This dissertation work focuses on the development and application of a tunable, scalable, and robust patterning methodology, based on colloidal lithography and plasma etching, to create graded-index, moth eye (ME)-like surface features and nanostructures to control reflection at IR and III-nitride material interfaces and realize novel, nanoscale light emitters. Silica colloidal mask particles were deposited on various substrates using Langmuir-Blodgett dip coating, followed by mask reduction, and mask pattern transfer into the underlying substrate using a combination of plasma and/or wet-chemical etching techniques. The resulting ME patterned surfaces and nanostructures were characterized and simulated using various experimental (SEM, AFM, photo/cathode/electro-luminescence, FTIR) and theoretical methods (finite difference time domain (FDTD) and Monte Carlo-based ray tracing), respectively, as well as incorporated into simple photonic and luminescent devices.

Hexagonal arrays of nanoscale moth eye features, i.e., conical frusta with tunable size, pitch, and shape, were realized in IR optical materials (CdTe, ZnS, and ZnSe) by isotropic etching of various size silica colloid masks before pattern transfer into the underlying substrate. Large single-side transmission enhancements (9-15% on CdTe thin films and 18% on bulk CdTe) were obtained over the short, mid, and far IR wavelength ranges (λ = 6-20 μm) by simply adjusting colloidal mask size (310-2530 nm). Substantial increases in broadband transmission were also achieved for ZnS and ZnSe across the 2–20 μm range (23% and 26% single-side transmission improvement and 92% and 88% absolute double-side transmission, respectively), in excellent agreement with FDTD optical simulations.

Moth eye surface structures were also implemented on the light outcoupling surface of GaN/InGaN light emitters to enhance light extraction efficiency. Features with aspect ratios of 3:1 were produced using silica masks (d = 170–2530 nm) and Cl2/N2-based plasma etching on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat device was achieved using a feature pitch of 2530 nm. FDTD and ray tracing calculations of light extraction enhancement were in excellent agreement with experimentally measured results.

Finally, nanoscale light-emitting diodes (nanoLEDs) with active and sacrificial multi-quantum well (MQW) layers were fabricated and released into solution using a combination of colloidal lithography and photoelectrochemical (PEC) etching of sacrificial MQW layers. Wafer-scale fabrication processes for both c-plane and semipolar nanoLEDs were developed. The PEC etch was optimized to minimize undercut roughness, and thus limit the damage to the active MQW layers. X-ray diffraction was employed to assess strain relaxation due to nanopatterning, which showed ~15% strain relaxation for 500 nm nanoLEDs. Overall, this work shows that colloidal lithography, combined with chemical release, is a viable route to produce solution processable, high efficiency nanoscale light emitters.

As our ability to engineer, design, and modify nanoscale systems continues to advance, characterization methods must keep pace. Light-matter interactions, in particular optical spectroscopy, provide a wealth of information on the vibrational and electronic structure of matter and can be directly related to physical properties such as phase, chemistry, charge transport, etc. However, the fundamental wave-like nature of light prevents radiation from being focused to arbitrarily small length scales using traditional optics. This is known as the diffraction limit, and is on the order of several hundred nanometers for optical wavelengths (~λ/2). One method of overcoming diffraction is to couple light to nanostructures (e.g., optical antennae) that support resonant oscillations of conduction electrons (plasmons). These charge oscillations generate intense optical fields that are spatially controlled by the antenna size, rather than the radiation wavelength. The set of related techniques utilizing nanoscale optical antennae to interrogate and image surfaces are known as tip-enhanced near-field optical microscopy (TENOM).

This work details the design, construction, and experimental validation of a TENOM instrument, and demonstrates specific applications in near-field spectroscopy and super-resolution chemical imaging. A commercial inverted optical microscope was integrated with a custom-built shear-force atomic force microscope (AFM). The inverted microscope geometry enables high excitation and collection efficiency of light from the antenna apex, while the shear-force AFM ensures the antenna is always positioned at the sample surface, allowing analytes to interact with the locally enhanced optical fields. Experimental validation of the completed TENOM instrument was accomplished using both copper (CuPc) and metal-free phthalocyanine (H2Pc) species. Chemical images of patterned CuPc and H2Pc were obtained with lateral spatial resolutions below 50 nm (<λ/10), unambiguously demonstrating the super-resolution capabilities of the instrument. Multimode imaging of H2Pc was performed with simultaneous collection of spatially correlated fluorescence, Raman, and topographic data. The combination of these measurements allowed nanoscale mapping of the H2Pc aggregation state across a wide range of surface coverages, including isolated molecules, molecular dimers, and continuous films.

