This work focuses on understanding the buckling deformation mechanisms of bending, wrinkling, and folding that occur on the surfaces and interfaces of polymer systems. We gained fundamental insight into the formation mechanism of these buckled structures for thin glassy films placed on an elastomeric substrate. By taking advantage of geometric confinement, we demonstrated new strategies in controlling wrinkling morphologies. We were able to achieve surfaces with controlled patterned structures which will have a broad impact in optical, adhesive, microelectronics, and microfluidics applications. Wrinkles and strain localized features, such as delaminations and folds, are observed in many natural systems and are useful for a wide range of patterning applications. However, the transition from sinusoidal wrinkles to more complex strain localized structures is not well understood. We investigated the onset of wrinkling and strain localizations under uniaxial strain. We show that careful measurement of feature amplitude allowed not only the determination of wrinkle, fold, or delamination onset, but also allowed clear distinction between each feature. The folds observed in this experiment have an outward morphology from the surface in contrast to folds that form into the plane, as observed in a film floating on a liquid substrate. A critical strain map was constructed, where the critical strain was measured experimentally for wrinkling, folding, and delamination with varying film thickness and modulus. Wrinkle morphologies, i.e. amplitude and wavelength of wrinkles, affect properties such as electron transport in stretchable electronics and adhesion properties of smart surfaces. To gain an understanding of how the wrinkle morphology can be controlled, we introduced a geometrical confinement in the form of rigid boundaries. Upon straining, we found that wrinkles started near the rigid boundaries where maximum local strain occurred and propagated towards the middle as more global strain was applied. In contrast to homogeneous wrinkling with constant amplitude that is observed for an unconfined system, the wrinkling observed here had varying amplitude as a function of distance from the rigid boundaries. We demonstrated that the number of wrinkles can be tuned by controlling the distance between the rigid boundaries. Location of wrinkles was also controlled by introducing local stress distributions via patterning the elastomeric substrate. Two distinct wrinkled regions were achieved on a surface where the film is free-standing over a circular hole pattern and where the film is supported by the substrate. The hoe diameter and applied strain affected the wavelength and amplitude of the free-standing membrane. Using discontinuous dewetting, a one-step fabrication method was developed to selectively deposit a small volume of liquid in patterned microwells and encapsulate it with a polymeric film. The pull-out velocity, a velocity at which the sample is removed from a bath of liquid, was controlled to observe how encapsulation process is affected. The polymeric film was observed to wrinkle at low pull-out velocity due to no encapsulation of liquid; whereas the film bent at medium pull-out velocity due to capillary effect as the liquid evaporated through the film. To quantify the amount of liquid encapsulated, we mixed salt in water and measured the size of the deposited salt crystals. The salt crystal size, and hence the amount of liquid encapsulated, was controlled by varying either the encapsulation velocity or the size of the patterned microwells. In addition, we showed that the deposited salt crystals are protected by the laminated film until the film is removed, providing advantageous control for delivery and release. Yeast cells were also captured in the microwells to show the versatility. This encapsulation method is useful for wide range of applications, such as trapping single cells for biological studies, growing microcrystals for optical and magnetic applications, and single-use sensor technologies.

Polymer materials have become important tools in nanomanufacturing due to their facile processing and ready attainment of the necessary feature sizes. The development of cleavable junctions has led to advances in the production of polymer nanotemplates. Photocleavage strategies have come to the forefront of the field because photons, as a cleavage stimulus, do not have the mass-transport limitations of chemical methods, and provide for targeted two- and three-dimensional feature control. This dissertation presents a method for producing photocleavable materials by one-pot copper-catalyzed azide-alkyne "click" chemistry (CuAAC), activator regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) and activated ester substitution methods that have each block labeled with a fluorescent dye, enabling exploration of the polymer physics of these systems by correlation fluorescence spectroscopy. It also introduces a novel photocleavable linker, the o-nitrobenzyl-1,2,3-triazole, its behavior on photocleavage, and a facile method for the production of the o-nitrobenzyl azides necessary for their synthesis. The synthesis and properties of a bulk photodegradable polytriazole are reported, as are proof of concept experiments demonstrating its potential as a directly photoetchable material. Lastly, this dissertation contains a perspective on possible avenues of new research on the topics presented.

