A quest of understanding complex behavior of flexible polyelectrolyte has been made in this thesis. A phase separating system, sodium polystyrene sulfonate and barium chloride, (NaPSS-BaCl2) has been studied with different quench depths around critical temperature. It was found that the complexity of polyelectrolyte system manifests itself as a non classical behavior of phase separation kinetics. One of the striking results was that even in deep temperature quenches, the induction time (lag time) has been always observed. Dynamic light scattering data revealed the formation of aggregates has an intermediate step at length scale of 50nm where the size did not increase appreciably as the population of the continuously increased. In the other part of the thesis, sodium polystyrene sulfonate and sodium chloride, NaPSS-NaCl solutions, can reveal very interesting patterns when a drop of mixture is placed on glass and evaporated. The crystallization of NaCl was mediated with the amount of NaPSS, temperature and humidity. Patterns can be modified and controlled by tuning these parameters.

This thesis reports the fabrication of nanophotonic structures by using electron beam lithography and using pattern transfer via self assembly with the aid of block copolymers. A theoretical and experimental basis was developed for fabricating anti-reflective coatings using block-copolymer pattern transfer. Block-copolymers were also used to fabricate plasmonic pattern arrays which form gold dots on glass surface. Electron-beam lithography was utilized to fabricate holey plasmonic structures from gold and silver films. Electron-beam exposure was used in block-copolymer lithography in selected regions. The exposure effects were studied for both thin and thick block-copolymer films. Reactive and ion beam etching techniques were used and optimized to fabricate those structures. This research required a great deal of development of new fabrication methods and key information is included in the body of the thesis.

Electron transport is a fundamental mechanism in a variety of biological systems such as photosynthesis and aerobic respiration. However, the transport has long been considered to occur only over short distances (< 1 μm), primary by metalloproteins. Recently, the conduction of electrons over large distances (> 10 μm) along networks of microbial pilin filaments known as microbial nanowires has been invoked to explain a wide range of important redox phenomena that influence carbon and mineral cycling in soils and sediments, bioremediation, corrosion, interspecies electron transfer and anaerobic conversion of organic wastes to methane or electricity. However, there has never been any direct experimental demonstration of this long-distance electron transport. In fact, previous measurements of microbial biofilms have noted just the opposite: that biofilms act as insulators, not conductors. In this thesis, we reconcile these confounding observations with the demonstration that biofilms of several species of commonly studied microorganisms do function as insulators, whereas biofilms of Geobacter sulfurreducens , common in soils and sediments can form a conductive matrix, with a conductivity comparable to synthetic conductive polymers. We show that biofilms are capable of conducting electrons over 1.25 cm, many thousands of times the size of a cell. Biofilm conductivity was found to be proportional to the abundance of pilin filaments and the conductivity of sheared pilins was comparable to biofilms. We also found that biofilm conductivity regulates fuel cell current density. We demonstrate that electron transport in the biofilms does not occur via localized charge carriers known as cytochromes, as almost universally predicted, but rather through delocalized electronic states. Moreover, we report a quantum mechanical interference phenomenon of weak localization in pilin nanowires. Additionally, we demonstrate that cytochromes can be used to store electrons with capacitance comparable to commercial supercapacitors. Furthermore, the degree of conductivity and capacitance within the films can be tuned via changes in gene expression or gate bias. This study demonstrates that pilin-associated long-distance electron transport through a microbial matrix is feasible, establishes approaches that could be used for evaluating the possibility of electron flow through natural microbial communities, and demonstrates the potential for developing novel bioelectronic materials.

