With continued shrinking of CMOS technology to reduce the gate delay times, an increase in the resistivity of the metal corresponding to the wire dimension is a concern. This phenomenon of increase in resistivity with decreasing dimension of the thin metallic film or interconnect is known as the "classical size effect". Various theories have been postulated to explain the phenomenon of classical size effect; these theories can be broadly classified as resistivity due to scattering arising from surface and grain boundaries. The total resistivity of metals depends on the electron scattering due to impurities, phonons, surfaces, grain boundaries, and other crystal defects. Managing the size effect in a practical and manufacturing way is of major concern to the microelectronics industry. Since each of the processes (phonon, surface and grain boundary scattering) adds to the resistivity and are interrelated, it further complicates managing the size effect. However, these effects have been separately studied. In this work, the effect of annealing on the classical size effect in Cu thin films deposited on SiO2 substrate is investigated. Polycrystalline Cu thin films having thicknesses in the range of 10nm to 200nm were ultra high vacuum sputter deposited on thermally grown SiO2 surfaces. The films were annealed at temperatures in the range of 150°C to 800°C in argon and argon+3% hydrogen gases. The un-annealed Cu thin films exhibit higher resistivity than the annealed films. The resistivities of un-annealed films were in good agreement with Mayadas and Shatzkes model. When annealed the films undergoes grain growth resulting in lowering the resistivities by about 20%-30% thereby confirming the role of grain size on resistivity of the film. However, there is a limit to annealing, i.e. agglomeration phenomenon. Agglomeration is a thermally activated process resulting in a reduction of the free energy of the film–substrate system and can occur well below the melting point of the material by surface and interfacial diffusion. The reduction of film-substrate interfacial energy, film-surface interfacial energy and stresses within the film are possible driving forces for agglomeration. This work also includes the study of agglomeration phenomenon. The agglomeration behavior of Cu is investigated and compared with that of Ru, Au and Pt thin films with thicknesses in the range of 10 nm to 100 nm UHV deposited on thermally grown SiO2 substrate. The films were annealed at temperatures in the range of 150°C to 800°C in argon and argon+3% hydrogen gases. Scanning electron microscopy was used to investigate the agglomeration behavior, and transmission electron microscopy was used to characterize the microstructure of the as-deposited and annealed films. The agglomeration sequence in all the films is found to follow a two step process of void nucleation and void growth. However, void growth in Au and Pt thin films is different from Cu and Ru thin films. Residual stress and adhesion were observed to play important part in deciding the mode of void growth in Au and Pt thin films. Lastly, it is also observed that the tendency for agglomeration can be reduced by encapsulating the metal film with an oxide overlayer, which in turn improves the resistivity of the thin film due to prolonged grain growth without film breakup.

The main focus of this dissertation is to characterize and improve the performance of thin-film piezoelectric-on-substrate (TPoS) lateral-mode resonators and filters. TPoS is a class of piezoelectric MEMS devices which benefits from the high coupling coefficient of the piezoelectric transduction mechanism while taking advantage of superior acoustic properties of a substrate. The use of lateral-mode TPoS designs allows for fabrication of dispersed-frequency filters on a single substrate, thus significantly reducing the size and manufacturing cost of devices. TPoS filters also offer a lower temperature coefficient of frequency, and better power handling capability compared to rival technologies all in a very small footprint. Design and fabrication process of the TPoS devices is discussed. Both silicon and diamond substrates are utilized for fabrication of TPoS devices and results are compared. Specifically, the superior acoustic properties of nanocrystalline diamond in scaling the frequency and energy density of the resonators is highlighted in comparison with silicon. The performance of TPoS devices in a variety of applications is reported. These applications include lateral-mode TPoS filters with record low IL values (as low as 2dB) and fractional bandwidth up to 1%, impedance transformers, very low phase noise oscillators, and passive wireless temperature sensors.

