Achieving tunability of two dimensional (2D) transition metal dichalcogenides (TMDs) functions calls for the introduction of hybrid 2D materials by means of localized interactions with zero dimensional (0D) materials. A metal-semiconductor interface, as in gold (Au) - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science as it constitutes an outstanding platform to investigate optical and electronic properties due to charge transfer. The applied aspects of such systems introduce new options for electronics, photovoltaics, detectors, catalysis, and biosensing. Here in this dissertation, we study the charge transfer interaction between Au nanoparticals and MoS2 flakes and its effect on Photoluminescence and electronic transport properties. The MoS2 was mechanically exfoliated from bulk single crystal. Number of layers in the flake was identified with the help of AFM and Raman Spectra. Au was deposited by physical vapor deposition method (PVD) in multiple steps to decorate MoS2 flakes. We first study the photoluminescence of pristine and Au decorated MoS2 and shows that in the presence of Au, the photoluminescence of MoS2 quenches significantly. We infer that the PL quenching can be attributed to a change in the electronic structure of the MoS2-Au system. The difference in Fermi level of a of MoS2 and Au results in a 0.4 eV energy level offset, which causes a band bending in the MoS2-Au hybrid. Upon illumination, the electrons in the excited state of MoS2 transfer to Au, leaving a hole behind, thus cause p-doping in MoS2. As electrons from MoS2 are transferred to Au, they do not decay back to their initial ground state, leading to PL quenching in the hybrid system. To study the effect of Au deposition on electronic properties of ultra-thin and multilayers MoS2 flakes, we have fabricated MoS2 FETs from (1) ultra-thin sample (2-4 MoS2 layers) and (2) multilayers samples (more than 20 layers). After each deposition of Au, we measured the electrical characteristics of FET at room temperature. We show that the threshold voltage shifts towards the positive gate voltage as we increase the thickness of Au. This shift in threshold voltage is indicative of p doping of the MoS2. We further show that the field effect mobility of MoS2 FET decrease with Au thickness. We have quantitatively estimated the charge transferring from MoS2 to Au.

This is Part A of the Thesis. Here is the Link to Part B: http://hdl.handle.net/11375/17845 / <p> This study was focused on the absorption spectra of tin dioxide thin films on optical quartz substrates. The films were doped with antimony from zero to ten percent, indium from zero to ten percent, and intrinsically, by heating undoped samples in vacuum and air. The surface resistances were also measured.</p> <p> The results for antimony doping show that the energy required for electron transitions from valence to conduction bands, the associated phonon energies, and the optical absorption by free carriers all increase while resistance decreases with increased doping. These results are consistant with antimony acting as a donor in SnO2 and elevating the Fermi level, which is in the conduction band for the undoped material, to a higher level, thus increasing the free carrier concentration. The results possibly also indicate a strain on the lattice caused by doping.</p> <p> The results for indium doping show a similar increase in energies along with a decrease in optical absorption by free carriers and an increase in resistance with increased doping. The indium acts as an acceptor and, in so doing, causes the Fermi level to drop into the valence band, at the doping levels used in this study. This is probably due to the formation of an acceptor band above or overlapping the valence band and resulting in a reduction of the free carrier concentration. The increase in phonon energy indicates that the doping imposes a strain on the lattice.</p> </p> Heating undoped films in vacuum appears to drive off oxygen resulting in reduced resistance and therefore higher free carrier concentration due to lattice defect doping, and reduced valence to conduction band transition energy possibly due to the formation of a conduction band in the forbidden band-gap. The changes were reversible by re-heating in air.</p> / Thesis / Master of Engineering (MEngr)

