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<p>In this thesis, a modern transmission electron microscope is used to perform high-resolution electron energy loss studies of metallic nanostructures and carbon nanotubes.</p> <p>The remarkable optical properties of metallic nanostructures arise from the excita- tion of surface plasmons. With improved instrumentation, surface plasmon resonances are imaged in a variety of nanostructures, enabling a greater understanding of their behaviour in nanoscale systems. It is shown that surface plasmons set up multiple high order resonances in silver nanorods, and they freely propagate around sharp corners in silver nanowires. It is also demonstrated that silver nanorice structures resonate in a similar manner to nanorods, despite the high density of stacking faults in the structure. Finally, a complementary structural pair is found to resonate in a complementary fashion, in agreement with Babinet’s principle.</p> <p>Carbon nanotubes exhibit unique physicochemical properties that have led to their use in a variety of novel materials science applications. Despite rapid progress in the theoretical and experimental investigation of carbon nanotubes, techniques capable of studying the structural and electronic properties of individual tubes are limited. Here, it is demonstrated that the spectral signature of carbon can be used to identify the electronic character of individual single-walled carbon nanotubes. In addition, a new technique is used to map bonding anisotropy in a multi-walled carbon nanotube.</p> <p>Also presented in this thesis is the design and construction of a unique laser-TEM system. Early results from the system include in-situ measurements of laser-induced structural and electronic distortions in individual carbon nanotubes.</p> / Doctor of Philosophy (PhD)

The work presented in this thesis centers on the application of atomic-resolution transmission electron microscopy to study Platinum-alloy nanoparticles. In particular, the thesis focusses on the platinum-iron and platinum-gold systems. Additionally, few other complementary structures based on Pt thin films and nanowires are also characterized. These materials are studied in the context of their catalytic application towards the oxygen reduction reaction in polymer electrolyte membrane fuel cells (PEMFCs). Here we report on the detailed investigation of many structural and compositional aspects of these catalyst nanoparticles including lattice strain, the surface and bulk atomic-structure, the surface/bulk chemical composition, surface segregation, and atomic ordering. In some cases we have even looked beyond the traditional characterization approaches. For instance, instead of observing the particle structures before and after a particular treatment (e.g., heating and degradation tests), we have captured the dynamics of structural evolution over the entire course of such treatments. These investigations were useful in interpreting their catalytic performances, which opened new perspectives towards further optimization of their material structure on the atomic-level. / Thesis / Doctor of Philosophy (PhD)

Graphene and its functionalized derivatives, such as nitrogen-doped graphene, have recently become a popular substrate material for the proton exchange membrane fuel cell (PEMFC) due to its enhanced electrical conductivity, electrochemical stability, and increased surface area when compared to the conventional, carbon black. In order to further develop the alternative fuel industry, the Pt catalyst within the PEMFC must also be considered. Single Pt atoms have a higher surface area to volume ratio when compared to nanoparticles, thus offering the potential to create a more affordable and efficient PEMFC. In this thesis, electrode materials comprising single Pt atoms and clusters, produced using atomic layer deposition (ALD) on various C derivatives, including graphene, N-doped graphene, carbon nanotubes (CNTs), and N-doped CNTs (NCNTs) are investigated through the utilization of aberration corrected transmission electron microscopy. Structural and chemical analysis was performed on thermally exfoliated N-doped graphene and CVD-produced graphene that was exposed to N+ ion sources. It was determined that the thermally exfoliated N-doped graphene maintained the short-range order of the graphene lattice; however, local inhomogeneities existed for the total N concentration, and the specific N-dopants within and between graphene sheets. More importantly, Pt atoms and clusters were observed and determined to be primarily stabilized at the edge of the N-doped graphene sheets. The stabilization of the Pt atoms and clusters resulted in a significantly higher mass and specific activity for the hydrogen evolution reaction, when compared to the use of a graphene substrate and Pt nanoparticles on C black. The N+ ion implantation in the CVD graphene showed the incorporation of N-dopants; however, electron energy loss spectroscopy revealed structural damage to thin sheets. NCNTs were also characterized in this thesis as possible gas containers, and as a substrate material to examine the effects of varying ALD conditions. It was determined that the NCNTs were an effective N2 gas conduit, wherein a decreasing pressure was observed with an increase to the inner diameter of the nanotubes. Using similar NCNTs, the effect of dosing time, temperature, and substrate on the Pt size were analyzed using ALD. While no singular condition resulted in the sole production of single Pt atoms, modifying both the substrate and dosing time were shown to provide the greatest potential for producing individual Pt atom catalysts. / Thesis / Doctor of Philosophy (PhD)