Additionally, finite-difference time-domain (FDTD) optical simulations were used to study the fundamental physics of plasmonic optical antennae relevant for near-field spectroscopy. For TENOM applications, tuning the optical properties of support structures with attached plasmonic nanocavities was shown to be critical for either enhancing or quenching local electric field strengths. Support structures with low extinction coefficients were found to produce the largest field enhancements, with the refractive index of the material being further tuned to optimize antenna performance as a function of the specific geometry considered. A quantitative comparison of several antenna designs was carried out, which has not been possible experimentally due to the low reproducibility of nanostructure fabrication procedures and variability in methods of measuring local optical fields. Two promising architectures were identified that both involve focused ion-beam milling a groove near the antenna apex. Methods of tuning the resonance energy of these structures over the full visible spectrum, using different plasmonic metals (Au/Ag) and by varying the groove positions relative to the apex, were also demonstrated.

FDTD simulations were also used to study pairs of plasmonic nanoparticles relevant for surface-enhanced Raman spectroscopy applications. Previous work on this system was shown to significantly overestimate field enhancements due to numerical effects present at nano-gap features. Metal bridging structures were used to halt the field divergence at physically relevant lengths scales, allowing accurate study of experimentally relevant parameters including the fused contact area and presence of a dielectric encapsulation layer. It was found that fused dimer antennae are capable of producing large enhancements at infrared energies, but may be challenging to reproducibly fabricate due to the high sensitivity of the supported plasmon resonances to changes in local morphology.

Advancements across multiple scientific and engineering disciplines are helping push the TENOM technique forward. Improvements in high-intensity broadband laser sources will enable flexible measurement of both the electronic and vibrational structure of materials, and general improvements in nano-manufacturing are expected to reduce the time and cost of producing high-enhancement resonant antennae with well-defined plasmonic structure. The future is bright for TENOM to find use as a versatile optical and physical surface characterization technique.

Controlling the reflection of visible and infrared (IR) light at interfaces is extremely important to increase the power efficiency and performance of optics, electro-optical and (thermo)photovoltaic systems. The eye of the moth has evolved subwavelength protuberances that increase light transmission into the eye tissue and prevent reflection. The subwavelength protuberances effectively grade the refractive index from that of air (n=1) to that of the tissue (n=1.4), making the interface gradual, suppressing reflection. In theory, the moth-eye (ME) structures can be implemented with any material platform to achieve an antireflectance effect by scaling the pitch and size of protuberances for the wavelength range of interest.

In this work, a bio-inspired, scalable and substrate-independent surface modification protocol was developed to realize broadband antireflective structures based on the moth-eye principle. Quasi-ordered ME arrays were fabricated in IR relevant materials using a colloidal lithography method to achieve highly efficient, omni-directional transmission of mid and far infrared (IR) radiation.

The effect of structure height and aspect ratio on transmittance and scattering is explored, with discussion on experimental techniques and effective medium theory (EMT). The highest aspect ratio structures (AR = 9.4) achieved peak single-side transmittance of 98%, with >85% transmission for lambda = 7-30 microns. A detailed photon balance constructed by transmission, forward scattering, specular reflection and diffuse reflection measurements to quantify optical losses due to near-field effects will be discussed. In addition, angle-dependent transmission measurements showed that moth-eye structures provide superior antireflective properties compared to unstructured interfaces over a wide angular range (0-60° incidence).

Finally, subwavelength ME structures are incorporated on a Si substrate to enhance the absorption of near infrared (NIR) light in PtSi films to increase Schottky-barrier detector efficiency. Absorbance enhancement of 70-200% in the lambda =1-2.5 micron range is demonstrated in crystalline PtSi films grown via electron beam evaporation of Pt and subsequent vacuum annealing. Low total reflectance (<10%) was measured in ME films, demonstrating the efficacy of the moth eye effect. Effective medium theory and transfer matrix calculations show that the large absorption enhancement at short wavelengths is partly due to light trapping, which increases the effective optical path length in PtSi. The demonstrated structures are promising candidates for efficient PtSi/p-Si Schottky barrier diode detectors in the NIR. Results further suggest a general method for relatively low-cost absorption enhancement of backside-illuminated detectors based on a wide variety of infrared absorptive materials.

The methods presented here to fabricate quasi-ordered ME structures provide a general platform for creating antireflective structures in many different materials, devices, and bandwidths. Furthermore, understanding the relationship between protuberance shape, height, aspect ratio, etc. and performance (antireflection, scattering loss, etc.) can guide the design of antireflective surfaces for different applications (for example, in certain applications, large amounts of forward scattering is desired, e.g. photovoltaics).

