Investigation of numerical atomic orbitals for first-principles calculations of the electronic and transport properties of silicon nanowire structures

This thesis is focused on the application of numerical atomic basis sets in studies of the structural, electronic and transport properties of silicon nanowire structures from first-principles within the framework of Density Functional Theory. First we critically examine the applied methodology and then offer predictions regarding the transport properties and realisation of silicon nanowire devices. The performance of numerical atomic orbitals is benchmarked against calculations performed with plane waves basis sets. After establishing the convergence of total energy and electronic structure calculations with increasing basis size we have shown that their quality greatly improves with the optimisation of the contraction for a fixed basis size. The double zeta polarised basis offers a reasonable approximation to study structural and electronic properties and transferability exists between various nanowire structures. This is most important to reduce the computational cost. The impact of basis sets on transport properties in silicon nanowires with oxygen and dopant impurities have also been studied. It is found that whilst transmission features quantitatively converge with increasing contraction there is a weaker dependence on basis set for the mean free path; the double zeta polarised basis offers a good compromise whereas the single zeta basis set yields qualitatively reasonable results. Studying the transport properties of nanowire-based transistor setups with p+-n-p+ and p+-i-p+ doping profiles it is shown that charge self-consistency affects the I-V characteristics more significantly than the basis set choice. It is predicted that such ultrascaled (3 nm length) transistors would show degraded performance due to relatively high source-drain tunnelling currents. Finally, it is shown the hole mobility of Si nanowires nominally doped with boron decreases monotonically with decreasing width at fixed doping density and increasing dopant concentration. Significant mobility variations are identified which can explain experimental observations.

Properties of molecules in tunnel junctions

Molecular tunnel junctions involve studying the behaviour of a single molecule sandwiched between metal leads. When a molecule makes contact with electrodes, it becomes open to the environment which can heavily influence its properties, such as electronegativity and electron transport. While the most common computational approaches remain to be single particle approximations, in this thesis it is shown that a more explicit treatment of electron interactions can be required. By studying an open atomic chain junction, it is found that including electron correlations corrects the strong lead-molecule interaction seen by the ΔSCF approximation, and has an impact on junction I − V properties. The need for an accurate description of electronegativity is highlighted by studying a correlated model of hexatriene-di-thiol with a systematically varied correlation parameter and comparing the results to various electronic structure treatments. The results indicating an overestimation of the band gap and underestimation of charge transfer in the Hartree-Fock regime is equivalent to not treating electron-electron correlations. While in the opposite limit, over-compensating for electron-electron interaction leads to underestimated band gap and too high an electron current as seen in DFT/LDA treatment. It is emphasised in this thesis that correcting electronegativity is equivalent to maximising the overlap of the approximate density matrix to the exact reduced density matrix found at the exact many-body solution. In this work, the complex absorbing potential (CAP) formalism which allows for the inclusion metal electrodes into explicit wavefunction many-body formalisms is further developed. The CAP methodology is applied to study the electron state lifetimes and shifts as the junction is made open.

Optimizing perioperative analgesia for patients undergoing operative fixation of hip fractures

This PhD thesis describes work carried out on investigation of various interventions with the aim to optimise the anaesthetic management of patients scheduled to undergo operative fixation of hip fractures. We analysed the perioperative effects of continuous femoral nerve block, single preoperative dose of i.v. dexamethasone, the intention to deposit local anaesthetic in different locations around the femoral nerve during ultrasound guided femoral nerve block, continuous spinal anaesthesia and peri-surgical site infiltration with local anaesthetic after surgical fixation of hip fractures. Continuous femoral nerve block provided more effective preoperative analgesia six hours after the insertion of the perineural catheter compared to a standard opiate-based regimen in patients undergoing operative fixation of fractured hip. A single low dose of preoperative dexamethasone in the intervention group decreased pain scores by 75% six hours after the surgery. Both interventions had no major effect on the functional recovery in the first year after the surgical fixation of fractured hip. The results of the ultrasound guided femoral nerve block trial showed no clinical advantage of intending to deposit local anaesthetic circumferentially during performing femoral nerve block. Using the Dixon and Massey’s “up- and-down” method, we demonstrated that intrathecal 0.26 ml of 0.5% bupivacaine provided adequate surgical anaesthesia within 15 minutes in 50% of patients undergoing operative fixation of hip fracture. Finally, we demonstrated that local anaesthetic infiltration had no effect on pain scores 12 hours after the surgical fixation of fractured neck of femur. In addition to this original body of work, a review article was published on femoral nerve block highlighting the use of ultrasound guidance. In conclusion, the results of this thesis offer an insight into interventions aimed at optimising perioperative analgesia in patients scheduled to undergo operative fixation of hip fractures.