No description available.

No description available.

No description available.

The wetting behavior of water on methacrylate copolymer films was studied on anodically oxidized and micro-roughened aluminium surfaces and also on smooth model surfaces. The copolymerization of tert-butyl methacrylate with a methacrylate containing a fluoroorganic side chain led to a considerable decrease of the surface free energy, but not to a superhydrophobic behavior of polymer-coated, micro-roughened aluminium surfaces. However, copolymers containing both hydrophobic and hydrophilic sequences are able to form superhydrophobic films. X-ray photoelectron spectroscopy showed that an enrichment of the interface between the solid phase and the air by fluorine-containing polymer components was the reason for the strong decrease of the surface free energy. The hydrophilic segments of the copolymers improved the ability to wet the highly polar aluminium surface and to form films of higher density.

This PhD project investigated the ramifications of air-cured and nitrogen-cured manufacturing processes during silicone hydrogel contact lens manufacture in terms of lens surface characterisation and clinical performance. A one-hour contralateral clinical study was conducted for ten subjects to compare the clinical performance of the two study lenses. The main clinical findings were reduced levels of subjective performance, reduced surface wettability and increased deposition. Contact angle analysis showed the air-cured lenses had consistently higher advancing and receding contact angle measurements, in comparison with the nitrogen-cured lens. Chemical analysis of the study lens surfaces in the dehydrated state, by x-ray photoelectron spectroscopy (XPS) and time-of-flight mass spectrometry (ToF-SIMS), showed no difference due to surface segregation of the silicone components. Analysis of frozen lenses limited surface segregation and showed a higher concentration of silicone polymer components and lower concentration of hydrophilic polymer components at the surface of the air-cured lens, in comparison with the nitrogen-cured lens. Scanning electron microscope (SEM) imaging showed the nitrogen-cured lens to have a surface typical of a hydrogel material, whereas the air-cured lens had regions of apparent phase separation. In addition, atomic force microscopy (AFM) showed the air-cured lens to have a rougher surface associated with greater adherence of contaminants (often observed in materials with reduced polymer cross-linking). In conclusion, clinical assessment of the study lenses confirmed the inferior performance of the air-cured lens. Surface analysis suggested that the non-wetting regions on the air-cured lenses were associated with elevated level of silicone components, reduced polymer cross-linking and polymer phase separation.

