Browsing by Subject "Silicon"

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Deformation mechanisms in nanoscale brittle materials.
(2011-05) Stauffer, Douglas Dean
In the past ten years nanotechnology has developed from a buzzword to an integral part of our modern life. The promise of bottom up devices has turned into better, faster, and stronger products utilizing nanoscale materials. Tires designed with carbon nanotubes, touchscreens, reformulated steel, self-cleaning fabrics, drug delivery systems, and semiconductor devices all rely on nanoscale materials. However, the mechanical property relationships are not fully understood, and the cross-roads of mechanical performance and electrical properties is still being explored. For example, the role of electrical contact in mechanical systems is important for reliability in systems that contain interconnect, switches, or relays. MEMS switches in particular can have reliability issues if the conducting area is decreased, or the switch fails due to plasticity. In this thesis, an attempt is made to characterize failure modes of several fundamental nanoscale materials using nanoindentation. In this thesis, ostensibly brittle materials such as alumina, chromia, and silicon are chosen as being archetypal examples of brittle materials. The use of conductive probe indentation is used here as a measure of plasticity under the indenter in constrained metal films with native oxide layers, as well as to determine the point of oxide fracture. In situ transmission electron microscope indentation is used to explore dislocation velocities and strain hardening in compressed silicon pillars. Dislocation velocities, in compression at room temperature, are found that approach that of those at 600°C in bulk tensile specimens. The dislocations, of unknown type, also contribute to strain hardening exponents of approximately 0.4 in pillars, and approach unity in silicon spheres.
Development of Novel Monocarboxylate Transporter Inhibitors as Potential Anticancer Agents
(2019-04) Nelson, Grady
The metabolic phenotype of cancer cells is dependent on the differential oxygen and nutrient distribution in the tumor microenvironment. These diminishing resources coupled with increased energetic and biosynthetic demands of tumor cells further encourage the upregulation of glycolysis with overexpression of related enzymes and transporters. This shift towards a more glycolytic character results in a disruption of the intracellular pH as the cell rapidly becomes more acidic. The accumulation of acidic byproducts like lactic acid requires new strategies to maintain cellular homeostasis resulting in the overexpression of transporters. These extracellular acidic byproducts are taken up by neighboring cells and are utilized for oxidative phosphorylation (OxPhos). Cancer cells often switch between glycolysis and OxPhos to meet the energy demands, termed as metabolic plasticity, which is driven by a necessity to avoid conditions that would induce apoptosis. For this reason, cancer cells express proton coupled monocarboxylate transporters 1-4 (MCT1-4). Specifically, these transporters have been found to be expressed in the most aggressive tumors and ultimately been linked to poor patient outcome. Hence, this transporter can be targeted for therapeutic intervention to treat a wide variety of cancers. One well known MCT1 inhibitor α-cyano-4-hydroxycinnamic acid (CHC) has been traditionally used to study the functions of these transporters and it has been found to reduce tumor growth in mouse xenograft models. The therapeutic potential of CHC is hindered by its lack of efficacy at low concentrations and very high dose requirement for significant anticancer efficacy in vivo. In this regard, we have modified the CHC template with alkyl and aryl silyl substitutions and also introduced nitric oxide donors. These structural modifications have resulted novel candidate compounds which exhibit potent MCT1 and MCT4 inhibition and higher cell proliferation inhibition on several cancer cell lines. These drug candidates have also been evaluated for their effects on glycolytic and mitochondrial metabolic parameters. These studies have shown that all the lead derivatives have significant effects on both metabolic processes. Further in vivo preclinical evaluation of lead candidate compounds indicate that these compounds are generally well tolerated in healthy mice and exhibit growth inhibition in MCT1 and MCT4 expressing tumor models.
Effects of nanocrystalline silicon inclusions in doped and undoped thin films of hydrogenated amorphous silicon.