This is Part B of the Thesis. Here is the Link to Part A: http://hdl.handle.net/11375/17844 / <p> This report deals with the effects of various parameters on the resistance and the temperature coefficient of resistance (or the T.C.) of tin dioxide films doped with antimony and boron. The films were produced on cylindrical ceramic substrates by the hydrolysis of SnCl4 and SbCls in the presence of HCl and H3BO3. The T.C. was measured over the range of 25 to 150°C and averaged.</p> <p> Under normal conditions, the films were produced at 950°C with an antimony concentration of 0.457 molar % and a boron concentration of 2.73 molar %. Varying this firing temperature (from 800-1100°C) was found to have no effect on the resistance but increased the T.C. by 2 to 3 ppm/°C per degree change. Varying the antimony content from 0 to about 1 molar % was found to have little effect on resistance. The effect on T.C. was to increase it at lower Sb levels and then to decrease the T.C. as the level increased.</p> <p> Varying the boron content (0 to 4.46 molar %) was also found to have little effect on resistance. A decrease in T.C. with boron content was noted when only the boron was varied, but an increase in T.C. was found when HCl and H2O volumes were varied with the boron. The introduction of additional air into the system was found to have no effect.</p> <p> Film thicknesses were varied by controlling the chemical flowrates. Thinner films were found to have dramatically higher resistances and reduced T.C. values. It was observed that below a certain flowrate resistive failure occurred in the films. It was found in this study that within the statistical distribution of film values, those samples with above average resistance had below average T.C. values and vice-versa. Annealing in vacuum at 500°C was found to produce samples of reduced resistance and increased T.C. while the opposite was found with air annealed samples. Quickly cooled samples were found to be more stable.</p> / Thesis / Master of Engineering (MEngr)

Silicon Carbide devices are proving to be most promising for high power and high-temperature application in recent times. Efficient and accurate characterization of the device characteristics is key to the fabrication of high quality devices and reproduction of the quality of the devices fabricated. Capacitance-Voltage profiling is one of the most commonly used techniques to measure the doping profiles of semiconductors. However, interpretation of C-V profiling in the presence of traps in the material becomes complicated. Various complications arising from compensation between donors and acceptors, partial ionization of dopants and presence of deep level impurities could yield anomalous measured profile. Silicon Carbide being a wide bandgap semiconductor, many impurities commonly found such as Boron and Aluminum are not completely ionized at Room temperature. This leads to complications in calculating doping profiles when the trap levels are deeper. Other complications arising due to series resistance effect and diode edge effect may also affect the measured profile. Accounting for these complications may be difficult by mere observation of the measured profile. Simulation can be an excellent tool to extract parameters of interest from experimental results that are influenced by non-ideal effects. Fitting of the experimentally obtained data with simulated profile using specific models may be a useful technique to quantitatively account for the deviations from the actual profiles.

No description available.

<p> A well characterized titanate system, Nd1_xTi03, has been studied by temperature dependent reflectance spectroscopy between 50 and 40 000 cm-1 at three different doping levels, x = 0.019, 0.046, and 0.095, which yield a Matt-Hubbard insulator, a semiconductor and a correlated metal, respectively. Two main issues are discussed regarding the optical properties of the current system. The first is the variation of the low-lying electronic structure with hole concentration. The doping-dependent optical conductivity of Nd1_xTi03 shows several obvious differences when compared to the superconducting cuprates. We observed mid-infrared absorption bands in the doped samples, suggesting that mid-gap states develop inside the Hubbard gap with hole doping in the context of a two-component model. A quantitative analysis of the spectral weight below 1.2 e V as a function of doping indicates that the evolution rate of the optical excitations below 1.2 eV is related to the electron correlation strength of the parent insulator, which has been observed in other titanates as well. The second issue addressed in this thesis is the temperature-dependent optical features of the correlated metallic sample with x = 0.095, a composition close to the metal-insulator transition at x '"'-~ 0.08. The optical conductivity shows an anomalous enhancement of spectral weight below leV, in both the Drude and midinfrared part, that develops with decreasing temperature. The dynamical mean field theory (DMFT) may explain this feature. Meanwhile, the metallic sample displays a Fermi-liquid like behavior in the low-frequency limit, which can be established from the spectra of the scattering rate as a function of both frequency and temperature. We found a good agreement between the experimental results extracted from the scattering rate and the Fermi-liquid theory. </p> / Thesis / Master of Science (MSc)

