III-V nitride based light-emitting diodes (LEDs) have experienced rapid developments during past decade, proving their potential to substitute conventional incandescent bulbs and fluorescent lamps to fulfil energy-efficient and sustainable lighting needs. Tremendous endeavours have been made to improve the performance of LEDs, most of which focused on enhancing the internal and external quantum efficiencies. However, other optical properties of LEDs remain to be explored for a more flexible way of using LEDs in various applications. Therefore, this thesis proposes two nanostructuring strategies through the use of nanospheres to tailor the optical properties of LEDs. The nanostructured LEDs are demonstrated enable light emission with reduced divergence, or becomes polarized. The monolithic modifications are free of external optics and thus eliminate light loss, meanwhile providing manipulability of optical emission from LEDs. Firstly, close-packed indium-tin-oxide (ITO) micron-lenses with dimension of the order of wavelength have been integrated onto InGaN LEDs aiming at reducing the emission divergence. The sub-micron lens arrays are patterned by nanosphere lithography with silica nanosphere serving as an etch mask on ITO layer, leaving the semiconductor layer damage-free. An enhancement of up to 63.5% on optical output power from the lensed LED has been observed. The LED with 500 nm lenses exhibits a 26.8° reduction in emission divergence (full width at half maximum) compared with the bare LED. Three-dimensional finite-difference time-domain simulations performed for light extraction and emission characteristics is found to be consistent with the observed results. Secondly, polarization behavior of light emitted from InGaN LEDs propagating through a self-assembled polystyrene nanosphere opal film has been studied. Angular-resolved optical transmission of transverse electric (TE) and transverse magnetic (TM) polarized light has been measured. An integrated p/s ratio of 2.16 is observed at a detection angle of 70°, attributed to the suppression of TE mode at particular frequencies by the three-dimensional photonic crystal. Polarization is found to depend strongly on both the photonic bandgap of the opal and the angle of incidence. Theoretical calculations by transfer matrix method yield results consistent with the experimental data. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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In this dissertation several aspects on one-dimensional edge states in grapheme are studied. First, a background in the history and development of graphitic forms is presented. Then some novel features found in two-dimensional bulk graphene are presented. Here, some focus is given to the chiral nature of the Dirac equation and the symmetries found in the grahene. Magnetism and interactions in graphene is also briefly discussed. Finally, the graphene nanoribbon with its two typical edges: armchair and zigzag is introduced. Gaps due to finite-size effects are studied. Next, the problem of determining the zigzag ground state is presented. Later, we develop this in an attempt to add the Coulomb interaction to the zigzag flat-band states. These nanoribbons can be stimulated with a tight-binding code on a lattice model in which many different effects can be added, including an A/B sublattice asymmetry, spin-orbit coupling and external fields. The lowest Landau level solutions in the different ribbon orientations is of particular current interest. This is done in the context of understanding new physics and developing novel applications of graphene nanoribbon devices. Adding spin-orbit to a graphene ribbon Hamiltonian leads to current carrying electronic states localized on the sample edges. These states can appear on both zigzag and armchair edges in the semi-finite limit and differ qualitatively in dispersion and spin-polarization from the well known zigzag edge states that occur in models that do not include spin-orbit coupling. We investigate the properties of these states both analytically and numerically using lattice and continuum models with intrinsic and Rashba spin-orbit coupling and spin-independent gap producing terms. A brief discussion of the Berry curvature and topological numbers of graphene with spin-orbit coupling also follows.

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Thesis (Ph. D.)--Tufts University, 2005. / Adviser: David R. Walt. Submitted to the Dept. of Chemistry. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Thesis (Ph. D.)--University of Hong Kong, 2007. / Also available in print.