Abstract In this thesis oxidation of a pure and alloyed Cu surface was studied using density functional theory based calculations. The mechanism and energetics behind experimentally observed missing row reconstruction on a pure Cu(100) surface were studied. Examination of the formation of the missing row was approached by studying the profitableness of vacancy formation to the surface at oxygen coverages below and equal to 0.5 ML. However, an ideal surface was in all cases more favourable in energy than vacancy included structures. Therefore an additional Cu atom was added to the simulation cell to promote vacancy formation. A consequence of this on-surface Cu addition was the identifying of a new energetically favoured Cu-O-Cu chain formation. This newly identified structure is a possible transition state in the path from an ideal surface to a reconstructed one. Copper(I)oxide formation through sub-surface adsorption was examined on a reconstructed Cu(100) surface. At low O coverage on-surface adsorption was found to be much more favoured than adsorption on sub-surface sites. However, when coverage increased to 0.75 ML structures with both on-surface and sub-surface adsorbates possessed the lowest energy. Furthermore, the diffusion barriers to the sub-surface adsorption sites were low. Both these facts support the conclusion that the transition from the reconstructed missing row structure to the copper(I)oxide takes place without any large energy effort. The adsorption induced segregation of copper on a Ag/Cu(100) surface was studied by calculating the surface energies of slabs where the silver layer was positioned at different depths with varying O coverage. In addition, the segregation energies were also calculated with two Ag compositions. It was discovered that without oxygen adsorbates silver atoms lying on top of the surface were favoured but the surface segregation probability of Cu increases with increasing O coverage. The same calculation procedure was expanded to study the segregation probability of five additional alloy metals with a Cu surface. Without oxygen present Mg, Al and Zn possess a weak tendency to segregate to the surface, however, V and Cr prefer to stay in the bulk. The addition of oxygen adsorbates on the surface causes the segregation of all the dopants to the top of the surface. With the Al dopant the effect of the surface orientation and the oxygen coverage were also studied. The bonds between adsorbates and the surface were strongest with the densest surface, (111), and weakest with the most opened surface, (110). Furthermore, the enrichment probability was largest in the case of the (111) surface and lowest with the (110) surface. Probability increased with increasing oxygen coverage. / Tiivistelmä Tässä väitöstyössä on tutkittu puhtaan ja seostetun kuparipinnan hapettumista tiheysfunktionaaliteoriaan perustuvilla laskuilla. Kokeellisesti Cu(100) pinnan on havaittu rekonstruoituvan hapen vaikutuksesta. Rekonstruoitumisen aikana ideaalisesta pinnasta irtoaa joka neljäs pintakuparirivi ja muodostuu niin kutsuttu missing row- rakenne. Tämän rakenteen muodostumismekanismia ja muodostumiseen kuluvaa energiaa tutkittiin vertaamalla pintavakansillisten rakenteiden pintaenergioita vastaaviin ideaalipinnan arvoihin vaihtelevalla happipeitolla. Koska ideaalipinta oli kaikilla peittoasteilla energeettisesti suotuisampi kuin vakanssirakenne, yksittäisiä kupariatomeja lisättiin pinnalle stabiloimaan vakanssirakenteita. Kuparilisäyksen seurauksena löydettiin uusi energeettisesti edullinen Cu-O-Cu ketjurakenne. Tämä rakenne on mahdollinen ideaalisen ja rekonstruoituneen pinnan välinen siirtymätila. Kupari(I)oksidin muodostumista pinnanalaisen happiadsorption kautta tutkittiin rekonstruoituneella Cu(100) pinnalla. Pienillä happi-peittoasteilla pinnanpäällisen adsorption havaittiin olevan huomattavasti suotuisampaa kuin pinnanalaisen adsorption. Kuitenkin happi peiton kasvaessa 0,75 ML:iin rakenteet, joissa oli adsorbaatteja sekä pinnan alla että päällä, tulivat muita suotuisimmiksi. Myös lasketut energiavallit hapen diffuusiolle pinnalta pinnan alle olivat hyvin pieniä. Kaikki tulokset viittaavat siihen, että rekonstruoitunut Cu(100) pinta hapettuu helposti kupari(I)oksidiksi. Happiadsorption aikaansaamaa kuparin rikastumista (segregaatio) hopeakupariseosmetallin pinnalle tutkittiin laskemalla vaihtelevilla happipeitoilla pintaenergiat kuparipinnoille, joissa ensimmäinen, toinen ja kolmas kuparikerros oli korvattu hopealla. Myös segregaatioenergiat laskettiin kahdella eri seoskoostumuksella. Saatujen tulosten mukaan ilman happiadsoptiota hopea segregoituu pinnalle, kun taas hapen peittoasteen kasvaessa kupari rikastuu pinnalle. Samaa tutkimustapaa käytettiin myös tarkasteltaessa viiden muun seosmetallin segregaatiotodennäköisyyttä kuparipinnalla. Ilman pintahappea magnesiumilla, alumiinilla ja sinkillä oli heikko taipumus rikastua pinnalle, kun taas vanadium ja kromi pysyivät syvemmällä metallissa. Sen sijaan happea lisättäessä kaikki tutkitut seosmetallit pyrkivät siirtymään kohti pintaa. Myös hapen peiton sekä pintaorientaation vaikutusta segregaatioon tutkittiin hopeakuparipinnalla. Sekä happiadsorbaattien ja pinnan välinen sidos että segregaatiotodennäköisyys oli sitä voimakkaampaa, mitä tiheämmästä pinnasta oli kyse. Segregaatiotodennäköisyys kasvoi pinta-adsorption lisääntyessä.

The wetting behavior of water on methacrylate copolymer films was studied on anodically oxidized and micro-roughened aluminium surfaces and also on smooth model surfaces. The copolymerization of tert-butyl methacrylate with a methacrylate containing a fluoroorganic side chain led to a considerable decrease of the surface free energy, but not to a superhydrophobic behavior of polymer-coated, micro-roughened aluminium surfaces. However, copolymers containing both hydrophobic and hydrophilic sequences are able to form superhydrophobic films. X-ray photoelectron spectroscopy showed that an enrichment of the interface between the solid phase and the air by fluorine-containing polymer components was the reason for the strong decrease of the surface free energy. The hydrophilic segments of the copolymers improved the ability to wet the highly polar aluminium surface and to form films of higher density.

No description available.

