Browsing by Subject "Nanostructures"
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Item Ab Initio Study of Nanostructures for Energy Storage(2014-05-07) Cristancho Albarracin, DahiyanaNanomaterials are expected to overcome the challenges imposed from bulk materials in the design of electronic devices. With the help of nanotechnology smaller, lighter, and more energy efficient materials can be used in the development of smart nanodevices. The goal of this research is to characterize the chemical, electrical, and mechanical properties of nanostructures for energy conversion and storage. In this dissertation, three materials are studied at nano level using theoretical calculations: carbon nanotubes (CNTs), lithium silicon (Li_(4n)Si_(n)), and polyvinyl alcohol (PVA). The coupling of mechanical and electronic properties of carbon nanotubes are studied, we estimate a modulus of elasticity of 1.3 TPa and find that the mechanism of CNT structure deformation is chirality dependent. Armchair and chiral nanotubes have ductile deformation fracture while zigzag have both ductile and brittle. Furthermore, the HOMO-LUMO gap of CNT increases under plastic deformation. We conclude that mechanical forces affect the electromagnetic absorption properties of CNTs. Silicon has been proposed as a promising material for anodes in Li ion batteries; a layer called: the solid electrolyte interphase (SEI) is formed on the electrodes during charging process that may restrict the ion mobility. Preliminary electrical characterization shows the external potential effects of SEI on electron transport as a function of SEI thickness. Furthermore, the rotation of the Li_(2)O molecules in SEI plays a big role in the electron transport in Li-Ion Batteries. Mechanical and thermal properties of polyvinyl alcohol (PVA) are characterized using in situ X-ray photoelectron spectroscopy (XPS) and theoretical calculations. It is found that the carbon peaks in PVA shifted under mechanical and thermal stretching. At different temperatures, the C-O bond was the most stable carbon group than others. We find that Hartree-Fock/10-31G (d) reproduces the binding energy of core carbon electrons, which is enough to characterize bonds and corroborate the spectroscopic analysis.Item Aspects of metal and Si-based nanomaterials: synthesis, stability and properties(2006) Elechiguerra Joven, José Luis; Yacaman, Miguel JoseMetal and Si-based nanostructures have drawn increasing interest due to their potential uses in catalysis, biological sensors, and nanoelectronics among others. Therefore, in the present work, several nanostructures were produced, characterized and tested. In particular, the conventional synthesis of noble-metal nanostructures through the polyol method was modified by replacing poly-vinyl pyrrolidone PVP with poly-diallyl dimethyl ammonium chloride PDDA. As PDDA is a cationic polyelectrolyte, the initial strong electrostatic interaction between PDDA and the anionic metal precursors produce the formation of stable ion pairs, so the reactivity of the different species can be tailored and particles with different internal structure, i.e. crystallinity, can be produced.Item A comprehensive study of 3D nano structures characteristics and novel devices(2008-12) Zaman, Rownak Jyoti; Banerjee, SanjaySilicon based 3D fin structure is believed to be the potential future of current semiconductor technology. However, there are significant challenges still exist in realizing a manufacturable fin based process. In this work, we have studied the effects of hydrogen anneal on the structural and electrical characteristics of silicon fin based devices: tri-gate, finFET to name a few. H₂ anneal is shown to play a major role in structural integrity and manufacturability of 3D fin structure which is the most critical feature for these types of devices. Both the temperature and the pressure of H₂ anneal can result in significant alteration of fin height and shape as well as electrical characteristics. Optimum H₂ anneal is required in order to improve carrier mobility and device reliability as shown in this work. A new hard-mask based process was developed to retain H₂ anneal related benefit while eliminating detrimental effects such as reduction of device drive current due to fin height reduction. We have also demonstrated a novel 1T-1C pseudo Static Random Access Memory (1T-1C pseudo SRAM) memory cell using low cost conventional tri-gate process by utilizing selective H₂ anneal and other clever process techniques. TCAD-based simulation was also provided to show its competitive advantage over other types of static and dynamic memories in 45nm and beyond technologies. A high gain bipolar based on silicon fin process flow was proposed for the first time that can be used in BiCMOS technology suitable for low cost mixed signal and RF products. TCAD-based simulation results proved the concept with gain as high 100 for a NPN device using single additional mask. Overall, this work has shown that several novel process techniques and selective use of optimum H₂ anneal can lead to various high performance