Doping in bulk semiconductors (e.g., n- or p-type doping in silicon) allows for precise control of their properties and forms the basis for the development of electronic and photovoltaic devices. Recently, there have been reports on the successful synthesis of doped semiconductor nanocrystals (or quantum dots) for potential applications in solar cells and spintronics. For example, nanocrystals of ZnSe (with zinc-blende lattice structure) and CdSe and ZnO (with wurtzite lattice structure) have been doped successfully with transition-metal (TM) elements (Mn, Co, or Ni). Despite the recent progress, however, the underlying mechanisms of doping in colloidal nanocrystals are not well understood. This thesis reports a comprehensive theoretical analysis toward a fundamental kinetic and thermodynamic understanding of doping in ZnO, CdSe, and ZnSe quantum dots based on first-principles density-functional theory (DFT) calculations. The theoretical predictions of this thesis are consistent with experimental measurements and provide fundamental interpretations for the experimental observations. The mechanisms of doping of colloidal ZnO nanocrystals with the TM elements Mn, Co, and Ni is investigated. The dopant atoms are found to have high binding energies for adsorption onto the Zn-vacancy site of the (0001) basal surface and the O-vacancy site of the (0001) basal surface of ZnO nanocrystals; therefore, these surface vacancies provide viable sites for substitutional doping, which is consistent with experimental measurements. However, the doping efficiencies are affected by the strong tendencies of the TM dopants to segregate at the nanocrystal surface facets, as indicated by the corresponding computed dopant surface segregation energy profiles. Furthermore, using the Mn doping of CdSe as a case study, the effect of nanocrystal size on doping efficiency is explored. It is shown that Mn adsorption onto small clusters of CdSe is characterized by high binding energies, which, in conjunction with the Mn surface segregation characteristics on CdSe nanocrystals, explains experimental reports of high doping efficiency for small-size CdSe clusters. In addition, this thesis presents a systematic analysis of TM doping in ZnSe nanocrystals. The analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets, as well as dopant-induced nanocrystal morphological transitions, and leads to a fundamental understanding of the underlying mechanisms of dopant incorporation into growing nanocrystals. Both surface kinetics (dopant adsorption onto the nanocrystal surface facets) and thermodynamics (dopant surface segregation) are found to have a significant effect on the doping efficiencies in ZnSe nanocrystals. The analysis also elucidates the important role in determining the doping efficiency of ZnSe nanocrystals played by the chemical potentials of the growth precursor species, which determine the surface structure and morphology of the nanocrystals.

Engineering nanostructures with complete control over the shape, composition, organization of the surface structures, and function remains a major challenge. In my work, I have fabricated nanostructures using functional polymer motifs and nanoparticles (NPs) via supramolecular and non-supramolecular interactions. In one of the approaches to generate nanostructures, I have integrated top-down approaches such as nanoimprint lithography, electron-beam lithography, and photolithography with the self-assembly (bottom-up) of NPs to provide nanostructures with tailored shape and function. In this strategy, I have developed a geometrically assisted orthogonal assembly of nanoparticles onto polymer features at precisely defined locations. This versatile NP functionalization method can be used to fabricate protein resistant patterned surfaces to provide essentially complete control over cellular alignment, making them promising biofunctional structures for cell patterning. In another approach, I have utilized self-assembly of dendrimers and NPs without preformed templates to generate nanostructures that can be used as chemoselective membranes for the separation of small and biomacromolecules.

Currently, around 28 million people globally suffer from the consequences of corneal blindness and most of them are part of a long waiting list; availability of donor tissue is highly limited. Furthermore, even those who are treated are in risk of developing post- surgery complications, mainly due to microbial infections. Hence, cell-free biomaterials with enhanced properties to prevent corneal associated infections would provide a safe alternative. We evaluated the efficacy of different peptides for the functionalization of collagen-based hydrogels through the in situ synthesis of silver nanoparticles (AgNPs). The produced biomaterials were characterized and evaluated in vitro for biocompatibility and potential antimicrobial activity. From the diverse strategies evaluated, the localized formation of AgNPs onto the periphery of cornea-shaped collagen hydrogels may represent a more promising option.

abstract: Nanomaterials that exhibit enzyme-like catalytic activity or nanozymes have many advantages compared to biological enzymes such as low cost of production and high stability. There is a substantial interest in studying two-dimensional materials due to their exceptional properties. Hafnium diboride is a type of two-dimensional material and belongs to the metal diborides family made of hexagonal layers of boron atoms separated by metal layers. In this work, the peroxidase-like activity of hafnium diboride nanoflakes dispersed in the block copolymer F77 was discovered for the first time. The kinetics, mechanisms and catalytic performance towards the oxidation of the chromogenic substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide are presented in this work. Kinetic parameters were determined by steady-state kinetics and a comparison with other nanozymes is given. Results show that the HfB2/F77 nanozyme possesses a unique combination of unusual high affinity towards hydrogen peroxide and high activity per cost. These findings are important for applications that involve reactions with hydrogen peroxide. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2019