Understanding and controlling carrier transport in conjugated polymer films and composites is critical to the development and application of plastic solar cells. Recent efforts have focused on "bulk heterojunction" structures where a conjugated polymer donor is mixed at the nanoscale with a fullerene acceptor to achieve large interfacial areas for exciton splitting. In these systems, fabrication protocols dramatically affect device efficiency and charge transport is intimately tied to film morphology through local order, domain formation, and compositional heterogeneity. We employ both far-field and confocal/near-field optical spectroscopy (absorbance, low-temperature photoluminescence, Raman) to study chain order (aggregation, π-stacking), photo-oxidation, and local morphology in conjugated polymer (PPV and polythiophene) - fullerene (PCBM) blends. Through quantitative analysis of exciton bandwidths, emission intensity, and vibronic lineshapes, we demonstrate that competition exists between the chemical "disordering" effect of photo-degradation and the physical "ordering" effect of aggregation, each of which dominate under different processing conditions. Large changes in photoluminescence and Raman show that PCBM begins to significantly hinder intra-chain planarization and inter-chain π-overlap at a critical PCBM weight fraction. Furthermore, the critical weight fraction is a function of the polymer regiochemistry, occurring at lower PCBM weight fractions for a more regio-random polymer. Mild thermal annealing of blended films was seen to restore order, which results from PCBM phase segregation (lower dispersion) and growth of polymer aggregates. Spatially resolved spectral analysis of photoluminescence was also used to map fullerene diffusion and agglomeration as well as detect local changes in interfacial contact between donor and acceptor domains due to thermal annealing.

Optical microscopy and spectroscopy are invaluable tools for the physical and chemical characterization of materials and surfaces in a wide range of scientific disciplines. However, the application of conventional optical methods in the study of nanomaterials is inherently limited by diffraction. Tip-enhanced near-field optical microscopy (TENOM) is a hybrid technique that marries optical spectroscopy with scanning probe microscopy to overcome the spatial resolution limit imposed by diffraction. By coupling optical energy into the plasmonic modes of a sharp metal probe tip, a strong, localized optical field is generated near the tip's apex and is used to enhance spectroscopic emissions within a sub-diffraction-limited volume. In this thesis, we describe the design, construction, validation, and application of a custom TENOM instrument with a unique attenuated total reflectance (ATR)-geometry excitation/detection system. The specific goals of this work were: (i) to develop a versatile TENOM instrument capable of investigating a variety of optical phenomena at the nanoscale, (ii) to use the instrument to demonstrate chemical interrogation of surfaces with sub-diffraction-limited spatial resolution (i.e., at super resolution), (iii) to apply the instrument to study plasmonic phenomena that influence spectroscopic enhancement in TENOM measurements, and (iv) to leverage resulting insights to develop systematic improvements that expand the ultimate capabilities of near-field optical interrogation techniques.

The TENOM instrument described herein is comprised of three main components: an atomic force microscope (AFM), a side-on confocal Raman microscope, and a novel ATR excitation/detection system. The design of each component is discussed along with the results of relevant validation experiments, which were performed to rigorously assess each component's performance. Finite-difference time-domain (FDTD) optical simulations were also developed and used extensively to evaluate the results of validation studies and to optimize experimental design and instrument performance. By combining and synchronizing the operation of the instrument's three components, we perform a variety of near-field optical experiments that demonstrate the instrument's functionality and versatility. ATR illumination is combined with a plasmonic AFM tip to show that: (i) the tip can quantitatively transduce the optical near-field (evanescent waves) above the surface by scattering photons into the far-field, (ii) the ATR geometry enables excitation and characterization of surface plasmon polaritons (SPPs), whose associated optical fields are shown to enhance Raman scattering from a thin layer of copper phthalocyanine (CuPc), and (iii) SPPs can be used to plasmonically excite the tip for super-resolution chemical imaging of patterned CuPc via tip-enhanced Raman spectroscopy (TERS). ATR-illumination TERS is quantitatively compared with side-on illumination. In both cases, spatial resolution was better than 40 nm and tip-on/tip-off Raman enhancement factors were >6500. Furthermore, ATR illumination was shown to provide similar Raman signal levels at lower "effective'' pump powers due to additional optical energy delivered by SPPs to the active region in the tip-surface gap.

