The experimental studies presented in this thesis deal with the photoluminescence intermittency of semiconductor quantum dots in different dielectric environments. Detailed analysis of intermittency statistics from single capped CdSe/ZnS, uncapped CdSe and water dispersed CdSe/ZnS QDs in different matrices provide experimental evidence for the model of photoionization with a charge ejected into the surrounding matrix as the source of PL intermittency phenomenon. The distribution of the dark state lifetimes can be described by a power law over a wide range while that of bright state can be described by a power law at shorter times followed by an exponential decay. The lifetimes of the bright and dark states are influenced by the dielectric properties of the surrounding environment. Our experimental results show that the lifetime of the dark state increases with the dielectric constant of the matrix. This is very clear from the linear correlation between αoff and f (ε). We propose a self-trapping model to explain the increase of dark state lifetimes with the dielectric constant of the matrix. A charge will be more stabilized in a medium with high dielectric constant. An energetically more favourable state for an electron in a high dielectric medium decreases the return probability which eventually increases the duration of the off-time. Moreover, the self-trapping model establishes a general model for distribution of states in a matrix. We like to mention, that in the case of bright states, a qualitative observation is the cross over of the on-time power law behavior to an exponential one. The power law part of the decay is nearly matrix independent while the exponential decay, which limits the maximum on-time, strongly depends on dielectric properties of the environment. The exponential part of the on-time probability decays much faster in a high dielectric medium and there exists a linear relation between the time constant of the exponential decay and f (ε). Theoretical background has been provided for the observed results using the recently published DCET model which correlates PL intermittency of QDs with properties of the environment. This supports our previous conjecture of a general model for matrix controlled blinking process. The disagreement between experimentally observed dependence of αoff and f (ε) for different matrices with that of the static tunnelling model proposed by Verberk is due to the fact that the tunneling model considers only an electron transfer between a QD and spatially distributed trap states in vacuum. These states are already stabilized states. It does not assume any medium in between. Therefore, matrix dependent blinking kinetics can not be explained quantitatively by tunneling model even though tunneling between a QD and spatially distributed trap states gives a power law distribution for the blinking kinetics. DCET is a more general (dynamic) model. The bright and dark state parabolas contain QD, charge and the matrix. Therefore, this model could in principle explain matrix dependent blinking kinetics in a better way, for example, the energy difference between the minima of the bright and dark state parabolas (-ΔG0) is defined by the stabilization energy of the system provided by the matrix. However, due to lack of the relevant intrinsic parameters we did not compare this relationship and dependence qualitatively. / Betrachtet man die Fluoreszenz einzelner Farbstoffmoleküle oder Halbleiternanokristalle bei kontinuierlicher Anregung, so stellt man fest, dass die im Zeitverlauf beobachtete Intensität einer stochastischen Variation unterliegt, d. h. dass das Chromophor zwischen emittierenden und nicht emittierenden Zuständen, auch Hell- und Dunkelzuständen genannt, hin- und herschaltet. Dieses als Blinken bekannte Phänomen ist physikalisch wie auch technologisch herausfordernd, lässt es doch einerseits die Realisierbarkeit einer Reihe von quantenoptischen Anwendungen, so z. B. auf dem Gebiet der Quantenkryptographie, dem Quantum Computing oder der optischen Schaltungstechnik auf Basis einzelner Quantenobjekte, in naher Zukunft möglich erscheinen. Andererseits setzt es gewissen Anwendungen, die auf die permanente Sichtbarkeit des Chromophors aufbauen, Grenzen, so zum Beispiel der Verwendung als Lumineszenzmarker in der medizinischen Diagnostik. Weiterhin ist festzustellen, dass das Blinken kritisch von den äußeren Bedingungen und von den Umgebungsparametern abhängt. Aus diesen und anderen Gründen ist ein fundamentales Verständnis der physikalischen Ursachen und der Wechselwirkungsprozesse unerlässlich. Die Forschung dazu steckt noch in den Kinderschuhen. Basierend auf umfangreiche Messungen der Fluoreszenzzeitreihen einzelner Nanokristalle aus CdSe und CdSe/ZnS in verschiedenen Umgebungen, zeigt diese Dissertation exemplarisch den Einfluss der Dielektrizitätsparameter auf das Blinken. Zur Erklärung des Sachverhalts wird ein so genanntes Self-Trapping-Modell zu Rate gezogen. Demnach kommt es zu einer Ionisation des Quantenobjekts und anschließender Ladungstrennung, woraufhin die abgetrennte Ladung für eine gewisse Zeit in der Umgebung lokalisiert bleibt. Die Dauer der Lokalisierung und damit der emittierenden und nicht emittierenden Perioden hängt von der dielektrischen Funktion des umgebenden Materials ab. Dies ist als direkter Nachweis für den photoinduzierten Ladungstransfer als Ursache des Fluoreszenzblinkens zu deuten. Die Arbeit demonstriert, dass die experimentellen Zeitreihen die charakteristischen Merkmale eines diffusionsgesteuerten Ladungstransferprozesses besitzen und nimmt dabei den gegenwärtigen wissenschaftlichen Diskurs über geeignete theoretische Modelle des Fluoreszenzblinkens auf.

