Over the last decade, geckos' remarkable ability to stick to and climb surfaces found in nature has motivated a wide range of scientific interest in engineering gecko-mimetic surface for various adhesive and high friction applications. The high adhesion and friction of its pads have been attributed to a complex array of hairy structures, which maximize surface area for van der Waals interaction between the toes and the counter-surface. While advances in micro- and nanolithography technique have allowed fabrication of increasingly sophisticated gecko mimetic surfaces, it remains a challenge to produce an adhesive as robust as that of the natural gecko pads. In order to rationally design gecko adhesives, understanding the contact behavior of fibrillar interface is critical.

The first chapter of the dissertation introduces gecko adhesion and its potential applications, followed by a brief survey of gecko-inspired adhesives. Challenges that limit the performance of the current adhesives are presented. In particular, it is pointed out that almost all testing of gecko adhesives have been on clean, smooth glass, which is ideal for adhesion due to high surface energy and low roughness. Surfaces in application are more difficult to stick to, so the understanding of failure modes in low energy and rough surfaces is important.

The second chapter presents a fabrication method for thermoplastic gecko adhesive to be used for a detailed study of fibrillar interfaces. Low-density polyethylene nanofibers are replicated from a silicon nanowire array fabricated by colloidal lithography and metal-catalyzed chemical etching. This process yields a highly ordered array of nanofibers over a large area with control over fiber diameter, length, and number density. The high yield and consistency of the process make it ideal for a systematic study on factors that affect adhesion and friction of gecko adhesives.

The following three chapters examine parameters that affect macroscale friction of fibrillar adhesives. Basic geometric factors, namely fiber length and diameter, are optimized on smooth glass for high friction. The test surfaces are then processed to intentionally introduce roughness or lower the surface energy in a systematic and quantifiable manner, so that the failure mechanisms of the adhesive can be investigated in detail. In these studies, observed macroscale friction is related to the nano-scale contact behavior with simple mechanical models to establish criteria to ensure high performance of fibrillar adhesives.

Chapter 6 presents various methods to produce more complex fiber structures. The metal-assisted chemical etching of silicon nanowires is studied in detail, where the chemical composition of the etching bath can be varied to produce clumped, tapered, tilted, and curved nanowires, which provide interesting templates for molding and are potentially useful for applications in various silicon nanowire devices. Hierarchical fiber structures are fabricated by a few different methods, as well as composite structures where the fibers are embedded in another material. A way to precisely control tapering of microfibers is demonstrated, and the effect of tapering on macroscale friction is studied in detail. The final chapter summarizes the dissertation and suggests possible future works for both further investigating fibrillar interfaces and improving the current gecko adhesive.

Supercapacitors represent a critical energy storage device technology for applications which require higher power density and/or cycle lifetime than existing battery technologies. Micro-scale supercapacitors, in particular, can enable novel applications in autonomous, wireless microsensors and microelectronics. If these micro-supercapacitors can be fabricated in a planar, on-chip geometry, the energy storage and the devices to be powered can be integrated on a single chip, improving scalability and reducing cost. The primary components of a supercapacitor are the electrodes and electrolyte. The properties of the electrode and electrolyte materials have a significant effect on device performance, and thus, there is significant opportunity for engineering materials to improve the energy density, power density, cycle lifetime, cost, safety, manufacturability, and harsh environment performance of micro-supercapacitors. Furthermore, these properties could be intelligently tailored for specific applications.

This manuscript provides some background on the principles underlying supercapacitor materials selection and testing, and presents several approaches to engineer these materials. Electrode materials which are investigated include photoresist derived porous carbon, vertically aligned carbon nanotubes, and 3D templated graphene. Electrolytes which are explored include various aqueous salts (and their impact on device cycle lifetime performance) as well as ionic liquid based gels (or ionogels). Furthermore, efforts to fabricate flexible micro-supercapacitors are discussed.

For on-chip energy storage involving high temperature operation, yttria-stabilized zirconia is investigated as the electrolyte. Silicon carbide (SiC)-based material for the electrode and its metal contact stability are also investigated, as the stability of these contacts during operation is an important consideration for the performance of high temperature micro-supercapacitors. Epitaxial graphene growth on SiC thin films is presented as one approach to stabilizing metal-to-SiC contacts.