Lithium-ion batteries have become a vital part of our modern life and play an essential role in electric vehicle development. One of the most feasible strategies to enhance the energy density of Li-ion batteries is to use layered, Ni-rich cathode materials. However, higher nickel content causes several problems and therefore, several methods, including doping and coating, have been utilized to stabilize their structure and boost their performance. This thesis aims to understand the microstructure of such engineered complex cathodes and provide valuable contributions by comprehensively understanding and establishing a link between the composition, structure, performance, and properties of these complex materials. In this regard, the most advanced electron- and photon-based techniques have been used to uncover the fundamental underlying reasons for the enhanced performance or degradation in these complex cathode structures. This study shows that introducing W cation inside the LiNiO2 results in new W-variants with a heterogeneous concentration on the top surface and through grain boundaries of the host secondary particles. These W-rich regions play a reinforcing role in grain boundaries and protect the outer surface of LiNiO2 particles. However, synthesis defects, such as porosities, could reduce these benefits by increasing the electrolyte infiltration inside the cathode particles. It is also demonstrated that the degradation process can be studied through the changes in electron energy loss near-edge structure spectra. The investigation of a coating approach on LiNi0.8Co0.15Al0.05O2 materials through the mechanofusion process illustrates more microscopic-scale details regarding the thickness unevenness of the coating and some degree of physical intermixing between the core (LiNi0.8Co0.15Al0.05O2) and coating (LiFePO4 and alumina) precursors. In addition to good physical contact between the core and coating materials, further analysis at higher resolution reveals some nanoscale grains and defective areas near the top surface of the secondary particles following the mechanofusion coating process. / Thesis / Doctor of Philosophy (PhD)

No / Make it connected! 2D close-packed layers of inorganic nanoparticles are interconnected by organic fibrils of oleic acid as clearly visualized by electron holography. These fibrils can be mineralised by PbS to transform an organic-inorganic framework to a completely interconnected inorganic semiconducting 2D array.

This study investigates the impact of Sn alloying on the structural, electronic, and optical properties of core-shell Ge-GeSn nanowires, with Ge as the core and GeSn as the shell. Using advanced transmission electron microscopy (TEM) techniques, including High-Resolution Scanning TEM (STEM) with High-Angle Annular Dark Field (HAADF) imaging, Energy-Dispersive X-Ray Spectroscopy (EDS), and Electron Energy-Loss Spectroscopy (EELS), we achieve high spatial, energy, and momentum resolution. These methods reveal complex morphological changes and bandgap transitions within the Ge_(1-x)Sn_x nanowires. The study demonstrates that defect-free Ge-Ge_(1-x)Sn_(x) core-shell nanowires can be successfully synthesized by overcoming challenging growth conditions, achieving stable structures even with varying Sn content. Detailed sub-angstrom investigations reveal that these nanowires maintain stability and defect-free characteristics despite the presence of strain, which is alleviated by their core-shell morphology. Our findings show that increasing Sn content from 8 to 18 at.\% leads to a notable transition from an indirect to a direct bandgap, with the bandgap energy decreasing to approximately 0.2 eV at high Sn concentrations. This research highlights the significant role of Sn alloying in altering the characteristics of Ge-Ge_(1-x)Sn_(x) core-shell nanowires and confirms the transition to a direct bandgap with increased Sn content. / Dissertation / Doctor of Philosophy (PhD)