Gallium antimonide (GaSb) is a promising low-bandgap binary substrate for the fabrication of various infrared-based optoelectronic devices, particularly thermophotovoltaics (TPV). In order to make GaSb-based technologies like TPV more widely available, non-epitaxial dop- ing methods for GaSb must be pursued. Ion implantation is relatively unexplored for GaSb, and can offer advantages over the more common method of zinc diffusion, including higher flexibility with regards to substrate type and control over the resulting doping profile. Pre- vious work has shown beryllium (Be+) implantation to be a suitable method for fabricating a diode in an n-type GaSb substrate, opening the possibility for other ions to be considered for implanting into both n-type and p-type substrates. This work identifies sulfur (S+) as another species to investigate for this purpose. To do so, material and electrical characterization was done on S+ and beryllium implanted GaSb films grown onto a semi-insulating gallium arsenide (GaAs) substrate. X-ray Diffraction spectroscopy (XRD) and Atomic Force Microscopy (AFM) indicate that the post-implant anneal of 600 for 10 s repaired the implant damage in the bulk material, but left behind a damaged surface layer composed of coalesced vacancies. While the beryllium implant resulted in moderate doping concentrations corresponding to an activation percentage near 15 %, Hall Effect data showed that implanting S+ ions induced a strongly p-type behavior, with hole concentrations above 1 × 19 cm^3 and sheet hole densities 3.5 times higher than the total implanted dose. This strong p-type behavior is attributed to the remaining lattice damage caused by the implant, which induces a large density of acceptor-like defect states near the valence band edge. This technique was used on an unintentionally-doped p-type GaSb substrate to create a + /p junction. The implant process succeeded in producing a potential barrier similar to that of a hole-majority camel diode with a thin delta-doped region suitable for collecting diffused carriers from the p-type substrate. A post-fabrication etching process had the effect of strongly increasing the short circuit current density to as high as 41.8 mA/cm^2 and the open circuit voltage as high as 0.21 V by simultaneously removing a high carrier recombination surface layer. This etching process resulted in a broadband spectral response, giving internal quantum efficiencies greater than 90 %. / Doctor of Philosophy / Thermophotovoltaics (TPV) is a technology that converts light and other forms of electromagnetic energy into electrical power, much like a typical solar panel. However, instead of sunlight, the energy source used in a TPV system is a terrestrial heat source at a temperature range of 1250–1750 ◦C, whose radiation is primarily infrared (IR). The IR-absorbing qualities and commercial availability of the compound semiconductor gallium antimonide (GaSb) have made it a key component in the development of absorber devices for TPV-related systems. GaSb-based devices have most often been fabricated using epitaxy, a method in which layer(s) of material are ‘grown’ in a layer-by-layer fashion atop a substrate GaSb wafer to induce an interface between negatively-charged (n-type) and positively-charged (p-type) regions. In order to improve upon the scalability of TPV production, device fabrication methods for GaSb that avoid the use of epitaxy are sought after as a lower-cost alternative. In this work, sulfur ion implantation is examined as one of these methods, in which elemental sulfur ions are injected at a high energy into a p-type GaSb substrate. The implanted ions then alter the charge characteristics at the surface of the material, producing an electric field from which a photovoltaic (PV) device can be fabricated. The results of this study showed that by implanting sulfur ions, an extremely p-type (p++) layer was formed at the surface of the GaSb substrate, which was attributed to residual damage induced by the implant process. The resulting interface between the p++ surface and the moderately p-type GaSb substrate was found to induce an electric field suitable for a PV device. Removing the excess surface damage away from the device’s metal contacts resulted in an improvement in the output electrical currents, with measured values being significantly higher than that of other devices made using more common non-epitaxial fabrication methods. The success of this work demonstrates the advantages of using a p-type GaSb substrate in place of an n-type substrate, and could help diversify the types of TPV-related devices that can be produced.