This dissertation undertakes theoretical and computational research to characterize and understand in detail atomic configurations and electronic structural properties of surfaces and interfaces at the nano-scale, with particular emphasis on identifying the factors that control atomic-scale diffusion and transport properties. The overarching goal is to outline, with examples, a predictive modeling procedure of stable structures of novel materials that, on the one hand, facilitates a better understanding of experimental results, and on the other hand, provide guidelines for future experimental work. The results of this dissertation are useful in future miniaturization of electronic devices, predicting and engineering functional novel nanostructures. A variety of theoretical and computational tools with different degrees of accuracy is used to study problems in different time and length scales. Interactions between the atoms are derived using both ab-initio methods based on Density Functional Theory (DFT), as well as semiempirical approaches such as those embodied in the Embedded Atom Method (EAM), depending on the scale of the problem at hand. The energetics for a variety of surface phenomena (adsorption, desorption, diffusion, and reactions) are calculated using either DFT or EAM, as feasible. For simulating dynamic processes such as diffusion of adatoms on surfaces with dislocations the Molecular Dynamics (MD) method is applied. To calculate vibrational mode frequencies, the infinitesimal displacement method is employed. The combination of non-equilibrium Green’s function (NEGF) and DFT is used to calculate electronic transport properties of molecular devices as well as interfaces and junctions.

Understanding the mechanisms that control the mechanical behavior of microscale actuators is necessary to design an actuator that responds to an applied actuation force with the desired behavior. Micro actuators which employ a diaphragm supported by torsional hinges which deform during actuation are used in many applications where device stability and reliability are critical. The material response to the stress developed within the hinge during actuation controls how the actuator will respond to the actuating force. A fully recoverable non-linear viscoelastic response has been observed in electrostatically driven micro actuators employing torsional hinges of silicon covered with thin metal films. The viscoelastic response occurs over a time period of 50 minutes at an operating temperature of 35°C. This viscoelastic phenomenon is similar to that reported in articles addressing anelastic behavior associated with viscous grain boundary slippage and dislocation bowing. In order to investigate this viscoelastic response as a function of metal film composition and thickness, bi-layer torsional hinge actuators consisting of Si with a deposited metal layer were designed to exhibit similar stress levels as the electrostatically driven micro actuators. The test devices were fabricated using common semiconductor fabrication techniques. The actuators were micromachined by deep etching 100mm diameter, 425µm thick, double side polished, single crystal (100) wafers to create a 4.5µm thick device layer. Subsequent etching of the device layer released the fixed-fixed torsional hinge test actuators. Physical vapor depositions of Au, Al and Al-Ti in two different thicknesses (100nm, and 150nm) were deposited in order to investigate the impact of metal film thickness and composition on the viscoelastic response. Grain sizes of the deposited films were estimated using backscattered electron images. Rotational actuation of the test actuators was achieved by using a modified Ambios XP-1 surface profiler that applies a constant force of 0.28mN while measuring the displacement of the actuator with respect to time. The viscoelastic response was observed in the test devices with Au and Al thin films indicating that this phenomenon is attributable to the stresses induced on the torsional hinge. Results indicate that the viscoelastic response was not observed in AlTi thin films consisting of 0.3at% titanium. Two theoretical models are presented that discuss the mechanism associated with the viscoelastic response as well as a method for inhibiting these mechanisms by the addition of an alloying element to form a second phase precipitate.

