Various techniques of fabricating surface patterns of small scales have been widely studied due to the potential applications of surface patterns in a variety of areas. It is a challenge to fabricate well-ordered surface area efficiently and economically. Evaporation-induced surface patterning is a promising approach to fabricate well-ordered surface patterns over a large area at low cost. In this study, the evaporation-induced surface patterns with controllable geometrical characteristics have been constructed. The dewetting kinetics on deformable substrate is also investigated. Using simple templates to control the geometry and the evaporation behavior of a droplet of volatile solvent, various gradient surface patterns, such as concentric rings, multiple straight stripes formed with a straight copper wire, etc. have been constructed on PMMA films. The wavelength and amplitude are found to decreases with the decrease of the distance to the objects used in templates. There is also a nearly linear relation between the amplitude and wavelength. The effects of several experimental parameters on the geometrical characteristics of the surface structures are studied, i.e. dimensions of the template, film thickness (solution concentration), substrate temperature, etc. The wavelength and amplitude increase with the increase of the film thickness (solution concentration), with the increase of the dimension of the template. However with the increase of the substrate temperature, the wavelength increases, while the amplitude decrease. Hexagonal network in pre-cast PMMA film have been fabricated by a “breath figure” approach at low humidity and low substrate temperature. The dimensions of the hexagonal holes are dependent on the template size and film thickness. The kinetics of the evaporative dewetting of a liquid (toluene) film on a deformable substrate (PMMA film) with the confinement of a circular copper ring is also studied. The liquid film first dewets from the outside towards the copper ring. When a critical volume is reached, an internal contact line appears, which dewets from the center to the copper ring smoothly with a constant velocity, then switches to a “stick-slip” motion. The average velocity of the smooth motion increases with the increase of the copper ring size and film thickness.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Organic semiconductors are used to manufacture thin film (smaller than 50nm) photovoltaic devices. Layer thicknesses are calibrated with the use of an AFM and QCM crystals. An in house method is prepared for solar cell comparison, and AM1.5G (one sun equivalent) testing is performed on manufactured solar cells. The importance of layer thickness and the exciton blocking layers are also highlighted. Numerical modelling of the optical electric field amplitude is done by the transfer matrix method, to take optical interference effects into consideration. The photo generated current was extracted as a function of absorption with varying position in the active layers, and used to excite a general model for organic photovoltaic cells. / AFRIKAANSE OPSOMMING: Organiese halfgeleiers word gebruik vir die vervaardiging van dun-film (kleiner as 50nm) fotovoltaïse toestelle. Laagdiktes is gekalibreer deur die gebruik van ’n AFM en QCM kristalle. ’n Inhuis metode is voorberei vir die vergelyking van vervaardigde selle. Daarna is AM1.5G (een son ekwivalente) toetse uitgevoer op die vervaardigde sonselle. Die belangrikheid van laag dikte en die “exciton” blok lae word ook beklemtoon. Numeriese modellering van die optiese elektriese veld amplitude word gedoen deur die oordrag matriks metode, om optiese interferensie gevolge in ag te neem. Die foto-gegenereerde stroom is as ’n funksie van absorpsie onttrek met wisselende posisie in die aktiewe lae, en is gebruik in ’n algemene model vir organiese fotovoltaïse selle.

The use of photonic crystals for light extraction from light-emitting diodes (LEDs) gives the possibility to shape the farfield emission pattern. This is of particular interest for étendue-limited LED applications that require a more directional farfield than state- of-the-art Lambertian emitters. However, the application of a photonic crystal in a LED results in directional emission only if the photonic crystal and the distribution of guided modes in the LED are tuned correctly. In this thesis, red- and blue-emitting thin-film PhC-LEDs in the AlGaInP and InGaN material systems were modelled, designed, fabricated and characterized. The first experimental results show that light extraction with photonic crystals from AlGaInP thin-film LEDs several microns thick is neither directional nor more efficient than state-of-the-art LEDs with a rough surface structure. Directional light extraction for AlGaInP PhC-LEDs is for the first time demonstrated in much thinner devices where the photonic crystal light extraction of guided modes is combined with the resonant-cavity effect. In an attempt to approach the ideal PhC-LED, strong photonic crystal farfield shaping is demonstrated in InGaN thin-film LEDs of sub-micron thickness. Analysis of their spectral farfields unexpectedly shows that high order diffraction contributes significantly to the light extraction efficiency if the mode absorption is sufficiently low. It is also demonstrated that directional photonic crystal light extraction is possible in InGaN thin-film LEDs several microns thick. The directionality stems from the modulation of the spontaneous emission caused by the proximity of the active region to the bottom mirror. Two new concepts for enhanced light extraction and high directionality are presented: Photonic crystals with two dominating lattice constants are found to outperform conventional photonic crystal LEDs. An alternative approach is the dielectric PhC-LED - FDTD simulations show that the high extraction efficiency of LEDs with surface roughness is combined with the higher directionality of photonic crystal light extraction.

