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Ziel der vorliegenden Arbeit war die Synthese und Charakterisierung von anisotropen Goldnanopartikeln in einer geeigneten Polyelektrolyt-modifizierten Templatphase. Der Mittelpunkt bildet dabei die Auswahl einer geeigneten Templatphase, zur Synthese von einheitlichen und reproduzierbaren anisotropen Goldnanopartikeln mit den daraus resultierenden besonderen Eigenschaften. Bei der Synthese der anisotropen Goldnanopartikeln lag der Fokus in der Verwendung von Vesikeln als Templatphase, wobei hier der Einfluss unterschiedlicher strukturbildender Polymere (stark alternierende Maleamid-Copolymere PalH, PalPh, PalPhCarb und PalPhBisCarb mit verschiedener Konformation) und Tenside (SDS, AOT – anionische Tenside) bei verschiedenen Synthese- und Abtrennungsbedingungen untersucht werden sollte.
Im ersten Teil der Arbeit konnte gezeigt werden, dass PalPhBisCarb bei einem pH-Wert von 9 die Bedingungen eines Röhrenbildners für eine morphologische Transformation von einer vesikulären Phase in eine röhrenförmige Netzwerkstruktur erfüllt und somit als Templatphase zur formgesteuerten Bildung von Nanopartikeln genutzt werden kann.
Im zweiten Teil der Arbeit wurde dargelegt, dass die Templatphase PalPhBisCarb (pH-Wert von 9, Konzentration von 0,01 wt.%) mit AOT als Tensid und PL90G als Phospholipid (im Verhältnis 1:1) die effektivste Wahl einer Templatphase für die Bildung von anisotropen Strukturen in einem einstufigen Prozess darstellt. Bei einer konstanten Synthesetemperatur von 45 °C wurden die besten Ergebnisse bei einer Goldchloridkonzentration von 2 mM, einem Gold-Templat-Verhältnis von 3:1 und einer Synthesezeit von 30 Minuten erzielt. Ausbeute an anisotropen Strukturen lag bei 52 % (Anteil an dreieckigen Nanoplättchen von 19 %). Durch Erhöhung der Synthesetemperatur konnte die Ausbeute auf 56 % (29 %) erhöht werden.
Im dritten Teil konnte durch zeitabhängige Untersuchungen gezeigt werden, dass bei Vorhandensein von PalPhBisCarb die Bildung der energetisch nicht bevorzugten Plättchen-Strukturen bei Raumtemperatur initiiert wird und bei 45 °C ein Optimum annimmt.
Kintetische Untersuchungen haben gezeigt, dass die Bildung dreieckiger Nanoplättchen bei schrittweiser Zugabe der Goldchlorid-Präkursorlösung zur PalPhBisCarb enthaltenden Templatphase durch die Dosierrate der vesikulären Templatphase gesteuert werden kann. In umgekehrter Weise findet bei Zugabe der Templatphase zur Goldchlorid-Präkursorlösung bei 45 °C ein ähnlicher, kinetisch gesteuerter Prozess der Bildung von Nanodreiecken statt mit einer maximalen Ausbeute dreieckigen Nanoplättchen von 29 %.
Im letzten Kapitel erfolgten erste Versuche zur Abtrennung dreieckiger Nanoplättchen von den übrigen Geometrien der gemischten Nanopartikellösung mittels tensidinduzierter Verarmungsfällung. Bei Verwendung von AOT mit einer Konzentration von 0,015 M wurde eine Ausbeute an Nanoplättchen von 99 %, wovon 72 % dreieckiger Geometrien hatten, erreicht.