Rapid scaling of silicon-based transistors has enabled remarkable performance enhancements in integrated electronics. However, silicon-based transistors are quickly reaching theoretical scaling limits as a result of short channel effects dictated by silicon’s intrinsic material properties. As such, there is a need for alternative materials that can transcend or better address these limitations. Scaling theory suggests that atomically thin, pristine semiconductors can enable more aggressive shrinking of the gate length. Given these requirements, two-dimensional materials (2DMs) have emerged as promising alternatives because they can be scaled down to single or few atomic layers, have no dangling bonds, and exhibit unique electrical properties.

Despite the burgeoning trove of proof-of-concept demonstrations of 2DM-based devices, large-scale fabrication and commercialization remain elusive. At this time, two of the most pressing shortcomings include (1) the transfer step requirement, wherein the 2DM must be physically transferred from a growth substrate to the target substrate, and (2) the high contact resistance between the 2DM and metal contacts. These device requirements create significant bottlenecks and challenges that limit the current feasibility of 2DM implementation in everyday devices. This work focuses on two particular classes of two-dimensional materials—graphene and transition metal dichalcogenides—and highlights some of the progress made towards addressing both of these device-related issues.

To start, this work details the development of improved transfer-free graphene synthesis by chemical vapor deposition (CVD) directly on SiO2. We outline an expansion of the parameter space that optimizes process conditions, using nickel and copper as metal catalysts and gaseous methane as the carbon precursor. We introduce a mechanism based on carbon permeability that provides deeper insight into the growth process. Low-energy electron microscopy (LEEM) is used to showcase some of the intrinsic differences between nickel and copper that lead to contrasting results. In the end, we demonstrate reproducible, monolayer graphene with low defect density using nickel as a catalyst, and reproducible, 2-3 layer graphene with uniform coverage using copper as a catalyst.

To address the high metal–2DM contact resistance, mild hydrogen plasma treatment is applied to WSe2. X-ray photoemission spectroscopy (XPS) indicates that H2 plasma treatment selectively induces selenium vacancies in the WSe2 lattice, resulting in controllable

n-doping with increasing plasma treatment times. WSe2 n-FETs fabricated with H2 treatment on the contact regions demonstrate two orders of magnitude decrease in contact resistance.

By addressing some of the challenges related to graphene transfer and TMDC contact resistance, these studies help establish a foundation towards scalable integration of 2DMs in beyond-silicon electronics.

An apocryphal quote attributed to Michelangelo regarding how he made his most famous sculptures is: “It is easy – just remove everything that isn’t David.” This idea can also be useful in materials science in design. In gas sensing, engineering the environment around an active site to have a more favorable environment for an analyte with metal-organic frameworks is becoming an attractive method of achieving high selectivity. In cements, the amount of porosity controls the way that the matrix will deform under a structural load and the way that heat will propagate through the cement matrix – and when porosity decreases below a certain level, the grains themselves start to yield. In this thesis, I will discuss my work with porous materials and how tuned porosity engenders favorable properties. In the first section, porosity was tuned towards a desired chemical adsorption property, using metal organic frameworks (MOFs) as chemical sensing material. MOFs are porous, crystalline materials comprising metal nodes joined by organic linkers. The porosity of the MOFs means that if the MOF’s substrate is also reactive to the analyte of interest then the surface should be made inert, which I showed how to do with self-assembled monolayers. I measured how the work function response of a MOF changed as molecules adsorbed to the interior surfaces. Work function is a measure of the electrical potential in a structure and is highly MOF – analyte specific. I determined the mechanism for work function responses by MOFs on a chemical-sensitive field-effect transistor (CS-FET). In comparing the response to adsorption isotherms and to spectroscopic studies, I found that adsorption at metal sites tended to generate larger changes in work function than adsorption at the linker. I showed that MOFs could also be used as a selective membrane over another active sensing material, such as PdO, yielding improved sensing performance for hydrogen. In the second section, porosity affects the thermal and mechanical properties of the main binding phase in Portland cement, calcium-silicate-hydrates (C-S-H). C-S-H has a nanocrystalline, lamellar structure of 2-D calcium oxide sheets decorated with 1-D silicate chains. The distance between the sheets is controlled by the chemistry of the material. The porosity makes comparing the properties of different structures challenging, because the measured property is a function of the porosity. Using high-pressure Raman spectroscopy, I measured the relative rate of thermal scattering, and found that structures with more spacing between the CaO sheets are more thermally insulating. The technique could also be used to refine atomistic simulations of C-S-H; I found that the current dominant force field CSH-FF underestimates the thermal conductivity of cross-linked silicate chains, and that the force field could be improved by including a covalent term to the Si – O interactions. I also studied C-S-H under deviatoric stress Raman spectroscopy and found the plastic deformation mechanism depends on the structure. In cross-linked silicate structures the grains slide on their calcium oxide sheets, but in non-cross-linked structures, the grains deform by sliding between the silicate chains. X-ray diffraction experiments also showed sliding defects induced by a deviatoric stress. X-ray diffraction experiments also showed a segregation effect, where in high-porosity C-S-H, the lowest-porosity aspect ratio (defined as thickness divided by diameter) grains condense to an ordered phase first. The number of sliding defects is inversely proportional to the volume fraction of the ordered phase and the thickness of water between calcium oxide sheets. My experiments informed the mechanism for hydrated cement creep at the grain level to improve models to inform engineers about the effect of new concrete formulations, such as the inclusion of graphitic waste from methane decarbonization.

