Catalyst is a key component in the chemical industry, with more than 90% of total chemical products reliant on their use. However, the working mechanisms are in many cases still not fully understood. For heterogeneous catalysts, in which the reactions normally occur on solid phase materials, a better understanding of the catalytic surfaces, and how they evolve under reactive environments is recognised as the next step forward in the field. This work presents a study utilising atom probe tomography (APT), combined with an in-situ reaction cell, to understand the initial oxidation processes of catalytic NiFe and NiCo model alloy systems. In order to improve reliability of results, a protocol was developed to clean the sample surfaces by field ion evaporation, eliminate sample surface contamination before in-situ oxidation was then performed. APT was successfully applied to these alloys to characterise oxide development as a function of exposure time and temperature. APT also demonstrated surface enrichment induced by oxide formation remained after reduction of the alloy. The successful application of APT on the model alloys led to the next goal which was to associate the data to real catalytic particles. To achieve this, work was extended into the field of nanoparticle catalysts. Nanoparticles with similar compositions to the model alloys were fabricated by chemical synthesis and were examined initially by transmission electron microscopy (TEM). The main goal of this phase was to investigate the surface segregation behaviour of the particles, identifying common behaviours with the model alloys. However, the presence of residual complex chemical environments around the particles following synthesis made APT analysis difficult. Therefore, an alternative method of particle fabrication was explored to better control the resulting materials for easier application of atom probe for nanoparticle analyses. Metallic nanoparticles of Ag, AuCu, AuNi, and AuNiMo were made by an inert gas condensation method, deposited on suitable support materials and were subsequently analysed by APT, facilitated by an improved sample preparation method. Surface segregation on individual nanoparticles was detected. Together with other complementary surface-probing techniques, a complete understanding of these particles from micrometre down to the level of individual particles was achieved. The potential for APT is highlighted to play a key role in this approach to realise a complete understanding of the chemical order, microstructure in multimetallic nanoparticles designed for catalysis.

The presence of and interactions between structural defects, fluids, and trace elements during deformation play a vital role in the manner in which materials respond to an applied stress. Although the links between crystal defects and trace element mobility have been lying at the frontier of research in Earth sciences, the role of fluids and the underlying physico-chemical processes linking them remain poorly understood. Investigation of these nanometer scale processes requires a correlative approach combining high-spatial resolution analytical techniques. This thesis integrates novel 2D and 3D structural and geochemical mapping methods such as electron channeling contrast imaging, electron backscatter diffraction, scanning transmission electron microscopy (STEM) and atom probe tomography (APT) to interrogate the atomic structure and composition of geomaterials in an attempt to better understand long-standing questions in Earth sciences and build bridges between materials science and geoscience. The processes investigated in this thesis include: 1) the underlying diffusion processes that mobilize trace elements into deformation-induced nanostructures; 2) the mechanisms of trace element segregation associated with fluid inclusions; 3) the influence of fluid inclusions on the mobility of structural defects and trace element mobility; and 4) the initial stages of bubble nucleation in the presence of nanoscale chemical heterogeneities. Ultimately, this research interrogates the feedbacks between deformation and trace element diffusion processes, fundamentally investigating their impact on rheology. More specifically, the thesis investigates the influence of deformation and associated nanostructures on the remobilization of trace elements and, in turn, the influence of trace elements on the nucleation and mobility of nanostructures. The combined work successfully identified two diffusion mechanisms for deformation-induced trace element mobility, characterized fluid-inclusions in APT data, documented two processes that led to proposing a new fluid inclusion-induced hardening model, and documented direct evidence of bubble nucleation on the surface of nanoscale chemical heterogeneities. This work not only pushes the limits of high-spatial resolution analytical techniques including STEM and APT, but the results have significant transdisciplinary implications in the fields of geoscience, materials science, engineering, and analytical microscopy.

Controlled decarburization experiments were carried out on ternary and quaternary iron alloys. The planar ferrite interfaces formed during decarburization were subsequently investigated using atom probe tomography (APT) to measure interfacial segregation. The segregation results for the Fe-Si-C, Fe-Mn-C, and Fe-Mo-C were used to improve the three-jump-model developed Zurob et al. These three systems were accurately modelled using interfacial binding energy values in agreement with the atom probe tomography results. Qualitative explanations for the modelling results of Sun et al. on the Fe-Mn-Mo-C system and Qiu et al. on the Fe-Mn-Si-C system have also been provided using the results from the atom probe tomography investigation. / Thesis / Master of Applied Science (MASc) / Controlled decarburization experiments were carried out on ternary and quaternary iron alloys. The planar ferrite interfaces formed during decarburization were subsequently investigated using atom probe tomography (APT) to measure interfacial segregation. The segregation results for the Fe-Si-C, Fe-Mn-C, and Fe-Mo-C were used to improve the three-jump-model developed Zurob et al. These three systems were accurately modelled using interfacial binding energy values in agreement with the atom probe tomography results. Qualitative explanations for the modelling results of Sun et al. on the Fe-Mn-Mo-C system and Qiu et al. on the Fe-Mn-Si-C system have also been provided using the results from the atom probe tomography investigation.