Atomically flat colloidal semiconductor CdSe nanoplatelets (NPLs) with precisely controlled thickness possess a range of unique optoelectronic properties. Here, we study the growth of CdSe, CdTe, and CdS NPLs with the aim of synthesizing thicker NPLs in order to extend their optical activity further into the lower energy/larger wavelength range. We employ cadmium halides, which lead to faster reaction kinetics as confirmed by control experiments with cadmium hydroxide as a Cd-precursor. Addition of halides in all cases led to the formation of thicker NPL species, as compared with the corresponding syntheses without these additives. Analysis of a recent theoretical model of the platelet growth mechanism confirms an earlier suggestion that reducing both the reaction enthalpy and the surface energy of CdSe, by replacing acetate ligands with chloride ions, should indeed lead to thicker NPLs as observed. We noticed a formation of Cd0-metal nanoparticles in the first stage of the synthesis by preparing the Cd-precursor, which is another key finding of our work. We assume that these particles can serve as an active cadmium source facilitating the growth of the NPLs. The resulting 6 ML CdSe NPLs exhibited bright photoluminescence with quantum yield of up to 50%, exceptionally narrow spectrum centered at 582 nm with full width at half-maximum of approx. 11 nm, and small Stokes shift of 2 nm. Moreover, we demonstrated the synthesis of heterostructured core/shell CdSe/CdS NPLs based on 6 ML CdSe platelets, which also exhibited bright fluorescence. This work shows the possibility to overcome energetic barrier limiting the size (thickness) control by using appropriate promoters of the growth of CdSe, CdTe, and CdS 2D structures.

Hauptziel dieser Arbeit ist die Synthese und Charakterisierung neuer Organosilicium-Chalcogen-Verbindungen (Chalcogen: Schwefel, Selen, Tellur), sowohl mit acyclischen, als auch mono- und polycyclischen Strukturen. Dabei konnten in vielen Fällen auch isostrukturelle Verbindungen mit Germanium- oder Zinnatomen anstelle von Silicium aufgebaut werden. Insgesamt wurden 42 der dargestellten Verbindungen auch durch Röntgenkristallstrukturanalysen charakterisiert. In cyclischen und polycyclischen Verbindungen war es damit auch möglich, die auftretenden Konformationen der Ringe- bzw. Ringsysteme zu bestimmen und die Resultate mit den Ergebnissen von DFT-Berechnungen zu vergleichen. Die NMR-Spektroskopie war parallel zu Kristallstrukturanalysen und GC/MS Messungen die Methode der Wahl zur Charakterisierung der dargestellten Verbindungen. Dabei boten sich neben der 1H und 13C NMR auch die 29Si, 119Sn, 207Pb, 77Se, und 125Te NMR Spektroskopie an, da man direkt die Elemente analysiert, die die Ringsysteme aufbauen, was zu einer weit höheren Strukturempfindlichkeit führt. Vor allem beim Vergleich von 77Se und 125Te NMR Daten analoger Selen- und Tellurverbindungen ergaben sich lineare Korrelationen sowohl der chemischen Verschiebungen als auch der Kopplungskonstanten 1JSiE bzw. 1JSnE mit Faktoren von ca. 2.5. Ähnliche Korrelationen konnten auch zwischen 29Si und 119Sn bzw. 119Sn und 207Pb NMR chemischen Verschiebungen gezogen werden. Von einigen Verbindungen konnten durch 29Si MAS NMR Spektroskopie auch die Tensorhauptwerte der chemischen Verschiebung ermittelt werden. Parallel dazu wurden mittels GIAO und IGLO Verfahren die 29Si NMR chemischen Verschiebungen ausgehend von den aus den Kristallstrukturanalysen zugänglichen Geometrien berechnet. Neben cyclischen Verbindungen konnten unter Verwendung des sperrigen Hypersilylrestes auch eine Reihe acyclischer Verbindungen mit der Sequenz Si–E–Si bzw. Si–E–Sn dargestellt und z. T. auch strukturell charakterisiert werden.