Magnetische Nanopartikel bieten ein großes Potential, da sie einerseits die Eigenschaften ihrer Bulk-Materialien besitzen und anderseits, auf Grund ihrer Größe, über komplett unterschiedliche magnetische Eigenschaften verfügen können; Superparamagnetismus ist eine dieser Eigenschaften. Die meisten etablierten Anwendungen magnetischer Nanopartikel basieren heutzutage auf Eisenoxiden. Diese bieten gute magnetische Eigenschaften, sind chemisch relativ stabil, ungiftig und lassen sich auf vielen Synthesewegen relativ einfach herstellen. Die magnetischen Eigenschaften der Eisenoxide sind materialabhängig aber begrenzt, weshalb nach anderen Verbindungen mit besseren Eigenschaften gesucht werden muss. Eisencarbid (Fe3C) kann eine dieser Verbindungen sein. Dieses besitzt vergleichbare positive Eigenschaften wie Eisenoxid, jedoch viel bessere magnetische Eigenschaften, speziell eine höhere Sättigungsmagnetisierung. Bis jetzt wurde Fe3C hauptsächlich in Gasphasenabscheidungsprozessen synthetisiert oder als Nebenprodukt bei der Synthese von Kohlenstoffstrukturen gefunden. Eine Methode, mit der gezielt Fe3C-Nanopartikel und andere Metallcarbide synthetisiert werden können, ist die „Harnstoff-Glas-Route“. Neben den Metallcarbiden können mit dieser Methode auch die entsprechenden Metallnitride synthetisiert werden, was die breite Anwendbarkeit der Methode unterstreicht. Die „Harnstoff-Glas-Route“ ist eine Kombination eines Sol-Gel-Prozesses mit einer anschließenden carbothermalen Reduktion/Nitridierung bei höheren Temperaturen. Sie bietet den Vorteil einer einfachen und schnellen Synthese verschiedener Metallcarbide/nitride. Der Schwerpunkt in dieser Arbeit lag auf der Synthese von Eisencarbiden/nitriden, aber auch Nickel und Kobalt wurden betrachtet. Durch die Variation der Syntheseparameter konnten verschiedene Eisencarbid/nitrid Nanostrukturen synthetisiert werden. Fe3C-Nanopartikel im Größenbereich von d = 5 – 10 nm konnten, durch die Verwendung von Eisenchlorid, hergestellt werden. Die Nanopartikel weisen durch ihre geringe Größe superparamagnetische Eigenschaften auf und besitzen, im Vergleich zu Eisenoxid Nanopartikeln im gleichen Größenbereich, eine höhere Sättigungsmagnetisierung. Diese konnten in fortführenden Experimenten erfolgreich in ionischen Flüssigkeiten und durch ein Polymer-Coating, im wässrigen Medium, dispergiert werden. Desweiteren wurde durch ein Templatieren mit kolloidalem Silika eine mesoporöse Fe3C-Nanostruktur hergestellt. Diese konnte erfolgreich in der katalytischen Spaltung von Ammoniak getestet werden. Mit der Verwendung von Eisenacetylacetonat konnten neben Fe3C-Nanopartikeln, nur durch Variation der Reaktionsparameter, auch Fe7C3- und Fe3N-Nanopartikel synthetisiert werden. Speziell für die Fe3C-Nanopartikel konnte die Sättigungsmagnetisierung, im Vergleich zu den mit Eisenchlorid synthetisierten Nanopartikeln, nochmals erhöht werden. Versuche mit Nickelacetat führten zu Nickelnitrid (Ni3N) Nanokristallen. Eine zusätzliche metallische Nickelphase führte zu einer Selbstorganisation der Partikel in Scheiben-ähnliche Überstrukturen. Mittels Kobaltacetat konnten, in Sphären aggregierte, metallische Kobalt Nanopartikel synthetisiert werden. Kobaltcarbid/nitrid war mit den gegebenen Syntheseparametern nicht zugänglich.
