Browsing by Author "Dr. Christopher B. Gorman, Committee Chair"

Now showing 1 - 7 of 7

Characteristics of Replacement Lithography and Desorption on Various Metal Substrates
(2004-07-06) Williams, James Alfred; Dr. Stefan Franzen, Committee Member; Dr. Edmond F. Bowden, Committee Member; Dr. Christopher B. Gorman, Committee Chair
The development of molecular electronic systems relies on the investigation of metal/molecule interactions. A popular methodology to forming metal/molecule interfaces uses the formation of self-assembled monolayers (SAMs), SAMs are capable of forming consistent, well order single to multiple layers of molecules oriented with respect to surface characteristics. While a plethora of techniques exist for exploring this phenomenon the utilization of replacement lithography presents a systematic approach to investigating the formation and stability of SAMs on various substrates. However, in authenticating investigations using lithographic techniques, the employment of electrochemistry forms conclusions on desorption and adsorption characteristics of the SAMs. While contemplating the contribution posed by metal/molecule interactions the facilitation of comparative methodologies in rationalizing processes of altering the characteristics of the monolayers, provides a speculative vantage of future endeavors into the development of molecular electronics.
Investigating Structure Property Relationships in Electroactive Molecules via Scanning Tunneling Microscopy
(2004-11-22) Wassel, Ronald Andrew; Dr. Daniel L. Feldheim, Committee Member; Dr. Edmond F. Bowden, Committee Member; Dr. T. Brent Gunnoe, Committee Member; Dr. Raymond E. Fornes, Committee Member; Dr. Christopher B. Gorman, Committee Chair
This dissertation will discuss issues related to determining structure property relationships that are relevant to molecular electronics. More specifically, an STM will be used to compare electronic measurements over insulating n-alkanethiolate SAMs and electroactive SAMs composed of ferrocenyl- terminated thiols and viologen terminated thiols, as well as multilayer SAMs created by alternating a metal dication and a thiolate. The surface characterization of these SAMs will be discussed. Current — voltage (I-V) measurements on the SAMs will be discussed as they pertain to the non-linear current voltage behavior of negative differential resistance (NDR). A mechanism for the nature of NDR will be discussed as well as the need to understand how the junctions used to measure I-V curves influence NDR. The attenuation of the apparent tunneling barrier of ferrocenyl-terminated thiolates via non-covalent binding of β-cyclodextrin will be shown. This attenuation will be shown to influence the peak to valley ratio of NDR Stochastic variation in electroactive molecules inserted into an insulating n-alkanethiolate background SAM was observed and is believed to be a general phenomenon. In this dissertation it will also be shown that the electric field generated by the STM tip increases the rate of replacement of electroactive molecules in to the background SAM.
Molecule-Based Negative Differential Resistance Across a Self-Assembled Monolayer: Its Dependence on Monolayer Coverage and Tunneling Gap
(2004-09-02) Robuck, Holly Elizabeth; Dr. Edmond F. Bowden, Committee Member; Dr. Christopher B. Gorman, Committee Chair; Dr. Daniel L. Feldheim, Committee Member
A scanning tunneling microscope (STM) was used to perform current-voltage (I-V) measurements over electroactive self-assembled monolayers (SAMs). It was hypothesized that the redox potential of an electroactive molecule influences the peak position of negative differential resistance (NDR). For this research, a ferrocene terminated alkanethiol, a ferrocenyl-ketone terminated alkanethiol, and a methyl viologen terminated alkanethiol were used, giving a range of redox potentials. Variability from the tip and the sample made the detection of any discernable differences impossible in the NDR peak position when the different SAMs were used. To address the variability, experiments were conducted where the order of the SAM was increased. The STM tip was also coated with 2,2,2-trifluoroethanethiol to make the tip surface more homogeneous. The results of the experiments discussed in this thesis suggest that the mechanism that leads to NDR is much more complicated than the resonant tunneling model suggests.
Structural Effects on Encapsulation as Probed in Solution - Based and Surface - Confined Redox - Active Core Dendrimers
(2004-03-31) Chasse, Tyson Lee; Dr. Christopher B. Gorman, Committee Chair; Dr. Edmond F. Bowden, Committee Member; Dr. David A. Shultz, Committee Member; Dr. Daniel L. Feldheim, Committee Member; Dr. Jack R. Edwards, Committee Member
The purpose of this research was to study structure — property relationships of iron sulfur core [Fe4S4(S-Dend)4]2- dendrimers. Previous studies have demonstrated that biasing dendrimer architecture increases the effective encapsulation of redox-active, paramagnetic, Fe4S4 clusters. To further examine structure-property relationships of iron-sulfur core dendrimers, studies were carried out to 1) probe the relationship between dendritic architecture and encapsulation via the study of solution-based and surface-confined constitutional isomers differing only in their benzyl substitution patterns, and 2) studying the effects of counterion concentration and permeability on the electronic properties of iron-sulfur core dendrimer thin films. Three pairs of isomeric, iron-sulfur core dendrimers were synthesized. Each isomer pair was distinguished by a 3,5-aromatic substitution pattern (extended) versus 2,6-aromatic substitution pattern (backfolded). Several observations were made supporting the hypothesis that the iron-sulfur cluster cores were encapsulated more effectively in the backfolded isomers as compared to their extended counterparts. The backfolded isomers were more difficult to reduce electrochemically, consistent with encapsulation in a less polar microenvironment. Furthermore, heterogeneous electron-transfer rates for the backfolded molecules were attenuated compared to the extended molecules. From diffusion measurements obtained by pulsed field gradient spin-echo NMR and chronoamperometry, the backfolded dendrimers were found to be smaller than the extended dendrimers. Comparison of longitudinal proton relaxation (T1) values also indicated a smaller, more compact dendrimer conformation for the backfolded architectures. These findings indicated that dendrimer size was not the major factor in determining electron-transfer rate. Instead, the effective electron-transfer distance, determined by the relative core position and mobility, is most relevant for encapsulation. In addition to solution studies, the electrochemical behavior of thin films composed of redox-active, iron-sulfur core dendrimers were studied as a function of the type of counterion available during reduction and re-oxidation. The rate of permeation/migration of counterions into the film appeared to be the bottleneck to electron transfer through the film. As the dendrimer is essentially non-polar, decreasing the relative polarity of the counterion increased the rate and extent of electron hopping within the films.
