Polymer-fullerene blend films are of increasing interest in the field of organic solar cells. This thesis describes the use of transient absorption spectroscopy (TAS) as a means to study charge generation and recombination in these blend films. In blend films of P3HT and PCBM, the absorption spectrum associated with the positive and negative charges have been identified and the transient decay kinetics analysed to reveal the mechanisms leading to charge recombination losses in the films. The effects of annealing treatment, along with the effects of decreasing polymer regioregularity or different polymer chain terminating end-groups on the blend film properties and morphology are also discussed. For films with a low concentration of PCBM in polystyrene, absorption and emission from the individual PCBM molecules are observed. As the PCBM concentration is increased, aggregation related changes causes the absorption in the visible region to increase disproportionately, an additional emission peak at higher energies to appear and a decrease in the lifetime of the PCBM triplet excited state. These changes are linked to electronic interactions between fullerenes in a molecular crystal form of PCBM. Blends of F8BT polymer with PCBM show poor photovoltaic device performance. This is explained by the quick deactivation of the F8BT excited state by Förster energy transfer to the PCBM. As the PCBM excited state is stable compared to the charge separated state, the PCBM triplet is formed via inter-system crossing from the singlet to be the final product of photoexcitation. The high energy of the charge separated state is interpreted as a result of the high F8BT HOMO energy.

Ce travail porte sur l'étude des propriétés optiques des composés à transition d'état de spin. Plus précisément, nous avons caractérisé le transition photo-induite de ce type de composé. Pour réaliser ces études, nous avons développé des montages basés sur la technique pompe-sonde résolue en temps. Nous avons alors caractérisé la cinétique de la transition photo-induite de composé du Fe(II) montant ainsi qu'un processus thermique est à l'origine de la transition et amorce un processus de type "nucléation/croissance". De plus, ces études ont été faites pour des poudres composées de particules dont la taille a été réduite. Par ailleurs, nous avons étudié les propriétés optiques de nanoparticules synthétisées sous forme de micelles. / This work deals with the study of optical properties of spin-state transition compounds. More precisely, we characterized the photo-induced transition of this complex. To make these studies, we have used two home-built experimental set ups based on time resolved pump-probe techniques. We have shown that thermal mechanism induces nucleation/growth process. Besides, we have studied the effect of particle size on the photo-induced transition. Finally, we have studied the optical properties of nanoparticles.

In this research, the polarization converters were manufactured by impinging intensity-gradient-distribution laser on DDLC (dye-doped liquid crystal) samples, which the gradient distributed light intensity was produced by applying linear variable ND filter. The performances of these polarization converters were demonstrated, which can convert a linear polarized incident light into a circular polarized light or an elliptic polarized light. The influence of cell gap and beam size of probe beam on the DDLC polarization converter has been discussed.