(2009-12) Blackwell, Charlie Pearman
Hydrogenated amorphous silicon has attracted considerable interest as a low-cost material for various large-area electronic devices, such as scanners, thin film transistors employed in flat panel displays, and photovoltaic devices. A major limitation of amorphous silicon is a light-induced degradation of the photoconductivity and dark conductivity, associated with the creation of metastable dangling bond defects. Recent reports that mixed phase thin films, consisting of silicon nanocrystallites embedded within a hydrogenated amorphous silicon matrix, display a resistance to this light-induced degradation have motivated the development of a novel deposition system to synthesize such materials. Conventional techniques to generate such amorphous/nanocrystalline mixed phase films involve running a Plasma Enhanced Chemical Vapor Deposition system very far from those conditions that yield high quality amorphous silicon. A dual-plasma co-deposition system has thus been constructed, whereby the silicon nanoparticles can be fabricated in one chamber, and then injected into a second plasma reactor, in which the surrounding amorphous silicon is deposited. The deposition process, as well as structural, optical, and electronic characterization of these films, including the dark conductivity, photoconductivity, infra-red absorption spectra, micro-RAMAN spectra, and the optical absorption spectra, will be discussed for these films.
Electrical and optical characterization of colloidal silicon nanocrystals
(2013-12) Li, Meng
Colloidal silicon nanocrystals (SiNCs), due to their high photoluminescence efficiency and tunable bandgap, can be used to fabricate highly efficient hybrid nanocrystal-organic light-emitting-devices (NC-OLEDs) that emit in red or near infrared spectrum. Despite reports of outstanding device performance, the underlying mechanism of this high efficiency remains unknown. Consequently, this thesis focuses on studying the electrical and optical properties of SiNCs. The electrical conductivity and mobility of electrons and holes are successfully extracted in order to explain the observed dependence of device efficiency on SiNC surface ligand coverage. Steady-state and transient photoluminescence is also examined to better understand the connection between surface ligand coverage and molecular photophysics. In addition, these measurements are used to better understand the mechanisms for non-radiative exciton decay in SiNCs. This work elucidates the relationship between SiNC properties and device performance, potentially guiding the design of future NC materials for high performance.
Electronic transport in mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films.
(2010-08) Adjallah, Yves Gbemonde
The opto-electronic properties of amorphous/nanocrystalline hydrogenated silicon (a/nc-Si:H) mixed-phase thin films are investigated. Small crystalline silicon particles (3-5 nm diameter) synthesized in a flow-through reactor are injected into a separate capacitively-coupled plasma (CCP) chamber where mixed-phase hydrogenated amorphous silicon is grown by Plasma Enhanced Chemical Vapor Deposition (PECVD) deposition techniques. This dual-chamber co-deposition system enables the variation of crystallite concentration incorporated into a series of a-Si:H films deposited simultaneously. The structural, optical and electronic properties of these mixed-phase materials are studied as a function of the silicon nanocrystal concentration. That is, we compare a sequence of films deposited in a single run, where the location of the substrate in the CCP chamber determines the density of embedded nanocrystals. Raman spectroscopy is used to determine the volume fraction of nanocrystals in the mixed phase thin films. At a moderate concentration of silicon crystallites, the dark conductivity and photoconductivity are consistently found to be up to several orders of magnitude higher than in mixed phase films with either low or heavy nanocrystalline inclusions. These results are interpreted in terms of a model whereby for low nanocrystal concentrations conduction is influenced by the disorder introduced into the a-Si:H film by the inclusions, while at high nanocrystal densities electronic transport is described by a heterojunction quantum dot model. The thermopower of the undoped a/nc-Si:H has a lower Seebeck coefficient, and similar temperature dependence, to that observed for undoped a-Si:H. In contrast, the addition of nanoparticles in doped a/nc-Si:H thin films leads to a negative Seebeck coefficient (consistent with n-type doping) with a positive temperature dependence, that is, the Seebeck coefficient becomes larger at higher temperatures. The temperature dependence of the thermopower of the doped a/nc-Si:H is similar to that observed in unhydrogenated a-Si grown by sputtering or following high-temperature annealing of a-Si:H, suggesting that charge transport may occur via hopping in these materials.