Thema dieser Arbeit ist die Synthese von Wurtzit M-plane (In,Ga)N(1-100)-Heterostrukturen auf g-LiAlO2(100) mittels plasmaunterstützter Molekularstrahlepitaxie (MBE). Der Einfluß der Wachstumsbedingungen auf die strukturellen, morphologischen, und optischen Eigenschaften von M-plane GaN-Filmen werden untersucht. Ferner werden M-plane (In,Ga)N/GaN Multiquantenwells (MQWs) hergestellt und deren strukturelle und optische Eigenschaften untersucht. Schließlich wird der Einbau von Mg in M-plane GaN untersucht, um p-Typ-Leitfähigkeit zu erreichen. Die Arbeit beginnt mit einer Einführung bezüglich der Verspannung und der elektrostatischen Polarisation in Nitriden. Die Motivation fuer das Wachstum in [1-100]-Richtung anstatt in der konventionellen [0001]-Richtung ist, dass die GaN(1-100)-Flaeche nichtpolar ist, da sie aus einer gleichen Anzahl dreifach koordinierter Ga- und N-Atome aufgebaut ist. GaN ist überdies nicht piezoelektrisch in der [1-100]-Richtung. Das daraus folgende Fehlen elektrostatischer Felder in dieser Richtung stellt einen klaren Vorteil für die Leistung von GaN-basierenden hocheffizienten Leuchtdioden (LEDs) dar. Entsprechende [0001]-orientierte Strukturen, die auf konventionellen Substraten wie Al2O3 und SiC abgeschieden werden, leiden unter einer verringerten Effizienz durch die Präsenz der spontanen und piezoelektrischen Polarisation in dieser Wachstumsrichtung. Die Eigenschaften des Substrats LiAlO2 in Bezug auf das MBE-Wachstum werden anschliessend diskutiert. Es wird gezeigt, daß die thermische Stabilität von LiAlO2 fuer das MBE-Wachstum von Heterostrukturen geeignet ist. Die Polaritaet von LiAlO2 hat einen entscheidenden Einfluß auf die Phasenreinheit der GaN-Filme, und die Wahl der richtigen Polaritaet ist Voraussetzung fuer die Herstellung von einphasigen M-plane GaN-Schichten. In Kapitel 4 wird der Einfluß der Nukleationsbedingungen auf die strukturellen und morphologischen Eigenschaften von M-plane GaN-Filmen systematisch untersucht. Ferner wird die Ga-Adsorption und -Desorption ausführlich untersucht. Optimale Wachstumsbedingungen werden etabliert, die es ermoeglichen, M-plane-GaN-Schichten hoher Qualitaet reproduzierbar zu erhalten. Die Mikrostruktur der M-plane-GaN-Schichten, untersucht mittels Transmissionselektronenmikroskopie, ist durch eine hohe Dichte an Stapelfehlern als dominierenden Defekt gekennzeichnet. Vollstaendige Fadenversetzungen, die die dominanten Defekte in C-plane GaN sind, werden dagegen nicht beobachtet. Die Korrelation zwischen den Stapelfehlern und den optischen Eigenschaften der Films wird untersucht. Eine intensive Emissionslinie wird in Tieftemperatur-Photolumineszenzspektren beobachtet, die an Stapelfehlern gebundenen Exzitonen zugeordnet wird. In Kapitel 6 wird die erfolgreiche Synthese von M-plane-(In,Ga)N/GaN-MQWs beschrieben. Das Zusammensetzungsprofil dieser Strukturen wird mittels Roentgendiffraktometrie und Sekundaerionenmassenspektrometrie untersucht. Die Ergebnisse belegen eine betraechtliche Oberflaechensegregation von In, die zu einem erniedrigten In-Gehalt sowie stark verbreiterten Quantenwells führt. Der erhaltene In-Gehalt von 7% ist niedriger als derjenige (15%), der in entsprechenden C-plane-Strukturen gefunden wird, die unter identischen Bedingungen hergesellt wurden. Dieses Resultat deutet auf eine niedrigere Einbaueffizienz von In auf (1-100) verglichen mit (0001) hin. Die Abhaengigkeit der Übergangsenergien von der Quantenwellbreite dieser M-plane-MQWs belegt die Abwesenheit interner elektrostatischer Felder entlang der Wachstumsrichtung. Die Rekombinationsdynamik in diesen MQWs wird im Detail untersucht. Sie ist stark von lokalisierten Zuständen beeinflußt. Im Gegensatz zu C-plane-Strukturen, wird in diesen M-plane MQWs eine starke Polarisation der spontanen Emission in der Filmebene mit einem energieabhängigen Polarisationsgrad von bis zu 96% beobachtet. In Kapitel 7 wird der Einfluß der Wachstumstemperatur und der Stoechiometrie auf den Mg-Einbau in GaN(1-100) zur p-Dotierung untersucht. Eine Mg-Konzentration bis zu 8×1020 cm-3 kann in M-plane-GaN-Schichten ohne beobachtbare Degradation der Kristallqualität erreicht werden. Es wird sowohl eine Diffusion als auch eine Segregation von Mg in M-plane GaN beobachtet. Zusaetzlich wird eine ausgepraegte Abhaengigkeit des O-Einbaus von der Mg-Dotierung beobachtet, was auf die hohe