The work reported in this dissertation describes the design and synthesis of different gold nanoshells with strong absorption coefficients at the near-infrared region (NIR) of the spectrum, and includes preliminary studies of their use for the photo-induced heating of pancreatic cancer cells and ex vivo tissues. As the emphasis was on gold nanoshells with maximum extinctions located at 800 nm, the methods explored for their synthesis led us to the preparation of silica-core and hollow gold nanoshells of improved stability, with maximum extinctions at or beyond the targeted within the near-infrared window. The synthesis of silica-core gold nanoshells was investigated first given its relevance as one of the pioneering methods to produce gold nanostructures with strong absorption and scattering coefficients in the visible and the near-infrared regions of the spectrum. By using a classical method of synthesis, we explored the aging of the precursor materials and the effect of using higher concentrations than the customary for the reduction of gold during the shell growth. We found that the aging for one week of the as-prepared or purified precursors, namely, the gold cluster suspensions, and the seeded silica particles, along with higher concentrations of gold in the plating solution, produced fully coated nanoshells of 120 nm in size with smooth surfaces and maximum extinctions around 800 nm. Additional work carried out to reduce the time and steps in the synthesis of silica-core gold nanoshells, led us to improve the seeding step by increasing the ionic strength of the cluster suspension, and also to explore the growth of gold on tin-seeded silica nanoparticles. The synthesis of hollow gold nanoshells (HGS) of with maximum extinctions at the NIR via the galvanic replacement of silver nanoparticles for gold in solution was explored next. A first method explored led us to obtain HGS with maximum extinctions between 650 and 800 nm and sizes between 30 and 80 nm from silver nanoparticles, which were grown by the addition of silver nitrate and a mild reducer. We developed a second method that led us to obtain HGS with maximum extinctions between 750 and 950 nm by adjusting the pH of the precursor solution of the silver particles without much effort or additional steps. The last part of this work consisted in demonstrating the photo-induced heating of two biological systems containing HGS. Photothermal therapy studies of immobilized PANC1 pancreas cancer cells in well-plates were carried out with functionalized HGS. We found that cells exposed to HGS remained viable after incubation. Moreover, the cells incubated with HGS modified with mercaptoundecanoic acid and folic acid turned non-viable after being irradiated with a laser at 800 nm. The other study consisted in the laser-induced heating between 750 and 1000 nm of ex vivo tissues of chicken and pork with nanoshells injected. In comparison with non-injected tissues, it was found that the temperature at the irradiated areas with HGS increased more than 10 °C. Moreover, the extent of the heated area was broader when the laser was used at wavelengths beyond 900 nm, suggesting that the heating was due to the radiation absorbed and transformed into heat primarily by the HGS and at a lesser extent by the water in the tissue.

Il y a un besoin de nouvelles technologies photovoltaïques avec une efficacité de conversion lumière-électricité augmentée, qui puissent être des alternatives aux dispositifs plus traditionnels d’efficacité limitée et couteux à base de jonctions p-n. Dans ce contexte, la recherche sur les pérovskites ferroélectriques inorganiques ouvre des possibilités pour le développement de nouvelles approches pour augmenter l’efficacité, par exemple grâce à leur aptitude à séparer les charges électriques photoexcitées par le champ électrique intrinsèque (associé à leur polarisation) qui existe dans chaque maille élémentaire de ces matériaux. Pour profiter de cet avantage, un matériau doit posséder plusieurs propriétés comme la ferroélectricité, une bande interdite d’énergie relativement petite pour pouvoir absorber la lumière et une mobilité des porteurs de charges élevée. Ici, on a synthétisé et étudié des solutions solides Ba(Sn,Ti)O3, qui ont comme composants parents un ferroélectrique BaTiO3 et un paraélectrique BaSnO3. Les transitions de phases cristallographiques et la modification des états des dipôles sont caractérisées par les méthodes de diffraction et la spectroscopie diélectrique. La spectrométrie des photoélectrons X montre une corrélation entre l’évolution non-linéaire de la bande interdite pour les différentes compositions et entre l’évolution des charges locales dynamiques. Les propriétés optiques en température sont dominées par l’arrangement des dipôles dans les compositions ferroélectriques. Pour les autres compositions les propriétés sont plutôt guidées par les défauts. Il a été possible de déterminer les températures critiques des différents mécanismes à partir des caractérisations optiques. Dans ce système Ba(Sn,Ti)O3, les propriétés optiques et le photocourant sont fortement reliés à la structure locale particulière et la nature de la liaison chimique, comme nous avons mis en évidence par la spectroscopie Raman et la spectrophotométrie photoélectronique X. / There is an active search for new photovoltaic technologies with improved efficiency, since the traditional p-n junctions have either the limited efficiency or the increased cost. The research on inorganic ferroelectric perovskites offers opportunities to develop new approaches and increase photovoltaic efficiency, for instance due to capability of these materials to more efficiently separate the photoexcited charges due to the existence of an internal electric field within their unit cell. To profit from this advantage, the material must combine properties like ferroelectricity, relatively small band gap and high charge mobility. In this work, we have synthesized and studied compounds from Ba(Sn,Ti)O3 solid solution, having as end members ferroelectric BaTiO3 and paraelectric BaSnO3. Crystallographic phase transitions and changes of the polar states were characterized by diffraction techniques and dielectric spectroscopy. The non-linear evolution of the band gap for different compounds has been correlated to arise from evolution of the local dynamic charge existing in these compounds, as deduced from X-ray photoelectron spectroscopy. The temperature-dependent optical properties are dominated by polar order in ferroelectric compositions, while for the other compositions the defect-related mechanisms prevail. The critical temperatures for different mechanisms can be determined from optical characterization. In these compounds, the optical properties and photocurrent are strongly related to particularities of the local structure and chemical bonding deduced from Raman and X-ray photoelectron spectroscopies.