Within this work, three physicochemical methods for the hydrophobization of initially hydrophilic solid particles are investigated. The modified particles are then used for the stabilization of oil-in-water (o/w) emulsions. For all introduced methods electrostatic interactions between strongly or weakly charged groups in the system are es-sential. (i) Short chain alkylammonium bromides (C4 – C12) adsorb on oppositely charged solid particles. Macroscopic contact angle measurements of water droplets under air and hexane on flat silica surfaces in dependency of the surface charge density and alkylchain-length allow the calculation of the surface energy and give insights into the emulsification properties of solid particles modified with alkyltrimethylammonium bromides. The measure-ments show an increase of the contact angle with increasing surface charge density, due to the enhanced adsorp-tion of the oppositely charged alkylammonium bromides. Contact angles are higher for longer alkylchain lengths. The surface energy calculations show that in particular the surface-hexane or surface-air interfacial en-ergy is being lowered upon alkylammonium adsorption, while a significant increase of the surface-water interfa-cial energy occurs only at long alkyl chain lengths and high surface charge densities. (ii) The thickness and the charge density of an adsorbed weak polyelectrolyte layer (e.g. PMAA, PAH) influence the wettability of nanoparticles (e.g. alumina, silica, see Scheme 1(b)). Furthermore, the isoelectric point and the pH range of colloidal stability of particle-polyelectrolyte composites depend on the thickness of the weak polye-lectrolyte layer. Silica nanoparticles with adsorbed PAH and alumina nanoparticles with adsorbed PMAA be-come interfacially active and thus able to stabilize o/w emulsions when the degree of dissociation of the polye-lectrolyte layer is below 80 %. The average droplet size after emulsification of dodecane in water depends on the thickness and the degree of dissociation of the adsorbed PE-layer. The visualization of the particle-stabilized o/w emulsions by cryogenic SEM shows that for colloidally stable alumina-PMAA composites the oil-water interface is covered with a closely packed monolayer of particles, while for the colloidally unstable case closely packed aggregated particles deposit on the interface. (iii) By emulsifying a mixture of the corrosion inhibitor 8-hydroxyquinoline (8-HQ) and styrene with silica nanoparticles a highly stable o/w emulsion can be obtained in a narrow pH window. The amphoteric character of 8-HQ enables a pH dependent electrostatic interaction with silica nanoparticles, which can render them interfa-cially active. Depending on the concentration and the degree of dissociation of 8-HQ the adsorption onto silica results from electrostatic or aromatic interactions between 8-HQ and the particle-surface. At intermediate amounts of adsorbed 8-HQ the oil wettability of the particles becomes sufficient for stabilizing o/w emulsions. Cryogenic SEM visualization shows that the particles arrange then in a closely packed shell consisting of partly of aggregated domains on the droplet interface. For further increasing amounts of adsorbed 8-HQ the oil wet-tability is reduced again and the particles ability to stabilize emulsions decreases. By the addition of hexadecane to the oil phase the size of the droplets can be reduced down to 200 nm by in-creasing the silica mass fraction. Subsequent polymerization produces corrosion inhibitor filled (20 wt-%) poly-styrene-silica composite particles. The measurement of the release of 8-hydroxyquinoline shows a rapid increase of 8-hydroxyquinoline in a stirred aqueous solution indicating the release of the total content in less than 5 min-utes. The method is extended for the encapsulation of other organic corrosion inhibitors. The silica-polymer-inhibitor composite particles are then dispersed in a water based alkyd emulsion, and the dispersion is used to coat flat aluminium substrates. After drying and cross-linking the polmer-film Confocal Laser Scanning Micros-copy is employed revealing a homogeneous distribution of the particles in the film. Electrochemical Impedance Spectroscopy in aqueous electrolyte solutions shows that films with aggregated particle domains degrade with time and don’t provide long-term corrosion protection of the substrate. However, films with highly dispersed particles have high barrier properties for corrosive species. The comparison of films containing silica-polystyrene composite particles with and without 8-hydroxyquinoline shows higher electrochemical impedances when the inhibitor is present in the film. By applying the Scanning Vibrating Electrode Technique the localized corrosion rate in the fractured area of scratched polymer films containing the silica-polymer-inhibitor composite particles is studied. Electrochemical corrosion cannot be suppressed but the rate is lowered when inhibitor filled composite particles are present in the film. By depositing six polyelectrolyte layers on particle stabilized emulsion droplets their surface morphology changes significantly as shown by SEM visualization. When the oil wettability of the outer polyelectrolyte layer increases, the polyelectrolyte coated droplets can act as emulsion stabilizers themselves by attaching onto bigger oil droplets in a closely packed arrangement. In the presence of 3 mM LaCl3 8-HQ hydrophobized silica particles aggregate strongly on the oil-water inter-face. The application of an ultrasonic field can remove two dimensional shell-compartments from the droplet surface, which are then found in the aqueous bulk phase. Their size ranges up to 1/4th of the spherical particle shell.
