abstract: Silicon nanowires were grown epitaxially on Si (100) and (111) surfaces using the Vapor-Liquid-Solid (VLS) mechanism under both thermal and plasma enhanced growth conditions. Nanowire morphology was investigated as a function of temperature, time, disilane partial pressure and substrate preparation. Silicon nanowires synthesized in low temperature plasma typically curved compared to the linear nanowires grown under simple thermal conditions. The nanowires tended bend more with increasing disilane partial gas pressure up to 25 x10-3 mTorr. The nanowire curvature measured geometrically is correlated with the shift of the main silicon peak obtained in Raman spectroscopy. A mechanistic hypothesis was proposed to explain the bending during plasma activated growth. Additional driving forces related to electrostatic and Van der Waals forces were also discussed. Deduced from a systematic variation of a three-step experimental protocol, the mechanism for bending was associated with asymmetric deposition rate along the outer and inner wall of nanowire. The conditions leading to nanowire branching were also examined using a two-step growth process. Branching morphologies were examined as a function of plasma powers between 1.5 W and 3.5 W. Post-annealing thermal and plasma-assisted treatments in hydrogen were compared to understand the influences in the absence of an external silicon source (otherwise supplied by disilane). Longer and thicker nanowires were associated with longer annealing times due to an Ostwald-like ripening effect. The roles of surface diffusion, gas diffusion, etching and deposition rates were examined. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011

Nanowires and nanotubes with single component such as gold and nickel were fabricated by electrochemical deposition method directed by the Anodic Alumina Membrane (AAO) as a template. A so called "polymer-free" method has been investigated to make striped nanowires with superlattice structure. Various methods have been used to characterize these nanomaterials, including SEM, FESEM TEM, STM and Optical Microscope. The aggregation of the nanowires and their alignment under the magnetic force were observed under the optical microscope.

The objective of this research was to develop the initial constituents of a highly scalable and label-free electronic biosensing platform. Current immunoassays are becoming increasingly incapable of taking advantage of the latest advances in disease biomarker identification, hindering their utility in the potential early-stage diagnosis and treatment of many diseases. This is due primarily to their inability to simultaneously detect large numbers of biomarkers. The platform presented here - termed the electronic microplate - embodies a number of qualities necessary for clinical and laboratory relevance as a next-generation biosensing tool. Silicon nanowire (SiNW) sensors were fabricated using a purely top-down process based on those used for non-planar integrated circuits on silicon-on-insulator wafers and characterized in both dry and in biologically relevant ambients. Canonical pH measurements validated the sensing capabilities of the initial SiNW test devices. A low density SiNW array with fluidic wells constituting isolated sensing sites was fabricated using this process and used to differentiate between both cancerous and healthy cells and to capture superparamagnetic particles from solution. Through-silicon vias were then incorporated to create a high density sensor array, which was also characterized in both dry and phosphate buffered saline ambients. The result is the foundation for a platform incorporating versatile label-free detection, high sensor densities, and a separation of the sensing and electronics layers. The electronic microplate described in this work is envisioned as the heart of a next-generation biosensing platform compatible with conventional clinical and laboratory workflows and one capable of fostering the realization of personalized medicine.

Thermal properties of one dimensional nanostructures are of interest for thermoelectric energy conversion. Thermoelectric efficiency is related to non dimensional thermoelectric figure of merit, ZT=S^2 o T/k, where S ,o , k and T are Seebeck coefficient, electrical conductivity, thermal conductivity and the absolute temperature respectively. These physical properties are interdependent. Therefore, making materials with high ZT is a very challenging task. However, nanoscale materials can overcome some of these limitations. When the size of nanomaterials is comparable to wavelength and mean free path of energy carriers, especially phonons, size effect contributes to the thermal conductivity reduction without bringing about major changes in the electrical conductivity and the Seebeck coefficient. Therefore, the figure of merit ZT can be manipulated. For example, the thermal conductivities of several silicon nanowires were more than two orders of magnitude lower than that of bulk silicon values due to the enhanced boundary scattering. Among the nanoscale semiconductor materials, Silicon-Germanium(SiGe) alloy nanowire is a promising candidate for thermoelectric materials The thermal conductivities of SiGe core-shell nanowires with core diameters of 96nm, 129nm and 177nm were measured using a batch fabricated micro device in a temperature range of 40K-450K. SiGe nanowires used in the experiment were synthesized via the Vapour-Liquid-Solid (VLS) growth method. The thermal conductivity data was compared with thermal conductivity of Si and Ge nanowires. The data was compared with SiGe alloy thin film, bulk SiGe, Si/SixGe1-x superlattice nanowire, Si/Si0.7Ge0.3 superlattice thin film and also with the thermal conductivity of Si0.5Ge0.5 calculated using the Einstein model. The thermal conductivities of these SiGe alloy nanowires observed in this work are ~20 times lower than Si nanowires, ~10 times lower than Ge nanowires, ~3-4 times lower than Si/SixGe1-x superlattice thin film, Si/SixGe1-x superlattice nanowire and about 3 time lower than bulk SiGe alloy. The low values of thermal conductivity are majorly due to the effect of alloy scattering, due to increased boundary scattering as a result of nanoscale diameters, and the interface diffuse scattering by core-shell effect. The influence of core-shell effect, alloy scattering and boundary scattering effect in reducing the thermal conductivity of these nanowires opens up opportunities for tuning thermoelectric properties which can pave way to thermoelectric materials with high figures of merit in the future.

