Browsing by Subject "Self-Assembly"

Now showing 1 - 6 of 6
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    Block copolymers in ionic liquids.
    (2009-06) Simone, Peter Mark
    In this thesis the self-assembly behavior of block copolymers diluted with ionic liquids has been investigated. Initial experiments involved characterizing the selfassembly of poly(styrene-b-methyl methacrylate) (PS–PMMA) and poly(butadiene-bethylene oxide) (PB–PEO) copolymers at dilute concentrations (~1 wt%) in the ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF6]) and 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). Dynamic light scattering and cryogenic transmission electron microscopy results showed that the ionic liquids behave as selective solvents for the PMMA and PEO blocks of the copolymers, and that the micelle morphology and self-assembly behavior of the block copolymers in the ionic liquids was analogous to that observed in conventional solvents. At increased solution concentrations (≥ 20 wt%) the lyotropic mesophase behavior for PB–PEO diluted with [BMI][PF6] and [EMI][TFSI], and poly(styrene-bethylene oxide) (PS–PEO) diluted with [EMI][TFSI] was investigated via small angle X-ray scattering. These experiments showed a microstructure phase progression with addition of ionic liquid that was analogous to that expected for an increase in the PEO volume fraction of the bulk copolymers. Additionally, an increase in the lamellar microstructure domain spacing with ionic liquid content indicated that both ionic liquids behave as strongly selective solvents for the PEO blocks of the copolymers. The ionic conductivity of the concentrated PS–PEO/[EMI][TFSI] solutions was measured via impedance spectroscopy, and found to be in the range of 10−3 S/cm at elevated temperatures (~100 °C). Additionally, the ionic conductivity of the solutions was observed to increase with both ionic liquid content and molecular weight of the PEO blocks of the copolymer. Finally, preliminary investigations of the microstructure orientation in thin films of a concentrated PS–PEO/[EMI][TFSI] solution were conducted. The copolymer microstructure was observed to align perpendicular to the film surface with short term (≤ 2 hours) thermal annealing. Longer term thermal annealing resulted in a transition to parallel alignment of the copolymer microstructure relative to the film surface.
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    Charge Transport and Contact Effects in Nanoscopic Conjugated Molecular Junctions Characterized by Conducting Probe Atomic Force Microscopy
    (2008-10) Kim, Bong Soo
    This thesis describes electrical characterization of nanoscale molecular junctions based on a small assembly of molecules. Gaining rigorous knowledge about nanoscopic molecular junctions is essential to the field of molecular electronics, a field that is driven by the potential of utilizing molecules as active elements in electronic circuits. Further advancement requires detailed understanding of factors that influence charge transport through molecules. Critical aspects include molecular length, molecular structure, contact effects, and energy level alignment. For example, the precise dependence of resistance (or conductance) on molecular length is subject to the electronic structure of the molecule and to the charge transport mechanisms. In addition, contact effects can be dominant in current-voltage characteristics due to the inherently small dimensions of these junctions. To address these issues, my research focused on understanding how currents flow through molecular assemblies in metal-molecule-metal junctions using conducting probe atomic force microscopy (CP-AFM). The CP-AFM technique allows us to form a molecular junction conveniently by contacting metal-coated AFM tips with self-assembled monolayers (SAMs) on metal substrates, and the current-voltage characteristics can then be recorded. Electrical measurements on several series of conjugated molecules revealed the length dependent tunneling efficiency of each molecular structure. In addition, spectroscopic measurements on the metal/molecule interfaces revealed a direct correlation between contact resistance and energy level alignment. In terms of transport mechanisms, a mechanistic transition from nonresonant tunneling to field emission was observed under high bias.