In dieser Arbeit wird ein chemisches Abgussverfahren für selbstorganisierte Strukturen in Lösung verwendet, das es ermöglicht definierte poröse Materialien mit Strukturierung auf der Nanometerskala herzustellen. Ähnlich wie beim Gussverfahren von Werkstücken wird die Vorlage durch ein geeignetes Material abgebildet. Durch Entfernen dieser Vorlage erhält man ein poröses (mit Hohlräumen durchsetztes) Negativ derselben. Die auf diese Weise erhaltenen Materialien sind in mehrerer Hinsicht interessant: So lassen sich aus ihrer Morphologie Rückschlüsse über die Natur der selbstorganisierten Strukturen erhalten, da der hier verwendete Abbildungsprozess selbst kleinste strukturelle Details erfasst. Die Hohlräume der synthetisierten porösen Stoffe hingegen können als winzige Reaktionsgefäße, sogenannte "Nano-Reaktoren" verwendet werden. Dies ermöglicht sowohl die Synthese von Nano-Partikeln, die auf anderem Wege nicht zugänglich sind, als auch die Möglichkeit Einflüsse der räumlichen Restriktion auf die Reaktion zu untersuchen. Besonders räumlich ausgedehnte Strukturen sollten hierbei Auffälligkeiten zeigen. Somit ist die Gliederung der Arbeit vorgegeben: - Die Herstellung und Charakterisierung von porösen Stoffen und selbstorganisierten Strukturen - Ihre Verwendung als "Nano-Reaktor"
Hybrid organic-inorganic perovskites have attracted attention in recent years, caused by the incomparable increase in efficiency in energy convergence, which implies the application as an absorber material for solar cells. A disadvantage of these materials is, among others, the instability to moisture and UV-radiation. One possible solution for these problems is the reduction of the size towards the nano world. With that nanosized perovskites are showing superior stability in comparison to e.g. perovskite layers. Additionally to this the nanosize even enables stable perovskite structures, which could not be achieved otherwise at
room temperature.
This thesis is separated into two major parts. The separation is done by the composition and the band gap of the material and at the same time the shape and size of the nanoparticles. Here the division is made by the methylammonium lead tribromide nanoplatelets and the caesium lead triiodide nanocubes.
The first part is focusing on the hybrid organic-inorganic perovskite (methylammonium lead tribromide) nanoplatelets with a band gap of 2.35 eV and their thermal behaviour. Due to the challenging character of this material, several analysis methods are used to investigate the sub nano and nanostructures under the influence of temperature. As a result, a shift of phase-transition temperatures towards higher temperatures is observed. This unusual behaviour can be explained by the ligand, which is incorporated in the perovskite outer structure and adds phase-stability into the system.
The second part of this thesis is focusing on the inorganic caesium lead triiodide nanocubes with a band gap of 1.83 eV. These nanocrystals are first investigated and compared by TEM, XRD and other optical methods. Within these methods, a cuboid and orthorhombic structure are revealed instead of the in literature described cubic shape and structure. Furthermore, these cuboids are investigated towards their self-assembly on a substrate. Here a high degree in self-assembly is shown. As a next step, the ligands of the nanocuboids are exchanged against other ligands to increase the charge carrier mobility. This is further investigated by the above-mentioned methods. The last section is dealing with the enhancement of the CsPbI3 structure, by incorporating potassium in the crystal structure. The results are suggesting here an increase in stability.
