The characterization of nanomaterials involved in dynamic processes are conventionally conducted using microscopy, spectroscopy and other physical/chemical methods through the pseudo-dynamic approach. In details, the dynamics processes are recorded by repeating or terminating the process multiple times. However, the above approach can lead to missing important transition information and inducing contamination for mechanistic studies. This motivates the efforts to develop real time characterization techniques which can probe the dynamic change of nanoparticles in their native operating environments. With the capability of probing structural change at the nanoscale, in situ transmission electron microscopy, has shown great potential in studies and applications of various processes. Targeting at conducting precise analysis, which has been limited by many uncertainties from electron beam effects and the miniaturized reaction cell used for TEM, the work presented herein pursues a quantitative characterization of a few electrochemical and biological processes through in situ liquid-phase transmission electron microscopy. In this work, the in situ transmission electron microscopy system is evaluated by comparing the in situ results with those from standard experiments to show its capabilities in studying dynamic processes. The in situ system is quantitatively calibrated to obtain the optimized observation conditions to avoid detectable electron beam interference, solution depletion and achieve sufficient resolution for analysis through micrometer thick liquid. These form the fundamentals for the in situ studies. Moreover, a comprehensive analysis protocol is established by incorporating multiple ex situ and post situ characterizations. Using this optimized in situ system, the mechanism of electrodeposition of gold on carbon electrode is studied. The in situ results allow quantitative analysis of the growth process. The prevailing diffusion limited three dimensional growth model is examined. A study of the effect of supporting electrolyte on the electrodeposition of palladium is also conducted. The self-limiting, surface diffusion and aggregation/recrystallization growth model is found to describe the early stage of growth, rather than the classical Volmer-Weber growth model. A further study is conducted on the structural evolution of palladium nanoparticles under electrochemical cycling. The mechanisms involved in this process, including electrodeposition, dissolution, hydrogen co-deposition and hydrogen desorption, are studied. The supporting electrolyte, HCl, is found to enhance the dissolution of deposited palladium clusters and induce movements and aggregation of the deposits during the hydrogen interaction process to form chain-like and irregular clusters, which provide direct experimental proof on the morphology formation of palladium with hydrogen involvement. Ultimately, the in situ technique is applied to the study of calcium phosphate biomineralization. Combined with multiple post situ characterization techniques, the study provides direct experimental evidence of the non-classical pre-nucleation and attachment growth of calcium phosphate structures. This demonstrates the potential of the in situ technique for studying the mechanisms involved in biological processes. / Dissertation / Doctor of Philosophy (PhD) / Nanostructured materials have been widely used in various fields. In situ transmission electron microscopy, a technique used to characterize nanomaterials involved in different dynamic processes in their operating environments, is an advanced tool over the traditional characterization methods such as ex situ microscopy and spectroscopy. However, there are several challenges in applying this in situ technique to processes occurring in liquid media. In this thesis, an in situ transmission electron microscopy system is applied to study the mechanisms of structural changes during different processes in liquids with both high spatial and temporal resolution. Protocols to evaluate and optimize the in situ system are developed to provide results comparable with those from their actual applications. Then in situ studies on the structural evolution of nanomaterials during electrochemical processes are performed and different theoretical models are applied to describe these processes. Finally, this technique is extended to investigate biomineralization to show its capabilities in future studies on biological processes.

Nanoparticles possess unique physical, chemical, optical and electronic properties stemming from their nanoscale dimensions and are currently used in catalysis, microelectronics, drug delivery, as well as other applications. However, due to their large surface area-to-volume ratio, nanoparticles have a strong tendency to coalesce and sinter during processing or usage over short time scales and at low temperatures, which lead to significant changes in behavior and performance. In this work, in-situ transmission electron microscopy (TEM) heating has been used to investigate the effects of particle size, temperature and carbon capping layers on sintering in face-centered cubic (FCC) metallic nanoparticles. For the first time, we make direct and real-time measurements of nanoparticle size, neck growth, dihedral angle and grain boundary motion during sintering, which are then used to calculate fundamental material transport parameters such as surface diffusivity and grain boundary mobility. We observe that carbon surface coatings typically present on most commercial nanoparticles can significantly inhibit sintering in nanoparticles. Also, a new mechanism for coalescence in nanoparticles is shown where small clusters on the support can initiate neck growth by forming a bridge between the nanoparticles consisting of individual atoms or small clusters of atoms. In-situ TEM experiments provide critical and valuable real-time dynamic information for direct investigation of the link between the evolution of sintering and controlling mechanisms, which conventional experiments such as post-mortem TEM observations are not capable of conveying. / text