Thermoelectric materials can convert thermal energy into electrical energy and vice-versa. They are widely used in energy harvesters, thermal sensors, and cooling systems. However, the low efficiency and high cost of the known material compositions limit their widespread utilization in electricity generation applications. Therefore, there is a strong interest in identifying new thermoelectric materials with high figure of merit. In response to this need, this dissertation works on the synthesis, structure, doping mechanism, and thermoelectric properties of zinc oxide (ZnO) and lead tellurium (PbTe). The main focus is on ZnO based materials and in improving their performance. The influences of micro- or nano-structures on thermal conductivity, as well as the correlation between the electrical property and synthesis conditions, have been systematically investigated. ZnO is a likely candidate for thermoelectric applications, because of its good Seebeck coefficient, high stability at high temperature, non-toxicity and abundance. Its main drawbacks are the high thermal conductivity (κ) and low electrical conductivity (σ). To decrease κ, two novel structures—namely, precipitate system and layered-and-correlated grain microstructure—have been proposed and synthesized in ZnO. The mechanisms iii governing the nature of thermal behavior in these structures have been explored and quantified. Due to strong phonon scattering, the nano-precipitates can reduce the thermal conductivity of ZnO by 73%. The ZnO with layered-and-correlated grains can further reduce κ by about 52%, which compares favorably with the dense ZnO with nanoprecipitates. The figure of merit of this ZnO based structure was 0.14×10⁻³ K⁻¹ at 573 K. In order to understand the electrical behavior in nanostructured ZnO, the impact of Al doping and chemical defects in ZnO under different synthesis conditions were studied. Under varying sintering temperatures, atmospheres and initial physical conditions, ZnO exhibited very distinct σ. High temperature, lack of oxygen, vacuum condition, and chemically synthesized powder can increase the carrier concentration and σ of ZnO. A promising alloy system, PbTe-PbS, undergoes natural phase separation by nucleation and growth, and spinodal decomposition depending on the thermal treatment. The correlation between the thermal treatment, structure, and the thermoelectric properties of Pb0.9S0.1Te has been studied. The nano-precipitates were incorporated in the annealed alloy resulting in a 40% decrease in κ. The PbS precipitation was shown to enhance the carrier concentration and improves the Seebeck coefficient. These concomitant effects result in a maximum ZT of 0.76 at 573 K. Throughout the thesis, the emphasis was on understanding the impact of the microstructures on thermal conductivity and the effect of the synthesis condition on thermal and electrical properties. The process and control variables identified in this study provide practical ways to optimize the figure of merit of ZnO and PbTe materials for thermoelectric applications. / Ph. D.

