http://dspace.cityu.edu.hk:80/handle/2031/777 2013-05-01T03:06:36Z 2013-05-01T03:06:36Z Leung, Ming Wai ( 梁明蕙) http://dspace.cityu.edu.hk:80/handle/2031/6537 2012-08-07T07:44:31Z 2011-01-01T00:00:00Z

Title: Inelastic electron tunneling spectroscopy of magnetic tunnel junctions Authors: Leung, Ming Wai ( 梁明蕙) Abstract: Magnetic tunnel junction (MTJ), which consists of an insulator of nano-size thickness sandwiched with two ferromagnetic electrodes, has become one of the most successful technologies applied to the read head in hard disk drive recording. MTJ can also be used as magnetic logic, and magnetoresistive random access memory (MRAM). Electrons, which tunnel through a MTJ, are determined by many parameters of a MTJ, such as the materials used in the stack, the crystallinity of the materials, the integrity of the layers and the interfaces between ferromagnetic electrodes and the insulating barrier. In other words, these parameters help us to understand the physics in electron tunneling mechanism in the MTJ which would help to generate some insights in the improvement in MTJ fabrication technique. In this project, a cryogenic (low temperature) electrical characterization technique known as inelastic electron tunneling spectroscopy (IETS) was set up to study the interface properties of the MTJ with 10Å magnesia (MgO) insulating barrier and 1.1-1.3Ωµm2 resistance-area (RA). After the successful set up of the IETS instrument, various tests were conducted to confirm its capability. Thereafter, the IETS was used to obtain and compare the IET spectra of two groups of industrial read head samples. From the IET spectra, the group with a complicated material content in the ferromagnetic layer was shown to contain slightly more impurities than the group with a simpler material content in ferromagnetic layer by observing the IETS peak intensities at around ±0-15mV. Besides, the crystallinity of MgO barrier of each read head was deduced from the IET spectra by observing the IETS peaks at around ±80mV. IETS spectra of the two groups of read head samples showed similar peak location and intensities when compared with results of past investigators. The results from this work indicated that although read heads from the same group were manufactured from the same wafer, different IET spectra were obtained. While the present instrument was able to distinguish their differences, it was also explained that the difference in the spectra in the form of different magnitudes of peak was due to the variation of back-end process. All the IETS expected peaks were present in our MgO barrier MTJ samples, which were the zero-bias anomaly (ZBA) at around ±0-15mV. It was also found that the magnon excitation at the ferromagnetic-barrier interface occurred at ±25 and ±50mV. The presence of phonon excitation was attributable to MgO barrier at around ±80mV. Notes: CityU Call Number: QC454.T75 L48 2011; xviii, 89 leaves : ill. (some col.) 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 71-80)

2011-01-01T00:00:00Z Yum, Hoi Wa (任凱華) http://dspace.cityu.edu.hk:80/handle/2031/6166 2011-05-25T01:16:08Z 2009-01-01T00:00:00Z

Title: Effects of alpha particles on zebrafish embryos in vivo Authors: Yum, Hoi Wa (任凱華) Abstract: It has been common to study DNA damage responses in vertebrates using cell cultures. However, such experiments cannot be used to study dynamic in vivo processes such as temporally and spatially regulated patterns of gene expression. In recent years, the zebrafish, Danio rerio, a small vertebrate from Southeast Asia, has become a preferred model for studying human disease, including carcinogenesis. The most important advantage is that the human and zebrafish genomes share considerable homology, including conservation of most DNA repair-related genes. Rapid embryonic development is another advantage in that major organ systems become evident within 48 hours postfertilization (hpf). Recently, a number of research works using the zebrafish embryo as an in vivo model to study the DNA damage response to ionizing radiation have emerged. Despite the success of using the zebrafish embryos to study the DNA damage response to ionizing radiation, only energetic photons (X-rays and gamma rays) were used. Studies using alpha particles will be of interest because alpha particles are also an ionizing radiation, and with high linear energy transfer (LET). Furthermore, alpha particles are emitted from radon and its progeny, which are ubiquitous in our natural environment, and constitute the largest natural radiation dose to human and can induce lung cancers. The objective of this study was to investigate the effect of alpha particles on zebrafish embryos. Hormetic effect and bystander effect were found in zebrafish embryos in vivo. Chapter 1 gives the introduction and the literature review. Chapter 2 describes the study of hormetic effect in zebrafish embryos in vivo. Dechorionated zebrafish embryos were irradiated at 1.5 hpf to low-dose alpha particles from an 241Am source, viz., 0.3, 0.6, 1.2, 2.4 mGy (determined using Monte Carlo simulations). At 24 hpf, these embryos were then examined for apoptotic cells through acridine orange staining. The mean number of apoptotic cells was found to decrease significantly from controls to 0.3-mGy irradiation, and then to increase almost linearly to 0.6, 1.2 and 2.4-mGy irradiation. This trend was a typical characteristic of a hormetic effect. Chapter 3 describes the study of bystander effect in zebrafish embryos in vivo. Dechorionated zebrafish embryos at 1.25 hpf were irradiated with alpha particles from an 241Am source. Thin polyallyldiglycol carbonate (PADC) films with a thickness of 16 m were used as support substrates for holding the embryos and recorded alpha-particle hit positions, and thus enabled calculation of the dose absorbed by the embryos. The irradiated embryos were subsequently incubated with naïve (unirradiated) embryos in such a way that the irradiated and naïve embryos were spatially separated but the medium was shared. Acridine orange was used to perform in vital staining to show cell deaths in the naive embryos at 24 hpf. The results gave evidence in supporting the existence of alpha-particle-induced bystander effects between zebrafish embryos in vivo, and a general positive correlation between the cell death signals in the naive embryos and the alpha-particle dose absorbed by the irradiated embryos. Chapter 4 gives the conclusions and discussion for further studies. Notes: CityU Call Number: QP82.2.I53 Y85 2009; v, 80 leaves : ill. (some col.) 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2009.; Includes bibliographical references (leaves 74-80)