Ein Spezialgebiet der modernen Mikroelektronik ist die Miniaturisierung und Entwicklung von neuen nanostrukturierten und Komposit-Materialen aus 3d-Metallen. Durch geeignete Zusammensetzungen können diese sowohl mit einer hohen Sättigungsmagnetisierung und Koerzitivfeldstärke als mit besserer Oxidationsbeständigkeit im Vergleich zu den reinen Elementen erzielt werden. In der vorliegenden Arbeit werden neue Methoden für die Herstellung von bimetallischen kolloidalen Nanopartikeln vor allem mit einer Kern-Hülle-Struktur (Kern@Hülle) präsentiert. Bei der überwiegenden Zahl der vorgestellten Reaktionen handelt es sich um die thermische Zersetzung von metallorganischen Verbindungen wie Kobaltcarbonyl, Palladium- und Platinacetylacetonate oder die chemische Reduktion von Metallsalze mit langkettigem Alkohol in organischem Lösungsmittel. Daneben sind auch Kombinationen aus diesen beiden Verfahren beschrieben. Es wurden Kolloide aus einem reinen Edelmetall (Pt, Pd, Ag) in einem organischen Lösungsmittel synthetisiert und daraus neue, bisher in dieser Form nicht bekannte Ag@Co-, Pt@Co-, Pd@Co- und Pt@Pd@Co-Nanopartikel gewonnen. Der Kobaltgehalt der Ag@Co-, Teilchen konnte im Bereich von 5 bis 73 At. % beliebig eingestellt werden. Der mittlere Durchmesser der Ag@Co-Partikel wurde von 5 nm bis 15 nm variiert. Bei der Herstellung von Pt@Co-Teilchen wurde eine unterschiedlich dicke Kobalt-Hülle von ca. 1,0 bis 2,5 nm erzielt. Im Fall des Palladiums wurden sowohl monodispere als auch polydisperse Pd-Nanopartikel mit einer maximal 1,7-2,0nm dicken Kobalthülle synthetisiert. Ein großer Teil dieser Arbeit befasst sich mit den magnetischen Eigenschaften der kolloidalen Teilchen, wobei die SQUID-Magnetometrie und Röntgenzirkulardichroismus (XMCD) dafür eingesetzt wurden. Weil magnetische Messungen alleine nur indirekte Schlüsse über die untersuchten Systeme erlauben, wurde dabei besonderer Wert auf die möglichst genaue strukturelle Charakterisierung der Proben mittels moderner Untersuchungsmethoden gelegt. Röntgendiffraktometrie (XRD), Röntgenabsorptionsfeinstruktur- (EXAFS) und UV-Vis-Spektroskopie sowie Transmissionselektronenmikroskopie (TEM) in Kombination mit Elektronen Energieverlustspektroskopie (EELS) und energiedispersive Röntgenfluoreszensanalyse (EDX) wurden verwendet.
Health effects, attributed to the environmental pollution resulted from using solvents such as benzene, are relatively unexplored among petroleum workers, personal use, and laboratory researchers. Solvents can cause various health problems, such as neurotoxicity, immunotoxicity, and carcinogenicity. As such it can be absorbed via epidermal or respiratory into the human body resulting in interacting with molecules that are responsible for biochemical and physiological processes of the brain.
Owing to the ever-growing demand for finding a solution, an Ionic liquid can use as an alternative solvent. Ionic liquids are salts in a liquid state at low temperature (below 100 C), or even at room temperature. Ionic liquids impart a unique architectural platform, which has been interesting because of their unusual properties that can be tuned by simple ways such as mixing two ionic liquids.
Ionic liquids not only used as reaction solvents but they became a key developing for novel applications based on their thermal stability, electric conductivity with very low vapor pressure in contrast to the conventional solvents.
In this study, ionic liquids were used as a solvent and reactant at the same time for the novel nanomaterials synthesis for different applications including solar cells, gas sensors, and water splitting.
The field of ionic liquids continues to grow, and become one of the most important branches of science. It appears to be at a point where research and industry can work together in a new way of thinking for green chemistry and sustainable production.