The advancement of energy and sensing technologies hinges on the development of materials with novel physical and chemical properties and the rational design of new configurations that leverage synergistic interaction between multiple materials. In the context of devices that generate or utilize energy or sense the environment, new materials or material configurations can offer unique or improved properties that can help resolve technological barriers introduced by stringent design parameters, necessary performance requirements, or harsh and extreme operating environments. This work discusses the existing material limitations of two separate technologies, thermionic energy converters and chemiresistive gas sensors, and details the investigation of new materials or material configurations aimed toward addressing device-related issues.

The first part of this work centers on microfabricated thermionic energy converters (TECs). TECs leverage the thermionic emission of electrons from materials at high temperature to directly convert thermal energy to electrical energy. For optimal efficiency and reliable performance, the emitter electrode, an essential component of the TEC, must demonstrate good mechanical, chemical, and surface stability during high-temperature operation. This work first demonstrates the protection of a tungsten (W) emitter from oxidation using a silicon carbide (SiC) passivation layer. The addition of functional layers such as titanium nitride (TiN) and tantalum carbide (TaC) diffusion barriers is investigated. A W/TiN/SiC multilayered emitter that exhibits stability at 1100 °C for up to 24 h under representative operating conditions is realized. The effects of TiN thickness and post-deposition annealing are examined, and a relationship between thermal stress and observed cracking patterns in the TiN is established. An alternative configuration involving W/TaC/SiC is also investigated, showing enhanced stability.

The second part of this work focuses on chemiresistive gas sensors. These devices detect gases by monitoring changes in the resistance of a material as a result of gas adsorption and desorption and are key components in health, environmental, and industrial process monitoring. While several classes of materials have been investigated for chemiresistive gas sensing, the transition metal diborides have been largely unexplored. The fabrication of sensors made from hafnium diboride (HfB2) and zirconium diboride (ZrB2) aerogels is presented and their sensing behavior probed. Of particular note is the sensitivity and selectivity of HfB2 toward NO2 and the relative insensitivity of ZrB2 to NO2. The disparity in their gas sensing behavior is discussed. Computations based on density functional theory indicate charge transfer between HfB2 and NO2. The requirements and advantages of printable gas sensors are detailed, and the printability of HfB2 is examined.

The studies presented in this dissertation illustrate the impact novel materials and material configurations can have in improving and expanding the capabilities of energy and sensing technologies.

