Date of Award

12-2015

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Dr. Asghar N. Kayani

Second Advisor

Dr. Muralidhar K. Ghantasala

Third Advisor

Dr. Sung G. Chung

Fourth Advisor

Dr. Paul Pancella

Keywords

Carbon nanotubes, chernical vapor deposition, physical vapor deposition, catalyst, polymer, sensing applications

Abstract

The advent of carbon nanotubes (CNTs) has opened up lot of novel applications because of their unique electrical and mechanical properties. CNTs are well known material for its exceptional electrical, mechanical, optical, thermal and chemical properties. A single-wall nanotube (SWNT) can be either semiconducting, metallic or semi-metallic, based on its chirality and diameter. SWNTs can be used in transistor device as active channels due to high electron mobility (~10000 cm2/(V s), electrical interconnects, nano-scale circuits, field-emission displays, light-emitting devices and thermal heat sinks due to low resistivity, high current density (~109A cm-2) and high thermal conductivity (~3500 W m-1). Further, their high Young’s modulus and fracture stress is suitable for various sensing applications such as strain/pressure and use in chemical/biological sensors. This work mainly involves the deposition of CNT-based films following two different methods via a conventional microwave chemical vapor deposition (MWCVD) and spinning CNT-composites, and explored the possibility of using CNT-based films in strain gauge applications. Deposited films are characterized and analyzed for their structure, microstructure, composition and electrical properties. Rutherford Backscattering Spectrometry (RBS), X-ray Reflectivity (XRR), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and electrical impedance measurement techniques are used to characterize the films prepared by both the above mentioned methods. The synthesis/deposition process is improved based on the observed films properties.

A carbon nanotube forest grown on the Si (100) substrate with Ni as a catalyst using CVD system shows an amorphous nature due to loss of catalytic activity of Ni nano-islands. XPS and RBS data show Ni nano-particles diffused into the Si substrate and surface layer of Ni particles turns out to nickel silicide. The diffusion of Ni nano-particles and nickel silicide formation occurs due to annealing and longer plasma treatment. Because of annealing and plasma treatment, the phase of Ni changes from amorphous to hexagonal close packed with p63/mmc space group and cubic Fm-3m with plasma treatment time. Further, the role of Ni diffusion into silicon in the catalyzing CNT formation has been investigated and discussed further.

Carbon nanotube-polymer nanocomposite films are synthesized using spin coating of a polyurethane and CNTs mixture. This produced porous films with the average pore size of around 10μm. The electrical conductivity of the CNTs-polymer composites increases with the carbon nanotube loading, and composites show frequency independent conductivity as frequency increases. Generally, ac conductivity as a function of signal frequency for the above composites shows conductive behavior. The parallel RC-circuit model reveals that composites without any cross-linkers are uniform and homogenous. The composite with low carbon nanotube content with cross-linkers shows the same trend. But composites with higher carbon nanotube content with cross-linkers shows poor dispersion and agglomeration of carbon nanotubes.

Access Setting

Dissertation-Open Access

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