Hopping Conduction and Metallic behavior in 2D Silicon Surface States induced by an Ionic Liquid
(2015-06) Nelson, JJ
Ionic liquids (ILs) are essentially molten salts with a melting point below room temperature. When used as the gate dielectric of a transistor, carrier densities on the order of $10^{15}\text{ cm}^{-2}$ can be achieved. These record high carrier densities are significantly higher than the maximum carrier density achievable with oxide dielectrics. The physical mechanism for inducing carriers to such a high carrier density is not well understood. Some groups have reported that the induced carriers are a result of electrostatic and electrochemical processes. Other groups have suggested that carriers induced with an IL may be entirely due to electrochemical reactions. Here we report on IL gated Si at carrier densities from $10^{11}\text{ cm}^{-2}$ to $10^{13}\text{ cm}^{-2}$. The experiment was designed to preferentially induce electrostatic carriers over electrochemical reactions. At low carrier densities, sample surface conductivity follows nearest neighbor hopping conduction. This form of conduction has also been observed in experiments where surface conductivity was induced by implanting $\text{Na}^{+}$ near the oxide surface interface. A surprising result of this work was that in some samples a 2D metallic state could be created on the surface of Si. The transition to metallic behavior occurred just below $10^{13}\text{ cm}^{-2}$. High quality Si transistors with oxide dielectric materials observe critical carrier densities around $10^{11}\text{ cm}^{-2}$. The critical carrier density observed in IL gated Si is the highest density reported to date. At carrier densities higher than $10^{13}\text{ cm}^{-2}$ it was observed that the sample conductivity decreased with increasing carrier density. The behavior was unexpected and not fully understood. Both metallic and non metallic samples show a similar reduction in conductivity that is not thought to be due to sample degradation by the IL. The reduction in the sample conductivity at high carrier densities is thought to be due to surface roughness scattering. Similar behavior has been observed in other IL gated experiments on different materials.
In-flight gas phase passivation of silicon nanocrystals for novel inorganic-silicon nanocrystal based electroluminescent devices.
(2009-10) Liptak, Richard William
Silicon nanocrystals (SiNCs) have become a heavily researched material over the past several years. Researchers envision that this material can be used in many diverse applications such as electronic devices, non-toxic biological tags, optical devices such as LEDs, lasers or displays, thermoelectrics, and photovoltaic (PV) applications. For many of these proposed applications one needs to properly control the NC size and the surface chemistry via passivation. Current passivation techniques allow for the creation of highly efficient SiNC optical emitters, however the emission of these NCs are fixed in the red- NIR range. To resolve this issue several novel in-flight passivation techniques were investigated. A novel dual-plasma setup which allows for the in-flight passivation of SiNCs through a thermal or LPCVD based nitridation process was developed first. FTIR and XPS analysis were used to study the surface chemistry on of the nitride passivated NCs while TEM was used to investigate whether or not a “shell” was grown on the surface. PL measurements and thermal stability tests were performed on the nitride passivated NCs to gain a further understanding of the stability (in both air as well as other ambients) of the NCs and their surface chemistry. Tunable full color emission from SiNCs was developed for the dual-plasma reactor utilizing CF4 as both an etching and passivating source. F radicals generated in the etching plasma remove Si from the surface of the NC, while at the same time CF2 radicals lead to the formation of a fluorocarbon passivation layer on the NC surface. By controlling the parameters of the reactor (CF4 flow rate, power), the NC size and thus its color can be controlled. Red to green luminescence was observed from SiNCs and is believed to be due to the quantum confinement effect. The blue emission observed from the NCs is appears to be related to oxide related surface states. Despite the defects, high QY was observed from these CF4-etched NCs. The fluorocarbon passivation layer, although stable, prevents further functionalization of the NCs. To counteract this problem another silicon-based dry etch chemistry, SF6 was investigated. Full-color emission was observed from SF6 etched NCs, with QY 2X higher than that of CF4-etched NCs. A maximum QY of nearly 55% at 700 nm was observed after several weeks in air, comparable to that observed with alkyl passivation. The native oxidation of the bare oxidized and SF6-etched NCs were also studied. Results show that the NC oxidation follows the Cabrera-Mott mechanism for low temperature oxidation. Inorganic-NC based LED structures were then investigated. Fabrication processes for the inorganic hole and electron transport layers were developed by RF sputtering and atomic layer deposition (ALD). Thorough characterization was performed on the metal-oxide films (ZnO, TiO2, NiO) to verify their stoichiometry as well as study their optical and electrical properties. Novel inorganic-NC device structures were fabricated. Inorganic NC devices which use a metal-oxide HTL but no ETL, emit light, however their emission is so weak. The addition of an ETL increases the light output by a factor of 4, but the device reproducibility is poor. To improve efficiency two insulating matrix layers were investigated. In both cases, the film deposited on the top of the NC is rough, porous, discontinuous, and potentially full of traps – certainly not the ideal film for a device. Therefore, more work is needed, specifically on the NC layer to improve the structure of the as-deposited NC film, but efficient device structures appear to be possible.