>Magister Scientiae - MSc / infarction (MI) affects many parts of the western world and in South Africa alone it is estimated that MI is responsible for 1 in 6 deaths (17.3%). Traditional diagnostic methods for MI include an electrocardiograms and blood tests. The problem with these diagnostic methods are that they are time consuming, require large sample volumes, expensive equipment and complicated machinery. To achieve early detection of MI the discovery of specific, sensitive and reliable biomarkers are required. Brain natriuretic peptide (BNP) has been identified as a reliable biomarker for MI due to the fact that it has a defined cutoff of 100 pg/ml and it is not susceptible to patient‘s age which could make early detection of BNP complicated. Early detection methods for BNP has been based on immunoradiometric assays but problems associated with immunoradiometric assays are that there is a restricted availability of antigens and incubation of the labeled antibody could take up to two weeks which affects the patients waiting time on results. Electrochemical biosensors are emerging as early detection method for MI because they can be designed to be sensitive, specific to BNP at a low cost. This research study reported for the first the successful fabrication and implementation of highly sensitive mercaptosuccinic acid capped nickel selenide quantum dots (MSA-NiSe2 QDs) aptasensor for the detection of BNP. The poly-dispersed MSA-NiSe2 QDs were synthesized via an inexpensive, simple and reproducible aqueous microwave assisted irradiation method. The prepared MSA-NiSe2 QDs were characterized by Ultraviolet spectroscopy (UV-Vis), X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), High Resolution Transmission/Scanning Electron Microscopy (HR TEM/SEM) and Small Angle X-ray Scattering (SAXSpace). The electrochemical properties of the MSA-NiSe2 QDs were investigated by Cylic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). HR-TEM revealed the formation of small sized MSA-NiSe2 QDs about 4 nm in diameter which was complemented by SAXSpace. UV-Vis studies showed absorption peaks in the ultraviolet region (100-400 nm) confirming the small size of these QDs as well confirming the direct and indirect bandgap of the QDs. XRD confirmed that the QDs are crystalline and belong to the bulk cubic MSA-NiSe2 QDs phase. FTIR studies confirmed the successful capping of MSA on the QDs due to the disappearance of the thiol peak at 2652 cm-1. Electrochemical studies revealed that the MSA-NiSe2 QDs showed good electrochemical properties on screen printed carbon electrodes (SPCE) which allowed them to be used as a mediating platform between the aptamer and SPCE. The successful detection of BNP was achieved by an incubation process between the aptamer drop coated on the MSA-NiSe2 QDs/SPCE surface overnight. The response of the MSA-NiSe2 QDs based aptasensor towards different concentrations of BNP was studied by differential pulse voltammetry (DPV). DPV showed a good linearly with correlation coefficient of R2 = 0.98. DPV also showed a high sensitivity (0.4513 μA/ pg/mL) towards detecting BNP with a detection limit of 11.93 pg/ml. The value of 11.93 pg/ml falls within the negative predictive value range of 10-100 pg/ml for early-stage diagnosis of BNP.

No description available.