This thesis discusses experimental research and theoretical analysis on exploring the physics and techniques of manipulation of magnetization states of ferromagnetic nanorings in both homogeneous and non-homogeneous applied magnetic field. Magnetization states and their switching processes are fundamental properties of magnetic systems. The ring shape is particularly interesting because of the existence of the closed-flux vortex state, which can be used to encode binary information. The understanding and control of the magnetization switching of ferromagnetic nanorings could lead to new designs of practical magnetic data storage devices. The work in this thesis is grouped into three main activities: theoretical analysis and micromagnetic simulation, fabrication techniques, and characterization of magnetization switching by applied magnetic field. Ferromagnetic rings with different geometric parameters were fabricated by electron beam lithography (EBL), electron beam evaporation and lift-off techniques. EBL patterning on double layer photo resist improved lithography and lift-off resolution. The experiments with applied homogeneous and non-homogeneous fields were able to manipulate the ferromagnetic nanorings through different magnetic configurations. The key accomplishment of this work is we experimentally achieved direct switching between two magnetic vortex states of opposite circulation of magnetization, by using an applied azimuthal (circular) Oersted magnetic field. Such field was generated by applying current through the center of a ring using a platinum atomic force microscopy tip. We used magnetic force microscope imaging to demonstrate the controllability of magnetic switching from onion state to vortex state and for the first time, direct switching between two opposite vortex states. Moreover, we investigated the switching mechanisms associated with nucleation, annihilation, and propagation of domain walls. The magnetic switching properties were found to be sensitive to the ring geometrical parameters. Smaller rings require less circular field to complete the switching than bigger rings. Asymmetric rings require less circular field to complete the switching than symmetric rings with the same dimensions. Theoretical analysis and micromagnetic simulations were conducted on symmetric and asymmetric nanorings, with the purpose of helping us better understand the physics behind the experimental results. Those systematic studies investigated the energy and stability of different magnetization states, the switching field required as a function of the ring geometric designs, as well as switching mechanism and the evolutions among different magnetic states, in both in-plane and azimuthal Oersted magnetic fields. We found the simulations results are in a good agreement with the characterization results.

This study focuses on the reactions leading to the organometallic vapor-phase epitaxy of gallium nitride from trimethylgallium (TMGa) and ammonia and of gallium arsenide from TMGa and trimethylarsenic (TMAs). Organometallic vapor-phase epitaxy involves both homogeneous and heterogeneous reactions. This process is used to form epitaxial layers on semiconductor surfaces. The chemistry involved is complex and many of the intermediate species leading to the deposited crystalline material remain unresolved. Heterogeneous processes proved to have a primary effect on the decomposition of the reactants, and surface infrared emission spectroscopy was developed as a method to observe these species in situ. All the reactants were observed to be adsorbed on the surface during in situ runs. The surface was between 400 and 600$\sp\circ$C during these experiments, indicating chemisorption. Specifically, sorbed methyl groups were observed above 500$\sp\circ$C, indicating that both TMGa and TMAs decompose heterogeneously. It was also found that TMAs reacts directly with the hydrogen diluent and thus transports beneficial hydrogen to the surface.

The results of nuclear magnetic resonance (NMR) on thin $\sp3\rm{He}$-$\sp4$He mixture films at temperatures 24mK $\leq T \leq$ 650mK which are adsorbed to Nucleopore are reported. The nuclear magnetic susceptibility, the relaxation times T$\sb1$ and T$\sb2$, and the spin diffusion coefficient, D, were measured used pulsed NMR techniques in a 2 Tesla field. The $\sp4$He coverages investigated ranged from 0.137 $\leq n\sb4\leq$ 0.534 atoms A$\sp2$ with a fixed submonolayer $\sp3$He coverage of 0.00746 $\leq n\sb3 \leq$ 0.00749 atoms A$\sp2.$ At $n\sb4$ = 0.391 atoms/A$\sp2$ measurements were taken with the $\sp3$He coverage ranging 0.00749 $\leq n\sb3 \leq$ 0.0179 atoms/A$\sp2$. We present the $\sp3$He magnetization as a function of the $\sp4$He coverage. The magnetization is degenerate for temperatures below the Fermi temperature, T$\sb{F}$, and from the degenerate magnetization the hydrodynamic mass over a range of $\sp4$He coverages is obtained. Variational and density functional descriptions of the film are considered. The diffusion data are seen to rise rapidly, from 10$\sp{-8}$ to 10$\sp{-3}\ \rm{cm}\sp2$/sec, as the $\sp4$He coverage is increased from 0.19 to 0.39 atoms/A$\sp2$, a range of just 2.5 layers. For $T < T\sb{F}$ the temperature dependence of a degenerate Fermi gas is not seen; $D \not= T\sp{-2}$. For all coverages T$\sb1$ is two orders of magnitude larger than T$\sb2$. Two regimes are seen. For coverages $n\sb4 <$ 0.23 the temperature dependence of T$\sb1$ and T$\sb2$ are consistent with $\omega\tau\sb{c} \gg$ 1. A signature of the completion of the second layer of the $\sp4$He is seen in T$\sb{1}$. For coverages $n\sb4 >$ 0.23, $\omega\tau\sb{c} \ll$ 1 and the temperature dependence correlates with the superfluid areal density. Activated behavior is seen which probes higher bound states of the $\sp3$He in the $\sp4$He film.