Reverse osmosis (RO) is widely used for water reclamation and is one of the most feasible technologies for addressing water scarcity around the world. RO membrane fabrication procedures are continually being optimized and modified to enhance the treatment performance and efficacy of the RO process. A review of the existing literature published on membrane fabrication revealed that a detailed and reproducible methodology consistent among prior studies was not available. Therefore, the primary objective of this study was to utilize techniques from prior research to develop a reliable lab-scale membrane fabrication process for studying the potable reuse applications of TFC RO membranes. Phase inversion was used to create a polysulfone (PSF) support layer on a non-woven fabric sheet. Then, the process of interfacial polymerization (IP) between amine and acyl chloride monomers was utilized to form a highly selective and ultrathin polyamide (PA) layer on the PSF support surface. The resulting membrane composition and performance was dependent on a wide range of parameters during the fabrication process. The optimal support materials, reactant types and concentration, and reaction conditions were determined through trial and error. The best performing membranes utilized N-methyl-2-pyrrolidone (NMP) as the solvent, Novatexx-2471 non-woven fabric for mechanical support, and 15 wt% PSF concentration for phase inversion. The optimal immersion duration was five minutes for the aqueous amine monomer solution during the IP process. The flux for membrane triplicates was 20.2  3.6 liters per square meter per hour (LMH) while the salt rejection was 96.8  2.0%. The relatively low standard deviation for flux and salt rejection indicates that the fabrication method developed herein is consistent. A commercial Dow Filmtec BW30 flat sheet PA-TFC RO membrane was tested for comparison and exhibited a flux of 44.9 LMH and a salt rejection of 98.5%. Thus, the membranes developed in this study achieved salt rejection on par with commercial membranes but exhibited a flux that was significantly lower. Furthermore, this study investigated modifications to the traditional TFC membrane using engineered silica nanomaterials with the goal of enhancing the membrane flux while maintaining high salt rejection. Two types of nonporous silicon dioxide nanoparticles (SDNPs), non-functionalized and amine functionalized, were dispersed in the aqueous and organic IP solutions. Ultrasonication of the non-functionalized SDNPs in the aqueous solution was observed to produce the most stable dispersion. Compared to the unmodified TFC membranes, the average flux of the SDNP-modified (TFC-NP) RO membrane triplicates was higher at 25.4  2.0 LMH with 0.1% (w/v) SDNPs incorporated in the PA layer. The salt rejection was lowered to 92.3  0.1% for the TFC-NP membranes. In addition, the membranes fabricated in this study were characterized using scanning electron microscopy (SEM), Fourier Transport Infrared Spectroscopy (FTIR), atomic force microscopy (AFM), and goniometry measurements. SEM images showed that the TFC-NP membranes contained larger spaces between ridges and valleys of the PA pore structure. FTIR confirmed the PA layer formation on the membranes fabricated herein but a spectral peak from the SDNPs was not observed for the TFC-NP membranes. AFM measurements indicated an increase in surface roughness of the modified membranes, likely because of the incorporation of SDNPs. The surface of TFC-NP membranes was found to be more hydrophilic than the unmodified TFC membranes based on contact angle measurements. Further optimization of the fabrication method developed herein is warranted before pursuing additional RO research topics, such as the disinfection byproduct precursor removal of TFC membranes.

The development of metals that can overcome the strength-ductility-weight trade-off has been an ongoing challenge in engineering for many decades. A promising option for making such materials are Metal matrix composites (MMCs). MMCs contain dispersions of reinforcement in the form of fibers, particles, or platelets that significantly improve their thermal, electrical, or mechanical performance. This dissertation focuses on reinforcement with two-dimensional (2D) materials due to their unprecedented mechanical properties. For instance, compared to steel, the most well-studied 2D material, graphene, is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa). Examples of reinforcement by graphene have achieved increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. However, the superior mechanical properties of graphene are not fully transferred to the matrix in conventional MMCs, a phenomenon known as the "valley of death." In an effort to develop key insight into how the relationships between composite design, processing, structure, properties, and mechanics can be used to more effectively transfer the intrinsic mechanical properties of reinforcements to bulk composite materials, nanolayered composite systems made of Ni, Cu, and NiTi reinforced with graphene or 2D hexagonal boron nitride h-BN is studied using experimental techniques and molecular dynamics (MD) simulations. / Doctor of Philosophy / The design of new metals with concurrently improved strength and ductility has been an enduring goal in engineering for many decades. The