Traditional sources of electrical energy are finite and can produce significant pollution. Solar cells produce clean energy from incident sunlight, and will be an important part of our energy future. A new nanostructured extremely thin absorber solar cell with 0.98% power conversion efficiency and maximum external quantum efficiency of 61% at 650 nm has been fabricated and characterized. This solar cell is composed of a fluorine-doped tin oxide base layer, n-type aluminum doped zinc oxide nanowires, a cadmium selenide absorber layer, poly(3-hexylthiophene) as a p-type layer, and thermally evaporated gold as a back contact. Zinc oxide nanowire electrodeposition has been investigated for different electrical environments, and the role of a zinc oxide thin film layer has been established. Cadmium selenide nanoparticles have been produced and optimized in-house and compared to commercially produced nanoparticles. Argon plasma cleaning has been investigated as a method to improve electronic behavior at cadmium selenide interfaces. The thermal anneal process for cadmium selenide nanoparticles has been studied, and a laser anneal process has been investigated. It has been found that the most efficient solar cells in this study are produced with a zinc oxide thin film, zinc oxide nanowires grown under constant -1V bias between the substrate material and the anode, cadmium selenide nanoparticles purchased commercially and annealed for 24 hours in the presence of cadmium chloride, and high molecular weight poly(3-hexylthiophene) spin-coated in a nitrogen environment.

No description available.

No description available.

D. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology. / Nanoscience receives a lot of attention in the 21 century and is one of the most advancing technology in our days. It provides many new and advanced technological opportunities. This field involves many disciplines which include chemical, physical, and biological related fields. The advancement of nanoscience makes life to be better and bring about new inventions which can solve many problems in our day to day life. Although there are reservations about the use of these materials in other fields. Some researchers believe that these materials can be a problem to the environment and humanity at large. Therefore, more research needs to be done to fully understand these materials. Polymer science is another field that has been advancing every day. Many problems in our lives require material which have properties from nanomaterials and polymers. The combination of these technologies can leads to new materials which have many possibilities in solving most problems. Some researchers have taken advantage of these two powerful fields and merge them. There has been a lot of work done that involves combination of nanotechnology and polymer science. The current project is an initiative to manufacture nanofibers. These fibers are prepared using polymer solution mixed with metal oxide and metal selenide nanomaterials. The polymer solution is incorporated with nanoparticles and electrospunned to make nanofibers. The electrospinning afford the material prepared to be at nanoscale. The fact that the material formed is at nanoscale opens many possibilities to be used in various fields. The study is about fabrication of polymer nanofibers embedded with metal chalcogenide nanoparticles. The metal oxide and metal selenide nanoparticles were prepared using complexes. These complexes contain both the metal and the chalcogenide of interest. The complexes are prepared from oxygen-based (urea), and selenium-based (diphenyldiselenide) ligands. The urea complexes co-ordinates with metal using oxygen for iron, however in silver complexes both nitrogen and oxygen are used. These complexes allow easy control of reaction parameters, and thermal decomposes to form metal oxide, metal selenide, and metal. The complexes are very stable and decomposes at about 200 °C. These compounds are thermal decomposed to form metal chalcogenides, and metal nanoparticles. The complexes are characterized with FTIR, TGA, and elemental analysis. The metal chalcoginedes (copper oxide, iron oxide, silver oxide, copper selenide, iron selenide, and silver selenide) nanoparticles were prepared using thermal decomposition of a single source (complexes or metal salts). The prepared chalcogenides nanoparticles have good absorption and emission properties consistent with small sizes. These nanoparticles are composed of various phased and stoichiometry. Some metal chalcogenides have a mixture of stoichiometry and phase. The metal chalcogenides nanoparticles are dominated by spheres, and other shapes such as rods. These metal chalcogenides have a particles size in the range of 1-36 nm. The metal chalcogenides nanoparticles were tested against bacteria and fungi. These nanoparticles show highest activity in gram positive compared to gram negative bacteria. Metal oxide nanoparticles show the highest activity compared to metal selenide. All the metal chalcogenides show the highest against fungi. The nanoparticles are able to inhibit the fungi at lowest concentration. The nanoparticles are characterized with various instruments which includes UV-Vis, PL, XRD, and TEM. Nanofibers of poly(methyl methacrylate) (PMMA) incorporated with metal selenide and metal oxide nanoparticles were prepared by electrospinning. The nanofibers incorporated with metal chalcogenide are more thermal stable than PMMA nanofibers. Therefore, incorporation of metal chalcogenides nanoparticles leads to more thermal stability nanofibers. The PMMA are coordinated to the metal oxide and metal selenide through carbonyl oxygen atom. The PMMA incorporated with metal oxide and metal selenide leads to the formation of nanofibers with uneven surface with a diameter in the range of 30 to 200 nm. The prepared fibers are characterized using FTIR, TGA, SEM.

No description available.