2009-01-01T00:00:00Z Choi, Wing-yan (蔡詠恩) http://dspace.cityu.edu.hk:80/handle/2031/6156 2011-05-25T01:15:45Z 2010-01-01T00:00:00Z

Title: Induction of radioadaptive response by high LET radiation in zebrafish embryo in vivo Authors: Choi, Wing-yan (蔡詠恩) Abstract: Radioadaptive response (RAR) refers to a biological response whereby an exposure of cells or animals to a low dose of radiation induces mechanisms that protect the cells or animals against the detrimental effects of a subsequent radiation exposure. The discovery of RAR has raised queries to the traditional methodology in estimating the radiation risk in the low-dose region, which is usually achieved through extrapolation from those determined at high doses. Since then, this important defense mechanism has stimulated a series of in vitro studies. However, relatively little has been studied using in vivo models. The present study focused on the study of RAR induced by high-linear-energy-transfer (LET) radiations which provided the priming adaptive dose in zebrafish embryos in vivo. The objectives of this study were to first design the experimental setup for investigating RAR in vivo, then to investigate the RAR induced through bystander effects, and finally to study the potential effect on RAR from the effectiveness in generating DNA double strand breaks (DSBs) during the priming exposure. Two types of high-LET radiations with different effectiveness in causing DNA DSBs were used, namely, (broad-beam) alpha particles from a radioactive source and (microbeam) protons from an accelerator. The studies on the biological effects of both alpha particles and protons have real-life applications. Alpha particles are ubiquitous in our natural environment, e.g., they are constantly emitted by the naturally occurring radon gas and its progeny, which contribute to the largest natural radiation dose to human beings. On the other hand, protons are dominant in solar-radiation spectra, so a study on the biological effects from exposure to protons is important for human interplanetary missions. Chapter 1 gives the introduction and literature review. Chapter 2 describes the design of the experimental setup and the associated procedures for alpha-particle-induced RAR. Dechorionated zebrafish embryos at 5 hours post fertilization (hpf) were irradiated by alpha particles using a planar 241Am source first to provide a priming exposure and subsequently to provide a challenging exposure, and the effect of the time interval between the priming and challenging exposures, chosen as 1, 2, 4 and 5 hours, was examined. Quantification of apoptotic signals was achieved using acridine orange staining. When compared with the control cases, the amount of apoptotic signals decreased significantly in the adapted embryos for the interval time of 5 h between the priming and challenging exposures while no significant decreases in the amount of apoptotic signals for the time intervals of 1, 2 and 4 h. The results gave evidence to support the existence of RAR induced by alpha particles with a 5 h interval between the priming and challenging exposures. The importance of DNA repair in the induction of RAR was also demonstrated by the absence of RAR if the priming dose was introduced to the embryos when their repair system had not yet started operative. Chapter 3 describes the study on RAR induced by the embryo-to-embryo bystander effect. In contrast to the work described in chapter 2, the priming exposure here for the studied embryos (adapted group) was established through sharing the medium with another group of embryos that had been irradiated by alpha particles. Other experimental conditions and the experimental endpoint described in the previous chapter were employed. Again, the zebrafish embryos were irradiated by alpha particles to provide a challenging exposure 5 hours after the priming exposure. In four out of six experiments, the number of apoptotic signals was significantly reduced in the adapted group. Chapter 4 explains the procedures of and summarizes the results from the experiments on the use of microbeam protons with an energy of 3.4 MeV to provide the priming exposure to the 5 hpf embryos. The microbeam facility enjoyed advantages of being able to deliver a selected number of protons to chosen positions. To exploit these advantages, 5, 10 or 20 protons were delivered to 10 chosen positions on each embryo. The subsequent challenging exposure was delivered through whole embryo irradiation with 2 Gy of X-ray photons, with a 5 h interval between the priming and challenging exposures. Terminal dUTP transferase-mediated nick end-labeling (TUNEL) assay was used as the end point to reveal the apoptotic events. The results showed a significant decrease in the number of apoptotic signals in the adapted group with the magnitude of adaptation ranging from 14 % to 21 % for the delivery of 5 and 20 protons to each embryo, respectively, as the priming exposure. RAR could also be induced through the delivery of 10 protons to each embryo as the priming exposure, however, only one statistically significant result was obtained out of 3 sets of experiments. Chapter 5 gives the conclusion and presents discussion and suggestions for further studies. Notes: CityU Call Number: QP82.2.R3 C45 2010; 5, 104 leaves : ill. (some col.) 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2010.; Includes bibliographical references.