The amniotic membrane is a lining along the inner aspect of the placenta that envelops a developing embryo (then fetus). This component is critical for the adequate growth and nutrition of the fetus and can greatly impact the viability of the fetus. This role in development has led scientists to explore its post-delivery uses in regenerative medicine. Specifically in this paper, current literature was reviewed to determine the applicability of amniotic membranes to neural tissue engineering. The amniotic membrane has been greatly characterized with respect to immune response (including inflammatory effects) and microbial influence. These preliminary characterizations of the amniotic membrane and its components (i.e. stem cells) demonstrated a promising future for clinical implementation. Some fields, such as cardiovascular and orthopedic research, have begun projects using either the amnion-derived stem cells or amniotic membrane as a central element in their research. Both elements have received extensive praise for their versatility and relatively easy implementation into multiple organs and systems in the body. In each of these systems, the amniotic membrane retained its optimal antimicrobial and anti-immunogenic characteristics. After researching the current applications, it was apparent that amniotic membranes could have a place in the future of neural tissue engineering whether it be axillary components (cerebral vessels or surrounding bone) or direct regeneration of nerves. The largest impediment was the lack of basic science understanding in neuroscience. As the specific mechanisms of normal brain behavior and disease states are uncovered amniotic membranes can be added to the pre-clinical testing for the neurological sciences. Similar to the other fields of medicine, amniotic membranes will be specifically useful due to their ability to not evoke an immune response, thereby mitigating the possibility of rejection and infection in the central nervous system. These elements are critical to the research in neurology. Overall, amniotic membrane research would be very valuable to neuroscientists and physicians when exploring the future of neural tissue engineering

Progress in nanotechnology is trending towards applications which require the integration of soft (organic or biological) and hard (semiconductor or metallic) materials. Many applications for functional nanomaterials are currently being explored, including chemical and biological sensors, flexible electronics, molecular electronics, etc., with researchers aiming to develop new paradigms of nanoelectronics through manipulation of the physical properties by surface treatments. This dissertation focuses on two surface modification techniques important for integration of hard and soft materials: thermal annealing and molecular modification of semiconductors. First, the effects of thermal annealing are investigated directly for their implication in the fundamental understanding of transparent conducting oxides with respect to low resistivity contacts for electronic and optoelectronic applications and the response to environmental stimuli for sensing applications. The second focus of this dissertation covers two aspects of the importance of molecular modification on semiconductor systems. The first of these is the formation of self-assembled monolayers in patterned arrays which leads explicitly to the directed self-assembly of nanostructures. The second aspect concerns the modification of the underlying magnetic properties of the preeminent dilute magnetic semiconductor, manganese-doped gallium arsenide. Tin oxide belongs to a class of materials known as transparent conducting oxides which have received extensive interest due to their sensitivity to environmental stimuli and their potential application in transparent and flexible electronics. Nanostructures composed of SnO2 have been demonstrated as an advantageous material for high performance, point-of-care nanoelectronic sensors, capable of detecting and distinguishing gaseous or biomolecular interactions on unprecedented fast timescales. Through bottom-up fabrication techniques, binary oxide nanobelts synthesized through catalyst-free physical vapor deposition are implemented in the field-effect transistor structure. We have discovered that conductivity is absent in as-grown devices. However, utilizing a process for thermal treatment in vacuum and oxygen environments is found to be instrumental in fabricating field-effect transistors with significant conductivity, up to five orders of magnitude above the as-grown devices, for field-effect transistor application. Further investigation by photoluminescence coupled with the annealing parameters reveals that the likely cause of conductance comes from the reduction of surface defect states in the material. Importantly, the annealed material maintains its response to an applied gate potential showing orders of magnitude switching from the 'off' to the 'on' state. In order to show the practical relevance of our improvements on the SnO₂ material, we show our results for implementing the annealed material in biomolecular sensing experiments to detect the presence of streptavidin and Hepatitis C virus. Surface modification was carried out on oxide-free gallium arsenide (in some cases doped with manganese or zinc) through self-assembly of thiol molecules. First, we investigate the ability to pattern via two complementary micro- and nanopatterning techniques, microcontact printing (μCP) and dip-pen nanolithography (DPN). DPN is a unique lithography tool that allows drawing of arbitrary patterns with a molecular ink on a complementary substrate. It is extremely useful in integration of molecular inks within a pre-defined structure. Here, DPN was used to investigate the diffusion of organic molecules from a point source for both a moving and stationary tip on oxide-free GaAs. The diffusion can be calibrated so that intricate patterns down to tens of nanometers can be arbitrarily drawn on the surface. μCP, a less complicated method for large-scale arrayed patterning, is utilized to investigate the deposition of different thiolated molecular inks on GaAs and (Ga,Mn)As. The patterns deposited by μCP provide the template for directed self-assembly of gold nanoparticles. The systems based on these techniques can be extended to many substrate-molecule-nanostructure systems for an incredible variety of applications. Finally, the thiol-(Ga,Mn)As system is studied to determine the effects of molecular modification on the substrates' magnetic properties via modulation of the hole concentration in the wafer. The results for two molecules, one an electron donor and one an electron acceptor, show opposite trends for modulation of both the Curie temperature and the saturation magnetization. We suggest that nanopatterning of electron donor or electron acceptor molecules could lead to the development of reconfigurable nanomagnetic systems in (Ga,Mn)As with potential applications in molecular spintronics or magnetic memory. / A Dissertation submitted to the Department of Physics in partial fulfillment of the Doctor of Philosophy. / Spring Semester 2017. / March 22, 2017. / Includes bibliographical references. / Peng Xiong, Professor Directing Dissertation; P. Bryant Chase, University Representative; David van Winkle, Committee Member; Per Arne Rikvold, Committee Member; Mark Riley, Committee Member.