The direct growth of graphene by scalable methods on magnetic insulators is important for industrial development of graphene-based spintronic devices, and a route towards substrate-induced spin polarization in graphene without spin injection. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction LEED, electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES) demonstrate the growth of Co3O4(111) and CoO(111) to thicknesses greater than 100 Å on Ru(0001) surfaces, by molecular beam epitaxy (MBE). The results obtained show that the formation of the different cobalt oxide phases is O2 partial pressure dependent under same temperature and vacuum conditions and that the films are stoichiometric. Electrical I-V measurement of the Co3O4(111) show characteristic hysteresis indicative of resistive switching and thus suitable for advanced device applications. In addition, the growth of Co0.5Fe0.5O(111) was also achieved by MBE and these films were observed to be OH-stabilized. C MBE yielded azimuthally oriented few layer graphene on the OH-terminated CoO(111), Co0.5Fe0.5O(111) and Co3O4(111). AES confirms the growth of (111)-ordered sp2 C layers. EELS data demonstrate significant graphene-to-oxide charge transfer with Raman spectroscopy showing the formation of a graphene-oxide buffer layer, in excellent agreement with previous theoretical predictions. XPS data show the formation of C-O covalent bonding between the oxide layer and the first monolayer (ML) of C. LEED data reveal that the graphene overlayers on all substrates exhibit C3V. The reduction of graphene symmetry to C3V – correlated with C-O bond formation – enables spin-orbit coupling in graphene. Consequences may include a significant band gap and room temperature spin Hall effect – important for spintronic device applications. The results suggest a general pattern of graphene/graphene oxide growth and symmetry lowering for graphene formation on the (111) surfaces of rocksalt-structured oxides.

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.<br /> <br /> 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.<br /> <br /> 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.<br /> <br /> 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. / Magnetic colloidal particles are attractive because of their possible application to ultra-high-density magnetic data storage media, sensors, electronic devices and medical diagnostics. The properties of small particles depend on their composition, shape, and method of preparation. The combination of 3d-metals (Fe, Co, Ni) with noble metals improves the stability of the colloids and leads to new properties of the magnetic systems, often distinct from those of the corresponding monometallic particles. Core-shell particles, where dia- or paramagnetic noble metal-cores are surrounded by a ferromagnetic Co-shell, are an interesting system to study surface and interfacial magnetism such as an induced polarization or a giant magnetoresistance effect. <br /> <br /> In this work, new synthetic routes for the preparation of monometallic (Pt, Pd, Ag) and bimetallic magnetic nanocrystals (Ag@Co, Pt@Co, Pd@Co) with core-shell structure are presented. Stable colloids with a narrow particle size distribution were obtained in organic solvents using methods of wet chemistry. The method of preparation of Ag@Co is based on the thermal decomposition of dicobalt octycarbonyl in combination with a transmetalation reaction with water free AgClO4. The cobalt amount in the Ag@Co system could be tuned from 5 to 73 at. %. The average diameter of the particles was varied from 5 to 15 nm. <br /> <br /> The reduction of platinum and palladium salts in organic solution using long chained alcohol as the reductant leads to stable metal nanostructures. Monodisperse Pd and Pt particles with average sizes of 1.7 to 7.0 nm were synthesized via thermal decomposition of metal-surfactant complexes too. Alkylamines and alkylphosphines were used in this procedure. The thickness of the Co-shell was controlled by a simple high-temperature thermolysis of dicobalt octacarbonyl at the presence of Pd and Pt seeds and was tunable from 0.5 to 2.5 nm. <br /> <br /> The crystalline structure of the samples was characterized by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX), UV-VIS and electron-energy loss spectroscopy (EELS). SQUID magnetometry, x-ray magnetic circular dichroism (XMCD) and extended x-ray absorption fine structure (EXAFS) measurements gave information about the magnetic properties of the bimetallic systems and revealed their dependency on the particle size and the chemical composition. A high spin to orbital moments ratio µL/µS of 0.26±0.06 for Ag@Co and 0.22±0.05 for Pt@Co nanocrystals was observed at XMCD measurements due to the lowered dimensionality the investigated systems.