Chalcogenide glasses and their use in a wide range of optical, electronic and memory applications, has created a need for a more thorough understanding of material property variation as a function of composition and in geometries representative of actual devices. This study evaluates compositional dependencies and photo-induced structural mechanisms in As-S-Se chalcogenide glasses. An effective fabrication method for the reproducible processing of bulk chalcogenide materials has been demonstrated and an array of tools developed, for the systematic characterization of the resulting material's physical and optical properties. The influence of compositional variation on the physical properties of 13 glasses within the As-S-Se system has been established. Key structural and optical differences have been observed and quantified between bulk glasses and their corresponding as-deposited films. The importance of annealing and aging of the film material and the impact on photosentivity and long term behavior important to subsequent device stability have been evaluated. Photo-induced structures have been created in the thin films using bandgap cw and sub-bandgap femtosecond laser sources and the exposure conditions and their influence on the post-exposure material properties, have been found to have different limitations and driving mechanisms. These mechanisms largely depend on both structural and/or electronic defects, whether initially present in the chalcogenide material or created upon exposure. These defect processes, largely studied previously in individual binary material systems, have now been shown to be consistently present, but varying in extent, across the ternary glass compositions and exposure conditions examined. We thus establish the varying photo-response of these defects as being the major reason for the optical variations observed. Nonlinear optical material properties, as related to the multiphoton processes used in our exposure studies, have been modeled and a tentative explanation for their variation in the context of composition and method of evaluation is presented.

No description available.