We also investigate the sensitivity of the TENOM instrument to changes in the plasmonic properties of the tip-surface system in the strongly-coupled regime at small tip-surface separations. Specifically, we demonstrate detection of a resonant plasmonic tip-surface mode (a gap plasmon) that dramatically influences the optical response of the system, and we use experimental results and FDTD simulations to support a hypothesized mechanism. Moreover, we confirm that the gap plasmon resonance has a strong effect on the enhancement of both fluorescence and Raman scattering, and we propose that this phenomenon could ultimately be exploited to improve sensitivity in super-resolution chemical imaging measurements. Finally, we recommend a straightforward modification to the TENOM instrument that could enable future application of these gap-mode plasmon resonances to increase spectroscopic enhancements by an order of magnitude.

This thesis entails the synthesis, automated catalytic testing, and in situ molecular characterization of supported Pt and Pt-alloy nanoparticle (NP) catalysts, with emphasis on how to assess the molecular distributions of Pt environments that are affecting overall catalytic activity and selectivity. We have taken the approach of (a) manipulating nucleation and growth of NPs using oxide supports, surfactants, and inorganic complexes to create Pt NPs with uniform size, shape, and composition, (b) automating batch and continuous flow catalytic reaction tests, and (c) characterizing the molecular environments of Pt surfaces using in situ infrared (IR) spectroscopy and solid-state 195Pt NMR. The following will highlight the synthesis and characterization of Ag-doped Pt NPs and their influence on C2H2 hydrogenation selectivity, and the implementation of advanced solid-state 195Pt NMR techniques to distinguish how distributions of molecular Pt environments vary with nanoparticle size, support, and surface composition.

Density functional theory (DFT) is undergoing a shift from a descriptive to a predictive tool in the field of solid state physics, with undertakings like the Materials Project, OQMD, and AFLOW leading the way in utilizing high-throughput data to predict and seek novel materials properties. However, methods to rigorously evaluate the \textit{validity} and \textit{accuracy} of these studies is lacking in both the availability and utilization of techniques. The natural disconnect between simulated and experimental length-scales and temperatures, combined with this lack of validation, raises serious questions when simulation and experiment disagree. In this thesis, we analyze several transition metal systems where simulations and experiments present unusual disagreements, and develop a new formalism for comparing high-temperature measurements to \textit{ab-initio} calculations. Our work aims to broaden the understanding not only of the specific systems discussed, but of how presently available \textit{ab-initio} methods perform for transition metal alloys across \textit{all} systems.

Recent high-throughput ab-initio studies of transition metal binaries have suggested a great number of undiscovered stable phases present in well-studied systems. Co-Pt alloys, especially, have a long experimental history demonstrating three stable mixed phases: L1$_0$ CoPt and L1$_2$ Co$_3$Pt and CoPt$_3$, but density functional theory suggests a set of yet-unobserved long-period $\beta_2$-like superstructures at Pt-rich compositions. We analyze the Co-Pt system in-depth, calculating the energy of over 1,400 structures to thoroughly explore the series of unusual superstructures suggested by DFT\@. Simulated diffraction patterns, analysis of magnetic behavior, and investigation of the density-of-states emphasize the stark differences between measured behaviors and \textit{ab-initio} predictions. By moving up the Jacob's Ladder of functionals, we show that we only replace one set of discrepancies for another, and even the introduction of vibrational degrees of freedom fails to solve the massive differences in predicted phase stability. By fitting the \textit{ab-initio} results to a cluster expansion Hamiltonian and performing Monte Carlo calculations, we show that the resulting high-temperature phase diagram is wholly incompatible with experimental results.

Like Co-Pt, Heusler compounds have unique magnetic properties, resulting in interest for their potential applications as spintronic materials. The pseudo-binary (Mn,Fe)Ru$_2$Sn, formed as a solid solution of the full Heuslers (Mn, Fe)Ru$_2$Sn, has been recently shown to exhibit exchange-hardening implicative of two \textit{magnetic} phases, despite the presence of only one \textit{chemical} phase. Using \textit{ab-initio} calculations we show that the magnetic behavior of this alloy arises from a competition between AFM-favoring Sn-mediated superexchange and FM-favoring RKKY exchange mediated by spin-polarized conduction electrons. Changes in valency upon replacement of Mn with Fe shifts the balance from superexchange-dominated interactions to RKKY-dominated interactions. Using our electronic structure calculations, we parameterize a mixed-basis chemical-and-magnetic cluster expansion, and use Monte Carlo simulations to demonstrate a ferromagnetic (FM) to antiferromagnetic (AFM) behavior dependent on composition with the experimental study. By examining the low-temperature ensemble averages of magnetic and chemical correlations, we identify the mechanism behind magnetic hardening in the solid solution.