Nonthermal Plasma Synthesis and Plasmonic Properties of Doped Silicon and Titanium Nitride Nanocrystals
(2017-08) Schramke, Katelyn
This work examines the nonthermal plasma synthesis of phosphorus and boron doped silicon and titanium nitride nanocrystals. The localized surface plasmon resonance (LSPR) of these materials was investigated. Titanium nitride has a plasmon resonance in the visible and near infrared, like gold nanorods, making it a viable alternative to gold for biological applications like photothermal therapy treatments. Titanium nitride is less expensive and more temperature stable than gold as well as biocompatible. The nonthermal plasma synthesis route is a continuous production method that eliminates the need for long processing and morphology modification steps required for gold nanorod production. The plasmon resonance, composition, and particle uniformity of the titanium nitride was found to be very dependent on the flow rate of ammonia during synthesis. Doped silicon has a plasmon resonance further into the infrared region and a tunable absorption controlled by the substitutional doping concentration in the nanocrystals. The presence of the plasmon absorption was used as a diagnostic tool to understand dopant behavior in doped silicon. The plasmon behavior supports the hypothesis of a uniform doping profile for phosphorus dopants and surface doping profile for boron dopants in silicon nanocrystals.
Stress localization and size dependent toughening effects in SiC composites.
(2010-08) Beaber, Aaron Ross
Coatings with high wear resistance have generated a great deal of interest due to a diverse range of applications, including cutting tools, turbine blades, and biomedical joint replacements. Ceramic nanocomposites offer a potential combination of high strength and toughness that is ideal for such environments. In the current dissertation research, silicon and silicon carbide based films and nanostructures were deposited using a hybrid of chemical vapor deposition and nanoparticle ballistic impaction known as hypersonic plasma particle deposition (HPPD). This included SiC/Ti-based multilayers and Si-SiC core-shell composite nanotowers. Using a combination of nanoindentation and confocal Raman microscopy, the role of plasticity and phase transformations was studied during fracture events at small length scales. In a parallel study, HPPD synthesized Si nanospheres and vapor-liquid-solid (VLS) Si nanotowers were compressed uniaxially inside the TEM. These experiments confirmed inverse length scale dependent relationships for strength and toughness in Si based on dislocation pile-up and crack tip shielding mechanisms, respectively. A transition was also identified in the deformation of Si under anisotropic loading below a critical size and used as the basis for a new toughening mechanism in Si-SiC composites. Overall, these results demonstrate the importance of nanoscale confinement and localized stress in the design of mechanically robust nanocomposites.