We investigated the modifying effects of ultraviolet (UV) light and chemical dispersant (Corexit 9500A) on crude oil toxicity in juvenile mysids (≤ 24 h) (Americamysis bahia) and larval red drum (24-72 hpf) (Sciaenops ocellatus). These results demonstrate that crude oil toxicity significantly increases with co-exposure to environmentally relevant UV levels in both species, indicating photo-induced toxicity. This toxicity was further exacerbated by the application of chemical dispersants which increased the dissolution and concentration of oil-derived polycyclic aromatic hydrocarbons (PAHs) in test solutions. To better understand the mechanisms and initiating events of this observed photo-induced toxicity, the incidence of apoptotic cell death and global transcriptomic changes were assessed in larval red drum (24-72 hpf) following co-exposure to crude oil and UV. These results showed that co-exposure to UV and low concentrations of crude oil (<1 µg/L ∑PAH50) induced apoptotic cell death in skin and eye tissue and altered transcriptomic pathways related to visual processing and dermatological disease. To link these cellular and molecular impacts of photo-induced toxicity to apical endpoints of ecological performance, sublethal impacts to growth, metabolic rate, and visually mediated behaviors were explored in larval red drum at 2 developmental stages. These results suggested that earlier life stages may be more sensitive to photo-induced toxicity and that growth and development, particularly of sensory systems, can be sensitive targets of photo-induced toxicity. Together, these studies provide novel insights into the photo-induced toxicity of crude oil in aquatic organisms and can be used to inform future ecological risk assessments.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or- dered states, when cooled to lower temperatures. These phases often show a periodic modulation of the charge density (called a charge density wave – CDW) coupled with a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD). Although many experiments have been performed and much has been learnt about CDW phases in low-dimensional materials, the reasons for their existence are still not fully understood yet. Many processes, involving either strong electron–electron or electron–lattice coupling, have been observed which all might play a role in explaining the formation of different phases under different conditions. With the availability of femtosecond lasers it has become possible to study materials under highly nonequilibrium conditions. By suddenly introducing a known amount of energy into the system, the equilibrium state is disturbed and the subsequent relax- ation processes are then observed on timescales of structural and electronic responses. These experiments can deliver valuable information about the complex interactions between the different constituents of condensed matter, which would be inaccessible under equilibrium conditions. We use time resolved electron diffraction to investigate the behaviour of a CDW system perturbed by a short laser pulse. From the observed changes in the diffraction patterns we can directly deduce changes in the lattice structure of our sample. A femtosecond electron diffraction setup was developed at the Laser Research In- stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec- tron pulses shorter than 500 fs at sample position can be achieved. Technical details of this system and its characterisation as well as sample preparation techniques and analysis methods are described in detail in this work. Measurements on two members of the quasi-two-dimensional transition metal di- chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating of the lattice. When the induced temperature rise heats the sample above a phase tran- sition temperature, a complete transformation into the new phase was observed. For 4Hb-TaSe2 we found that the recovery to the original state is significantly slower if the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we could not only study the suppression of the original phase but also the formation of the higher energetic CDW phase. Long (100 ps) time constants were found for the tran- sition between the two phases. These suggest the presence of an energy barrier which has to be overcome in order to change the CDW phase. Pinning of the CDW by de- fects in the crystal structure result in such an energy barrier and consequently lead to a phase of domain growth which is considerably slower than pure electron or lattice dynamics. / AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge- ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri- odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van die verskillende fases onder verskillende omstandighede. Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel- heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar- opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu- rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer oor die komplekse interaksies tussen die verskillende atomiese komponente van ge- kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes. Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls. Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse veranderinge die kristalstruktuur van ons monster ondergaan. ’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors- ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef- fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters asook analise metodes word volledig in hierdie tesis beskryf. Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik 4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon- de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe- ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2 het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met 1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie, maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van ’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver- ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat heelwat stadiger as pure elektron of kritalrooster dinamika is.

Light-addressable potentiometric sensors (LAPS) and scanning photo-induced impedance microscopy (SPIM) have been extensively applied as chemical sensors and biosensors. This thesis focuses on the investigation of LAPS and SPIM for electrochemical imaging based on two different semiconductor substrates, silicon on sapphire (SOS) and indium tin oxide (ITO) coated glass. Firstly, SOS substrates were modified with 1,8-nonadiyne self-assembled organic monolayers (SAMs), which served as the insulator. The resultant alkyne terminals provided a platform for the further functionalization of the sensor substrate with various chemicals and biomolecules by Cu(I)-catalyzed azide alkyne cycloaddition (CuAAC) 'click' reactions. The CuAAC reaction combined with microcontact printing (μCP) was successfully used to create chemical patterns on alkyne-terminated SOS substrates. The patterned monolayers were found to be contaminated with the copper catalyst used in the click reaction as visualized by LAPS and SPIM. Different strategies for avoiding copper contamination were tested. Only cleaning of the silicon surfaces with an ethylenediaminetetraacetic acid tetrasodium salt (EDTA) solution containing trifluoroacetic acid after the 'click' modification proved to be an effective method as confirmed by LAPS and SPIM results, which allowed, for the first time, the impedance of an organic monolayer to be imaged. Furthermore, the 1,8-nonadiyne modified SOS substrate was functionalized and patterned with an RGD containing peptide, which was used to improve the biocompatibility of the substrate and cell adhesion. By seeding cells on the peptide patterned sensor substrate, cell patterning was achieved. Single cell imaging using LAPS and SPIM was attempted on the RGD containing peptide modified SOS substrate Finally, an ITO coated glass substrate was used as a LAPS substrate for the first time. The photocurrent response, the pH response, LAPS and SPIM imaging and its lateral resolution using ITO coated glass without any modification were investigated. Importantly, single cell images were obtained with this ITO-based LAPS system.