Synthesis and Characterization of Nanoparticle Assemblies for Electronic Applications
(2009-07-27) Ayres, Jennifer Ann; Dr. Stefan Franzen, Committee Member; Dr. Edmond Bowden, Committee Member; Dr. Gregory Parsons, Committee Member; Dr. Christopher B. Gorman, Committee Chair
While significant effort has been made to synthesize molecular wires for electronic applications, the ability to insert these molecules between two metallic contacts with directional control has yet to be demonstrated. Control over molecular orientation is critical to the development of molecular devices such as diodes, capacitors and transistors. In this study, directional control is achieved using orthogonal self-assembly to synthesize electronic junctions between nanoparticles of different compositions. Phenyl ethynylene oligomers were synthesized with different end groups. One molecule was functionalized with a thiol which exhibits preferential binding to gold and an isocyanide which exhibits preferential binding to platinum. The other was functionalized with a thiol for binding to gold and a carboxylic acid which exhibits preferential binding to metal oxides. One of the major challenges of this work was the synthesis of nanoparticle building blocks that were suitable for the formation of these heterodimeric structures. Metal and metal oxide particles were synthesized with capping ligands that provided stability yet did not sterically hinder heterodimer formation. Once appropriate nanoparticles had been identified, preliminary studies indicated heterodimer formation. However, characterizing these structures presented additional challenges. Several characterization techniques, including transmission electron microscopy (TEM), size-exclusion chromatography (SEC), several types of electrophoresis and small-angle x-ray scattering (SAXS), were evaluated for their ability to characterize these structures with statistical accuracy. While all of these techniques did indicate the presence of dimers or larger aggregates in solution, accurate statistical information was not obtained using any single method.
Synthesis and Characterization of Orthogonally Self-Assembled Nanoparticle Heterodimers
(2007-05-16) Walker, Brandon; Dr. Christopher B. Gorman, Committee Chair; Dr. David A. Shultz, Committee Member; Dr. Edmond F. Bowden, Committee Member
This project involved the synthesis of novel bifunctional linear linker molecules that were used to bind gold and platinum nanoparticles using orthogonal self-assembly. These heterodimers were created as a model for the possibility of using similar bimetallic structures as molecular electronic components. The binding affinities of the terminal thiol and isonitrile functional groups on planar surfaces and nanoparticles were analyzed using surface and transmission Fourier-transform infrared spectroscopy. It was found, qualitatively, that the binding affinity of the binding groups differed significantly depending on whether the functional group was binding to a planar surface or a nanoparticle. The linking of two nanoparticles to form a heterodimer was also studied using transmission electron microscopy (TEM) and size-exclusion chromatography (SEC). While unable to provide quantitative results on heterodimer formation, TEM provided a straight-forward, though limited, method for demonstrating that some heterodimer formation did occur. The main limiting factor of TEM was the relative size difference of the particles in the heterodimer. SEC provided a more quantitative view of the heterodimer sample, but this technique introduced many variables that control the separation of nanoparticles and heterodimers. Results from the SEC experiments support the formation of heterodimers, but many of the variables inherent to the technique must be overcome for it to become a viable technique for routine analysis of nanoparticle arrays.
Synthesis of Amphiphilic Iron-Sulfur Core Dendrimers and Investigations into Their Encapsulation Abilities in Different Solutions
(2005-10-30) Cameron, Christopher Stan; Dr. Christopher B. Gorman, Committee Chair; Dr. David Shultz, Committee Member; Dr. Edmond Bowden, Committee Member; Dr. James D. Martin, Committee Member
Several novel amphiphilic iron-sulfur core dendrimers were synthesized by modifying the tips of the dendrons to install hydrophilic peripheries. These dendrimers were synthesized via standard convergent and a 'modular' synthetic strategy. The electrochemistry of the dendrimers was investigated in dimethylformamide and dimethylformamide/water solutions. Positive shifts in the redox potential of the iron-sulfur cores corresponding to dendrimer size and periphery type were recorded. Secondary peaks in the anodic wave of cyclic voltammograms of dendrimers in dimethylformamide/water solutions were possible indications of limited access of water molecules to the iron-sulfur cores of some of the dendrimers.