Structural and surface correlations to the optical properties of nonthermal plasma-produced silicon nanoparticles
(2011-06) Anthony, Rebecca Joy
Nanomaterials have diverse capabilities to enable new technology and to deepen our understanding of our world, providing exciting prospects for scientists and the public alike in a vast span of uses. In the past decade, however, the potential held by nanotechnology has been reframed in the context of helping to slow global climate change and to alter the ways in which we use our energy to reflect more efficient technology and renewable energy sources. Silicon is a standout material in this new framework: as a nanomaterial, silicon can emit light when exposed to an applied voltage or ultraviolet optical excitation source. Silicon nanocrystals also exhibit size-dependent light emission, due to quantum confinement. This thesis is an exploration of the synthesis and processing parameters that affect the optical performance of silicon nanocrystals produced in a nonthermal plasma reactor. The efficiency of this light emission is sensitive to both synthesis environment and post-synthesis treatment. The work presented here is an attempt to deepen our understanding of the effects of different reactor and treatment parameters on the light emission efficiency from silicon nanoparticles, such that the luminescence behavior of the nanoparticles can be specifically engineered. Being able to fine-tune the structure, surface, and optical characteristics of the silicon nanocrystals is key in maximizing their use in luminescence applications. For all of the experiments described here, a nonthermal plasma flow-through reactor has been used to create the silicon nanoparticles. Silane gas is dissociated in the plasma and fragments come together to form silicon clusters, then grow to create nanoparticles. The nanoparticles were collected from the reactor for further processing, characterization, and experiments. The first discovery in this project was that by adjusting the power to the plasma reactor, the crystallinity of the silicon particles can be tuned: low power results in amorphous silicon nanoparticles, and high power yields crystalline nanoparticles. Even more important, the crystallinity of a nanoparticle ensemble relates directly to the photoluminescence (PL) efficiency, or quantum yield, from the ensemble: crystalline silicon nanoparticle samples, after alkyl functionalization, exhibit PL efficiencies of 40% or greater, while amorphous samples emit light with very poor efficiency (<2%). Additional studies of the plasma reactor revealed the importance of injecting a flow of hydrogen gas into the afterglow of the plasma, which turns out to have dramatic implications for the ultimate PL quantum yields of the nanocrystals. This injection scheme was systematically studied by varying the injected gas and its position. Hydrogen injected directly into the plasma afterglow was found to be vital for achieving high quantum-yield silicon nanocrystals, likely due to a reduction in surface trap states due to additional hydrogen passivation at the nanocrystal surface.Further investigations into the nanocrystal surface and how it relates to PL quantum yield showed that the photoluminescence from silicon nanocrystals is not only dependent on synthesis parameters, but also on processing temperatures and procedures following synthesis. While the highest PL efficiencies are found for silicon nanocrystals capped with alkyl chains, the PL efficiency of a nanocrystal ensemble can also be improved simply by heating the sample to temperatures between 150-200° C. This heating step also leads to a change in the hydride structure at the nanocrystal surface, which appears to be brought about by the effusion of silyl (or disilane) groups. Finally, details of the construction of a silicon-nanocrystal-based LED will be discussed. The LED project is part of a collaboration, and while the majority of device-specific aspects of the project were carried out in the lab of Professor R. Holmes by his Ph.D. student Kai-Yuan Cheng, the processing and alterations made to the nanocrystals used in the LED were all the responsibility of the author. The details of the project and a summary of the results bear discussion here in this thesis, as well as outlining of a novel scheme for deposition of SiNCs for device construction.
Study of Heat Losses in Crystalline Silicon and Perovskite Solar Cells
(2023-08) Tisha, Zakia Tamanna
Energy from the sun is plentiful and sustainable, making it an excellent alternative to fossil fuels. Photovoltaic (PV) solar cells can directly convert this solar energy into electricity. However, PV solar cells face challenges in achieving high efficiency as some of the captured energy is lost as heat or through other means, reducing efficiency and performance. Researchers are constantly trying to improve the efficiency of solar cells. Silicon-based solar cells are widely used and have practical efficiency that keeps improving, reaching close to the theoretical limit of around 30%. One approach to increase the output of solar cells is converting the heat losses back into electricity, consequently boosting the overall efficiency of solar conversion. This heat recycling can be achieved by integrating photovoltaic (PV) devices with thermoelectric materials, which capture and recycle wasted heat. This thesis aims to lay the groundwork required for achieving this objective by studying the heat loss mechanisms and conducting evaluations of some of those mechanisms.This research focuses on understanding and categorizing the losses in solar cells, particularly the below bandgap energy and thermalization losses, which are responsible for more than half of the total losses. Two types of solar cells, crystalline silicon (c-Si) and CH3NH3PbI3 perovskite (C-P), are studied to analyze their loss characteristics.
