Morgan Stewart, H; (2016) Density Functional Studies of Nanomaterials with Applications in Electronic Devices and Hydrogen Storage. Doctoral thesis , UCL (University College London).

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Abstract

We present density functional calculations investigating two different nanomaterials: a titanium carbide nanocluster and few-layered black phosphorus. The titanium carbide nanocluster, Ti₈C₁₂, has properties that are well suited to applications in hydrogen storage, while few-layered black phosphorus has recently been used in the fabrication of novel field effect transistors. Chapter 1 provides some background information regarding hydrogen storage and electronic devices, with Chapter 2 introducing the computational methods used throughout subsequent chapters. In Chapter 3, we investigate the thermodynamic and kinetic profile of H₂ dissociation by Ti₈C₁₂ under a range of conditions. Our results show that that Ti₈C₁₂ is able to reversibly dissociate H₂ with an unusually low activation barrier. In Chapter 4, we introduce few-layered black phosphorus, dubbed phosphorene. The use of black-phosphorus exfoliates in FETs is potentially important given the fast approaching limits of transistor miniaturization using current technologies. Phosphorene appears to have properties necessary for use in next generation FETs, and has therefore attracted enormous experimental and theoretical attention. Our work on phosphorene contributes to an ever growing body of information, with Chapter 5 investigating the effects of deforming monolayer and bilayer phosphorene and Chapter 6 investigating the properties of phosphorene nanoribbons. In Chapter 5, we show that compressing bilayer phosphorene normal to its surface dramatically increases n-type mobility and modulates the band gap. The compressions required to increase n-type mobility by a factor of 10² are modest, meaning that our results are experimentally relevant. We also investigate the effects of bending of phosphorene, which has a highly anisotropic bending modulus. Our work on phosphorene nanoribbons in Chapter 6 shows that in-plane quantum confinement effects lead to a significant increase in the band gap. We replicate this effect by applying periodic boundary conditions to the bulk and derive a formula relating the band gap of phosphorene nanoribbons to phosphorene’s band edge effective masses. We also show that the band gap and mobility of phosphorene nanoribbons can be modified through the application of linear strain. II Chapter 7 concludes the main body of this thesis, summarising its outcomes and giving a direction for future work. We also include a brief investigation into a family of semiconducting quaternary oxynitride compounds in the Appendix. These compounds are of interest given that their band gaps fall within the visible light region.

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