chemical partitioning of Cobalt-containing soft magnetic amorphous and nanocomposite materials has been investigated with particular focus on its consequences on these materials’ nanostructure and electrical resistivity. Theory, models, experiment, and discussion in this regard are presented on this class of materials generally, and are detailed in particular on alloys of composition, (Fe65Co35)79.5+xB13Si2Nb4-xCu1.5, for X={0- 4at%}, and Co-based, Co76+YFe4Mn4-YB14Si2Nb4, for Y={0-4at%}. The context of this work is within the ongoing efforts to integrate soft magnetic metal amorphous and nanocomposite materials into electric motor applications by leveraging material properties with motor topology in order to increase the electrical efficiency and decrease the size, the usage of rare-earth permanent magnets, and the power losses of electric motors. A mass balance model derived from consideration of the partitioning of glass forming elements relates local composition to crystal state in these alloys. The ‘polymorphic burst’ onset mechanism and a Time-Temperature- Transformation diagram for secondary crystallization are also presented in relation to the partitioning of glass forming elements. Further, the intrinsic electrical resistivity of the material is related to the formation of virtual bound states due to dilute amounts of the glass forming elements. And lastly, a multiphase resistivity model for the effective composite resistivity that accounts for the amorphous, crystalline, and glass former-rich amorphous regions, each with distinct intrinsic resistivity, is also presented. The presented models are validated experimentally on the Co-containing alloys by Atom Probe Tomography performed through collaboration with Pacific Northwestern National Laboratory.

Hydrogen embrittlement (HE), which results in an unpredictable failure of metals, has been a major limitation in the design of critical components for a wide range of engineering applications, given the near-ubiquitous presence of hydrogen in their service environments. However, the exact mechanisms that underpin HE failure remain poorly understood. It is known that hydrogen, when free to diffuse in these materials, can tend to concentrate at a crack tip front. In turn, this facilitates crack propagation. Hence one of the proposed strategies for mitigating HE is to limit the content of freely diffusing hydrogen within the metal atomic lattice via the introduction of microstructural hydrogen traps. Further, it is empirically known that the introduction of finely-dispersed distribution of nano-sized carbide hydrogen traps in ferritic steel matrix can improve resilience to HE. This resilience has been attributed to the effective hydrogen trapping of the carbides. However, conclusive atomic-scale experimental evidence is still lacking as to the manner by which these features can impede the movement of the hydrogen. This lack of insight limits the further progress for the optimisation of the microstructural design of this type of HE-resistant steel. In order to further understand the hydrogen trapping phenomenon of the nano-sized carbide in steel, an appropriate characterisation method is required. Atom probe tomography (APT) has been known for its powerful combination of high 3D spatial and chemical resolution for the analysis of very fine precipitates. Furthermore, previous studies have shown that the application of isotopic hydrogen (²H) loading techniques, combined with APT, facilitates the hydrogen signal associated to fine carbides to be unambiguously identified. However, the considerable experimental requirements as utilised by these previous studies, particularly the instrumental capability necessary for retention of the trapped hydrogen in the needle-shaped APT specimen, limits the study being reproduced or extended. In this APT study, a model ferritic steel with finely dispersed V-Mo-Nb carbides of 10-20 nm is investigated. Initially, existing specialised instrumentation formed the basis of a cryogenic specimen chain under vacuum, so as to retain loaded hydrogen after an electrolytic charging treatment for APT analysis. This work confirms the importance of cryogenic treatment for the retention of trapped hydrogen in APT specimen. The quality of the obtained experimental data allows a quantitative analysis on the hydrogen trapping mechanism. Thus, it is conclusively determined that interior of the carbides studied in this steel acts as the hydrogen trapping site as opposed to the carbide/matrix interface as commonly expected. This result supports the theoretical investigations proposing that the hydrogen trapping within the carbide interior is enabled by a network of carbon vacancies. Based on the established importance of the specimen cold chain in these APT experiments, this work then successfully develops a simplified approach to cryo-transfer which requires no instrumental modification. In this approach there is no requirement for the charged specimen to be transferred under vacuum conditions. The issue of environmental-induced ice contamination on the cryogenic sample surface in air transfer is resolved by its sublimation in APT vacuum chamber. Furthermore, the temperature of the transferred sample is able to be determined independently by both monitoring changes to vacuum pressure in the buffer chamber and also the thermal response of the APT sample stage in the analysis chamber. This simplified approach has the potential to open up a range of hydrogen trapping studies to any commercial atom probe instrument. Finally, as an example of the use of this simplified cryo-transfer technique, targeted studies for determining the source of hydrogen adsorption during electropolishing and electrolytic loading process are demonstrated. This research provides a critical verification of hydrogen trapping mechanism of fine carbides as well as an achievable experimental protocol for the observation of the trapping of individual hydrogen atoms in alloy microstructures. The methods developed here have the potential to underpin a wide range of possible experiments which address the HE problem, particularly for the design of new mitigation strategies to prevent this critical issue.