and low cost devices and memory cells those are much better than the devices using current conventional 3D fin based process techniques.Item Electronic noise in nanostructures: limitations and sensing applications(Texas A&M University, 2007-04-25) Kim, Jong UnNanostructures are nanometer scale structures (characteristic length less than 100 nm) such as nanowires, ultra-small junctions, etc. Since nanostructures are less stable, their characteristic volume is much smaller compared to defect sizes and their characteristic length is close to acoustical phonon wavelength. Moreover, because nanostructures include significantly fewer charge carriers than microscale structures, electronic noise in nanostructures is enhanced compared to microscale structures. Additionally, in microprocessors, due to the small gate capacitance and reduced noise margin (due to reduced supply voltage to keep the electrical field at a reasonable level), the electronic noise results in bit errors. On the other hand, the enhanced noise is useful for advanced sensing applications which are called fluctuation-enhanced sensing. In this dissertation, we first survey our earlier results about the limitation of noise posed on specific nano processors. Here, single electron logic is considered for voltage controlled logic with thermal excitations and generic shot noise is considered for current-controlled logic. Secondly, we discuss our recent results on the electronic noise in nanoscale sensors for SEnsing of Phage-Triggered Ion Cascade (SEPTIC, for instant bacterial detection) and for silicon nanowires for viral sensing. In the sensing of the phage-triggered ion cascade sensor, bacteriophage-infected bacteria release potassium ions and move randomly at the same time; therefore, electronic noise (i.e., stochastic signals) are generated. As an advanced model, the electrophoretic effect in the SEPTIC sensor is discussed. In the viral sensor, since the combination of the analyte and a specific receptor located at the surface of the silicon nanowire occurs randomly in space and time, a stochastic signal is obtained. A mathematical model for a pH silicon nanowire nanosensor is developed and the size quantization effect in the nanosensor is also discussed. The calculation results are in excellent agreement with the experimental results in the literature.Item Electronic transport under strong optical radiation and quantum chaos in semiconductor nanostructures(2003) Li, Wenjun; Reichl, L. E.Item Enhanced Catalytic Activities of Nanostructured Materials(2014-10-31) Martinez De La Hoz, Julibeth MilenaCatalysis has enabled the development of very important industrial processes, especially those related to the petroleum and chemical industries. This has led to a significant influence in the worldwide economy, with 20% of it depending on catalysis. Current reliance of the industrial world on catalysis and rapidly increasing worldwide energy prices have motivated the search for improved catalysts allowing more energy-efficient processes. Catalysts performance is affected by the shape, structure, and chemical composition of the catalysts. Fortunately, the development of nanotechnology has allowed researchers to control the structure and morphology of catalyst nanoparticles, as well as that of solid supports. Even though, these approaches have enhanced the reactivity of materials towards specific reactions, there is still much more room for improvement. In this work, the incorporation of electron-rich environments into the structure of nanocatalysts is proposed as a new approach for the enhancement of the catalytic activity of nanomaterials. This study is conducted in its entirety using computational quantum-based simulations. The effect of electron-rich regions on activation barriers for the dissociation of diatomic molecules is studied using metallic slit-type pores, finding that electron-rich environments enhance the reactivity of nanomaterials by reducing activation barriers required for the dissociation of molecules. The influence of electronic and geometric effects in the pores is also evaluated. It is found that local geometric characteristics, such as stacking planes forming the pore, and the presence of step-like defects influence adsorption energies and barriers for dissociation of molecules. Additionally, electrons inside the metallic pores have energies close to the Fermi-energy of the metal surfaces, which may allow tuning their energies for interactions with LUMO anti-bonding orbitals of specific molecules. Subsequently, electron-rich regions are incorporated into a 3D nanostructured material (Pt22/NPG). This proposed catalyst shows enhanced reactivity towards the dissociation of gas-phase molecules. Additionally, Pt22/NPG may display enhanced reactivity, even when electron-rich regions do not interact with the molecules of interest, due to the good dispersion of Pt-clusters. Therefore, the incorporation of electron-rich environments into nanocatalysts is shown to be an efficient approach for the enhancement of the catalytic activity of nanomaterials.Item Experimental