Surface Engineering of Colloidal Group IV Nanocrystals for Optoelectronics
(2014-06) Wheeler, Lance
Colloidal nanocrystals (NCs), often synonymous with"quantum dots," represent a burgeoning class of next-generation optoelectronic materials. The promise of NCs is twofold: (i) Their optical properties are tunable and offer unique opportunities for enhanced energy conversion due to quantum confinement effects. (ii) The NCs can be processed into thin films using cost-efficient roll-to-roll printing techniques for large-scale integration into devices. Taken together, these two attributes enable a new platform for optoelectronic technology where energy-efficient devices can be produced at low costs. There is an array of research efforts to produce NC-based optoelectronic devices such as photovoltaic cells, light emitting devices, and photodetectors. Much of the recent progress in this direction hinges on the ability to manipulate the NC surface. Conventional solution synthesis yields NCs with ligands bound to metal surface atoms through a labile acid-base complex. The electrically-insulating native ligands are thus routinely exchanged to produce conductive NC arrays for devices integration. Just as surface manipulation has launched metal-based NCs to the forefront of optoelectronic technology, it is the inability to do so with the covalent surface of group IV (germanium and silicon) NCs that has greatly hindered progress. The motivation of this research is to bridge the gap between group IV and metal-based NCs in order to establish an abundant, non-toxic alternative to NCs that contain toxic lead or cadmium. The bridge is built by developing new Si NC surface chemistries, understanding how they interact with molecules, and applying chemical and physical models to uncover the mechanism of NC colloidal stability. The research begins by developing nonthermal plasma synthesis of Si NCs from a new precursor, silicon tetrachloride. This work builds on previous studies on chlorine-terminated germanium NCs synthesized from germanium tetrachloride, which were observed to form stable colloids without covalent ligand attachment. Synthesis from silicon tetrachloride offers the same flexibility for tuning size and crystallinity as typical silane synthesis but yields a new chlorinated surface chemistry. Si-Cl surface groups of the NCs are shown to be crucial for achieving the same colloidal stability observed in Ge NCs. It was determined spectroscopically the polarized Si-Cl surface bond renders the surface Si atoms Lewis acidic and capable of hypervalent interactions with Lewis basic molecules. The NCs were thus dispersible in select Lewis basic solvents. Interestingly, these interactions are also shown to be responsible for a reversible "surface doping" effect, which was also explored spectroscopically and by electrical characterization of a thin film device. The notion of a Lewis acidic surface gave rise to the development of a more robust Si NC surface chemistry. In this work, plasma synthesis that includes diborane is applied. The resulting Si NC surface is then terminated by a classic Lewis acid, boron, which is demonstrated to be an even more versatile chemistry than the Si-Cl surface. These NCs are also used as a model system for uncovering the mechanism of colloidal stability due to these surface interactions with solvent molecules. It is found that conventional theory cannot account for the stability observed, and a simple alternative model is developed. In light of this model, we are able to demonstrate stable Si NC colloids in media that spans hexane to water. The thesis concludes with a peripheral effort on Ge NCs, a material lacking in maturity even to Si NCs. In this work, the NC surface is modified to enhance the optical properties of the material as opposed to the ability to process the NCs into films from solution. Size-tunable band gap emission is demonstrated for the first time in gas-phase synthesized Ge NCs by applying Grignard chemistry to the Ge-Cl surface groups. The emission is narrower than any previous report, and emission near the bulk band gap of Ge is attained for the first time.