utilization of components made with these new materials would reduce the weight of structures without sacrificing their performance. Such materials have the potential to revolutionize many industries, from electronics to aerospace. Traditional methods of improving the properties of metals by thermomechanical processing have approached a point where only minor performance improvements can be achieved. The development of Metal matrix composites (MMCs) is among the best approaches to achieving the strength-ductility goal. Metal matrix composites are a class of materials containing reinforcements of dissimilar materials that significantly improve their thermal conductivity, electrical conductivity, or mechanical performance. Reinforcements are typically in the form of dispersed fibers, particles, or platelets. The ideal reinforcement materials have superior mechanical properties compared to the metal matrix, a high surface area, and a strong interfacial bond with the matrix. Two-dimensional (2D) materials (materials made up of a single to a few layers of ordered atoms) are attractive for reinforcement in composite materials because they possess unprecedented intrinsic properties. The most well-studied 2D material, graphene, is made of a single layer of carbon atoms arranged in a hexagonal honeycomb pattern. It is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa) than steel. Examples of graphene reinforcing have shown increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. Despite their exceptional mechanical properties, the superior mechanical properties of graphene are not fully transferred to the matrix when incorporated into conventional metal matrix composites. This phenomenon, known as the "valley of death," refers to the loss of mechanical performance at different length scales. One cause of this phenomenon is the difficulty of evenly dispersing the reinforcements in the matrix using traditional fabrication techniques. Another is the presence of dislocations in the metal matrix, which cause very large local lattice strains in the graphene. This atomistic-scale deformation at the interface between the metal and the graphene can significantly weaken it, leading to failure at low strains before reaching its intrinsic failure stress and strain. This dissertation aims to provide insight into how the relationships between composites' design, processing, structure, properties, and mechanics can be used to transfer intrinsic mechanical properties of reinforcements to bulk composite materials more effectively. For this, nanolayered composite systems of Ni and Cu reinforced with graphene or 2D h-BN were studied using experimental techniques and molecular dynamics (MD) simulations to elucidate the underlying mechanisms behind the composites' material structure and mechanical behavior. Additionally, we explore the incorporation of graphene in a metallic matrix that does not deform through dislocations (or shear bands), such as the shape memory alloy nickel-titanium ( Nitinol or NiTi), to avoid low strain failure of the metal/graphene interface. This theoretical strengthening mechanism is investigated by designing and fabricating NiTi/graphene composites.

Lead Halide Perovskite is emerging quickly as a promising material for the future solar cellsthanks to their inherent good optoelectrical properties along with their cheap and facile fabri-cation. However, their main drawback before commercialization is their weak stability. In thiswork, a novel carbon-coated perovskite quantum dot has been synthesized, and is to the extentof our knowledge, for the first time. The coated perovskite quantum dots show a remarkable in-creased stability under different conditions while in solution. Their photoluminescence intensityalso increased as time went on, exceeding that of the uncoated perovskite quantum dots aftera few weeks. These coated perovskite quantum dots, while not fully characterized and thusnot fully understood show a promising way on how to combat the low stability in perovskites.Further, Copper/Copper(I)Iodide core/shell nanowires were synthesized as a transparent inte-grated hole transport layer/electrode for solar cells. While limited due to the low controlledfabrication process used, they providing a solid base for further research on the material to beused in solar cells. / Bly-halid perovskite har snabbt utvecklats och visar sig vara ett lovande material till framti-dens solceller tack vare dess optoelektriska egenskaper samt dess billiga och lätta tillverkn-ingsprocess. Dock, så brister materialet på grund av dess dåliga stabilitet innan de kan kommer-sialieras fullt ut. I detta projekt har kol-överdragen perovskite kvantprickar framställts, och ärtill den omfattning vi besitter, för första gången. De coatade perovskite kvantprickarna visade enförvånadsvärd stabilitet under olika tillstånd, och även en ökning i dess photoluminescens efternågra veckor. Dessa kvantprickar, dock ej ännu fullt förstådda och mer karaktärisering krävs, ärett lovande alternativ till att lösa perovskitens låga stabilitet. Vidare har koppar/koppar(I)jodidcore/shell nanotrådar tillverkats som en transparent integrerar håltransport material/elektrodför solceller med goda egenskaper. Trotts sin begränsning i den framställningsprocess som an-vändes ger dem en lovande bas för framtida forskning på materialet.