Optoelectronic devices fabricated from nitride semiconductors include blue and green light emitting diodes (LEDs) and laser diodes (LDs). To design efficient devices, the structure and composition of the constituent materials must be well-characterised. Traditional microscopy techniques used to examine nitride semiconductors include transmission electron microscopy (TEM), and atomic force microscopy (AFM). This thesis describes the study of nitride semiconductor materials using these traditional methods, as well as atom probe tomography (APT), a technique more usually applied to metals that provides three-dimensional (3D) compositional information at the atomic scale. By using both APT and correlative microscopy techniques, a more complete understanding of the material can be gained, which can potentially lead to higher-efficiency, longer-lasting devices. Defects, such as threading dislocations (TDs), can harm device performance. An AFM-based technique was used to show that TDs affect the local electrical properties of nitride materials. To investigate any compositional changes around the TD, APT studies of TDs were attempted, and evidence for oxygen enrichment near the TD was observed. The dopant level in nitride devices also affects their optoelectronic properties, and the combination of APT and TEM was used to show that Mg dopants were preferentially incorporated into pyramidal inversion domains, with a Mg content two orders of magnitude above the background level. Much debate has been focused on the microstructural origin of charge carrier localisation in InGaN. Alloy inhomogeneities have often been suggested to provide this localisation, yet APT has revealed InGaN quantum wells to be a statistically random alloy. Electron beam irradiation in the TEM caused damage to the InGaN, however, and a statistically significant deviation from a random alloy distribution was then observed by APT. The alloy homogeneity of InAlN was also studied, and this alloy system provided a unique opportunity to study gallium implantation damage to the APT sample caused during sample preparation by the focused ion beam (FIB). The combination of APT with traditional microscopy techniques made it possible to achieve a thorough understanding of a wide variety of nitride semiconductor materials.

In the present work atom probe tomography (APT) was applied to analyze thin films used in semiconductor industry to investigate the capability of atom probe tomography as well as the dopant redistribution in thin silicide layer stacks. Different titanium silicide layer stacks are investigated and titanium diboride precipitates are identified by APT. Arsenic grain boundary segregation is verified by APT in cobalt silicide layer stacks. Furthermore APT measurements are compared to commonly used methods such as TEM and SIMS and found in good agreement. Each method exhibits its own advantages depending on the sample and the question. Atom probe tomography offers some unique features enabling three-dimensional analysis on the nanometer scale as shown on the mentioned thin film layer stacks.

Epitaxially grown quantum dots (QDs) make up a significant portion of nanoscale semiconductor research, yet precise solutions for their eigenstates in complex geometries are often unknown. Eigenstates are extremely relevant as they impact the emission wavelength, performance, and stability of many optoelectronic devices. In this thesis, atomic force microscopy, transmission electron microscopy, and atom probe tomography (APT) are used to assess and compare QD size and core concentration. APT by means of isosurface reconstruction provides the most accurate ensemble averaged quantum dot size and core concentration. High-angle annular dark-field imaging quantifies core concentration very well, but fails in comparison to precisely quantify QD size. Ensemble averaging is discarded in favour of using the raw APT data to devise a model that can solve the Schrödinger equation in 3-dimensional space and can be expanded upon to include non-trivial quantum dot geometries of any kind. The electron and hole eigenstates for an entire quantum dot ensemble are solved using this model. Hybridized eigenstates between neighbouring quantum dots are realized and found to experience both bonding and anti-bonding of the charge carriers. The existence of a degenerate state is also discovered. The simulated eigenenergies are compared to the photoluminescence emission spectrum and found to accurately represent the exciton recombination energy. This makes it possible to obtain very realistic 3-D eigenstate representations for a variety of complex structures. The modelling technique outlined in this thesis is not constrained to just QDs, but can also be applied to an array of many other nanoscale structures. / Thesis / Master of Applied Science (MASc)