Synthesis and Doping of Silicon Nanocrystals for Versatile Nanocrystal Inks
(2015-05) Kramer, Nicolaas
The impact of nanotechnology on our society is getting larger every year. Electronics are becoming smaller and more powerful, the “Internet of Things” is all around us, and data generation is increasing exponentially. None of this would have been possible without the developments in nanotechnology. Crystalline semiconductor nanoparticles (nanocrystals) are one of the latest developments in the field of nanotechnology. This thesis addresses three important challenges for the transition of silicon nanocrys- tals from the lab bench to the marketplace: A better understanding of the nanocrystal synthesis was obtained, the electronic properties of the nanocrystals were characterized and tuned, and novel silicon nanocrystal inks were formed and applied using simple coating technologies. Plasma synthesis of nanocrystals has numerous advantages over traditional solution-based synthesis methods. While the formation of nanoparticles in low pressure nonthermal plasmas is well known, the heating mechanism leading to their crystallization is poorly understood. A combination of comprehensive plasma characterization with a nanoparticle heating model presented here reveals the underlying plasma physics leading to crystallization. The model predicts that the nanoparticles reach temperatures as high as 900 K in the plasma as a result of heating reactions on the nanoparticle sur- face. These temperatures are well above the gas temperature and sufficient for complete nanoparticle crystallization. Moving the field of plasma nanoparticle synthesis to atmospheric pressures is impor- tant for lowering its cost and making the process attractive for industrial applications. The heating and charging model for silicon nanoparticles was adapted in Chapter 3 to study plasmas maintained over a wide range of pressures (10 − 10^5 Pa). The model considers three collisionality regimes and determines the dominant contribution of each regime under various plasma conditions. Strong nanoparticle cooling at atmospheric pressures necessitates high plasma densities to reach temperatures required for crystallization of nanoparticles. Using experimentally determined plasma properties from the literature, the model estimates the nanoparticle temperature that is achieved during synthesis at atmospheric pressures. It was found that temperatures well above those required for crystallization can be achieved. Now that the synthesis of nanocrystals is understood, the second half of this thesis will focus on doping of the nanocrystals. The doping of semiconductor nanocrystals, which is vital for the optimization of nanocrystal-based devices, remains a challenge. Gas phase plasma approaches have been very successful in incorporating dopant atoms into nanocrystals by simply adding a dopant precursor during synthesis. However, little is known about the electronic activation of these dopants. This was investigated with field-effect transistor measurements using doped silicon nanocrystal films. It was found that, analogous to bulk silicon, boron and phosphorous electronically dope silicon nanocrystals. However, the dopant activation efficiency remains low as a result of self-purification of the dopants to the nanocrystal surface. Next the plasmonic properties of heavily doped silicon nanocrystals was explored. While the synthesis method was identical, the plasmonic behavior of phosphorus-doped and boron-doped nanocrystals was found the be significantly different. Phosphorus-doped nanocrystals exhibit a plasmon resonance immediately after synthesis, while boron-doped nanocrystals require a post-synthesis annealing or oxidation treatment. This is a result of the difference in dopant location. Phosphorus is more likely to be incorporated into the core of the nanocrystal, while the majority of boron is placed on the surface of the nanocrystal. The oxidized boron-doped particles exhibit stable plasmonic properties, and therefore this allows for the production of air-stable silicon-based plasmonic materials which is very interesting for certain applications. Finally the boron atoms were used to form a Lewis acidic nanocrystal surface chemistry allowing for the creation of ligand-less silicon nanocrystal solutions. This represents an immense step towards an abundant, non-toxic alternative to Pb and Cd-based nanocrystal technologies. The lack of long ligand chains enables the production of dense films with excellent electrical conductivity. This was demonstrated by forming uniform nanocrystal thin-films using simple and inexpensive spray coating techniques.
Understanding and Improving Plasma Synthesized Silicon Germanium Films for Thermoelectric Applications
(2017-10) Mork, Kelsey C
Laser sintering and Phenyl Acetylene surface functionalization of plasma synthesized silicon germanium nanoparticle films and powders were studied to improve electrical conductivities of these materials. Laser sintering greatly improved the electrical conductivity in thin films with peak values of 70.42 S/cm. Phenyl Acetylene functionalization was successful in both doped and undoped silicon germanium nanoparticles. Further studies should be performed to quantify the electrical conductivity values in bulk, compacted pellets of the functionalized nanoparticles. Finally, research into activation energies of compacted silicon germanium films showed drastic differences between energies obtained when using Seebeck coefficient and energies obtained when using electrical conductivity. This is attributed to traps on the surface of the nanoparticles and the potential barrier between nanoparticles.

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