The self-assembly of magnesium aluminate spinel as a result of dewetting an overlaid thin film of (chiefly) gold was investigated. Thin films of gold were deposited on single-crystalline spinel substrates and were heat-treated to dewet gold film which led to self-assembly of intricate structures consisting of faceted spherical particles atop of frustums. The current work was conducted in continuation of previous studies which reported formation of such intricate structures. The most recent studies had evidently overruled a pure gold self-assembly scenario as was pointed out in preliminary investigations. It was in fact proven that these structures consist of three distinct parts: (i) a single or polycrystalline gold faceted sphere, (ii) quasi-phase interfacial bilayer, and (iii) a crystalline MgAl2O4 necking structure spontaneously risen from spinel substrate. In the current work, samples were produced through different film deposition methods of sputter, thermal evaporation, and e-beam evaporation coating which underwent thermal annealing to induce dewetting of gold film and subsequent self-assembly of intricate structures. Several characterization methods such as electron microscopy, X-ray energy dispersive spectroscopy, electron energy loss spectroscopy, and atom probe tomography were utilized to survey the different features of the intricate structures with focus on chemical analysis of the gold-spinel interface. The results rejected the previous findings about formation of interface complexion at the boundary of gold-spinel. It was found out that gold-spinel interface is in fact an ordinary metal-oxide boundary with sharp atomic distinction and no inter-diffusion or formation of interfacial complexion. It was further discovered that dewetting pure gold thin films does not result in formation of spinel self-assembly and existence of elemental impurities of copper (Cu) in the initial film is vital in development of such structures. Finally, it was concluded that chemical composition of metallic overlayer and the heat treatment parameters most fundamentally influence formation and physical characteristics of those self-assembled structures. / Thesis / Master of Applied Science (MASc)

<p dir="ltr">Catalysis will play a pivotal role in the transformation of the current chemical and fuel industries, driving efforts to mitigate greenhouse gas emissions, curbing the release of hazardous waste, and efficiently utilizing energy resources. Hence, it is crucial to establish a fundamental understanding of the active sites that drive chemical reactions and the transformation of these active sites under varying reaction conditions. A particular class of catalysts that are extensively used in industrially relevant reactions, but not well understood, are metal nanoparticles supported over transition metal oxides. Under specific conditions, the metal nanoparticles are believed to be partially covered by reduced, ultrathin oxide films, which can drastically transform the physical, chemical, and electronic properties of the catalyst surface. These transformations are often referred to as the Strong Metal Support Interactions (SMSI). The structure and chemical properties of the encapsulating SMSI overlayers can determine the reactivity, selectivity, and stability of the catalyst. To explore these phenomena, the encapsulating overlayers on metal nanoparticles are most effectively studied using ultrathin film models supported on single crystal transition metal substrates. In this thesis, periodic density functional theory (DFT) calculations, along with surface science experiments in collaborators’ groups, are carried out to systematically study the molecular-level underpinnings of the metal oxide transformations.</p><p dir="ltr">As a starting point, we analyze the Pd/ZnO system. This is a potential methanol synthesis catalyst, and since ZnO is an irreducible oxide, it provides a test of the traditional hypothesis that partial reduction of support cations is necessary to exhibit SMSI. In order to compare our calculations with surface science experiments, where the ultrathin films are not in equilibrium with bulk species, we developed a mixed canonical – grand canonical phase diagram scheme. The scheme, when combined with exhaustive DFT calculations of many different ultrathin ZnO_xH_y film structures and stoichiometries, permits direct comparison of the calculated free energies of these disparate films. Although, the thin film models provide more well-defined conditions for studying SMSI, there are thermodynamic differences with the real SMSI system. These differences can be described by changing the thermodynamic ensemble used to analyze the DFT results and extrapolating to deduce the stability of films at realistic SMSI conditions. Using this formalism, we have discovered that ZnO_xH_y films on Pd, which don’t exist in bulk, may form, and promote SMSI in irreducible oxides. This behavior is traced to both hydrogen incorporation in the films and strong stabilization of the films by the Pd substrates.