Diffusion Process In Hollow Silver Oxide Nanotubes And Magnetization Reversal Process In Nickel Three-dimensional Anti-sphere Arrays

Nanomaterials and nanotechnology is still the most productive research field in modern times. Unlike the random attempts in early research, present studies in this field intentionally assemble new systems to acquire desired properties. The characterization methods also require innovation and expansion in order to meet the necessary of the development of nanomateirals. In this thesis, two different hollow nanomaterial systems are studied; in one system we focus on the diffusion kinetics, while in the other we focus on the magnetization reversal mechanisms. Some expansions to the characterization methods are also introduced.In this thesis, several tasks and progresses have been made:1. It is the first-time quantitative imaging of hollowing kinetics of silver nanowires is presented in the course of oxidation using in-situ transmission x-ray microscopy (TXM). The real-time TXM images reveal the dynamic details of morphological evolution as well as the mass diffusion kinetics involved in the Kirkendall process in silver nanowires.In the initial stage a diffusion couple is formed as the photon-induced oxidation layer is formed on the surface of the silver nanowire. Because the inward diffusion of the oxygen is slower than the outward diffusion of the Ag atoms from the core, vacancies form at the interface between the oxide layer and the Ag core via Kirkendall effect. As the diffusion carries on, these vacancies accumulate, reach supersaturation, and form Kirkendall voids at the interface. As diffusion and oxidation continue, the Kirkendall voids enlarge and coalesce, result in the separation of the inner core and the outer oxide shell, finally form a whole hollow cavity.2. By quantitatively analyzing time-dependent variation of nanowires morphology, the diffusion coefficient of silver in its oxide at nanometer scale is determined, for the first time, to be 1.2×10-13cm2/s.The calculation is based on the Fick's laws and the radial diffusion, provides quantitatively detailed information of the mass diffusion kinetics in the Kirkendall process, enables rational design in this system.3. Based on the extracted X-ray absorption rate, the two different diffusion processes, radial diffusion and longitudinal diffusion, can be distinguished.Besides the radial diffusion process, the longitudinal diffusion along the nanowire also exists, which can be distinguished from the extracted X-ray absorption information. The longitudinal diffusion process is not obvious during the formation of a perfect hollow nanotube; however, when the shell is broken and an incomplete hollow nanotube is formed, the longitudinal diffusion dominates. From the X-ray absorption rate extracted from the TXM images, one can distinguish the two kinds of diffusion process.4. The first-order reversal curve (FORC) and micromagnetic simulation study of Ni three-dimensional anti-sphere arrays (3DAAs) is presented in this thesis. The transition of magnetization reversal mechanism as sample thickness increases has been found.Probing the micromagnetic properties in a realistic 3D nanostructure system has been a long-standing challenge. Extensive work has been done on 2D magnetic antidot arrays, however, research to date on their 3D counterparts (i.e. multilayered 3D magnetic anti-sphere arrays) has been limited to conventional magnetometry. The first-order reversal curve (FORC) method is employed to obtain detailed information about magnetization reversal behavior. The FORC distribution is shown to transform from a left-bending boomerang-like feature to a ridge, oriented parallel to the local coercivity Hc axis, as the thickness of 3DAAs samples increases. This transformation signals a change in the reversal from a domain nucleation and growth reversal mechanism to a localized, weakly-interacting reversal.5. Micromagnetic simulations reveal the thickness dependent domainpropagation behaviors and confirm our interpretation of the FORC features.The bottom part and the upper part of the Ni 3DAAs show different reversal behaviors. The bottom part shows domain propagation, corresponding to the exchange dominated domain growth reversal mechanism; the upper part with voids shows localized reversal features. As the sample thickness increases the upper part become larger, the ratio of the localized reversal behaviors increases as well; meanwhile the pinning effect is also enhanced as the thickness increases, which impedes the interaction between, resulting in domain wall pinning reversal.On one side, it is demonstrated that the in-situ TXM is suitable for the quantitative imaging of hollowing kinetics. The quantitative analysis process, as well as the foundation for the rational design, is established via the calculation of the nanoscale silver diffusion coefficient in the silver oxide. The TXM reaction cell provides enough space to tune in-situ reaction conditions (e.g. pressure, flow, gas and solution, etc.), making in-situ TXM to be capable of studying a broad range of nanostructures in reactive environments. Meanwhile, the X-ray absorption extracted from the TXM images characterizes the mass diffusion process in the sample.On the other side, the FORC method acquires the distribution of local coercivity and bias field, as well as the magnetization reversal mechanisms from a series of local magnetic hysteresis loop measurements. It overcomes the limitation in conventional magnetometry, solves the long-standing issue in characterizing three-dimensional nanostructures. The micromagnetic simulation is employed to study the magnetic domain configuration, which takes a deeper look into the magnetization reversal mechanisms of the 3D A As.