For applications in mobile and remote sensing platforms, microsupercapacitors are attractive energy storage devices due to their robust lifetimes and high specific power capacity. Utilization of green electrolytes in these devices reduces environmental impact and simplifies packaging by avoiding the stringent oxygen and moisture free conditions required for organic and ionic liquid based electrolytes. Porous silicon nanowire based microsupercapacitor electrode materials are promising for on chip applications using an environmentally benign aqueous electrolyte, 1 M KCl, however they are prone to oxidation. A silicon carbide coating was found to mitigate this issue. The fabrication techniques, involving low-temperature electroless etching of silicon, are compatible with current integrated circuit processing methods and may be readily integrated at the micro device level. The electrode materials are in good electrical contact with the underlying substrate and require no additional current collector. The base porous silicon nanowires are coated with a thin silicon carbide passivation layer by low pressure chemical vapor deposition. The demonstrated capacitance of the electrode materials, ~1700 μF/cm2 projected area, is comparable to other carbon based microsupercapacitor electrodes, remains stable over many charge/discharge cycles, and maintains capacitive behavior over a wide range of charge/discharge rates.

An improved passivation method for the porous silicon nanowires has also been developed. The selective coating procedure deposits an ultra-thin (~ 1-3 nm) carbon sheath over the nanowires and passivates them. The ultra-thin nature of the coating enables solvent access to the pore area and hence a large improvement of active specific surface over the SiC coated PSiNWs discussed above. The electrochemical performance of these coated nanowires is characterized in both an aqueous electrolyte and an ionic liquid electrolyte. Specific capacitance values reaching 325 mF cm 2 are achieved in ionic liquid, and calculations indicate that the theoretical maximum capacitance of the pristine wires is reached. TEM studies confirm the coating thickness and its conformality. Raman spectroscopy indicates that the carbon in the coating is mainly sp2 hybridized, with corresponding high conductivity. At the time of writing, these materials represent the largest specific energy microsupercapacitor electrode published. A test device is prepared and demonstrated powering an LED.

The testing results of silicon carbide (SiC) nanowires (NW) as an electrode material for micro-supercapacitors is described. SiC NWs are grown on a SiC thin film coated with a thin Ni catalyst layer via chemical vapor deposition. A specific capacitance of ~240 µF cm-2 is demonstrated. Charge-discharge studies demonstrate the SiC nanowires exhibit exceptional stability, with 95% capacitance retention after 2×105 charge/discharge cycles in an environmentally benign, aqueous electrolyte. Doping of the nanowires with nitrogen through the addition of 5 at% ammonia to the precursor gas flow rate improves the conductivity of the nanowire films by over an order of magnitude leading to increased power capabilities.

A method to transfer silicon and silicon carbide nanowire arrays to arbitrary substrates while maintaining electrical contact through the entire array is elucidated. The nanowires are grown on graphene sheets on SiO2 coupons. The graphene acts as both the flexible material for maintaining structural continuity and electrical contact through the array during transfer. The SiO2 acts as the sacrificial growth substrate which is etched after growth in order to release the nanowire/graphene hybrid. The nanowire/graphene hybrids are structurally characterized by XRD and electron microscopy. Good electrical contact is confirmed through testing of the SiCNW/graphene hybrids as supercapacitor electrode materials in an aqueous electrolyte. The specific capacitance, ~340 mF cm-2, is similar to SiCNW arrays grown on oxide while the electrical conductivity is improved and cycling stability tests show less than a 1% decrease in capacitance after 10,000 cycles.

Silicon carbide (SiC) based electronics and sensors hold promise for pushing past the limits of current technology to achieve small, durable devices that can function in high-temperature, high-voltage, corrosive, and biological environments. SiC is an ideal material for such conditions due to its high mechanical strength, excellent chemical stability, and its biocompatibility. Consequently, SiC thin films and nanowires have attracted interest in applications such as micro- and nano-electromechanical systems, biological sensors, field emission cathodes, and energy storage devices.

In terms of high-temperature microdevices, maintaining low-resistance electrical contact between metal and SiC remains a challenge. Although SiC itself maintains structural and electrical stability at high temperatures, the metallization schemes on SiC can suffer from silicide formation and oxidation when exposed to air. The second chapter presents efforts to develop stable metallization schemes to SiC. A stack consisting of Ni-induced solid-state graphitization of SiC and an atomic layer deposited layer of alumina is shown to yield low contact resistivity of Pt/Ti to polycrystalline n-type 3C-SiC films that is stable in air at 450 ºC for 500 hours.

