Electron microscopes offer scientists an invaluable tool in probing matter at a very small scale. Rapid advancements over the past several decades has allowed electron microscopes to routinely image samples at the atomic scale. These advancements have been in all aspects of electron microscope design – such as more stable control voltages and currents, brighter and more coherent sources, beam aberration correction, and direct electron detectors, to name a few. One very recent advancement is in shaping the electron beam to provide an almost arbitrary set of possible beam profiles. Following the demonstration of electron vortex beams in 2010, there has been a surge of interest in the potential shaping electron beams. Utilizing holographic electron diffraction gratings, an almost arbitrary set of electron beams can be generated. These diffraction gratings are challenging to create due their tiny size and the precision with which they must be fabricated. We present a comprehensive study on the fabrication and design of electron diffraction gratings with the aim of being able to produce optimal gratings that result in bright, well separated beams which closely match a desired beam profile. We have developed and optimized fabrication of these gratings with focused ion beam milling, and have been able to use the fabricated gratings in a number of important experiments. These electron diffraction gratings have allowed us to perform various experiments such as aberration correction, electron helical dichroism, advanced phase-contrast imaging, and multi-beam interferometric techniques. Holographic beam shaping will continue to be an important tool for electron microscopists.

Liang, Hui = 纳米线器件的制备和表征 / 梁慧. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 45-48). / Abstracts in English and Chinese. / Liang, Hui = Na mi xian qi jian de zhi bei he biao zheng / Liang Hui. / Chapter Chapter 1 --- Nanowire-based devices --- p.1 / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.1.1 --- Properties of various nanowires --- p.2 / Chapter 1.1.2 --- Nanowire growth methods --- p.3 / Chapter 1.1.3 --- Introduction to EBL --- p.4 / Chapter 1.1.4 --- Properties of nanowire and the arrays and related devices --- p.6 / Chapter Chapter 2 --- Experimental --- p.9 / Chapter 2.1 --- Nanowire preparation --- p.9 / Chapter 2.1.1 --- ZnS nanowire growth --- p.9 / Chapter 2.1.2 --- Sb2S3 nanowire growth --- p.10 / Chapter 2.2 --- Device fabrication --- p.10 / Chapter 2.2.1 --- Single-nanowire device --- p.10 / Chapter 2.2.2 --- Multiple-nanowire device --- p.17 / Chapter 2.2.3 --- Silicon device --- p.17 / Chapter 2.3 --- Characterizations --- p.18 / Chapter 2.3.1 --- Morphological and structural characterizations of the nanowires --- p.18 / Chapter 2.3.2 --- Two-probe measurements --- p.18 / Chapter 2.3.3 --- Four-probe measurements --- p.19 / Chapter Chapter 3 --- Results and Discussion --- p.21 / Chapter 3.1 --- Optimal factors for sample preparation --- p.21 / Chapter 3.1.1 --- Trial of spin coating --- p.21 / Chapter 3.1.2 --- Trial of Coating thickness --- p.21 / Chapter 3.1.3 --- Trial of e-beam lithography --- p.22 / Chapter 3.1.4 --- Trial of dosage --- p.23 / Chapter 3.1.5 --- Trial of development time --- p.26 / Chapter 3.2 --- Electrical Properties of devices made --- p.28 / Chapter 3.2.1 --- UV-visible response of single ZnS nanowire devices --- p.28 / Chapter 3.2.2 --- The optoelectronic characteristics of single Sb2S3 nanowire devices --- p.32 / Chapter 3.2.3 --- The optoelectronic characteristics of multiple-nanowire devices --- p.35 / Chapter 3.2.4 --- Temperature dependent resistance and magnetoresistance of the silicon device --- p.41 / Chapter Chapter 4 --- Conclusions --- p.44 / Chapter Chapter 5 --- References --- p.45

An energy analyzer for electron beams, based on a retarding-potential method, has been studied both experimentally and theoretically. In this method a potential energy barrier at right-angles to the beam acts as a high-pass energy filter allowing electrons having energies greater than the barrier height to pass through while turning back electrons of lower energy. The potential barrier in the present study was created by applying a negative potential to an electrode consisting of a 600/inch hexagonal copper mesh or an electrode having a pair of single apertures in series (compound aperture). Two different compound apertures, of diameters 0.4 mm and 1.0 mm, were used. The retarding electrode was mounted between two grounded outer electrodes having openings centered on the axis for entrance and exit of electrons. The barrier height was adjusted by means of a small variable bias voltage between the electron gun cathode and the retarding electrode. Auxiliary electron lenses external to the retarding module were used to satisfy the conditions that the beam be normal to the retarding electrode and, for an imaging filter, that an image plane be focused at the retarding electrode. A beam having a narrow distribution of energies was used to calibrate the energy analyzer as a function of bias voltage for the three different configurations of retarding electrode. The calibration curves were then compared with the transmission curve for a beam having a broadened energy distribution. The feasibility of obtaining a filtered image was explored by observing the image of a fine mesh focused into an aperture of the retarding electrode. The experiments were carried out for a beam voltage of 15 kV. At this beam voltage the energy resolution is poor, being 3. 6 volts for the smaller compound aperture, 3. 7 volts for the mesh, and 5.2 volts for the larger compound aperture. Typically, electrostatic analyzers operate on a beam which has been decelerated to low energies before entering the analyzer, in which case the energy resolution is proportionately better. For example, at 150 V the above resolution figures would be 0.036, 0.037, and 0.052, respectively. In view of the potential for high energy-resolution and the attractive features of in-line, rotationally-symmetric systems, it is felt that the retarding-potential analyzer is a promising candidate for further development.