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    Impact Of Oil Loading On Lyotropic Liquid Crystal Phase Behavior Of Carboxylate Surfactants
    (2019-07) Baez-Cotto, Carlos
    Aqueous lyotropic liquid crystals (LLCs) are formed by solvating amphiphilic molecules with minimal amounts of water. Manipulating the extent of hydration, as well as other external conditions such as temperature, pressure, absence or presence of salt or oil, etc. drives formation of different nanoscale morphologies with important consequences for their physical properties. Particular interest has focused on bicontinuous cubic network (N) LLCs, the percolating channels of which are useful in the development of selective separations media and therapeutic delivery vehicles. However, the inability of surfactant molecules to adopt the negative Gaussian curvature that characterizes these systems hinders access to these useful phases and limits their applications. The unique twin-tail and twin-head architecture of gemini surfactants favors their lyotropic self-assembly into N phases. While reports on this surfactant class have previously demonstrated their ability to stabilize larger LLC N composition windows than their single-tail counterparts, only binary surfactant/water LLCs have been studied to date with little attention to the impact of additives on their unusual phase behaviors. In this thesis, we seek to understand the impact of the hydrophobic additive n-decane on the aqueous supramolecular self-assembly of gemini dialkanoate amphiphiles. We first describe the LLC behavior of the single-tail surfactant tetramethylammonium decanoate swelled with 40 weight percent n-decane (relative to surfactant mass). These control experiments revealed that n-decane enables access to previously unreported LLC mesophases, including a normal double diamond N LLC and a hexagonal C14 micellar LLC Laves phase. We further explored the aqueous self-assembly of gemini dialkanoate amphiphiles, in which two single-tail alkanoate amphiphiles are covalently linked through a hydrocarbon chain. By modifying linker length and counterion identity, we develop structure-self-assembly relationships to rationalize how the gemini dialkanoate architecture drives aqueous LLC microemulsion phase selection. Gemini dialkanoates with odd carbon linkers (x = 3, 5) are found to stabilize ordered network microemulsions, whereas systems with even carbon linkers (x = 4, 6) favor self-assembly into low-symmetry, complex spherical micelle packings. The observed phase behaviors may be additionally tuned by modulating counterion identity. N LLC phase windows widen as the degree of headgroup-counterion association increases. On the other hand, a highly dissociated counterion-headgroup pairs foster wide micellar LLC windows. These observations are rationalized in terms of a cooperative self-assembly that involves the unique, linker-length dependent conformational preferences of the hydrated surfactants, ion pair correlations, and the packing of the lipidic tails at constant density with minimal water-hydrophobic contacts.
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    Investigations of Self-Assembling Guanosine Phosphoramidate Based Hydrogels
    (2023-07) Bentz, Nicole
    Peptides and nucleobases are supramolecular building blocks that have been widely investigated for their self-assembling properties and ability to form supramolecular structures. These high-aspect ratio structures eventually form entangled matrices that result in supramolecular hydrogels. Previous work in our lab incorporated enzymatic activity in the regulation of peptide self-assembly, where we investigated Histidine Triad Nucleotide Binding Protein 1 (Hint1) as a modulator of supramolecular self-assembly.1 In this work, we developed a panel of self-assembling nucleoside phosphoramidates (SANPs) capable of Hint1 triggered hydrogelation. SANPs are low molecular weight molecules that incorporate a self-assembling peptide conjugated to a nucleoside phosphoramidate group through a short PEG linker. A surprising observation in the development of these Hint1 responsive molecules was the spontaneous assembly of the SANPs into highly ordered nanofibers without enzymatic activity, and the unique ability of the guanosine SANPs to form supramolecular hydrogels.Guanosine and its derivatives have been of particular interest for researchers due to its unique ability to form G-quadruplexes that drive self-assembly, but their instability and tendency to precipitate from solution have limited the applications of these biomaterials. Herein, we report the unique interplay of two self-assembling moieties to drive supramolecular hydrogelation: an ultrashort self-assembling peptide and a guanosine nucleobase. This body of work details the fundamental investigation of our panel of SANPs and the influence of nucleobase identity on the critical aggregation concentration, stability, and hydrogelation ability of these molecules. Specifically, we examine the synergistic self-assembly of the guanosine SANPs and tease out the interactions that ultimately result in supramolecular hydrogelation. Characterization of these dual moiety guanosine SANPs reveal that the resulting hydrogels integrate principal properties of both supramolecular building blocks, resulting in a novel biomaterial that addresses the caveats of the traditional guanosine-based hydrogels. We further examined their innate biological activity and demonstrate the promising application of these novel biomaterials for controlled drug release with the potent chemotherapeutic Doxorubicin and assess the activity of our drug depot in vitro.
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    Nanoscale Self-Assembly: Energetic Irradiation Triggering And In Situ Monitoring
    (2020-04) Dai, Chunhui