Thermal conductivity measurements were performed on $\sp4$He films adsorbed on two substrates to test the role that substrate geometry may have on dynamic measurements at the Kosterlitz-Thouless Transition. For films having thicknesses less than the transition thickness, the evolution of the temperatures as a function of film thickness, at fixed heater power, were observed to be qualitatively similar for both substrates. For each substrate, the thermal conductivity of superfluid films followed the power law dependence on heater power consistent with the measurements of Maps and Hallock and suggests that the basic features of the Kosterlitz-Thouless transition are not fundamentally dependent on the two geometries. In a second experiment a search for wetting anomalies in a saturated helium film at temperatures near the bulk transition temperature is made using a capacitive technique. The wetting transition observed by Taborek and Senator is not observed, and under optimal operating conditions, the film thickness is constant as a function of temperature to within one atomic layer for temperature sweeps spanning the bulk transition temperature. Film thinning consistent with that observed by Taborek and Senator does occur when thermal gradients are imposed on the experimental cell.

A new effect leading to enhanced electroabsorption in quantum wells under biaxial tension is investigated through theoretical models and spectroscopic experiments. This effect is the electric field-induced merging of the light- and heavy hole absorption edges in an initially "light-hole-up" quantum well. The "absorption-edge-merging" or AEM effect is demonstrated for the first time through low temperature photocurrent spectroscopy. The modulation performance of reflection multiple quantum well (MQW) modulators with tensile-strained active regions utilizing AEM is compared with lattice-matched GaAs-Al$\sb{0.35}$Ga$\sb{0.65}$As modulators designed to operate over the same wavelength range. A theoretical model of electroabsorption in strained quantum wells is developed first. A variational trial function approach is used in conjunction with a Rayleigh-Ritz procedure in calculating the electronic structure of strained quantum wells. The electric-field dependent excitonic and interband absorption coefficients are then calculated. Calculations showing the AEM and electroabsorption enhancement for In$\sb{x}$Ga$\sb{1-x}$As-InP and GaAs$\sb{x}$P$\sb{1-x}$-Al$\sb{0.35}$Ga$\sb{0.65}$As are then described. Tradeoffs involving the advantages of AEM are identified through theoretical comparisons of tensile-strained modulators with analogous lattice-matched structures. Optimal structures for operation at 1.55$\mu$m in In$\sb{x}$Ga$\sb{1-x}$As-InP and 0.77$\mu$m in GaAs$\sb{x}$P$\sb{1-x}$-Al$\sb{0.35}$Ga$\sb{0.65}$As are identified and the sensitivity of their electroabsorption characteristics to material and structural parameters are examined. The experimental studies of the AEM effect are considered next. The field-induced merging of the electron-to-light hole (e-lh) and electron-to-heavy-hole (e-hh) excitonic absorption edges in tensile-strained quantum wells is demonstrated. Photocurrent spectra at 77K of a 95A GaAs$\sb{0.92}$P$\sb{0.08}$-Al$\sb{0.37}$Ga$\sb{0.63}$As MQW structure embedded in a p-i-n diode show reduction and eventual elimination of a zero-bias $\sim$ 7meV splitting between the e-lh and e-hh exciton peaks with increasing reverse bias. The modulation performance tensile-strained GaAs$\sb{x}$P$\sb{1-x}$ -Al$\sb{0.37}$Ga$\sb{0.63}$As reflection modulators is then compared to that of similar lattice-matched GaAs -Al$\sb{0.37}$Ga$\sb{0.63}$As devices operating over the same wavelength range. The tensile strained modulators utilize 95A GaAs$\sb{0.92}$P$\sb{0.08}$-Al$\sb{0.37}$Ga$\sb{0.63}$As quantum wells, while the lattice matched devices utilize $\sim$46 A GaAs-Al$\sb{0.37}$Ga$\sb{0.63}$As quantum wells yielding similar excitonic gaps. Room temperature differential reflection spectra demonstrating increased modulation depths at low drive voltages in the tensile-strained devices are presented, consistent with theory.