Nanostructured, porous materials offer great promise for application in areas such as energy storage, photovoltaics, and catalysis. These materials are often difficult to characterize because they are structurally and compositionally inhomogeneous, and disordered with features to small to be resolved by scanning probe techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM). These shortcomings require that new techniques be developed that can be applied to real world systems to elucidate how the interplay of material composition and structure alters their performance. Towards this end, the development of a hybrid quartz crystal microbalance/ ellipsometric porosimetry (QCM/EP) technique is being pursued to facilitate the determination of a number of material parameters such as porosity, pore size distribution, and surface area. Additionally, the use of adsorbate probe molecules of varying polarity gives further information about adsorbate-surface interactions and surface chemistry characteristics. Simultaneous acquisition of both mass-based and refractive index based adsorption isotherms fosters mechanistic understanding about the behavior of adsorbates confined in mesopores while at the same time reducing the uncertainty in the analysis of the optical parameters acquired via ellipsometry. To highlight the power of this approach, studies of TiO₂ and TiC, electrode materials as model systems will be presented that have helped us validate measurement and modeling protocols for extracting physical properties. / text

Our group has previously shown that small changes to molecular structure result in large changes to device properties and stability in organic electronic applications. By functionalizing aromatic heteroacenes with group 14 and group 16 elements, it is possible to control morphology and improve stability for a variety of applications such as thin film transistors and solar cells. Functionalization within the heteroacene core led to changes in electronic structure as observed by electrochemistry and light absorption. By substituting down the periodic table, the carbon heteroatom bond length increased, leading to subtle changes in crystal packing. Absorption maxima were red-­‐shifted and stability to light decreased. Substitution of group 14 elements to the solubilizing ethynyl groups attached to the heteroacene also had an effect on crystallization and stability. Substitution of silicon with carbon decreased solubility as well as stability to light. Substitution with germanium also decreased stability to light, but close contacts within the crystal structure and solubility in nonpolar organic solvents increased.

In this dissertation, three types of microfabricated solid-state sensors had been designed and developed on silicon wafers, aiming to detect hydrogen gas at elevated temperatures. Based on the material properties and sensing mechanisms, they were operated at 140°C, 500°C, and 300°C. The MOS-capacitor device working at 140°C utilized nickel instead of the widely-used expensive palladium, and the performance remained excellent. For very-high temperature sensing (500°C), the conductivity of the thermally oxidized TiO2 thin film based on the anodic aluminum oxide (AAO) substrate changed 25 times in response to 5 ppm H2 and the response transient times were just a few seconds. For medium-high temperatures (~300°C), very high sensitivity (over 100 times’ increment of current for H2 concentration at 10 ppm) was obtained through the reversible reduction of the Schottky barrier height between the Pt electrodes and the SnO2 nano-clusters. Fabrication approaches of these devices included standard silicon wafer processing, thin film deposition, and photolithography. Materials characterization methods, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), surface profilometry, ellipsometry, and X-ray diffractometry (XRD), were involved in order to investigate the fabricated nano-sized structures. Selectivities of the sensors to gases other than H2 (CO and CH4) were also studied. The first chapter reviews and evaluates the detection methodologies and sensing materials in the current research area of H2 sensors and the devices presented this Ph.D. research were designed with regard to the evaluations.

Soluble organic semiconductors have attracted interest due to their potential in making flexible and cheap electronics. Though their use is being implemented in electronics today, the conduction mechanism is still under investigation. In order to study the charge transport, this study examines the position, voltage, and frequency dependence of charge induced changes in far infrared absorption in soluble organic semiconductors in thin-film transistor structures. Measurements are compared to a simple model of a one-dimensional conductor which gives insight into the charge distribution and timing in devices. Main results of the study are dynamic measurements of charge taken by varying the frequency of the applied gate voltage while observing signal at one position within the transistor; mobility values obtained from a comparison to the one-dimensional model compare well with standard current-voltage measurements. Two small molecule soluble organic semiconductors were studied: 6,13 bis(triisopropylsilylethynyl)-pentacene and fluorinated 5,11 bis(triethylsilylethynyl) anthradithiophene.

