Studies of the low frequency dielectric constants and conductance of two molecular systems : carbon nanotubes and polyaromatic molecular wires

Abstract

This thesis deals with two projects on the electronic properties of molecular systems. Presented first is an experiment designed to characterize the direct current charge transport characteristics of polyaromatic molecular wires, specifically oligo(phenylene ethynylene) or a five benzene ring variant 1,4-bis(2-(4-(2-(4-thioacetylphenyl)ethynyl)phenyl)ethynyl)-2,5-didodecylbenzene. The techniques and methods used to make and contact these molecular wires are given and followed up a discussion of the results and conclusions. The molecular wires were contacted by adsorbing them into a prefabricated device. A gold wire, approximately 100nm in diameter and 10µm in length was grown electrochemically with a self assembled monolayer spacer grown into the wire at 5 µm. These gold wires were contacted photolithographically resulting in a device that could be electrically contacted allowing measurement of the material in the middle of the wire. The spacer could then be removed by volatilizing away the self assembled monolayer used leaving a contacted gold wire with a nanometer scale gap at its centre. The molecular wires could then be adsorbed across this gap allowing a measurement of their properties. Using the test bed developed a measurement of resistance of oligo(phenylene ethynylene) of 44GΩ in close agreement with other groups contact atomic force microscopy methods.

The second part to this thesis details the measurement of carbon nanotubes suspended in 1,2 - dichlorobenzene using impedance spectroscopy. A brief account of the theory used to understand the observed effects is presented followed by a description of the methods and results. The nanotubes were suspended in the 1,2 - dichlorobenzene at a maximum concentration of 90mg/L without a surfactant and measured across a frequency range from 10⁷Hz to 1Hz at varying electric field strengths. This was performed in a cell that was very large compared to the nanotubes themselves and had plates with a large surface area compared to their separation to keep the field as uniform as possible. The frequency spectra were run at increasing field strengths and a drop in the real part of the impedance of the solution between the plates from 2x10⁷Ω to 1x10⁵Ω was observed as the electric field strength was increased. This drop in impedance did not continue as the field strength was increased further but saturated at 1x10⁵Ω, with further decreases unobservable at the electric field strengths available to us. However it was found that the rate at which this threshold was reached depended strongly on the electric field strength and the concentration of the nanotubes in the suspension. Further work detailed in this projects show that this change in impedance is semi-permanent with no measurable decay to the original impedance observable over 8 hours. The system can be made to return towards is original state by agitation and the project shows that the more vigorous the agitation the closer to the original value of the impedance the solution becomes. This leads to the projects conclusion that chain formation or aggregation is the ultimate cause of the observed decrease in impedance although the details of the mechanism that causes this decrease in impedance remain unclear.

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