Nanocrystalline metals exhibit a high strength/hardness but generally poor ductility during deformation regardless of their crystal structure which is often called the strength-ductility trade-off relationship and generally appears in most ultrafine-grained metals. The ultrafine-grained (UFG) high manganese austenitic twinning-induced plasticity (TWIP) steels have been found to overcome the strength-ductility trade-off but their underlying mechanism of discontinuous yielding behavior has not been well understood. In this study, our systematic TEM characterization suggests that the plastic deformation mechanisms in the early stage of deformation, around the macroscopic yield point, show an obvious association with grain size and nucleation of deformation twin was promoted rather than suppressed in UFG. More specifically, the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 m. We also provide insights into the atomistic process of deformation twin nucleation at 3{111} twin boundaries, the dominant type of grain boundary in the UFG-TWIP steel of interest. In response to the external tensile stresses, the structure of coherent 3{111} twin boundary changes from atomistically smooth to partly defective by the grain boundary migration mechanism thus the "kink-like" defective step can act as a nucleation site for deformation twin, which deformation process is different from the one induced by dislocation pile-ups in coarse-grained counterparts and explain why UFG TWIP steel can retain the moderate ductility. In addition to the effect of grain size on deformation twin nucleation, grain boundary character was also taken into account. In coarse-grained TWIP steel, we experimentally reveal that deformation twin nucleation occurs at an annealing twin () boundary in a high-Mn austenitic steel when dislocation pile-up at boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as or  A periodic contrast reversal associated with a sequential stacking faults emission from boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism. The correlation between grain boundary character and deformation behavior was discussed both in low- and high-sigma value grain boundaries. On the other hand, localized strain concentration causes the nucleation of deformation twins at grain boundaries regardless of the grain boundary misorientation character in UFG TWIP steel. The invisibility of stacking fault (zero contrast) was also observed to be emitted at 3{111} boundaries in the coarse-grained TWIP steel, which deformation twin nucleation mechanism is found to be identical to UFG Fe-31Mn-3Si-3Al TWIP steel. / Doctor of Philosophy / High manganese (Mn) twin-induced plasticity (TWIP) steel is a new type of steels which exhibit pronounced strain hardening rate so that offering an extraordinary potential to adjust the strength-ductility relationship. This key advantage will help implement the current development of lightweighting components in automobile industry due to a considerable reduction of material use and an improved press formability. Such outstanding ductility can be contributed by the pronounced strain hardening rate during every such deformation processes, which is highly associated with several different controlling parameters, i.e., SFE, grain orientation, grain size, and grain boundary characters. In this study, we take particular attention to the effect of grain size and grain boundary characters on deformation twinning behavior besides well-known parameters such as SFE and grain orientation. The effect of grain size on deformation twinning behavior was found to be deeply associated with the yielding behavior in TWIP steel, i.e., a discontinuous yielding behavior with a unique yield drop was observed in ultrafine-grained TWIP while a continuous yielding behavior was observed in coarse-grained counterpart. Our TEM characterization indicates that the microstructural features of grains >10 m are different from the microstructural features in grains < 1 m. In over-10 m grains, normal dislocation slips and the formation of in-grain stacking faults are the main deformed microstructure. However, in the under-1 m grains, the in-grain dislocation slip is inhibited, but the deformation twinning is promoted at grain boundaries. This deformation transition from in-grain slip to twinning at grain boundary appears to be responsible for the discontinuous yielding behavior observed in stress-strain curve. The effect of grain boundary character on deformation twinning was examined in both coarse- and ultrafine-grained TWIP steels. In coarse-grained TWIP steel, we found that deformation twinning behavior varies as the function of boundary structure, i.e., different atomic configuration. Coherent twin boundary can act as a nucleation site for deformation twin as a localized strain concentration was introduced by dislocation pile-ups. On the other hand, incoherent boundaries can act as a deformation twin nucleation site by a boundary relaxation mechanism, i.e., grain-boundary dislocations can dissociate into partial dislocations to both side of boundary to accommodate the misfit between grains. In UFG TWIP steel, we found that the coherent twin boundary can act as a deformation twin nucleation site without presence of dislocation pile-ups. Alternatively, twin boundary becomes defective with a "kink-like" step by boundary migration. As a result, this defective step would progressively accumulate localized strain field thus stimulate the nucleation of deformation twin. Such study provides a novel insight into the UFG TWIP steel and a roadmap toward controlling TWIP effect.

Cette thèse porte sur la synthèse et la caractérisation de nouvelles phases bidimensionnelles par microscopie électronique in-situ, notamment la silice (SiO2), des cages nanométriques de carbone similaire à des molécules C20 et le chalcocite (β-Cu2S). Ces études ont permis de mettre en évidence les conditions préalables de croissance afin que celle-ci soit reproductible. La caractérisation de ces structures a été réalisée par imagerie haute résolution (HRTEM) ainsi que par spectroscopie de perte d’énergie (EELS). La première partie de la thèse est consacrée à l’étude de la nucléation et la croissance in-situ d’une phase cristalline 2D ordonnée et désordonnée sur différents métaux de transition (Co, Ru, Fe) ainsi qu’une phase 1D qui croît le long des marches atomiques sur la surface métallique. La seconde partie est consacrée à la croissance in-situ de cages de carbone d’un diamètre de l’ordre de 0.36 nm en présence d’un catalyseur métallique, tel que Co, Fe et Ru. La dernière partie est consacrée à l’étude de la croissance de la plus fine structure cristalline du β-Cu2S sur la surface du graphène. Toutes ces études ont été appuyées par des simulations d’images de microscopie. / The aim of this thesis is the synthesis and characterization of new two-dimensional phases in an in-situ transmission electron microscopy experiment. These studies concerned the nucleation and growth of three deferent materials: quasi-two-dimensional silica (SiO2), the smallest possible carbon cages with the size of C20, and two-dimensional chalcocite (β-Cu2S). The characterization of these structures has been performed using high resolution imaging (HRTEM) and electron energy-loss spectroscopy (EELS). The first part of this thesis is devoted to the study of the nucleation and growth of an ordered or disordered 2D crystalline phase of silica on different substrates (Co, Ru, Fe) and a 1D silica phase grown at atomic steps of a metal surface. The second part illustrates the in-situ growth of the smallest possible carbon cages with a diameter of about 0.36 nm on catalytically active metal surfaces such as Co, Fe, or Ru. The last part is devoted to the growth of the thinnest stable layer of β-Cu2S on a graphene surface. All these studies were accompanied by image simulations.