In Germany more than 200.000 persons die of cancer every year, which makes it the second most common cause of death. Chemotherapy and radiation therapy are often combined to exploit a supra-additive effect, as some chemotherapeutic agents like halogenated nucleobases sensitize the cancerous tissue to radiation. The radiosensitizing action of certain therapeutic agents can be at least partly assigned to their interaction with secondary low energy electrons (LEEs) that are generated along the track of the ionizing radiation. In the therapy of cancer DNA is an important target, as severe DNA damage like double strand breaks induce the cell death. As there is only a limited number of radiosensitizing agents in clinical practice, which are often strongly cytotoxic, it would be beneficial to get a deeper understanding of the interaction of less toxic potential radiosensitizers with secondary reactive species like LEEs. Beyond that LEEs can be generated by laser illuminated nanoparticles that are applied in photothermal therapy (PTT) of cancer, which is an attempt to treat cancer by an increase of temperature in the cells. However, the application of halogenated nucleobases in PTT has not been taken into account so far. In this thesis the interaction of the potential radiosensitizer 8-bromoadenine (8BrA) with LEEs was studied. In a first step the dissociative electron attachment (DEA) in the gas phase was studied in a crossed electron-molecular beam setup. The main fragmentation pathway was revealed as the cleavage of the C-Br bond. The formation of a stable parent anion was observed for electron energies around 0 eV. Furthermore, DNA origami nanostructures were used as platformed to determine electron induced strand break cross sections of 8BrA sensitized oligonucleotides and the corresponding nonsensitized sequence as a function of the electron energy. In this way the influence of the DEA resonances observed for the free molecules on the DNA strand breaks was examined. As the surrounding medium influences the DEA, pulsed laser illuminated gold nanoparticles (AuNPs) were used as a nanoscale electron source in an aqueous environment. The dissociation of brominated and native nucleobases was tracked with UV-Vis absorption spectroscopy and the generated fragments were identified with surface enhanced Raman scattering (SERS). Beside the electron induced damage, nucleobase analogues are decomposed in the vicinity of the laser illuminatednanoparticles due to the high temperatures. In order to get a deeper understanding of the different dissociation mechanisms, the thermal decomposition of the nucleobases in these systems was studied and the influence of the adsorption kinetics of the molecules was elucidated. In addition to the pulsed laser experiments, a dissociative electron transfer from plasmonically generated ”hot electrons” to 8BrA was observed under low energy continuous wave laser illumination and tracked with SERS. The reaction was studied on AgNPs and AuNPs as a function of the laser intensity and wavelength. On dried samples the dissociation of the molecule was described by fractal like kinetics. In solution, the dissociative electron transfer was observed as well. It turned out that the timescale of the reaction rates were slightly below typical integration times of Raman spectra. In consequence such reactions need to be taken into account in the interpretation of SERS spectra of electrophilic molecules. The findings in this thesis help to understand the interaction of brominated nucleobases with plasmonically generated electrons and free electrons. This might help to evaluate the potential radiosensitizing action of such molecules in cancer radiation therapy and PTT.
The formation of colloids by the controlled reduction, nucleation, and growth of inorganic precursor salts in different media has been investigated for more than a century. Recently, the preparation of ultrafine particles has received much attention since they can offer highly promising and novel options for a wide range of technical applications (nanotechnology, electrooptical devices, pharmaceutics, etc). The interest derives from the well-known fact that properties of advanced materials are critically dependent on the microstructure of the sample. Control of size, size distribution and morphology of the individual grains or crystallites is of the utmost importance in order to obtain the material characteristics desired. Several methods can be employed for the synthesis of nanoparticles. On the one hand, the reduction can occur in diluted aqueous or alcoholic solutions. On the other hand, the reduction process can be realized in a template phase, e.g. in well-defined microemulsion droplets. However, the stability of the nanoparticles formed mainly depends on their surface charge and it can be influenced with some added protective components. Quite different types of polymers, including polyelectrolytes and amphiphilic block copolymers, can for instance be used as protecting agents. The reduction and stabilization of metal colloids in aqueous solution by adding self-synthesized hydrophobically modified polyelectrolytes were studied in much more details. The polymers used are hydrophobically modified derivatives of poly(sodium acrylate) and of maleamic acid copolymers as well as the commercially available branched poly(ethyleneimine). The first notable result is that the polyelectrolytes used can act alone as both reducing and stabilizing agent for the preparation of gold nanoparticles. The investigation was then focused on the influence of the hydrophobic substitution of the polymer backbone on the reduction and stabilization processes. First of all, the polymers were added at room temperature and the reduction process was investigated over a longer time period (up to 8 days). In comparison, the reduction process was realized faster at higher temperature, i.e. 100°C. In both cases metal nanoparticles of colloidal dimensions can be produced. However, the size and shape of the individual nanoparticles mainly depends on the polymer added and the temperature procedure used. In a second part, the influence of the prior mentioned polyelectrolytes was investigated on the phase behaviour as well as on the properties of the inverse micellar region (L2 phase) of quaternary systems consisting of a surfactant, toluene-pentanol (1:1) and water. The majority of the present work has been made with the anionic surfactant sodium dodecylsulfate (SDS) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) since they can interact with the oppositely charged polyelectrolytes and the microemulsions formed using these surfactants present a large water-in-oil region. Subsequently, the polymer-modified microemulsions were used as new templates for the synthesis of inorganic particles, ranging from metals to complex crystallites, of very small size. The water droplets can indeed act as nanoreactors for the nucleation and growth of the particles, and the added polymer can influence the droplet size, the droplet-droplet interactions, as well as the stability of the surfactant film by the formation of polymer-surfactant complexes. One further advantage of the polymer-modified microemulsions is the possibility to stabilize the primary formed nanoparticles via a polymer adsorption (steric and/or electrostatic stabilization). Thus, the polyelectrolyte-modified nanoparticles formed can be redispersed without flocculation after solvent evaporation.