The subsequent chapters focus on the growth and structural characterization of SiC nanowires. In addition to its structural stability in harsh-environments, there is interest in controlling SiC crystal structure or polytype formation. Over 200 different polytypes have been reported for SiC, with the most common being 3C, 4H, and 2H. In terms of SiC nanowire growth, the 3C or cubic phase is the most prevalent. However, as the stacking fault energy for SiC is on the order of a few meV, it is common to have a high density of stacking faults within a given SiC crystal structure. Thus, to enable reliable performance of SiC nanowires, a growth method that can promote a specific polytype or reduce stacking faults is of importance. Ni-catalyzed chemical vapor deposition method is employed for the growth of the nanowires. The effects of substrate structure and quality as well as the various growth parameters such as temperature, pressure, and post-deposition annealing are investigated. Most significant has been the growth and characterization of vertically aligned hexagonal phase (or 4H-like) SiC nanowires grown on commercially available 4H-SiC (0001).

The studies presented in this thesis tackle issues in SiC metallization and nanowire growth in efforts to expand the versatility of SiC as a material platform for novel devices.

Silicon's chemical stability, high natural abundance (as the second most common element in the earth's crust), mechanical stiffness, and semiconducting behavior have made it the subject of extensive scientific investigation and the material of choice for both the microelectronics and microelectromechanical device industries. The success of Moore's Law that demands continual size reduction has directed it to a central place in emerging nanoscience and nanotechnology as well. Crystalline nanowires (NWs) are one nanostructured form that silicon may take that has sparked significant interest as they can exhibit considerable confinement effects and high surface-to-volume ratios, but may be interfaced simply along one direction for the determination of material properties and implementation into new technologies. The expense and difficulty involved in the creation of semiconductor nanowires using the "top down" fabrication techniques of the microelectronics industry has promoted an explosion of chemical synthetic "bottom up" techniques to produce high quality crystalline nanowires in large quanitities. Nevertheless, bottom up synthesized Si NWs retain a new set of challenges for their successful integration into reliable, high-performance devices, which is hindered by an incomplete understanding of the factors controlling their material properties.

The first chapter of this dissertation introduces the motivation for studying semiconductor NWs and the benefits of limiting the scope to silicon alone. A brief survey of the current understanding of thermal conductivity in silicon nanowires provides prime examples of how confinement effects and surface morphology may dramatically alter nanowire properties from their bulk crystal counterparts. The particular challenges to bottom up silicon nanowire device integration and characterization are noted, especially related to Si nanowires that are grown epitaxially on crystal silicon substrates, and Raman spectroscopy is introduced as a promising optical characterization and metrology tool for semiconductor nanowire based devices.

Chapter two describes the vapor-liquid-solid (VLS) mechanism for the synthesis of very high quality, single-crystal silicon nanowires using Au and Pt catalyst nanoparticles. A new technique is presented for the simplified synthesis of branched silicon nanowires based on the migration of Au catalyst during an hydrogen anneal intermediate between growth stages, and the faceting behavior at synthetic stages is revealed by the analysis of electron microscope images. Synthesis of solid and porous Si nanowires based on Ag mediated electrochemical silicon etching is described as well.

The third chapter specifies new processing techniques developed with future device integration of epitaxially VLS-grown Si nanowires in mind. Epitaxially bridging nanowires are shown to provide an excellent platform for single-wire electrical and mechanical property measurements. Galvanic displacement through block copolymer micelle/homopolymer surface templates is demonstrated as a means to deposit catalyst nanoparticles with controlled sizes and areal densities in a variety of geometries and with registration to photolithographic patterns. Ex situ boron doping by the direct hydrogen reduction of boron tribromide is shown to achieve active concentrations exceeding 10¹⁹ cm⁻³ with high axial uniformity, while avoiding the adverse impact on nanowire morphology that is often observed with in situ boron doping of silicon nanowires.

Chapter four describes the characteristics of Raman spectroscopy that are relevant to studying individual semiconductor nanowires. Careful spectral measurements show that the anharmonic dependence of Raman spectra on temperature for individual Si nanowires remains unchanged from the bulk crystal for diameters down to 30 nm, regardless of surface morphology. Using this result, a new technique for measuring the thermal conductivity of individual semiconductor nanowires is then outlined based on Raman thermal mapping of individual cantilevered nanowires.