and theoretical investigation of thermal and thermoelectric transport in nanostructures(2010-05) Moore, Arden Lot, 1982-; Shi, Li, Ph. D.; Ezekoye, Ofodike; Ferreira, Paulo; Howell, John; Tutuc, EmanuelThis work presents the development and application of analytical, numerical, and experimental methods for the study of thermal and electrical transport in nanoscale systems, with special emphasis on those materials and phenomena which can be important in thermoelectric and semiconductor device applications. Analytical solutions to the Boltzmann transport equation (BTE) using the relaxation time approximation (RTA) are presented and used to study the thermal and electrical transport properties of indium antimonide (InSb), indium arsenide (InAs), bismuth telluride (Bi₂Te₃), and chromium disilicide (CrSi₂) nanowires. Experimental results for the thermal conductivity of single layer graphene supported by SiO₂ were analyzed using an RTA-based model and compared to a full quantum mechanical numerical BTE solution which does not rely on the RTA. The ability of these models to explain the measurement results as well as differences between the two approaches are discussed. Alternatively, numerical solutions to the BTE may be obtained statistically through Monte Carlo simulation for complex geometries which may prove intractable for analytical methods. Following this approach, phonon transport in silicon (Si) sawtooth nanowires was studied, revealing that thermal conductivity suppression below the diffuse surface limit is possible. The experimental investigation of energy transport in nanostructures typically involved the use of microfabricated devices or non-contact optical methods. In this work, two such approaches were analyzed to ascertain their thermal behavior and overall accuracy as well as areas for possible improvement. A Raman spectroscopy-based measurement design for investigating the thermal properties of suspended and supported graphene was examined analytically. The resulting analysis provided a means of determining from measurement results the thermal interface conductance, thermal contact resistance, and thermal conductivity of the suspended and supported graphene regions. Previously, microfabricated devices of several different designs have been used to experimentally measure the thermal transport characteristics of nanostructures such as carbon nanotubes, nanowires, and thin films. To ascertain the accuracy and limitations of various microdevice designs and their associated conduction analyses, finite element models were constructed using ANSYS and measurements of samples of known thermal conductance were simulated. It was found that designs with the sample suspended were generally more accurate than those for which the sample is supported on a bridge whose conductance is measured separately. The effects of radiation loss to the environment of certain device designs were also studied, demonstrating the need for radiation shielding to be at temperatures close to that of the device substrate in order to accurately calibrate the resistance thermometers. Using a suspended microdevice like those analyzed using finite element analysis, the thermal conductivities of individual bismuth (Bi) nanowires were measured. The results were correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk Bi in the same crystal direction, the thermal conductivity of a single-crystal Bi nanowires of 232 nm diameter was found to be 3 - 6 times smaller than bulk between 100 K and 300 K. For polycrystalline Bi nanowires of 74 nm to 255 nm diameter the thermal conductivity was reduced by a factor of 18 - 78 over the same temperature range. Comparable thermal conductivity values were measured for polycrystalline nanowires of varying diameters, suggesting a grain boundary scattering mean free path for all heat carriers in the range of 15 - 40 nm which is smaller than the nanowire diameters. An RTA-based transport model for both charge carriers and phonons was developed which explains the thermal conductivity suppression in the single-crystal nanowire by considering diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 10¹⁹ cm⁻³. Using a similar experimental setup, the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of higher manganese silicide (HMS) nanostructures were investigated. Bulk HMS is a passable high temperature thermoelectric material which possesses a complex crystal structure that could lead to very interesting and useful nanoscale transport properties. The thermal conductivities of HMS nanowires and nanoribbons were found to be reduced by 50 - 60 % compared to bulk values in the same crystal direction for both nanoribbons and nanowires. The measured Seebeck coefficient data was comparable or below that of bulk, suggesting unintentional doping of the samples either during growth or sample preparation. Difficulty in determining the amorphous oxide layer thickness for nanoribbons samples necessitated using the total, oxide-included cross section in the thermal and electrical conductivity calculation. This in turn led to the determined electrical