Gold at the nanoscale
(2020)
In this cumulative dissertation, I want to present my contributions to the field of plasmonic nanoparticle science. Plasmonic nanoparticles are characterised by resonances of the free electron gas around the spectral range of visible light. In recent years, they have evolved as promising components for light based nanocircuits, light harvesting, nanosensors, cancer therapies, and many more.
This work exhibits the articles I authored or co-authored in my time as PhD student at the University of Potsdam. The main focus lies on the coupling between localised plasmons and excitons in organic dyes. Plasmon–exciton coupling brings light–matter coupling to the nanoscale. This size reduction is accompanied by strong enhancements of the light field which can, among others, be utilised to enhance the spectroscopic footprint of molecules down to single molecule detection, improve the efficiency of solar cells, or establish lasing on the nanoscale. When the coupling exceeds all decay channels, the system enters the strong coupling regime. In this case, hybrid light–matter modes emerge utilisable as optical switches, in quantum networks, or as thresholdless lasers. The present work investigates plasmon–exciton coupling in gold–dye core–shell geometries and contains both fundamental insights and technical novelties. It presents a technique which reveals the anticrossing in coupled systems without manipulating the particles themselves. The method is used to investigate the relation between coupling strength and particle size. Additionally, the work demonstrates that pure extinction measurements can be insufficient when trying to assess the coupling regime. Moreover, the fundamental quantum electrodynamic effect of vacuum induced saturation is introduced. This effect causes the vacuum fluctuations to diminish the polarisability of molecules and has not yet been considered in the plasmonic context.
The work additionally discusses the reaction of gold nanoparticles to optical heating. Such knowledge is of great importance for all potential optical applications utilising plasmonic nanoparticles since optical excitation always generates heat. This heat can induce a change in the optical properties, but also mechanical changes up to melting can occur. Here, the change of spectra in coupled plasmon–exciton particles is discussed and explained with a precise model. Moreover, the work discusses the behaviour of gold nanotriangles exposed to optical heating. In a pump–probe measurement, X-ray probe pulses directly monitored the particles’ breathing modes. In another experiment, the triangles were exposed to cw laser radiation with varying intensities and illumination areas. X-ray diffraction directly measured the particles’ temperature. Particle melting was investigated with surface enhanced Raman spectroscopy and SEM imaging demonstrating that larger illumination areas can cause melting at lower intensities. An elaborate methodological and theoretical introduction precedes the articles. This way, also readers without specialist’s knowledge get a concise and detailed overview of the theory and methods used in the articles. I introduce localised plasmons in metal nanoparticles of different shapes. For this work, the plasmons were mostly coupled to excitons in J-aggregates. Therefore, I discuss these aggregates of organic dyes with sharp and intense resonances and establish an understanding of the coupling between the two systems. For ab initio simulations of the coupled systems, models for the systems’ permittivites are presented, too. Moreover, the route to the sample fabrication – the dye coating of gold nanoparticles, their subsequent deposition on substrates, and the covering with polyelectrolytes – is presented together with the measurement methods that were used for the articles.