Finally, the dissertation is concluded with suggestions for possible future experiments. One avenue is to probe more deeply the morphology of faceted silicon nanowires and nanotrees and its impact on their transport physics. Another possible route for further study would be to explore new characterization and metrological applications of Raman spectrocopy for semiconductor nanowires.

Gas sensors can provide information about the presence of dangerous gases in industrial and residential sites, allowing for improved environmental protection and human health and safety. In order to enable ubiquitous wireless monitoring of combustible and toxic gases, sensor elements with low power consumption are required. Two common types of gas sensors, namely calorimetric and conductometric sensors, rely on heated sensor elements to activate the appropriate reactions with the gas of interest, resulting in high power requirements to maintain the necessary operating temperatures. Miniaturized gas sensors made with microfabricated heater platforms and nanoscale sensing materials can lead to low power sensor elements with high performance. The minimization of the power consumption cannot be at the expense of the sensor’s ability to detect dangerous combustible and toxic gases. The sensing performance is impacted by the properties of both the microheater and the sensing material, as well as the integration of the two elements, creating significant opportunity for research and development to improve the performance parameters, such as sensitivity, detectable concentration range, selectivity, response and recovery time, stability, size, manufacturability, and cost, as well as power consumption.

The development approach described in the following thesis relies on an optimized microheater platform as well as novel sensing materials. The design, fabrication, and characterization of a low-power microheater platform is described, with polycrystalline silicon as the heater material for moderate temperature and polycrystalline silicon carbide for improved reliability at high temperature operation. A small, isolated heated area decreases the required power consumption and a closed membrane facilitates easier deposition of sensing materials. Sensing materials for combustible and toxic gas detection have been developed based on ultra high surface area aerogels. For combustible gas sensing, graphene or boron nitride aerogel acts as a scaffold for catalytic platinum or palladium nanoparticles. Bare graphene aerogel and graphene aerogel coated with single to few-layer molybdenum disulfide sheets are used for toxic gas detection.

The microheater platform with fast response and recovery times provides opportunities for localized, in-situ synthesis of high surface area sensing materials. The use of the microheater platform for localized growth of a nanocrystalline tin oxide sensing film is described. The fast thermal response of the microheater leads to rapid heating of the precursor, creating a highly porous film that is proven beneficial for toxic gas sensing.

The transduction of electrical energy into chemical bonds represents one potential strategy for storing energy derived from intermittent sources such as solar and wind. Driving the electrochemical reduction of carbon dioxide using light requires (1) developing light absorbers which convert photons into electron-hole pairs and (2) catalysts which utilize these electrons and holes to reduce carbon dioxide and oxidize water, respectively. For both the light absorbers and catalysts, the use of nanoscale particles is advantageous, as charge transport length scales are minimized in the case of nanoscale light absorbers and catalytic surface-area-to-volume ratio is maximized for nanoscale catalysts. In many cases, although semiconductors and metals in the form of thin films and foils are increasingly well-characterized as photoabsorbers and electrocatalysts for carbon dioxide reduction, respectively, the properties of their nanoscale counterparts remain poorly understood.

This dissertation explores the nature of the light absorption mode of non-stoichiometric semiconductors which are utilized as light absorbers and the development of catalysts with enhanced stability, activity, and selectivity for carbon dioxide reduction. Chapter 1 provides an overview of the state of development of methods of transducing the energy of photons into chemical bonds.

Chapters 2 and 3 investigate the development of stable, active, and selective catalysts for the electrochemical reduction of carbon dioxide. Chapter 2 examines how copper nanoparticles have enhanced activities and selectivities for methanation compared to copper foils. Chapter 3 focuses on the development of strategies to stabilize high-surface-area catalysts to prevent surface area loss during electrochemical carbon dioxide reduction.

Chapters 4 and 5 entail a fundamental understanding of the light absorption mode of nanoscale photoabsorbers used in both photoelectrochemical cells and in photovoltaics. Chapter 4 focuses on the nature of the light absorption mode of non-stoichiometric tungsten oxide, a material which has been explored as a photoanode for the photon driven oxidation of water. Chapter 5 examines the tunability of the light absorption mode of nanoscale copper sulfide, a material which has been explored as a photoabsorber for photovoltaics. An understanding of the light absorption mode of non-stoichiometric oxides and sulfides at the nanoscale is critical for the use of these materials in redox active environments.