conductivity values representing the lower bound on the actual electrical conductivity of the HMS core. From this approach, the measured electrical conductivity values were comparable or slightly below the lower end of bulk electrical conductivity values. This oxide thickness issue affects the determination of the HMS nanostructure thermoelectric figure of merit ZT as well, though the lower bound values obtained here were found to still be comparable to or slightly smaller than the expected bulk values in the same crystal direction. Analytical modeling also indicates higher doping than in bulk. Overall, HMS nanostructures appear to have the potential to demonstrate measurable size-induced ZT enhancement, especially if optimal doping and control over the crystallographic growth direction can be achieved. However, experimental methods to achieve reliable electrical contact to quality four-probe samples needs to be improved in order to fully investigate the thermoelectric potential of HMS nanostructures.Item First-principles atomistic modeling for property prediction in silicon-based materials(2010-12) Bondi, Robert James; Hwang, Gyeong S.; Mullins, C. B.; Ekerdt, John G.; Chelikowsky, James R.; Banerjee, Sanjay K.The power of parallel supercomputing resources has progressed to the point where first-principles calculations involving systems up to 10³ atoms are feasible, allowing ab initio exploration of increasingly complex systems such as amorphous networks, nanostructures, and large defect clusters. Expansion of our fundamental understanding of modified Si-based materials is paramount, as these materials will likely flourish in the foreseeable cost-driven future in diverse micro- and nanotechnologies. Here, density-functional theory calculations within the generalized gradient approximation are applied to refine configurations of Si-based materials generated from Metropolis Monte Carlo simulations and study their resultant structural properties. Particular emphasis is given to the contributions of strain and disorder on the mechanical, optical, and electronic properties of modified Si-based materials in which aspects of compositional variation, phase, strain scheme, morphology, native defect incorporation, and quantum confinement are considered. The simulation strategies discussed are easily extendable to other semiconductor systems.Item Gas transport properties of reverse selective nanocomposite materials(2007-12) Matteucci, Scott Tyson, 1976-; Freeman, B. D., (Benny D.)The effect of dispersing discreet periclase (magnesium oxide) or brookite (titanium oxide) nanoparticles into poly(1-trimethylsilyl-1-propyne) (i.e., a super glassy polymer) and 1,2-polybutadiene (i.e., a rubbery polymer) has been examined. Particle dispersion has been investigated using atomic force microscopy and transmission electron microscopy to determine particle/aggregate size and distribution. Titanium dioxide nanoparticles dispersed into aggregates on the order of nanometers, as did magnesium oxide in 1,2-polybutadiene. However, the magnesium oxide filled poly(1-trimethylsilyl-1-propyne) did not exhibit nanoparticle aggregates below approximately one micron in characteristic dimensions. Nanocomposite transport properties were studied, where permeability and solubility coefficients were determined for light gases with increasing pressure, and diffusion coefficients were calculated from the solution-diffusion model. The permeability of light gases in the heterogeneous films increased with increasing particle loading. Depending on particle loading, brookite filled nanocomposite light gas permeability increased to over four times that of the unfilled polymer, whereas at high periclase loadings the nanocomposites exhibited light gas permeabilities in excess of an order of magnitude higher than the unfilled materials. Even at these high loadings the light gas selectivities were higher than predicted for films containing transmembrane defects. Solubility was relatively unaffected by the void volume concentration, although it did increase to some extent depending on the nanoparticle concentration. Wide angle X-ray diffraction, nuclear magnetic resonance, and Fourier transform infra-red experiments were used to determine if the nanoparticles remained stable during film preparation. TiO₂ nanoparticles did not appear to react with water, the polymer matrixes or test gases used in this research. However, under certain circumstances, periclase reacted with adventitious water to form brucite. A desilylation reaction occurred when brucite was exposed to polymers or small molecule compounds that contained a trimethylsilyl group attached to a conjugated organic backbone. This reaction caused certain disubstituted polyacetylenes to become insoluble in common organic solvents.Item Local electrostatic potential and strain characterization of semiconductor nanostructures(2006) Chung, Jayhoon; Rabenberg, LlewellynMaterials characterization techniques that determine the local charge transport properties of electronic devices are of increasing importance. Dopant profiles, electrostatic