</p><p dir="ltr">The computational framework, initially developed for the Pd/ZnO system, is subsequently extended to conduct thermodynamic investigations across different metal substrates. We found that linear scaling relationships (SRs) exist for the ultrathin films on metal surfaces that correlate the film formation energies with the combination of oxide cation and anion binding energies. However, these SRs deviate from classic bond order conservation principles. To provide an explanation for these deviations, and to enhance the predictive capabilities of the SRs, we introduced a generalized bonding model for oxy-hydroxy films supported on metal surfaces. By combining the SRs with grand canonical phase diagrams, we can precisely predict the stability of encapsulated films under specific reaction conditions. To validate the computational scheme, we apply it to the traditional SMSI system involving TiO₂-supported metal nanoparticles. Our calculations accurately predict which metals are prone to exhibit SMSI-like behavior and align well with available experimental results.</p><p dir="ltr">In order to analyze how these structures affect important real-world chemistries and identify key descriptors that influence their reactivity, we studied the adsorption behavior of common intermediates on oxide-decorated metal surfaces. We first investigated two types of ultrathin films, the compact graphite-like ZnO and the open honeycomb-like Zn₆O₅H₅ on Pt(111). We found that the graphite-like ZnO islands barely affect the electronic properties of the Pt surface, while the honeycomb-like Zn₆O₅H₅ network tunes the surface electron density of Pt such that the binding site for CO shifts from on-top to the bridge site. The findings enhance our understanding of metal-hydroxide interactions, potentially paving the way for innovative designs of highly efficient catalytic systems.</p><p dir="ltr">The SMSI effect is not confined to oxides used as supports. We confirmed the existence of a closely related phenomenon in Pt alloys, which are an important system for the oxygen reduction reaction (ORR). We identified elements that form stable oxy-hydroxy moieties on Pt surfaces under ORR conditions. Remarkably, elements like Cr, Mo, and Ir can form stable hydroxide 0d and 2d structures on Pt and can resist dissolution by preferentially covering the Pt edge and kink sites, which are otherwise susceptible to degradation. These nanoscale structures exhibit properties different from their bulk counterparts and can effectively tune the reactivity of the surface by introducing an inhomogeneous strain field into the Pt terrace sites.</p><p dir="ltr">The overarching goal of this dissertation is to formulate design principles applicable to metal nanoparticle catalysts coated with surface oxides. Given the pivotal role of these systems in industrially significant catalysts, the development of strategies aimed at engineering novel active sites using surface oxides is of great importance. The comprehensive molecular-level understanding of metal-metal oxide interactions, established through these studies, thus serves as a foundation for the study of these effects across a wider spectrum of reactions beyond ORR and CO oxidation. Through such studies, combined with rigorous experimental confirmation, it may ultimately be possible to engineer new classes of metal/oxide interfaces for desired catalytic applications.</p>

Ultra-thin films (UTFs) are important in many applications, seen in the semiconductor industry, in chromatography, in sensing, in microfluidics, in aerospace, and in robotics. They also protect materials from corrosion, change surface energies, limit water intrusion into materials, allow material self-cleaning and self-healing, provide scratch resistance, and impart other specific chemical properties. In many cases, UTFs drastically alter surface properties and therefore their applications. It is imperative that proper and consistent characterization be performed on coatings to confirm and understand their desired properties. In Chapter two, Al oxidation under MgF2 protective layers is studied using real time X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). These tools allowed me to monitor Al oxidation for both short (hours) and long (months) periods of time. XPS revealed the chemical changes that took place in these materials as a function of time, and these changes were verified with SE. These studies help increase an understanding of aluminum changes under MgF2 protective layers. The third chapter demonstrates ab initio calculations guided X-ray photoelectron spectroscopy (XPS) analysis of surfaces functionalized with fluorinated silanes. This study addresses deficiencies in the literature where CF2:CF3 ratios from experimental XPS data do not match theoretical CF2:CF3 ratios. In a systematic approach, I developed semi-empirical models directed both by ab initio calculations and adjustable, empirical parameters. These models were effective in describing the raw data and exceeded fitting methods used in literature. In Chapter four, SiO2 UTFs with variable thicknesses deposited on Eagle XG® glass substrates are characterized. Challenges associated with this work consisted of similar optical functions of the film and substrate as well as backside reflections from the substrate. These obstacles were met using a multi-sample analysis (MSA), a variable angle spectroscopic ellipsometric approach, and mechanical abrasion/roughening of the substrate backside. With these approaches, I developed a model that precisely fit the data collected from all the samples and gave the correct optical function of the material along with thickness values for each film. Surface characterization represents a commitment of resources. It takes time to make measurements, and it takes time to analyze and understand the results. As presented in this work, I increase understanding of ultra-thin films at interfaces using both a multi-tool approach as well as using multiple analytical methods on data collected from each tool.