    Origami inspired assembly of three-dimensional (3D) micro and nano-structures arise to be a broad topic in the past two decades due to their ability of property engineering, 3D space utilization, and controlled motion, which have been widely used in the applications of metamaterial and plasmonic devices, electronics, and biomedical devices. However, the present techniques for the assembly of 3D nanostructures, such as by using DNA technology, reactive ion etching, atomic layer deposition, and metal-assisted etching, do not allow real-time visualization, which bring great challenges in controlling the shape with nanoscale precision, resulting in an extremely low yield and significant geometric and topological constrains. To address the issues of the traditional nanoscale self-assembly, my Ph.D. work involves developing novel in situ monitored self-assembly techniques triggered by energetic irradiation, such as ion and electron beam. The energetic irradiations offer two functions in the self-assembly: 1) on one hand, an excited ion or electron beam is able to deliver energy to the specific irradiated material, triggering localized material phase change, such as Sn grain coalescence, crystallization of amorphous material, or polymer reflow and shrinkage. Associated with the material phase changes, stress is induced in the thin film, folding the suspended 2D thin film up to 3D nanostructures; 2) on the other hand, the imaging capability of ion or electron beam enables real-time monitoring of the self-assembly process, making it possible to precise tune the energy delivery to reach the desired assembly status. Because of the localized energy delivery and real-time imaging, ultra-high self-assembly process with sub-10 nm scale precision is achieved. With further understanding of the material-irradiation interaction and careful design of the 2D patterns and material layout, more advanced functions have been achieved, leading to programmable, sequential, multidirectional, and reversible self-assembly in nanoscale. Further, the energetic irradiation triggered self-assembly processes have been used to build functional materials with advanced properties. I develop a strategy to build 3D graphene based nanostructures (i.e. nanocube and nanotube) via self-assembly process, which is one of the pioneer works in this field. By transforming graphene into 3D format, its amazing properties could be modified by the extra dimensionality, achieving enhanced or novel behaviors that does not exist in 2D. For instance, the plasmonic near-field enhancement of planar graphene undergoes severe exponential decay in the vertical direction away from the surface of the graphene, resulting in a relatively small spatial overlap between the specimens and the volume of high field confinement. I find that 3D graphene nanostructures exhibit novel plasmon hybridizations, which result in a near-field enhancement across the entire surface of these 3D structures as well as within their spatial volume. As the sensitivity is directly related to the field intensity in the vicinity of the analyte, the strong volumetric electric field confinement in these 3D nanostructures are proposed to be candidates for high sensitivity detection of proteins and other biological specimens. In addition, self-assembled nanocylinders with automatically formed plasmonic nanogaps have been developed into a nanofluidic based plasmonic sensor. The sequential and reversible self-assembly processes enable the realization of nano-machine and nano-robotics. Overall, this in situ monitored self-assembly technique provides a solid foundation to build 3D nanostructures with various advanced functions, which push the limit for further exploration of the next generation devices.
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    Remote-Controlled Self-Assembly of Three-Dimensional Micro Structures for Ultra-Sensitive Sensors and Three-Dimensional Metamaterials
    (2018-10) Liu, Chao
    Self-assembly has been widely used to fabricate micro-scale three-dimensional (3D) structures for various applications like sensors, drug delivery systems, and advanced robotics (e.g., micro-actuators, micro-machines). Self-assembly is always driven by external sources (e.g., heat, solvent, pH), which makes the assembly process hard to control and leads to extremely low yield. Direct contact of heat or chemicals is usually required to trigger a self-assembly process, which limits the applications of self-assembly and decreases the manipulative capability of the process. To address the issues of the traditional direct triggered self-assembly, my Ph.D. work involved in developing novel remote-controlled self-assembly techniques with microwave and induction energies, combining the self-assembly technique with advanced metamaterial (MM) designs, and exploring their potential applications as 3D sensors and devices. The goal of the work is to achieve advanced remotely controlled self-assembly to improve the yield and manipulative capability of the assembly process and discover new aspects of the assembly technique (e.g., biocompatible assembly, multiple and sequential assembly) and its applications (e.g., 3D sensors, 3D MM devices). For remotely controlled self-assembly, electromagnetic waves can be remotely applied to the metal thin films within the microstructures. Eddy current can be created inside the thin films and generate heat to melt the polymeric hinges. The molten hinges generate surface tension force to transform the two-dimensional (2D) net into 3D microstructures. Induction heating can trigger self-assembly without harming live organs or tissues, which is suitable for biomedical applications. Remote-controlled self-assembly also allows multiple and sequential self-assembly. The movements of each part of structure can be precisely controlled by adjusting the energy sources in a remote location, increasing manipulative capability of the 3D assembly process. The achievement of sequential self-assembly and multiple folding angles in a single structure is essential for building complex microstructures and micro-actuators. One important application for remote-controlled 3D self-assembled structure is to build 3D MM devices. Split ring resonators (SRRs) and closed ring resonators (CRRs) can be patterned on each face of the self-assembled structures to achieve 3D MMs with fully anisotropic and isotropic behaviors. However, the quality factor (Q-factor) of conventional MMs is low (typically under 10), results in low sensitivity and selectivity. To increase Q-factor of the MMs, we developed novel nanopillar-based MMs driven by displacement current. The nanopillar-based MMs contain thousands of metallic nanopillars with nanoscale dielectric gaps between them. Forming the MMs with nanopillars and nano gaps decreases the Ohmic energy loss in the resonator and increases the energy storage in the dielectric nano gaps, thus an enhanced Q-factor up to 14000 can be achieved. The ultra-high Q nanopillar-based MM can be patterned on each face of the self-assembled 3D structures to realize ultra-high Q 3D MM structures. Novel ultra-sensitive THz MMs and 3D MMs combined with remote-controlled self-assembly opens a new area of creating diverse sensors and devices for 3D optoelectronic, 3D MMs, and ultra-high sensitive biomedical sensors. This thesis will be roughly divided into two parts. We begin with part one by introducing the novel remotely controlled self-assembly using electromagnetic energies that I have developed over my Ph.D. program as well as its unique properties and benefits over traditional self-assemblies. The second part involves my design and theory of ultra-high Q nanopillar-based MM and the 3D MM devices by combining the nanopillar-based MM with self-assembly technique.