potential distributions near interfaces, strains, and defects within nanoscale devices should be characterized very locally, two-dimensionally and quantitatively for a complete understanding of device characteristics. Transmission electron microscopy (TEM) has the ability to resolve the atomic structure of materials, but conventional TEM methods do not give all the information that would be desirable for complete device characterization. This dissertation examines two advanced phase reconstruction techniques that can be used to characterize electrostatic potentials and strains in semiconductor nanostructures. Electron holography was utilized to measure the electrostatic potentials associated with charges and their distribution within a core/shell nanowire. Electron holography was optimized for the nanowire geometry using a dual-lens imaging configuration. Using this method, the mean inner potential of intrinsic germanium and its oxide were determined to be 15.50 ± 0.44 V and 9.10 ± 0.42 V, respectively. The B concentration within the B-doped shell of the core/shell nanowire was determined through a comparison of measured and simulated phase profiles. Fermi level pinning at interface states between the doped shell and the inner germanium oxide was also observed by electron holography. The screening length and the potential in the interface charge region were quantitatively measured. These characteristics compared favorable with the values obtained from numerical solution of Poisson’s equation. The local strain in a strained silicon (sSi) wafer was characterized using geometric phase analysis of high-resolution TEM (HRTEM) images. The method enables the reconstruction of strain maps from HRTEM images by digital image processing alone, when the HRTEM images were taken under careful controlled imaging conditions. Using specimens with known strain values, this method was confirmed to give a reliable, quantitative measure of strains in a sSi structure. Geometric phase analysis was also applied to real sSi layers grown on relaxed SiGe alloys containing either 43.9 or 17.7 atomic percent Ge. The defects and stress relaxation of these wafers were also analyzed.Item Molecular Imaging of Amyloid-Beta Proteins by Polymeric Anoparticles in Mouse Models of Alzheimer's Disease(2006-12-20) Roney, Celeste Anita; Bonte, Frederick J.Alzheimer's disease (AD) is the most common cause of dementia among the elderly, affecting 5% of Americans over age 65, and 20% over age 80. An excess of senile plaques (beta -amyloid protein) and neurofibrillary tangles (tau protein), ventricular enlargement, and cortical atrophy characterizes it. In vivo detection of aggregated amyloid peptides (Abeta ) (amyloid plaques) presents targets for development of biological markers for Alzheimer's disease. In an effort to fabricate in vivo probes, polymeric n-butyl-2-cyanoacrylate (BCA) nanoparticles (NPs) were encapsulated with the radiolabelled amyloid affinity drug 125Iclioquinol (CQ, 5-chloro-7-iodo-8-hydroxyquinoline). 125ICQ was initially selected as a tracer of interest because it chelates transition metals, crosses the BBB, and is easily labeled with radioisotopes of iodine (e.g. 123I, 124I and 125I). Preliminary studies with 125ICQ showed that the agent crossed the BBB, but was retained too briefly for effective chelation. Therefore, a drug carrier is required to improve the extravascular retention of 125ICQ; BCA NPs were chosen as the drug carrier. 125I-CQ discriminately binds to AD post-mortem brain tissue homogenates, versus control. Additionally, 125I-CQ-BCA NPs labeled the Abeta plaques from AD human post-mortem frontal cortical sections, on paraffin-fixed slides. Storage phosphor imaging verified preferential uptake by the AD brain sections, compared to cortical control sections. Additionally, 125I-CQ-BCA NPs cross the BBB in the wild type mouse, with an enhanced brain uptake (%ID/g, significant with 95% confidence (p=0.05)), compared to 125I-CQ. Moreover, brain uptake of 125I-CQ-BCA NPs is enhanced in AD transgenic mice (APP/PS1 and APP/PS1/Tau), and in mice intracranially injected with the aggregated Abeta peptide, versus age-matched wild type controls. Thus, 125I-CQ-BCA NPs act as targeted drug carriers with an affinity for amyloid plaques. Brain entry of 125I-CQBCA NPs was rapid, demonstrating ideal in vivo imaging characteristics for small animal modalities; good clearance of free and non-specifically bound radioisotope affords high-quality temporal resolution, and good signal-to-noise. 