Reaktivitaet von Mg mit O zurückgeführt wird. Sowohl optische als auch elektrische Messungen weisen darauf hin, daß Mg in diesen M-plane GaN-Schichten als Akzeptor eingebaut wird. / In this thesis, we investigate the synthesis of wurtzite M-plane (In,Ga)N(1-100) heterostructures on g-LiAlO2(100) by plasma-assisted molecular beam epitaxy (MBE). We examine the impact of growth conditions on the structural, morphological, and optical doping properties of M-plane GaN. Furthermore, we fabricate M-plane (In,Ga)N/GaN multiple quantum wells and investigate their structural and optical properties. Finally, the incorporation of Mg in $M$-plane GaN is studied to achieve p-type conductivity. We start by giving an introduction concerning strain and electrostatic polarization fields. The motivation of growth along the [1-100] direction, instead of along the conventional [0001] direction is presented. The GaN(1-100) plane is nonpolar since it is composed of equal numbers of three-fold coordinated Ga and N atoms. Furthermore, GaN is not piezoelectrically active along the [1-100] direction. The resulting absence of electrostatic fields in this direction constitutes a distinct advantage for fabricating high-efficiency light-emitting diodes(LEDs). Corresponding [0001]-oriented structures grown on conventional substrates such as Al2O3(0001) and SiC(0001), suffer from a degradation of luminescence efficiency by the presence of both spontaneous and piezoelectric polarization along the growth direction. The properties of the LiAlO2 substrate with respect to MBE growth are discussed next. The thermal stability of LiAlO2 is demonstrated to be suitable for MBE-growth of heterostructures. The polarity of LiAlO2 is found to have a crucial influence on the phase-purity of the GaN films. The synthesis of pure M-plane GaN is preferentially achieved on one face of the substrate. The impact of nucleation conditions on the structural and morphological properties of M-plane GaN films is systematically investigated. Furthermore, a comprehensive study of Ga adsorption and desorption on the M-plane is presented. Optimum growth conditions are established, and high quality M-plane GaN can be obtained reproducibly. Concerning the microstructure of our M-plane GaN layers, stacking faults are found by transmission electron microscopy (TEM) to be the dominant defects, while perfect threading dislocations, which are the dominant defects (108-1010 cm-2) in C-plane GaN, are not observed by TEM. The correlation between the stacking faults and the optical properties of the films is explored. A strong transition from excitons bound to stacking faults is observed by low temperature photoluminescence measurements. The successful synthesis of M-plane (In,Ga)N/GaN multiple quantum wells (MQWs) is demonstrated. The composition profiles of these structures are investigated by both x-ray diffractometry and secondary ion-mass spectrometry. The results reveal significant In surface segregation, resulting in a reduced In content and much wider wells than intended. The resulting In content of ~7% is lower than that obtained (~15%) for corresponding C-plane structures grown under identical conditions, suggesting a lower In incorporation efficiency on the (1-100) plane compared to the (0001) plane. The dependence of the transition energies on the well thickness of these M-plane quantum wells evidences the absence of internal electrostatic fields along this growth direction. The recombination dynamics in these MQWs is investigated in detail, and is found to be strongly influenced by localized states. Furthermore, in contrast to C-plane (0001) structures, a strong in-plane anisotropy of the spontaneous emission with an energy-dependent polarization degree of up to 96% is observed in the M-plane (In,Ga)N/GaN MQWs. Finally, the impact of the growth temperature and stoichiometry on the Mg incorporation in GaN(1-100) is investigated. Mg doping levels up to 8×1020 cm-3 can be obtained in M-plane GaN, with no observed degradation in crystal quality. Both Mg diffusion and surface segregation in M-plane GaN are observed. In addition, a pronounced dependence of the O incorporation on the Mg doping is observed, and attributed to the high reactivity of Mg with O. Both optical and electrical measurements indicate that Mg acts as an acceptor in the Mg-doped M-plane layers.