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Molecular nonlinear optical (NLO) properties are extensively studied using both theory and experiment because of their use in myriad applications. Experimental measurements of the most interesting molecules’ NLO properties are difficult, so experimental data for molecules with desirable NLO properties is scarce. Theoretical tools don’t suffer from the same limitations and can provide significant insights into the physico-chemical phenomena underlying the nonlinear responses, can help in interpreting response behaviour of molecules, and can guide design the materials with desirable response properties. Here, I present my work on developing methods for accurately calculating the NLO properties of molecules using the finite field (FF) approach. The first chapter provides a background for the finite field and electronic structure methods used in this dissertation. Chapter two is a thorough investigation of the finite field method. The limitations of the method are highlighted and the optimal conditions for overcoming its drawbacks and obtaining meaningful and accurate results are described. Chapter three presents the first systematic study of the dependence of optimal field strengths on molecular descriptors. The first protocol for predicting the optimal field for the second hyperpolarizability is presented and successfully tested, and the dependence of the optimal field strength for the first hyperpolarizability on the molecular structure is investigated. Chapter four is an assessment of various DFT functionals in calculating the second hyperpolarizabilities of organic molecules and oligomers. This study shows the limitations of conventional DFT methods and the importance of electron correlation to response properties. In chapter five we present a new method of calculating NLO properties using a rational function model that is shown to be more robust and have lower computational cost than the traditional Taylor expansion. Finally, chapter six includes a summary of the thesis and an overview of future work. / Thesis / Doctor of Philosophy (PhD)

The research performed aims to develop a deeper understanding and prediction of behaviour of complex chemical and physical systems using density functional theory (DFT) modelling complemented by experimental techniques. We focus on phenomena relevant to practical applications of semiconducting materials. Semiconductor nanowires, produced by the vapor-liquid-solid method are being considered for applications in photo sensors, field effect transistors, light emitting diodes (LEDs) and energy harvesting devices. In particular, semiconductor nanowire based photovoltaic devices show potential for lower cost due to less material utilization and greater energy conversion efficiency arising from enhanced photovoltage or photocurrent due to hot carrier or multiexciton phenomena enhanced light absorption, compared to conventional thin film devices. Further, freedom from lattice matching requirements due to strain accommodation at the nanowire surfaces enable compatibility with a wide variety of substrates including Silicon. Thus understanding and improving the optoelectronic properties of nanowires is of great interest. In the first paper, we study the effect of planar defects on optoelectronic properties of nanowire based semiconductor devices. Specifically, we were interested in investing the origin of various features observed in the photoluminisence (PL) spectrum of GaP nanowire using density functional modelling, which are not well understood. In the second paper, we work to model bonding characteristics during a chemical synthesis. We focus on the synthesis of nanoparticles for supercapacitor application. In the past decade, comprehensive research has been emphasized on manganese oxides for electrochemical supercapacitor (ECS) applications. Mn3O4 has gained significant interest due to its compatibility with capping agents and the unique spinel structure allows for potential modifications with other cations. Many metal oxide synthesis techniques are based on aqueous processing. The synthesized particles are usually dried and redispersed in organic solvents to incorporate water-insoluble additives such as binders to fabricate films and devices. However, during the drying step nano-structures are highly susceptible to agglomeration, which can be attributed to the condensation reactions occurring between particles and reduction in surface energy. Poor electrolyte access due to agglomeration and low intrinsic conductivity of Mn3O4 are detrimental to the performance of Mn 3O4 electrode especially at high active mass loadings. Numerous attempts have focused on controlling size and morphology of Mn3O4 nanostructures using capping agents, which have strong adhesion to particles surface to inhibit agglomeration. Catechol containing molecules have been used for dispersion of metallic nanoparticles and fabrication of composite thin films, resulted in narrow size distribution of nanoparticles and strong adhesion to substrates. Despite the experimental results showing good adsorption of catechol group to metal atoms, the mechanism is unclear since it is highly influenced by synthesis parameters. We use Infrared spectroscopy in conjugation with density functional modelling to understand the binding mechanism of 3,4 dihydroxy benzaldehyde onto Mn3O4 surface. / Thesis / Master of Applied Science (MASc)