nitride nanotubes has been achieved by gentle plasma treatment. Firstly, it was shown that an amorphous surface layer found on the outside of the nanotubes can be removed without damaging the nanotube structure. Secondly, it was shown that an oxygen plasma creates nitrogen vacancies that then allow oxygen atoms to be successfully substituted onto the surface of BNNTs. The percentage of oxygen atoms can be controlled by changing the input plasma energy and by the Ar plasma pre-treatment time. Finally, it has been demonstrated that nitrogen functional groups can be introduced onto the surface of BNNTs using an N(2) + H(2) plasma. The N(2) + H(2) plasma also created nitrogen vacancies, some of which led to surface functionalization while some underwent oxygen healing.

Control of stress and microstructure in cathodic arc deposited films

The almost fully ionized cathodic arc plasma is a versatile source for the deposition of thin films. Ion energies impinging on the growth surface can easily be controlled by applying substrate bias. The natural energy of the depositing ions is moderate (tens of electron volts) and generates substantial compressive stress in most materials. In hard materials (such as tetrahedral-carbon and titanium nitride), the high-yield stress makes the problem particularly severe. Recent work has shown that stress relaxation can be achieved by pulses of high ion-energy bombardment (/spl sim/10 keV) applied to the substrate during growth. In this paper, we describe the variation of intrinsic stress as a function of applied pulsed bias voltage (V) and pulse frequency (f) for deposition of carbon and titanium nitride films. We found that stress relaxation depends on the parameter Vf, so it is possible to achieve the same level of stress relief for a range of voltages by selecting appropriate pulsing frequencies. With the right choice of parameters, it is possible to almost completely eliminate the intrinsic stress and deposit very thick coatings. Our experimental results showed correlations between intrinsic stress and film microstructures, such as the preferred orientation. This leads to the possibility of controlling microstructure with high energy ion pulsing during growth. Molecular dynamics computer simulations of isolated impacts provide insight into the atomic-scale processes at work. Using the results of such simulations, we describe a model for how stress relief might take place, based on relaxation in thermal spikes occurring around impact sites of the high-energy ions.

Structural analysis of carbonaceous solids using an adapted reverse Monte Carlo algorithm

We present microstructural analysis of a disordered carbonaceous solid using simulations that employ a modified reverse Monte Carlo (RMC) algorithm. This algorithm incorporates an accurate environment dependent interaction potential (EDIP) in addition to commonly used constraints derived from experimental data, such as the sp2/sp3 bonding ratio. Our approach improves the microstructural description for carbon, alleviating the common problem in standard RMC of generating structures with large proportions of highly strained and physically unreasonable small rings. We also compare the electron diffraction data used in the modified RMC algorithm to our recent results from a neutron diffraction investigation of the carbonaceous material under consideration.

Mechanism for the Amorphisation of Diamond

The breakdown of the diamond lattice is explored by ion implantation and molecular dynamics simulations. We show that lattice breakdown is strain-driven, rather than damage-driven, and that the lattice persists until 16% of the atoms have been removed from their lattice sites. The figure shows the transition between amorphous carbon and diamond, with the interfaces highlighted with dashed lines.

Ion‐beam induced compaction in glassy carbon

Cross‐sectional transmission electron microscopy has been used to investigate the implanted layer in glassy carbon irradiated with 50 keV C ions to a dose of 5×1016 ions/cm2. It was found that in addition to the formation of an amorphous surface layer approximately 100 nm deep, the ion‐beam modified layer was compacted from the unirradiated density of 1.5 to 2.4±0.2 g/cm3. Ion implantation was also found to increase the refractive index of glassy carbon from 1.8±0.1 to 2.4±0.1 which is also consistent with the proposition that an increase in the density of the implanted layer has occurred. The formation of a dense, amorphous carbon surface layer could explain the observed increase in wear resistance of glassy carbon following ion implantation.

Effect Of Intrinsic Stress On Preferred Orientation In Aln Thin Films

We examine the effect of ion impact energy on the intrinsic stress and microstructure of aluminum nitride thin films deposited using a filtered cathodic arc. The dependence of intrinsic stress on ion impact energy is studied over the range from 0 to 350 V using dc bias and up to several kV for a fraction of the ions using pulse bias. For dc bias, the stress reaches a maximum at 200 V and decreases with further increase in ion bias. The preferred orientation of the crystallites was studied by cross-section transmission electron microscopy and diffraction. We found that there is a preference for the c crystallographic axis to lie in the plane of the film under high intrinsic stress conditions (4 GPa), whereas a c-axis orientation perpendicular to the plane of the film was observed for low intrinsic stress (0.25 GPa). We carried out calculations of the expected distribution of intensity in cross-sectional electron diffraction patterns to predict the effect of rotation freedom of crystallites with the c axis pinned. The calculated patterns agreed well with experiment.We examine the effect of ion impact energy on the intrinsic stress and microstructure of aluminum nitride thin films deposited using a filtered cathodic arc. The dependence of intrinsic stress on ion impact energy is studied over the range from 0 to 350 V using dc bias and up to several kV for a fraction of the ions using pulse bias. For dc bias, the stress reaches a maximum at 200 V and decreases with further increase in ion bias. The preferred orientation of the crystallites was studied by cross-section transmission electron microscopy and diffraction. We found that there is a preference for the c crystallographic axis to lie in the plane of the film under high intrinsic stress conditions (4 GPa), whereas a c-axis orientation perpendicular to the plane of the film was observed for low intrinsic stress (0.25 GPa). We carried out calculations of the expected distribution of intensity in cross-sectional electron diffraction patterns to predict the effect of rotation freedom of crystallites with the c axis ...

Minimisation of intrinsic stress in titanium nitride using a cathodic arc with plasma immersion ion implantation

An important issue in thin film coatings is the intrinsic stress in the coatings. The quality of the film can strongly be affected by stress as it may cause films to delaminate from the substrate. Because of this it is important to investigate ways of controlling the build-up of the intrinsic stress in growing films. The approach reported in this work is to combine the plasma immersion ion implantation technique with conventional physical vapour deposition. We have used such a combined system to produce a series of titanium nitride films on silicon. The aim was to observe the stress of the film as a function of the applied substrate voltage and frequency of the PIII pulse. Our results indicate that the stress in the film is related to the voltage-frequency product (V.f), and that an optimum value of V.f can be found to produce high quality films of TiN without excessive build-up of stress.