High temperature thermal energy harvesting has attracted much attention recently. In order to achieve stable operation at high temperatures there is emerging need to develop efficient and oxidation-resistant materials. Most of the well-known materials with high dimensionless figure of merit (ZT) values such as Bi2Te3, PbTe, skutterudites, and half-Heusler alloys, are not thermally stable at temperatures approaching 500°C or higher, due to the presence of volatile elements. Oxide thermoelectric materials are considered to be potential candidates for high temperature applications due to their robust thermal and chemical stability in oxidizing atmosphere along with the reduced toxicity, relatively simpler fabrication, and cost. In this dissertation, nanoscale texturing and interface engineering were utilized for enhancing the thermoelectric performance of oxide polycrystalline Ca3Co4O9 materials, which were synthesized using conventional sintering and spark plasma sintering (SPS) techniques. In order to tailor the electrical and thermal properties, Lu and Ga co-doping was investigated in Ca3Co4O9 system. The effect of co-doping at Ca and Co sites on the thermoelectric properties was quantified and the anisotropic behavior was investigated. Because of the effective scattering of phonons by doping-induced defects, lower thermal conductivity and higher ZT were achieved. The layered structure of Ca3Co4O9 has strong anisotropy in the transport properties. For this reason, the thermoelectric measurements were conducted for the samples along both vertical and horizontal directions. The ZT value along the vertical direction was found to be 3 to 4 times higher than that along the horizontal direction. Metallic inclusions along with ionic doping were also utilized in order to enhance the ZT of Ca3Co4O9. The texturing occurring in the nanostructured Ca3Co4O9 through ion doping and Ag inclusions was studied using microscopy and diffraction analysis. Multi-length scale inclusions and heavier ion doping in Ca3Co4O9 resulted in higher electrical conductivity and reduced thermal conductivity. The maximum ZT of 0.25 at 670°C was found in the spark plasma sintered Ca2.95Ag0.05Co4O9 sample. In literature, limited number of studies have been conducted on understanding the anisotropic thermoelectric performance of Ca3Co4O9, which often results in erroneous estimation of ZT. This study addresses this limitation and provides systematic evaluation of the anisotropic response with respect to platelet microstructure. Textured Ca3Co4O9/Ag nanocomposites were fabricated using spark plasma sintering (SPS) technique and utilized for understanding the role of microstructure towards anisotropic thermoelectric properties. The thermoelectric response was measured along both vertical and horizontal direction with respect to the SPS pressure axis. In order to achieve enhanced degree of texturing and increase electrical conductivity along ab planes, a two-step SPS method was developed. Ag nanoinclusions was found to increase the overall electrical conductivity and the thermoelectric power factor because of improved electrical connections among the grains. Through two-step SPS method, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C in Ca3Co4O9/Ag nanocomposites was achieved. Lastly, this dissertation provides significant progress towards understanding the effect of synthesis method on thermoelectric properties and evolution of textured microstructure. The anisotropy resulting from the crystal structure and microstructural features is systematically quantified. Results reported in this study will assist the continued progress in developing Ca3Co4O9 materials for practical thermoelectric applications. / PHD / Among the wide range of renewable energy sources, wasted thermal energy has attracted worldwide interest as it is freely available from most of the industrial and natural processes. Among various choices for converting thermal energy into electricity, thermoelectric devices are attractive as they are solid state, noiseless, no moving parts, and can be easily integrated with most of the heat sources. Thus, there has been significant efforts to develop high efficiency thermoelectric energy harvesting devices. However, currently available thermoelectric materials are not thermally stable in oxidizing environments because of heavy metals’ evaporation and reactivity. In order to overcome this limitation of thermoelectric materials, in this dissertation, the focus is on developing calcium cobalt oxide (Ca₃Co₄O₉) materials through innovation in the processing, composition design, and modulation of the thermal transport mechanism by exploiting the anisotropy. Ca₃Co₄O₉ is promising candidate for high temperature thermoelectric applications due to its thermal and chemical stability in oxidizing atmosphere, reduced toxicity, easy fabrication, and low cost. Its main disadvantages are the high thermal conductivity and low electrical conductivity. In order to tailor the electrical and thermal properties, Lu and Ga co-doped Ca₃Co₄O₉ were synthesized and characterized. The thermoelectric measurements were conducted along both vertical and horizontal directions with respect to pressure axis during spark plasma sintering. Layered structure of Ca₃Co₄O₉ induces strong anisotropy in the transport properties which indicates that textured microstructure will result in better properties. Texturing and interface engineering were employed to control the grain orientation and thereby improve the electrical and thermal properties. In textured and nanostructured Ca₃Co₄O₉, Ag inclusions along with ionic doping was utilized to enhance the thermoelectric performance. In literature, the importance of the anisotropy in Ca₃Co₄O₉ has been less emphasized, which has restricted accurate thermoelectric evaluation of this material for practical application. In order to address this issue, first textured Ca₃Co₄O₉/Ag nanocomposites were fabricated using spark plasma sintering (SPS) techniques and next detailed investigation was conducted on correlation between microstructure and anisotropic thermoelectric properties. The power factor of the Ca₃Co₄O₉/Ag nanocomposites at high temperatures was almost 50% enhanced, as compared to the pure Ca₃Co₄O₉, which resulted in 50% improvement in ZT both horizontal and vertical directions. The samples with texturing along the vertical direction were used to perform the long-term durability test and almost same value of resistivity was maintained after a long-term heating. Two-step SPS method was developed to improve the in-plane electrical conductivity. Through this newly proposed synthesis process, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C was obtained in Ca₃Co₄O₉/Ag nanocomposites. Using a wide range of composition and synthesis process, the anisotropy and microstructural effects clarified in this study provides promising pathway towards enhance the thermoelectric performance of Ca₃Co₄O₉ materials.

Sport as a kind of physical activity should, above all, be a way of entertainment for people. However, one cannot forget to mention the other aspects, which sport provides to people, whether from the point of view of health or psychology. It particularly concerns the improvement of both physical and mental form, prevention against health problems, relaxation, improvement of communication, cooperation etc. Sport has become a phenomena of these days. Shining careers, tremendous popularity and last but not least sometimes huge amounts of money offered to the best sportsmen lead to swindles as a matter of course. Nowadays, the desire for success can overcome the limits of fair play, and therefore a lot of sportsmen tend to the use of illegal aids in order to have an advantage over their rivals. In spite of that, doping is not the term only connected with top-performance sports, but also with recreation sports. Dope substances are used on all sporting levels at the present. The main aim of this work is to introduce readers to the problems of using dopes and their detection. The work describes the ways in which doping checks proceed, and in which sports and on which levels doping checks can be anticipated. Keywords Doping, blood tests, testing, doping controls , prohibited substances , prohibited metods...