Les semi-conducteurs transparents de type oxyde, communément appelés TCO (Transparent Conducting Oxides) sont utilisés comme électrodes transparentes dans des nombreux d'applications telles que les cellules solaires, les écrans à cristaux liquides, les écrans tactiles et autres. Toutefois, les applications technologiques sont actuellement limitées puisque les TCO possédant des propriétés électriques et optiques satisfaisantes sont uniquement des semi-conducteurs de type n. Les oxydes de cuire de structures delafossite ACuO2 ou du type SrCu2O2, présentent des prometteuses avec un comportement de semi-conduction de type P et une faible absorption optique dans le spectre visible. Dans cette thèse, le systèm MOCVD (Dépôt chimique en phase vapeur du métal organique) a été utilisé pour le dépôt des couches minces de SrCu2O2. Cette phase est obtenue après quelques étapes de recuit sous oxygène puis argon, ou azote uniquement avec en particulier la nécessité de réalier des recuit rapaides. Les propriétés électriques et optiques mesurées pour la couche mince de SrCu2O2 ont un ordre de grandeur similaire à ce qui est publié dans la littérature.

The present thesis addresses two research areas related to film growth in a highly ionized magnetron sputtering system: plasma characterization, and thin film growth and analysis. The deposition technique used is called high power pulsed magnetron sputtering (HPPMS). Characteristic for this technique are high energy pulses (a few Joules) of length 50-100 µs that are applied to the target (cathode) with a duty time of less than 1 % of the total pulse time. This results in a high electron density in the discharge (>1x1019 m-3) and leads to an increase of the ionization fraction of the sputtered material reaching up to 70 % for Cu. In this work the spatial and temporal evolution of the plasma parameters, including the electron energy distribution function (EEDF), the electron density and the electron temperature are determined using electrostatic Langmuir probes. Electron temperature measurements reveal a low effective temperature of 2-3 eV. The degree of ionization in the HPPMS discharge is explained in light of the self-sputtering yield of the target material. A simple model is therefore provided in order to compare the sputtering yield in HPPMS and that in dc magnetron sputtering (dcMS) for the same average power. Thin Ta films are grown using HPPMS and dcMS and their properties are studied. It is shown that enhanced microstructure and morphology of the deposited films is achieved by HPPMS. The Ta films are also deposited at a number of substrate inclination angles ranging from 0o (i.e., facing the target surface) up to 180 o (i.e., facing away from the target). Deposition rate measurements performed at all inclination angles for both techniques, reveal that growth made using HPPMS resulted in an improved film thickness at higher inclination. Furthermore, the high ionization of the Ta atoms in HPPMS discharge is found to allow for phase tailoring of the deposited films at all inclination angles by applying a bias voltage to the substrate. Finally, highly ionized magnetron sputtering of a compound MAX-phase material (Ti3SiC2) is performed, demonstrating that the HPPMS discharge could also be used to tailor the composition of the growing Ti-Si-C films. / On the day of the public defence of the doctoral thesis, the status of articles III and IV was Submitted. The titles of papers VI and VII changed between their manuscript forms and when they were published.

VO2-based films are thermochromic and exhibit high or low infrared transmittance when the temperature is below or above a critical temperature. The thermochromic switching is passive and reversible, and therefore VO2 based films are promising for energy-efficient window appli­cations. However the practicaluse of VO2 for energy-efficient windows has long been hampered by low luminous transmittance and low solar energy transmittance modulation. The main goal of this dissertation work is to address these issues. The first half of the work proposes the concept of nanothermochromics for simultaneous improvement of luminous transmittance and modulation of solar energy throughput. nanoth­ermochromics considers VO2 nanoparticle composite layers, whose optical properties were modeled by effective medium theories. Calculations on VO2 spheroids have shown that VO2 nanoparticles, especially nanospheres, can offer dramatically improved luminous transmittance and solar transmittance modulation that are not possible for films. Calculations done on coreshell nanoparticles showed comparable improvements and offer an opportunity to reduce the material costs. It was also found that the composite of In2O3:Sn (ITO) and VO2 can yield moderately high luminous transmittance, solar transmittance modulation and low-emittance properties. In the second half of the dissertation work, Mg-doped VO2 films were sputter deposited. Their band gaps and Mg-content were investigated by means of optical absorption measurement and Rutherford backscattering spectrometry, respectively. The band gaps of VO2 were found to increase by ∼3.9±0.5 eV per unit of atom ratio Mg/(Mg+V) for 0<Mg/(Mg+V)<0.21. Computations based on effective medium theory were done to estimate the performance of Mg­-doped VO2 films and nanoparticle composite layers. The results suggest that moderately doped VO2 films with 0<Mg/(Mg+V)<0.06 perform better than un-doped films and that the perfor­mance can be further enhanced with one layer of antireflection coating. The best results were achieved by un-doped VO2 nanospheres, closely followed by the VO2 nanospheres with low Mg-content. Furthermore, the an experimental study on sputter deposited VO2 nanorods has identified the geometry of the oxygen gas inlet, the type of substrate, the substrate temperature and the layer thickness as important factors that influence the growth morphology. Taken as a whole, nanothermochromics offered by VO2 nanoparticles was shown to be the best solution for VO2 based thermochromic energy-efficient window coatings.