The increasing technological need to push service conditions of structural materials to higher temperatures has motivated the development of several alloy systems. Among them, superalloys are an excellent candidate for high temperature applications because of their ability to form coherent ordered precipitates, which enable the retention of high strength close to their melting temperature. The accelerated kinetics of solute diffusion, with or without an added component of mechanical stress, leads to coarsening of the precipitates, and results in microstructural degradation, limiting the durability of the materials. Hence, the coarsening of precipitates has been a classical research problem for these alloys in service. The prolonged hunt for an alternative of nickel base superalloys with superior traits has gained hope after the recent discovery of Co-Al-W based alloys, which readily form high temperature g precipitates, similar to Ni base superalloys. In the present study, coarsening behavior of g precipitates in Co-10Al-10W (at. %) has been carried out at 800°C and 900°C. This study has, for the first time, obtained critical coarsening parameters in cobalt-base alloys. Apart from this, it has incorporated atomic scale compositional information across the g/g interfaces into classical Cahn-Hilliard model for a better model of coarsening kinetics. The coarsening study of g precipitates in Ni-14Al-7 Cr (at. %) has shown the importance of temporal evolution of the compositional width of the g/g interfaces to the coarsening kinetics of g precipitates. This study has introduced a novel, reproducible characterization method of crystallographic study of ordered phase by coupling of orientation microscopy with atom probe tomography (APT). Along with the detailed analysis of field evaporation behaviors of Ni and Co superalloys in APT, the present study determines the site occupancy of various solutes within ordered g precipitates in both Ni and Co superalloys. This study has explained the role of structural and compositional gradients across the precipitates (g)/matrix (g) interfaces on the coarsening behavior of coherent precipitates in both Ni and Co-base superalloys. The observation of two interfacial widths, one corresponding to a structural order-disorder transition, and the other to the compositional transition across the interface, raises fundamental questions regarding the definition of the interfacial width in such systems. The comparative interface analysis in Co and Ni superalloy shows significant differences, which gives insights to the coarsening behaviors of g precipitates in these alloys. Hence, the principal goal of this work is to compare and contrast the Co and Ni superalloys and also, to accommodate atomic scale information related to transitions across interfaces to coarsening models for a better practical applicability of coarsening laws to various alloys.

Titanium nitride (TiN) films are widely applied as diffusion barrier layers in microelectronic devices. The continued miniaturization of such devices not only poses new challenges to material systems design, but also puts high demands on characterization techniques. To gain understanding of diffusion processes that can eventually lead to failure of the barrier layer and thus of the whole device, it is essential to develop routines to chemically and structurally investigate these layers down to the atomic scale. In the present study, model TiN diffusion barriers with a Cu overlayer acting as the diffusion source were grown by reactive magnetron sputtering on MgO(001) and thermally oxidized Si(001) substrates. Cross-sectional transmission electron microscopy (XTEM) of the pristine samples revealed epitaxial, single-crystalline growth of TiN on MgO(001), while the polycrystalline TiN grown on Si(001) exhibited a [001]-oriented columnar microstructure. Various annealing treatments were carried out to induce diffusion of Cu into the TiN layer. Subsequently, XTEM images were recorded with a high-angle annular dark field detector, which provides strong elemental contrast, to illuminate the correlation between the structure and the barrier efficiency of the single- and polycrystalline TiN layers. Particular regions of interest were investigated more closely by energy dispersive X-ray (EDX) mapping. These investigations are completed by atom probe tomography (APT) studies, which provide a three-dimensional insight into the elemental distribution at the near-interface region with atomic chemical resolution and high sensitivity. In case of the single-crystalline barrier, a uniform Cu-enriched diffusion layer of 12 nm could be detected at the interface after an annealing treatment at 1000 °C for 12 h. This excellent barrier performance can be attributed to the lack of fast diffusion paths such as grain boundaries. Moreover, density-functional theory calculations predict a stoichiometry-dependent atomic diffusion mechanism of Cu in bulk TiN, with Cu diffusing on the N-sublattice for the experimental N/Ti ratio. In comparison, the polycrystalline TiN layers exhibited grain boundaries reaching from the Cu-TiN interface to the substrate, thus providing direct diffusion paths for Cu. However, the microstructure of these columnar layers was still dense without open porosity or voids, so that the onset of grain boundary diffusion could only be found after annealing at 900 °C for 1 h. The present study shows how to combine two high resolution state-of-the-art methods, TEM and APT, to characterize model TiN diffusion barriers. It is shown how to correlate the microstructure with the performance of the barrier layer by two-dimensional EDX mapping and three-dimensional APT. Highly effective Cu-diffusion barrier function is thus demonstrated for single-crystal TiN(001) (up to 1000 °C) and dense polycrystalline TiN (900 °C).