This study details the dielectrophoretic assembly and mechanical characterization of boron nitride nanotubes on silicon chips with gold electrodes. The chips were fabricated from 4in round silicon wafers with a 100nm-thick low stress silicon nitride insulating layer on the top and bottom. The electrodes were patterned using photo- and electron-beam lithography and dry etching, and the wafers were cut into 4 x 6mm chips. The boron nitride nanotubes studied were obtained from NIA and were synthesized via a unique pressurized vapor/condensor method, which produced long, small-diameter BNNTs without the use of a catalyst. These nanotubes were studied due to their desirable mechanical and electrical properties, which allow for unique applications in various areas of science, engineering, and technology. Applications span from magnetic manipulation to the formation of biocomposites, from nano-transistors to humidity and pH sensors, and from MRI contrast agents to drug delivery. The nanotubes and nanotube bundles characterized were suspended over gaps of 300 to 500nm. This study was unique in that assembly was performed using dielectrophoresis, allowing for batch fabrication of chips and devices. Also, stiffness measurements were performed using AFM, eliminating the reliance of other methods upon electron microscopes, and allowing for imaging and measurements to occur simultaneously and at high resolution. It was found that DEP parameters of V = 2.0Vpp, f = 1kHz, and t = 2min provided the best results for mechanical testing. The nanotubes tested had suspended lengths of 300nm, the width of the electrode gap, and diameters of 15–65nm. Chips were imaged with both scanning electron microscopy and atomic force microscopy. Force-displacement measurements with atomic force microscopy were used to find stiffness values in the range of 1–16N/m. These stiffness values, when plugged into a simple double-clamped beam model, indicated Young’s moduli of approximately 1–1600GPa. Within this wide range, it was shown that a decrease in diameter strongly correlated exponentially to an increase in Young’s modulus. Work in this study was divided between assembly and characterization. Therefore, a lot of time was spent working to optimize dielectrophoresis parameters, followed by SEM and AFM imaging. Parameters that were adjusted included DEP voltage and time, pre-DEP sonication times, as well as adding a centrifuging procedure to attempt to better separate nanotube bundles in solution. Another method discussed but not pursued was the use of surfactants to agitate the solution, thus separating the nanotubes. The reason this material in particular was so difficult to separate was twofold. First, the small size of the nanotubes—individual BNNTs have diameters on the order of ∼5 nanometers—generates very strong nanoscale van der Waals forces holding the nanotubes together. Larger nanotubes—with diameters on the order of 50 to 100nm or more—suffer less from this problem. Also, the dipoles created by the boron-nitrogen bonds cause attraction between adjacent nanotubes. The results shown in this thesis include DEP parameters, SEM and AFM images, and force- displacement curves leading to nanotube stiffness and Young’s modulus values. The force-displacement tests via AFM are also detailed and explained.

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Low-dimensional systems in condensed matter physics exhibit a rich array of correlated electronic phases. One-dimensional systems stand out in this regard. Electrons cannot avoid each other in 1D, enhancing the effects of interactions. The resulting correlations leave distinct spatial imprints on the electronic density that can be imaged with scanning probes. Disorder, however, can destroy these delicate interacting states by breaking up the electron liquid into localized pieces. Thus, to generate fragile interacting quantum states, one requires an extremely clean system in which disorder does not overcome interactions, as well as a high degree of tunability to design potential landscapes. Furthermore, to directly measure the resulting spatial correlations, one requires an exceptionally sensitive scanning probe, but the most sensitive probes presently available are also invasive, perturbing the system and screening electron-electron interactions. / Engineering and Applied Sciences

Nanobelts are a new class of semiconducting metal oxide nanowires with great potential for nanoscale devices. The present research focuses on the manipulation of SnO₂ nanobelts suspended in ethanol using microfluidics and electric fields. Dielectrophoresis (DEP) was demonstrated for the first time on semiconducting metal oxide nanobelts, which also resulted in the fabrication of a multiple nanobelt device. Detailed and direct real-time observations of the wide variety of nanobelt motions induced by DEP forces were conducted using an innovative setup and an inverted optical microscope. High AC electric fields were generated on a gold microelectrode (~ 20 µm gap) array, patterned on glass substrate, and covered by a ~ 10 µm tall PDMS (polydimethylsiloxane) channel, into which the nanobelt suspension was introduced for performing the DEP experiments. Negative DEP (repulsion) of the nanobelts was observed in the low frequency range (< 100 kHz) of the applied voltage, which caused rigid body motion as well as deformation of the nanobelts. In the high frequency range (~ 1 MHz - 10 MHz), positive DEP (attraction) of the nanobelts was observed. Using a parallel plate electrode arrangement, evidence of electrophoresis was also found for DC and low frequency (Hz) voltages. The existence of negative DEP effect is unusual considering the fact that if bulk SnO₂ conductivity and permittivity values are used in combination with ethanol properties to calculate the Clausius Mossotti factor using the simple dipole approximation theory; it predicts positive DEP for most of the frequency range experimentally studied. A fluidic nanobelt alignment technique was studied and used in the fabrication of single nanobelt devices with small electrode gaps. These devices were primarily used for conducting impedance spectroscopy measurements to obtain an estimate of the nanobelt electrical conductivity. Parametric numerical studies were conducted using COMSOL Multiphysics software package to understand the different aspects of the DEP phenomenon in nanobelts. The DEP induced forces and torques were computed using the Maxwell Stress Tensor (MST) approach. The DEP force on the nanobelt was calculated for a range of nanobelt conductivity values. The simulation results indicate that the experimentally observed behavior can be explained if the nanobelt is modeled as having two components: an electrically conductive interior and a nonconductive outer layer surrounding it. This forms the basis for an explanation of the negative DEP observed in SnO₂ nanobelts suspended in ethanol. It is thought that the nonconductive layer is due to depletion of the charge carriers from the nanobelt surface regions. This is consistent with the fact that surface depletion is a commonly observed phenomenon in SnO₂ and other semiconducting metal oxide materials. The major research contribution of this work is that, since nanostructures have large surface areas, surface dominant properties are important. Considering only bulk electrical properties can predict misleading DEP characteristics.