125I-CQ-BCA NPs have specificity for the Abeta plaques in post-mortem tissue, and have a rapid brain entry. This combination makes radioiodinated CQ-BCA NPs a promising candidate as an in vivo SPECT (123I), or PET (124I) amyloid imaging agent.Item Real-space pseudopotential calculations for the electronic and structural properties of nanostructures(2010-08) Han, Jiaxin; Chelikowsky, James R.; Demkov, Alex; MacDonald, Allan; Korgel, Brian; Kleinman, LeonardNanostructures often possess unique properties, which may lead to the development of new microelectronic and optoelectronic devices. They also provide an opportunity to test fundamental quantum mechanical concepts such as the role of quantum confinement. Considerable effort has been made to understand the electronic and structural properties of nanostructures, but many fundamental issues remain. In this work, the electronic and structural properties of nanostructures are examined using several new computational methods. The effect of dimensional confinement on quantum levels is investigated for hydrogenated Ge <110> using the plane-wave density-functional-theory pseudopotential method. We present a real-space pseudopotential method for calculating the electronic structure of one-dimensional periodic systems such as nanowires. As an application of this method, we examine H-passivated Si nanowires. The band structure and heat of formation of the Si nanowires are presented and compared to plane wave methods. Our method is able to offer the same accuracy as the traditional plane wave methods, but offers a number of computational advantages such as the ability to handle large systems and a better ease of implementation for highly parallel platforms. Doping is important to many potential applications of nano-regime semiconductors. A series of first-principles studies are conducted on the P-doped Si <110> nanowires by the real-space pseudopotential methods. Nanowires of varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the wires, energetics of P atoms in different doping positions and core-level shift of P atoms. Defect wave functions of P atoms are also analyzed. In addition, we study the electronic properties of phosphorus-doped silicon <111> nanofilms using the real-space pseudopotential method. Nanofilms with varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the films, and energetics of P atoms in different doping positions. Quantum confinement effects are compared with P-doped Si nanocrystals and as well as nanowires. We simulate the nanofilm STM images with P defects in varied film depths, and make a comparison with the experimental measurement.Item Schrödinger equation Monte Carlo simulation of nano-scaled semiconductor devices(2004) Chen, Wanqiang; Register, Leonard F.Semiconductor devices have been continuously scaled into the deep submicron regime. As a result, quantum effects which were neglected in semiclassical models become more and more important. Meanwhile, scattering still remains important down to the gate length around 10 nm. Accurate quantum transport simulators with scattering will be needed to explore the essential device physics. The work of this dissertation project is aimed at developing an accurate quantum transport simulation tool for deep submicron device modeling, as well as utilizing this newly developed simulation tool to study the quantum transport and scattering effects in ultra-scaled semiconductor devices. The quantum transport simulator “Schrödinger Equation Monte Carlo” (SEMC) provides a physically rigorous treatment of quantum transport and phasebreaking inelastic scattering (in 3D) via real (actual) scattering processes such as optical and acoustic phonon scattering. SEMC has been used to simulate carrier transport in nano-scaled devices in order to gauge the potential reliability of semiclassical models, phase-coherent quantum transport, and other limiting models as the transition from classical to quantum transport is approached. SEMC has also been successfully applied to study the carrier capture and transport in tunnel injection lasers. In this work, a 2D version of SEMC − SEMC-2D − has been developed. The quantum transport equations are solved self-consistently with Poisson equation. SEMC-2D has been used to simulate quantum transport in nano-scaled double gate MOSFETs. Simulation results serve not only to demonstrate the capability of this new quantum transport simulator, but also to illuminate the importance of physically accurate simulation of scattering for predictive modeling of transport in nano-scaled MOSFETs.Item Self-Assembly of Organic Nanostructures(2012-10-19) Wan, AlbertThis dissertation focuses on investigating the morphologies, optical and photoluminescence properties of porphyrin nanostructures prepared by the self-assembly method. The study is divided into three main parts. In the first part, a large variety of porphyrin nanostructures, including nanoplates, nanofibers, nanoparticles and nanowires, were obtained through direct acidification of tetra(p-carboxyphenyl)porphyrin (TCPP) in aqueous solution. Protonation of the carboxylate groups of TCPP resulted in the formation of nanoplates through the J-aggregation of the porphyrin. Further protonating the core nitrogens of TCPP formed the porphyrin diacids which organized into well-defined structures through their interactions with counter-anions in the solution. The structures of the resulting assemblies were found to be counterion dependent. In the second part of this work, we explored the optical memory effect of the porphyrin thin film. We found that the morphology and the emission of the porpyrin thin film on Si can be changed by varying