Our multiple successes in utilizing cluster expansions, both to deeply analyze failures and successfully describe complex chemical-magnetic interactions, motivates an experiments-driven approach to lattice Hamiltonians. For alloys, cluster expansion Hamiltonians reduce the complex, many-body electron problem of density functional theory to a series of simple site-wise basis functions (e.g., products of site occupancy variables) on an atomic scale. The resulting energy polynomial is computationally inexpensive, and hence suitable for the (tens of) thousands of calculations of large systems required by stochastic methods. We present a new method to run the statistical mechanics problem ``in reverse'', using high-temperature observations and thermodynamic connections to construct an effective Hamiltonian and thereby predict the 0 Kelvin energy spectrum and associated ground states. By re-examining the cluster expansion coefficients as thermodynamic state variables and utilizing entropy-maximization approaches, we develop an algorithm to select clusters and determine cluster interactions using only a few, high-temperature experiments on disordered phases. We demonstrate that our approach can recover not only the stable ground states at 0 Kelvin, but also the full phase behavior for three realistic two-dimensional and three-dimensional alloy test-cases.

Ni-Fe oxides have received significant interest from the scientific community because they have attractive magnetic and electrochemical properties for use in next generation data storage and energy conversion technologies. For example, the NiFe2O4/NiO nanogranular system exhibits the exchange bias effect, a magnetic phenomenon occurring at the interface of a ferro- or ferrimagnet (FM or FiM) and an antiferromagnet (AFM), where the AFM acts to increase the magnetic hardness of the corresponding FM or FiM. Additionally, doping of NiO with Fe has resulted in remarkably high catalytic activities for water splitting, a potential clean energy alternative to fossil fuels. A key challenge in implementing these Ni-Fe oxides for magnetic and electrocatalytic applications is the ability to control film morphology, crystallinity, composition, chemical phase, and doping during synthesis. Moreover, how these physiochemical properties effect magnetic and electrochemical behavior in the Ni-Fe oxide system is not fully understood.

This dissertation focuses on the development and use of a novel synthesis technique, known as microplasma (MP) jet-based deposition, for the fabrication of biphasic NiFe2O4 (FiM)/NiO (AFM) and Fe-doped NiO nanostructured films for fundamental studies of exchange bias and electrocatalysis, respectively. The goal of this work was to understand how MP operation and deposition conditions (e.g., precursor composition, flux, substrate temperature, and post-deposition heat treatment) influence Ni-Fe oxide growth and film microstructure. Specifically, the role of composition, phase fraction, grain size, temperature, and interfacial density on exchange bias phenomena in NiFe2O4/NiO nanogranular films was investigated. MP jets were also used to realize metastable Fe-doped NiO films with high surface area to assess how doping affects the electrochemical properties of NiO for the oxygen evolution reaction (OER).

Biphasic NiFe2O4/NiO films of different composition were synthesized using MP jets and post-deposition annealing. The exchange bias effect (HE) and enhanced coercivity (HC) were seen at 300 K, which was the first time that HE has been reported at room temperature in the NiFe2O4/NiO nanogranular system. These values increased with Ni incorporation, and were rationalized as due to increased NiFe2O4/NiO interfacial density. Moreover, MP jet deposition of NiFe2O4/NiO films on heated substrates was explored to realize higher interfacial densities. HE was observed at low temperatures in these films, but not at room temperature, which was attributed to spin glass coupling arising from structurally disordered interfaces. Through systematic post-deposition heat treatments, it was found that spin glass-like phases disappeared after annealing, and the observed HE was due to direct exchange coupling between the NiO and NiFe2O4 phases.

MP jets were also used to deposit high-surface area, metastable Fe-doped NiO films of different composition (up to 20% Fe on a metals basis) at room temperature on indium tin oxide (ITO) substrates for OER catalysis. It was seen that Fe fully incorporated into the NiO rocksalt lattice, decreasing the overpotential for OER (i.e., 360 to 310 mV at 10 mA/cm2 for NiO and Ni0.95Fe0.05O, respectively). Turnover frequency (TOF) calculations demonstrated an improvement in the catalytic activity of the NiO surface with Fe doping, and chronopotentiometry measurements verified that Fe-doped NiO films were mechanically and chemically robust during extended operation under OER conditions.

Overall, this work demonstrates the potential of MP jet deposition as a versatile, one-step approach to realize multi-phase and doped nanostructured oxide films with high interfacial densities and surface areas for a variety of magnetic and energy conversion applications.