by Chen Yunjie. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references (p. 141). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web.

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.

Rosetta is a European spacecraft that will rendevouz with andfollow the comet 67P/Churyumov-Gerasimenko from 2014 to 2015. Among other instruments are two Langmuir probes that measure certain plasma parameters. This study aims to describe the photoemssionon Rosetta and in particular on the Langmuir probes. More than 7800 bias voltage sweeps were analyzed and parameters such as photosaturationcurrent, spacecraft potential, photoelectron temperature and offset current were calculated from these sweeps. It is found that the photoemission is stronger on Probe 1 than Probe 2. The photosaturation currents for dierent mission phases align rather well when normalized to distance to the Sun and the solar activity, when correcting for the offset currents found on the probes the alignment became even better. The typical electron density in the photoelectron cloud is 25 cm^-3 at 1 AU and the electron temperature varies from 1 - 2 eV.

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Saturn’s largest moon Titan (2575 km radius) is the second largest in the Solar system. Titan is the only known moon with a fully developed nitrogen-rich atmosphere with ionosphere extending to ~2000 km altitude, hosting complex organic chemistry. One of the main scientific interests of Titan’s atmosphere and ionosphere is the striking similarity to current theories of those of Earth ~3.5 billion years ago. The Cassini spacecraft has been in orbit around Saturn since 2004 and carries a wide range of instruments for investigating Titan’s ionosphere, among them the Langmuir probe, a “space weather station”, manufactured and operated by the Swedish Institute of Space Physics, Uppsala. This thesis reviews the first half of the PhD project on the production of pre-biotic molecules in the atmosphere of Titan and early Earth, focusing on the ion densities and dynamics in Titan’s ionosphere derived from the in-situ measurements by the Cassini Langmuir probe. One of the main results is the detection of significant, up to ~2300 cm-3, charge densities of heavy (up to ~13000 amu) negative ions in Titan’s ionosphere below 1400 km altitude. On the nightside of the ionosphere at altitudes below 1200 km, the heavy negative ion charge densities are comparable to the positive ion densities and are in fact the main negative charge carrier, making this region of the ionosphere exhibit properties of dusty plasma. The overall trend is the exponential increasing of the negative ion charge densities towards lower altitudes. Another important result is the detection of ion drifts that between 880-1100 km altitudes in Titan’s ionosphere translate to neutral winds of 0.5-5.5 km/s. Ion drifts define three regions by altitude, the top layer (above ~1600 km altitude) where the ions are frozen into the background magnetic field, the dynamo region (1100 – 1600 km altitudes) where the ions are drifting in partly opposing directions due to ion-neutral collisions in the presence of the magnetic and electric fields and the bottom layer (below 1100 km altitude) of the ionosphere, where the ions are coupled to neutrals by collisions.

The Kelvin Probe Force Microscope is a type of scanning probe instrument that is used to discern the different work functions of a sample. A sharp probe at the end of a cantilever is lowered onto a substrate where electrostatic forces, caused by the difference in work function cause the cantilever to oscillate at the modulated frequency. Using this instrument, high resolution images can be obtained, mapping the surface electronic characteristics. However, developments of this instrument have generally been limited to obtaining higher resolution images as well as reducing noise in the output, limiting the widespread appeal of this expensive instrument. There exist many applications where extremely cheap, low footprint and easy-to-use Kelvin Probe Force Microscopes would be beneficial. In order to cheaply produce this microscope in batch, a post-processed CMOS-MEMS device is utilized. The polysilicon resistors act as a strain gauge such that a conventional optical system will not have to be employed. The ability to use integrated bimorph actuators on chip allow for movement of the cantilever without the employment of large piezoelectric stages with creep effects. Embedded electronics can be fabricated with the CMOS process alongside the MEMS device, allowing full integration of an on board amplifier and read out system. In general, a large table top system can be minimized onto the size of a <1 mm2 area, a microcontroller and a computer. In this work, a Kelvin Probe Force Microscope is designed, fabricated and validated. A MEMS device was designed following similar characteristics of a generic cantilever beam. The stiffness, length, resonant frequency, and other tip characteristics can be mimicked with careful design. The resultant designs were fabricated using a CMOS-MEMS process. In order to obtain a sharper tip with modified characteristics, various methods were employed; such as gallium-aluminum alloy tip formation as well as electroless plating onto the tip of the device. Finally, the resultant device is tested against a sample. It was seen that the MEMS device followed similar characteristics of the conventional microscope itself, validating the equations that define the method. Bimorph actuators were tested to show movement, allowing the integration of the cantilever with the XYZ-stage. Work function changes are observed while scanning different materials. It is shown throughout the course of this thesis, that a true Kelvin Probe Force Microscope can be designed, fabricated and validated using CMOS-MEMS technology.

This thesis describes the development of a new type of Magnetic Force Microscope (MFM) probe based on a unique electromagnetic design. In addition the design, construction and testing of a new MFM system, complete in both hardware and software, is also described. The MFM allowed initial tests on prototypes of the new probe, and is to provide a base for future new probe integration. The microscope uses standard MFM micro-cantilever probes in static modes of imaging. A new computer hosted DSP control system, software, and its various interfaces with the MFM have been integrated into the system. The system has been tested using standard probes with various specimens and satisfactory results have been produced. A novel probe has been designed to replace the standard MFM magnetic coated tip with a field generated about a sub-micron aperture in a conducting film. The field from the new probe is modelled and its imaging capability investigated, with iterative designs analysed in this way. The practical construction and potential problems therein, of the probe are also considered. Test apertures have been manufactured, and an image of the field produced when operating is provided as support to the theoretical designs. Future methods of using the new probe are also discussed, including the examination of the probe as a magnetic write mechanism. This probe, integrated into the MFM, can provide a new method of microscopic magnetic imaging, and in addition opens a new potential method of magnetic storage that will require further research.

Thesis (PhD) -- University of South Australia, 1998

Thesis (doctoral)--Karolinska Universitet, Stockholm, 1987. / Added t.p. with thesis statement inserted. Includes bibliographical references.