The molecular dynamics is employed to investigate the dynamical behavior of helical multi-shell gold nanowire with diameter < 2nm . The study can be arranged into two parts, which are part I ¡§The investigation of the dynamical behavior of 7-1 gold nanowire on different axial strain¡¨ and part II ¡§the investigation of the self-assembly of crossed multi-shell gold nanowires ¡¨. In part I: We investigate the dynamical behavior of 7-1 gold nanowire on different axial strain. Some physical properties can also be determined during the tensile process, which including the strain-stress relationship, yield stress, and bond length. Moreover, vibrational properties under different tensile strains also are discussed . In part II: The aim of this work is to investigate the effect of fixed and flexible boundary conditions during the self-assembly of crossed multi-shell gold nanowires. The atomic trajectory and deformation morphology have been discussed during the assembly process. In addition, the structure transformation has also been observed on the junction by estimating the Angular Correlation Function (ACF).

¡@Silver nanowires prepared by the the reduction of AgNO3 at low temperature with thermocatalystic biphase (anatase and brookite phases) TiO2 nanoparticles are described. Furthermore, the possible mechanism to grow silver nanowires without the help of the Ag seed and capping reagent is proposed. ¡@Firstly, the amorphous TiO2 nanoparticles prepared by sol-gel method were spin-coated on the silicon wafer to form amorphous TiO2 matrix. Then an aqueous AgNO3 (1 &#x00B5;L 0.7 M) solution was dropped on the amorphous TiO2 matrix. Following the heat treatment at 200 ¢XC for 8 h, the silver nanowires (length~10 &#x00B5;m, line width~100 nm) were grown on the silicon wafer. We found that amorphous phase of TiO2 was changed to the anatase and brookite phases during the thermal reduction of the aqueous solution of AgNO3. ¡@Silver nanowires were characterized as f.c.c. structure by XRD. The TiO2 particles play an important role in providing electrons and holes for redox reaction and nucleation. With the controlling of the heating temperature and the amount of AgNO3, the silver nanowires were selectively grown in one dimension with large energetic surface. A combination of HR-TEM imaging and selected area electron diffraction reveals that the growing direction for the Ag wires is <011>.

Nanowires are sensitive to external influences such as surface charges or external electric fields. An Atomic Force Microsope (AFM) is modified to perform back gating and tip gating measurements in order to understand the interaction between an external field, and surface charge and nanowire conductance. A 2D finite element method (FEM) model is developed to simulate the measured conductance. The model shows that surface states play a critical role in determining nanowire conductance. A 3D FEM model is developed to examine the influence of the AFM tip on the lateral resolution of the AFM tip in the electrostatic measurement. The radius of the AFM tip determines the lateral resolution of the tip. However, carrier concentration in the nanowire establishes a lower limit on the lateral resolution, for small tip radii. These results enable one to optimize Scanning Probe Microscopy experiments as well as inform sample preparation for nanowire characterization.

Nanowires are sensitive to external influences such as surface charges or external electric fields. An Atomic Force Microsope (AFM) is modified to perform back gating and tip gating measurements in order to understand the interaction between an external field, and surface charge and nanowire conductance. A 2D finite element method (FEM) model is developed to simulate the measured conductance. The model shows that surface states play a critical role in determining nanowire conductance. A 3D FEM model is developed to examine the influence of the AFM tip on the lateral resolution of the AFM tip in the electrostatic measurement. The radius of the AFM tip determines the lateral resolution of the tip. However, carrier concentration in the nanowire establishes a lower limit on the lateral resolution, for small tip radii. These results enable one to optimize Scanning Probe Microscopy experiments as well as inform sample preparation for nanowire characterization.

Flexibility, low cost, versatility, miniaturization and multi-functionality are key aspects driving research and innovation in many branches of the electronics industry. With many anticipated emerging applications, like wearable, transparent and biocompatible devices, interest among the research community in pursuit for novel multifunctional miniaturized materials have been amplified. In this context, multiferroic polymer-based nanocomposites, possessing both ferroelectricity and ferromagnetism, are highly appealing. Most importantly, these nanocomposites possess tunable ferroelectric and ferromagnetic properties based on the parameters of their constituent materials as well as the magnetoelectric effect, which is the coupling between electric and magnetic properties. This tunability and interaction is a fascinating fundamental research field promising tremendous potential applications in sensors, actuators, data storage and energy harvesting. This dissertation work is devoted to the investigation of a new class of multiferroic polymer-based flexible nanocomposites, which exhibits excellent ferromagnetism and ferroelectricity simultaneously at room temperature, with the goal of understanding and optimizing the origin of their magnetoelectric coupling. The nanocomposites consist of high aspect ratio ferromagnetic nanowires (NWs) embedded inside a ferroelectric co-polymer, poly(vinylindene fluoride-trifluoroethylene), P(VDF-TrFE) matrix. First, electrochemical deposition of ferromagnetic NWs inside anodic aluminum oxide membranes is discussed. Characterization of electrodeposited iron, nickel and highly magnetostrictive iron-gallium alloy NWs was done using XRD, electron and magnetic force microscopy. Second, different nanocomposite films have been fabricated by means of spin coating and drop casting techniques. The effect of incorporation of NWs inside the ferroelectric polymer on its electroactive phase is discussed. The remanent and saturation polarization as well as the coercive field of the ferroelectric phase are slightly affected. Third, effects of NW alignment on the magnetic properties of nanocomposites are discussed. Nanocomposites with aligned NWs showed anisotropic magnetic properties while the ones without showed isotropic properties. Forth and last, the effects of NWs loading, alignment and material on the magnetoelectric properties of the nanocomposites are analyzed. Low NW concentrations are found to promote the electroactive phase of the nanocomposite, whereas high concentrations lower it. Nanocomposites with aligned NWs showed an anisotropic magnetoelectric effect. Higher magnetostrictive NWs exhibited a higher magnetoelectric coupling, demonstrating the advantage of galfenol-based nanocomposites, which are reported in this thesis for the first time.

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