Kohn-Sham Density Functional Theory (DFT) has been the most successful tool to probe the electronic structure, mainly the ground-state total energies and densities of many condensed matter systems has led to the development of various databases such as Materials Project (MP), Inorganic Crystal Structure Database (ICSD), and many others. These databases ignited the interest of the material science community towards Machine Learning (ML), leading to the development of a new sub-field in material science called material-informatics, which aims to uncover the interrelation between known features and material properties. ML techniques can handle and identify relationships in complex and arbitrarily high-dimensional spaces data, which are almost impossible for human reasoning. Unlike DFT, the ML approach uses data from past computations or experiments. In many cases, ML models have shown their superiority over DFT in terms of accuracy and efficiency in predicting various physical and chemical properties of materials. The incorporation of material property data obtained from atomistic simulations is crucial important to make continuous progress in data-driven methods. In this direction, we use DFT with Perdew-Burke-Ernzerhof (PBE), and Heyd–Scuseria–Ernzerhof (HSE) functionals, to introduce a family of mono-layer isostructural semiconducting tellurides M2N2Te8, with M = {Ti, Zr, Hf} and N = {Si, Ge}. These compounds have been identified to possess direct band gaps that are tunable from 1.0 eV to 1.3 eV, which are well suited for photonics and optoelectronics applications. Additionally, in-plane transport behavior is observed, and small electron and hole (0.11-015 Me) masses are identified along the dominant transport direction. High carrier mobility is found in these compounds, which shows great promise for applications in high-speed electronic devices. Detailed analysis of electronic structures reveals the presence of metal center bicapped trigonal prism as the structural building blocks in these compounds; a common feature in most of the group V chalcogenides helps to understand the atomic origins of promising properties of this unique class of 2D telluride materials. Atomistic simulations based on DFT theory played a vital role in the development of data-driven materials discovery process. However, the resource-based constraints have limited the high-throughput discovery process by using DFT. The main motivation of our work towards the application of machine learning in material science is to assist the discovery process using available material property data in various databases. Incorporation of physical principles in a network-based machine learning (ML) architecture is a fundamental step toward the continued development of artificial intelligence for materials science and condensed matter physics. In this work, as inspired by the Pauling’s rule, we propose that structure motifs (polyhedral formed by cations and surrounding anions) in inorganic crystals can serve as a central input to a machine learning framework for crystalline inorganic materials. We demonstrated that an unsupervised learning algorithm Motif2Vec is able to convert the presence of structural motifs and their connections in a large set of crystalline compounds into unique vectors. The connections among complex materials can be largely determined by the presence of different structural motifs, and their clustering information is identified by our Motif2Vec algorithm. To demonstrate the novel use of structure motif information, we show that a motif-centric learning framework can be effectively created by combining motif information with the recently developed atom-based graph neural networks to form an atom-motif hybrid graph network (AMDNet). Taking advantage of node and edge information on both atomic and motif level, the AMDNet is more accurate than a single graph network in predicting electronic structure related material properties such as band gaps. The work illustrates the route toward the fundamental design of graph neural network learning architecture for complex materials properties by incorporating beyond-atom physical principles. Due to the limitations in resources, it is not feasible to synthesize hundreds of thousands of materials listed in various databases by experiment or compute their detailed properties by using various electronic structure codes and state-of-the-art computational tools. Hence, the identification of an alternative route to screen such databases is very desirable. If identified, this route would be very helpful in reducing the material search space for any application. Categorizing materials based on their structural building blocks is very important to study the underlying physics and to understand the possible mechanisms for any application. Based on structure motifs, we purpose a novel way to categorize, analyze, and visualize the material space called a material network. The connection between any two nodes in this network is determined by using the calculated similarity value (Tanimoto-coeffecient) between each motif and its surrounding information, encoded in terms of a feature vector of length 64. By mapping a known compound, the network thus constructed can be used to screen compounds for the desired application. All the connections of the mapped compound are identified and extracted as a subgraph for further analysis. In our test screening for the transparent conducting oxides (TCO), the proposed network is successful in identifying compounds that are already listed as TCO in the literature. Thus, this indicates its usefulness in reducing the search space for the new TCO materials and various applications. This motif-based material network can serve as an alternate route for functional material discovery and design. / Physics

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