Nanoscience and nanotechnology research has provided us with new paradigms of technologies to improve human life, but still there is plenty of room to expand its frontiers. In order to do so, we need to pursue the development and study of novel nanostructures with the main goal of understanding the physical properties and finding potential applications. Understanding the physics of low-dimensional systems is the first step to fostering the corresponding technological applications. Considering this premise, the goal of this dissertation is to study two distinct classes of heterogeneous nanowires (NWs): phosphorous-doped Si NWs with an axial doping gradient and metal NWs grown on DNA templates. The Si NWs grown by vapor-liquid-solid chemical vapor deposition were utilized to fabricate Schottky barrier-limited field-effect transistors (FETs), which have shown significant promise in the areas of electronics and sensing because of their unique characteristics. The idea of utilizing the modulation of the nano Schottky junction at a metal-semiconductor interface promises higher performance for chemical and biomolecular sensor applications when compared to conventional FETs with Ohmic contacts (exponential versus linear responses). However, the fabrication of such asymmetric FETs presents challenges such as reproducibility through complications in the fabrication processes. We have been able to circumvent the fabrication difficulties and reproducibility problems by utilizing our Si nanowires synthesized by a chemical vapor deposition process which yields a pronounced doping gradient along the length of the NWs. This inhomogenous doping in NWs has typically been seen as a detrimental characteristic; however, we have taken advantage of this doping profile as the basis of our approach. The graded doping profile facilitates definition of a series of metal contacts on a single NW that systematically evolve from Ohmic to Schottky with increasing effective barrier height along the axial direction. The study of this systematic variation is presented in this dissertation as a proposal to obtain devices for sensing and electronic applications. The main results of our research is recently published. The fabrication and study of metal NWs is the second effort discussed in this dissertation. The main motivation is to address the fundamental question of whether a true superconducting state could exist in one dimension. The answer to this question lies in the nature of superconducting fluctuations of the order parameter that describe the coherent behavior of the Cooper pairs. In a superconducting system, the order parameter has a well-defined amplitude and phase. The superconducting fluctuations occur in the form of phase slips which can be either thermally activated or quantum mechanical. Although much experimental and theoretical work has been done on the topic, an unambiguous resolution of this issue remains elusive mainly due to the challenge of producing NWs having the dimensions of the cross-section of the NW smaller than the superconducting coherence length or the size of the Cooper pairs. Our approach to overcome the fabrication difficulties to reach the true 1D limit is a unique combination of DNA templates and low temperature quench-condensation for in situ fabrication and measurement of superconducting NWs with a width of just a few nanometers. In this dissertation, details on the fabrication and our initial results demonstrating the capability of our DNA molecular templates to reach small cross-section metal NWs are presented; also, we present systematic characterizations of the electrical properties of metal nanowires with respect to in situ variation of the geometry of the nanowire. This effort has laid a full foundation for a comprehensive examination of superconductivity in 1D reaching unprecedentedly small cross-sections. A manuscript summarizing these results is in preparation. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2017. / April 11, 2017. / DNA templates, metal nanowires, Schottky energy barrier, silicon nanowires / Includes bibliographical references. / Peng Xiong, Professor Directing Dissertation; Jingjiao Guan, University Representative; Nicholas E. Bonesteel, Committee Member; Irinel Chiorescu, Committee Member; Jorge Piekarewicz, Committee Member.

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