2010-01-01T00:00:00Z Cheung, Wing Sze (張穎思) http://dspace.cityu.edu.hk:80/handle/2031/6155 2011-05-25T01:15:43Z 2010-01-01T00:00:00Z

Title: Studies of ZnO nanostructure-based dye sensitized solar cells (DSSCs) Authors: Cheung, Wing Sze (張穎思) Abstract: The energy crisis and soaring oil price have triggered the development of photovoltaic technology. Silicon (Si) based solar cell devices have dominated the market for the past decades. However, the high demanding and processing cost of crystalline Si have limited their applications and at the same time encouraged the development of simple and low cost alternatives. Dye-sensitized solar cells (DSSCs) utilizing nanostructured materials have been considered as one of the promising candidates for photovoltaic devices. In this dissertation, a low temperature hydrothermal growth method to synthesize one dimensional ZnO nanorods arrays (ZNAs) on indium doped tin oxide (ITO) substrates from a zinc salt for DSSCs application is demonstrated. By adjusting the reaction times, ZNAs with different morphologies resulted. The length of nanorods (NRs) increased directly proportional to the total reaction times along the [0001] direction of its hexagonal wurtizite (WZ) structure, whereas the diameters of NRs increased at a slower rate. However, the thickening effect of NRs reduced the surface area for dye loading in DSSCs application and worsened the performance of the cell. The morphologies of hydrothermal growth NRs were not only affected by the conditions of hydrothermal growth but also the ZnO seed layer which acts as homogeneous nucleation sites for the growth of NRs. Through the tuning of the Ar:O2 ratio in the seed layer deposition process using a pure Zn target by radio frequency (r.f.) magnetron sputtering, ZnO seed layers with different properties resulted and subsequently influenced the NRs based films. The growth rate of NRs was found to be increased with the oxygen flow rate during the seed layer deposition process. The orientation of the NRs along the [0001] direction perpendicular to the ITO substrate was improved at the same time. In a relatively short period of the hydrothermal growth reaction, hybrid NRs and nanosheets (NSs) complexes could result. To further optimize the morphologies for DSSCs application, a highly branched polymer additive, poly(ethylenimine) (PEI), was added to the hydrothermal growth solution bath. The growth of NRs along the c-axis perpendicular to the substrate was promoted and the existence of NSs structure in the film remained for a longer reaction time. The above ZnO nanostructures based films grown on the ITO substrates served as the photoelectrode for fabricating DSSCs. They were dipped into a commercial ruthenium dye, known as N719, for the dye adsorption process. After the application of an iodinebased electrolyte, with a platinum catalyst and fluorine doped tin oxide (FTO) as the counter-electrode, the sandwiched DSSCs were sealed and tested using simulated sunlight of AM 1.5. The current-voltage responses of the cells were recorded. The overall conversion efficiencies ranged from 0.3% to more than 1.2% and were found to be highly dependent on the morphologies and nanocrystalline structures of the ZnO based photoelectrodes. These findings helped in optimizating the performance of ZnO nanostructures based DSSCs fabricated by a low cost and simple route. Notes: CityU Call Number: TK2960 .C45 2010; xiv, 100 leaves : ill. (some col.) 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2010.; Includes bibliographical references (leaves 86-99)

2010-01-01T00:00:00Z