the pH of its surrounding solution. The changing in morphology and light emission of the thin film resulted from the protonation or deprotonation of TCPP'S core nitrogens. By selectively deprotonating the TCPP dications in a confined region utilizing the water meniscus between an AFM tip and the surface, Fluorescence patterns can be generated on the thin film. The fluorescence patterns can be easily erased by re-protonating the porphyrin. In the third part of this study, porphynoid nanoparticles were deposited on a surface energy gradient, and then characterized by AFM in order to investigate how the surface energy influences thier morphologies. The surface energy gradient was prepared by selectively oxidizing a self-assembly monolayer of octadecyltrichlorosilane (OTS) by UV-ozone. The nanoparticles disassemble into smaller nanoparticles with narrower size distribution on the surface with higher surface energy. Lastly, we engaged in characterizing the morphologies of polymer nanocomposites prepared by layer-by-layer assembly for wettability control. The surface roughness of the nanocopmosite in air and in salt solutions was also measured to study the correlation between the wettability of the polymer surface and its surface roughness.Item Synthesis and characterization of III-V semiconductor nanowires and fabrication of colloidal nanorod solar cells(2006) Davidson, Forrest Murray; Korgel, Brian A.Nanowires have attracted intensive research efforts due to their one-dimensional quantum confinement and their ability to serve as the building blocks for and functional components of future semiconductor devices. Widespread use of nanowires in bottom-up device fabrication will require a general method for the controllable synthesis of nanowires with regards to shape, size, composition, and interfacial properties. Gallium arsenide and gallium phosphide nanowires as small as 8 nm in diameter were synthesized in supercritical hexane and seeded by alkanethiol-stabilized 7 nm gold nanocrystals. The wires are single crystal with a zinc-blende structure and grow exclusively in the <111> direction. The importance of precursor degradation kinetics was explored. Multiple lamellar {111} twins are observed in GaAs, GaP and InAs nanowires synthesized by supercritical fluid-liquid-solid (SFLS) and solution-liquid-solid (SLS) approaches. All of these nanowires have zinc blende (cubic) crystal structure and were grown in the <111> direction. The twins cross-section the nanowires to give them a “bamboo”-like appearance in TEM images. In contrast, Si and Ge nanowires with <111> growth direction do not exhibit {111} twins, even though this is a common twin plane with relatively low twin energy in diamond cubic Ge and Si. However, Si and Ge nanowires with <112> growth directions typically have several {111} twins extending down the length of the nanowires. A semi-quantitative model that explains the observed twinning in III-V and IV nanowires is presented. Heterojunction solar cells were fabricated from colloidal solutions of CdTe and CdSe nanorods by sequential spin casting onto ITO coated glass substrates. A broad range of factors impacting the success of the solar cell fabrication were explored; including the method of nanorod synthesis, choice of capping ligand, method of active layer application, and use of hydrazine treatment. It is necessary to maximize the stability of the nanords in solution to ensure even application into thin films. However, electrical properties of the nanorod films are improved by using weaker stabilizing agents. The devices exhibit good diode behavior.Item Synthesis and field emission studies of 1-D nanostructures(2005) Kulkarni, Niraj Narasinha; Shih, Chih-Kang; Yao, Zhen, Ph. D.1-D Nanostructures are attractive candidates for electron emitters in vacuum microelectronic devices because of their sharp tip radii and high aspect ratio. With advances in nanotechnology, various strategies have been reported for controlled synthesis of nanostructures including 1-D variants (nanowires and nanotubes). While various functional electronic/optoelectronic devices and circuits have been demonstrated using nanostructures, this work is focused on the synthesis and field emission studies of 1-D nanostructures of three materials systems, namely carbon nanotubes, silicon nanowires and graphitic nanocones. The potential applications of 1-D nanostructures as electron emitters are varied and include displays, microwave amplifiers, x-ray sources, holography, multiple e-beam lithography, electronic cooling. The carbon nanotubes (CNTs) are grown in anodic alumina templates by thermal chemical vapor deposition (CVD). The Silicon Nanowires (Si NWs) are grown by atmospheric pressure CVD (APCVD) via hydrogen reduction of silicon tetrachloride with an Au thin film acting as the catalyst for the Vapor-Liquid-Solid (VLS) process. Further post-growth processing was employed in the case of Si NWs, namely in-situ annealing and cesiation, to improve the field emission characteristics. Finally, field emission characterization of individual tubular graphitic nanocones (TGCs) was carried out. The TGCs were grown on iron needle by microwave plasma assisted CVD of C2H2 + N2. An individual nanocone emitted a current as high as 80 µA, corresponding to a current density of ~ 108 A/cm2 , without breakdown. Individual emitters would be of interest for applications in holography and as coherent electron sources.Item Synthesis and thermoelectric properties of higher manganese silicides for waste heat recovery(2014-12) Chen, Xi, active 21st century; Shi, Li, Ph. D.; Zhou, JianshiThermoelectric (TE) materials, which can convert temperature gradients directly into electricity and vice versa, have received renewed interest for waste heat recovery and refrigeration applications. Higher manganese silicides (HMS) are promising p-type TE materials due to the abundance of the constituent elements, environmental friendliness, and good chemical stability. The objective of this dissertation is to establish a better understanding of the structure-TE properties relationship of HMS with a complex Nowotny chimney ladder structure. The focus of this work is on the investigations of the phonon dispersion of HMS crystals and the effects of chemical doping and nanostructuring on the TE properties of polycrystalline HMS. HMS crystals have been synthesized by the Bridgeman method for inelastic neutron scattering measurements of the phonon dispersion. In conjunction with density functional theory calculations, the results clearly show the presence of numerous low-lying optical phonon branches, especially an unusually low-energy optical phonon polarization associated with the twisting motions of the Si helical ladders in the Mn chimneys. The obtained phonon dispersion can be used to explain the low and anisotropic thermal conductivity of HMS crystals. (Al,Ge) double doping was found to be effective in modifying the electrical properties of HMS polycrystals. The peak thermoelectric power factor occurs at an optimized hole concentration of 1.8~2.2×10²¹ cm⁻³ at room temperature. On the other hand, Re substitution can suppress the lattice thermal conductivity to approach the calculated minimum value corresponding to the amorphous limit. Meanwhile, the thermoelectric power factor does not markedly change at low Re content of x ≤ 0.04 although it drops considerably with increasing Re content. Hence, the peak ZT has been improved to ~0.6 in both systems. The effects of nanostructuring on the TE properties have been studied in the cold-pressed samples and ball-milled samples. The thermal conductivity was reduced remarkably by decreasing the grain size. It is found that the grain size effects are more significant at low temperature. However, it is difficult to reduce the grain size to less than 50 nm without the formation of impurity phases by ball milling. These facts limit the ZT enhancement of the nanostructured HMS at high temperatures in this study.Item Synthesis of silicon/germanium nanowires and field emission studies of 1-D nanostructures(2007-05) Bae, Joonho, 1972-; Shih, Chih-KangUsing the vapor-liquid-solid (VLS) growth method, silicon nanowires and germanium nanowires are grown. We find the high growth rate is responsible for the silicon nanowires with less growth defects when they are grown by use of silicon tetrachloride as a precursor and hydrogen as a carrier gas. Based on this funding, large area, high aspect ratio, h111i oriented silicon nanowires are successfully grown on Si (111) and Si (100). Novel growth mechanisms of VLS growth method were discovered in SiOx nanoflowers and silicon nanocones. In SiOx nanoflowers grown at the tip of silicon nanowires, it is found that they are produced via the enhanced oxidation of silicon at the gold-silicon interface. Furthermore, the analysis of the flower pattern reveals that it is the observation of the dense branching morphology on nanoscale and on spherical geometry. For the silicon nanocones, they are grown by the in situ etching of the catalysts of Ga/Al by HCl during the growth. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) reveal that the nanocones are composed of amorphous silicon oxides and crystalline Si. Based on the similar chemistry of hydrogen reduction of SiCl₄ for the growth of silicon nanowires, single crystalline germanium nanowires are grown by use of GeCl4 as a precursor and H₂ as a carrier gas. As one of important application of one dimensional nanostructures, the field emission properties of 1-D nanostructures are explored. The field emission properties of a single graphite nanocone are measured in SEM. The inter-electrode separation is controlled using scanning tunneling microscopy (STM) approach method, allowing the precise and ne determination of the separation. Its Fowler-Nordheim plot shows it emits currents in accordance with the Fowler-Nordheim field emission. Its onset voltage, field enhancement factor show that its basic field emission parameters are comparable to those of a single carbon nanotube. It is observed that single nanocone is damaged after emitting a current of about 100 nA, which seems to be due to its hollow interior structure.