Suspended carbon nanotubes are of particular interest for optical and optoelectronic applications because of their superior optical properties. The purpose of this work is to explore sample preparation and chemical vapour deposition (CVD) process parameters to develop methods of fabricating large quantities of freely suspended single walled carbon nanotubes (SWNTs). Parameters including CVD hydrogen soak temperature, growth temperature, choice of catalyst, gas flow rate, and gas composition were examined. Various techniques were tested for their practical capability to suspend SWNTs. A simple procedure was used to suspend SWNTs over networks of randomly organized thick MWNTs on unpatterned substrates. In a more versatile procedure SWNTs were suspended between adjacent pillars of vertically aligned MWNT forests on patterned substrates. Large quantities of SWNTs were also suspended over pre-fabricated trenches in the substrate. All of these laterally suspended SWNT were found to be useful for optical studies.

The mechanisms by which groups of neurons interact is an important facet to understanding how the brain functions. Here we study stochastic neural networks with delayed feedback. The first part of our study looks at how feedback and noise affect the mean firing rate of the network. Secondly we look at how the spatial profile of the feedback affects the behavior of the network. Our numerical and theoretical results show that negative (inhibitory) feedback linearizes the frequency vs input current (f-I) curve via the divisive gain effect it has on the network. The interaction of the inhibitory feedback and the input bias is what produces the divisive decrease in the slope (known as the gain) of the f-I curve. Our work predicts that an increase in noise is required along with increase in inhibitory feedback to attain a divisive and subtractive shift of the gain as seen in experiments [1]. Our results also show that, although the spatial profile of the feedback does not effect the mean activity of the network, it does influence the overall dynamics of the network. Local feedback generates a network oscillation, which is more robust against disruption by noise or uncorrelated input or network heterogeneity, than that for the global feedback (all-to-all coupling) case. For example uncorrelated input completely disrupts the network oscillation generated by global feedback, but only diminishes the network oscillation due to local feedback. This is characterized by 1st and 2nd order spike train statistics. Further, our theory agrees well with numerical simulations of network dynamics.

The present thesis consists of a number of studies on the elastic properties of covalently bonded crystals, the ideal strengths of a selection of face-centred cubic (FCC) metals and the anomalously large shear modulus of iridium. Semi-empirical formulae for both bulk modulus B and shear modulus G of covalent crystals were elaborated in terms of bond length and ionicity fraction of the bonding. The resulting equations can be applied to a broad selection of covalent materials and their predictions are in good agreement with the experimental data and the results from first-principles calculations. Furthermore, the correlation between the ratio G/B and the aforementioned bonding parameters was investigated. The analysis of this relationship demonstrates that the bond length is the predominant parameter responsible for the brittle features of covalent materials. The ideal shear and tensile strengths of FCC transition metals were examined through density functional theory (DFT)-based computations. These results allowed establishing a new indicator of ductility, namely the ratio of ideal shear strength to ideal tensile strength taum/sigma <111>. Moreover, it was found that palladium can sustain a surprisingly long range of deformation. Based on the analysis of density of states (DOS) curves, it is suggested that creation of angular features is responsible for the notable amount of distortion suffered by palladium. The electronic origin of the anomalously large elastic modulus and intrinsic brittleness of FCC iridium were studied using ab initio DFT-based calculations. The electron localisation function (ELF) and bond orders (BO) of iridium and a selection of FCC metals were calculated and then used to evaluate the directionality and the strength of the bonds. The analysis of the trend of bond strength versus elastic moduli, Cauchy pressure and the ratio of shear modulus over bulk modulus suggests that the bond strength is the primary factor that causes the abnormally high modulus of iridium and its intrinsic brittleness.

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