Using electron beam evaporation for the production of polycrystalline silicon (pc-Si) thin-film solar cells is an attractive alternative to PECVD deposition. Due to its faster deposition rate, using evaporation technology could significantly reduce module production costs. Other advantages are lower running costs, and the fact that no toxic gases are involved. However, currently no on-shelf equipment is available, and research in this field often relies on in-house designed systems. These can have various problems with reliability, deposition uniformity, and due to their custom design require frequent maintenance. In this work, a newly purchased electron beam evaporation system was upgraded and redesigned to be capable of depositing amorphous Si diodes for the fabrication of pc-Si thin-film solar cells. The main goal of the upgrade was to provide a safe and reliable tool which allows for the deposition of high purity semiconductor material. Reliable and safe operation was accomplished by designing the entire electrical supply circuit and incorporating various safety interlocks. Source cross-contamination issues were addressed by installing a specially designed shroud (source housing). To provide uniform substrate temperatures up to 600??C, a heater was specially designed, fabricated, installed and tested. Accurate design of all mechanical system components was realised by using 3D product design software (ProEngineer). The new evaporator was commissioned, which included testing and calibration of all the system components required for depositing on substrate sizes of up to 10x10cm2. Over this area a Si film thickness uniformity of +/-2%, performed with a maximum deposition rate of 7nm/s was achieved. Initial experiments using solid phase crystallisation and rapid thermal annealing revealed a sheet resistance uniformity of +/-4% for the Phosphorus and +/-7% for the Boron dopant effusion cell. Experimentation via Raman spectrometry and X-ray diffraction has revealed good crystalline properties, of the crystallised Si films, which is comparable to those of existing evaporation systems. Although the system was upgraded to achieve deposition pressures below 3x10-7 mbar, experiments have shown that this quality of vacuum may not be necessary for the fabrication of low impurity films. The system is now ready for further research in the field of thin-film photovoltaics, and the first functioning devices have been fabricated.

The short range of low energy electrons from 2 to 9 MeV has made them useful for the treatment of superficial lesions covering large areas of the body, such as mycosis fungoides and other cutaneous lymphomas. At these electron energies, the beam penetration falls off rapidly beyond a shallow depth. Thus superficial lesions can be treated up to few millimeters without exceeding the tolerance of the bone marrow. The purpose of this project was to study the effect of the beam modifiers on the characteristics of the Varian 2100C 6 MeV beam using high dose rate total skin electron mode (HDTSe). The technique developed in the study was a modified Stanford Technique. In this technique, the patient is treated with dual six fields using + 17.5o angle above and below the horizontal line at 350 cm SSD. The patient is rotated every 60o intervals so that the whole skin surface is covered with the beam. The scattering filter used in the study was two strips of non-exposed developed radiographic films. The filter was mounted on the HDTSe applicator. The dose uniformity within a rectangle of 160 cm x 60 cm was found to be + 3% along the vertical direction and + 4% along the horizontal direction which meets the beam requirements recommended by the AAPM report (23) [61]. The use of the scattering filter has improved the dose uniformity, but it increased the x-ray contamination beyond the 30 mm depth to about 1.5% which makes the technique

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--Brigham Young University. Dept. of Chemistry and Biochemistry, 2006. / Includes bibliographical references (p. 191-200).

No description available.

Immersion energies for an impurity in a homogeneous electron gas with a uniform positive background charge density have been calculated numerically using density functional theory. The numerical aspects of this problem are very demanding and have not been properly discussed in previous work. The numerical problems are related to approximations of infinity and continuity, and they have been corrected using physics based on the Friedel sum rule and Friedel oscillations. The numerical precision is tested extensively. Immersion energies are obtained for non-spin-polarized systems, and are compared with published data. Numerical results, such as phase shifts, density of states, dielectric constants, and compressibilities, are obtained and compared with analytical theories. Immersion energies for excited systems are obtained by varying the number of electrons in the bound states of an impurity. The model is extended to spin-polarized systems and is tested in detail for a carbon impurity. The spin-coupling with an external magnetic field is considered mainly for a hydrogen impurity. These new results show very interesting behavior at low densities. / Graduation date: 2005

Much experimental research has been performed in the equilibrium regime on individual quantum dots and quantum point contacts (QPCs). The focus of the research presented here is electron transport in the nonequilibrium regime in coupled quantum dot and QPC systems fabricated on AlGaAs/GaAs material using the split gate technique. Near equilibrium magnetoconductance measurements were performed on a quantum dot and a QPC. Oscillations were seen in the conductance of the sensor which corresponded to Aharonov-Bohm oscillations in the quantum dot, to our knowledge the first such observation. Sudden jumps in the conductance of the QPC were observed under certain gate biases and under certain magnetic fields. When the gate biases and magnetic field were held constant and the conductance was observed over time, switching was observed with the form of a random telegraph signal (RTS). RTS switching is usually attributed to charging of a single impurity. However, in this case switching may have been due to tunneling via edge states in the dot. Nonequilibrium transport in single quantum dots was investigated. A knee or kink was observed in the current-voltage characteristics of two dots on different material. The bias conditions under which the knee occurred point to electron heating as the physical mechanism for the observed behavior. However, the data can not be fit accurately over all bias ranges with an energy balance hot electron model. Modifications to the model are needed to accurately represent the devices studied here. Finally, the effect of nonlinear transport through a one dimensional (1D) QPC on the equilibrium conductance of an adjacent OD quantum dot was explored. This was the first attempt to observe Coulomb drag between a OD and 1D system. It was observed that the equilibrium conductance peaks in the quantum dot were broadened as the current in the QPC increased. This apparent electron heating effect in the dot can be explained by a simple ballistic phonon model. However, reasonable phase coherence times can be estimated from peak fitting using a Breit- Wigner formula which points to a Coulomb interaction. More detailed numerical calculations should illuminate the dominant scattering processes. / Graduation date: 1999