Defense Date

2014

Document Type

Thesis

Degree Name

Master of Science

Department

Mechanical and Nuclear Engineering

First Advisor

Arunkumar Subramanian

Abstract

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.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

8-18-2014

Included in

Engineering Commons

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