ZnO Quantum Dots : Size Dependent Optical, Vibrational and Photoelectrochemical Properties

Sammanfattning: This thesis is devoted to quantum dots of ZnO in the size regime 2.5-10 nm.The focus is directed towards their size dependent properties with specialemphasis on the optical, vibrational and photocatalytic properties. The particleswere synthesized by hydrolysis in alkaline zinc acetate solution.Analysis and characterisation were performed on both growing particles insolution as well as on thin particle films. The main characterisation techniqueswere UV-vis, florescence, XRD, Raman, electrochemical- and photoelectrochemicalmethods.Based on a large set of XRD and UV-vis measurements an empirical relationbetween the band gap and the particle size were developed. This enabledsize resolved measurements of the visible fluorescence on growing particlesin solution. We report evidence of mobile trap states responsible for the fluorescenceand discuss a possible surface dependent mechanism for this.The quantum confinement of the phonon modes were investigated withRaman spectroscopy and molecular dynamic simulations. We report on asize depended suppression of the phonon modes and the contribution fromsurface effects.The absolute position of the conduction band edge was determined as afunction of particle size with different photoelectrochemical methods that wedescribe in detail. We demonstrate that most of the size dependent shift inthe band gap occurs by a change in the position of the conduction band. Wealso show that the parabolic band approximation is valid in a region ofslightly less than 0.1 eV from the conduction band edge, and this even underan external electric field. An interesting electroabsorption phenomenonwhere the absorption locally increases under an applied potential are describedand explained in terms of the quantum confined Stark effect and theFranz-Keldysh effect.The produced films of the particles show antireflective properties on conductingglass substrates, which could be interesting from a technologicalperspective. The films are demonstrated to show photocatalytic activity withrespect to degradation of organic dyes and for solar water splitting.