This thesis present the fabrication of 2 types of soft wearable electrical devices, utilizing the 3D printing technique. The devices are capable to detect human heart pulse waves and sound waves for health evaluation and speech recognition.

<div>Nature has evolved efficient strategies to make materials with hierarchical internal structure that often exhibit exceptional mechanical properties. One such example is found in cellulose, which has achieved a high order of functionality and mechanical properties through a hierarchical structure with an exceptional control from the atomic level all the way to the macroscopic level. Cellulose is present in a wide variety of living species (trees, plants, algae, bacteria, tunicates), and provides the base reinforcement structure used by organisms for high mechanical strength, high strength-to-weight ratio, and high toughness. Additionally, being the most abundant organic substance on earth, cellulose has been used by our society as an engineering material for thousands of years, and are prolific within our society, as demonstrated by the enormity of the world-wide industries in cellulose derivatives, paper/packaging, textiles, and forest products.</div><div><br></div><div><div>More recently, a new class of cellulose base particles are being extracted from plants/trees, cellulose nanocrystals (CNCs), which are spindle-shaped nano-sized particles (3 ̶ 20 nm in width and 50 ̶ 500 nm in length) that are distinct from the more traditional cellulose materials currently used (e.g. molecular cellulose and wood pulp). They offer a new combination of particle morphology, properties and chemical functionalities that enable CNCs for use in applications that were once thought impossible for cellulosic materials.</div></div><div><br></div><div><div>CNCs have shown utility in many engineering applications, for example, biomedical, nanocomposites, barrier/separation membranes and cementitious materials. To gain greater insight as to how best use CNCs in various engineering application areas, a comprehensive understanding of the mechanics of CNCs is needed. The characterization of the mechanical properties of nanomaterials via experimental testing has always been challenging due to their small size, resulting in large uncertainties related to testing near sensitivity limits of a given technique, the same is true when characterizing CNCs. For CNCs, to help offset limitations in experimental testing, numerical modeling has been useful in predicting the mechanical properties of CNCs. We present a continuum-based structural model to study the mechanical behavior of cellulose nanocrystals (CNCs), and analyze the effect of bonded and non-bonded interactions on the mechanical properties under various loading conditions. In particular, this model assumes the uncoupling between the bonded and nonbonded interactions and their behavior is obtained from atomistic simulations.</div></div><div><br></div><div><div>For large deformations and when there is interaction and dynamics of many particles involved, continuum models could become as expensive as MD simulations. In addition, it has been shown that traditional material models in the continuum mechanics context, cannot model all the mechanical properties of CNC, especially for large deformation. To overcome these setbacks and to be able to model real size of CNC, 50-1000 nm, and/or to increase the number of particles involved in the simulation, a so called ‘‘coarse-grained’’ (CG) model for mechanical and interfacial properties of CNC is proposed. The proposed CG model is based on both mechanical properties and crystal-crystal interactions. Parametrization of the model is carried out in comparison with all-atom (AA) molecular dynamics and experimental results of some specific mechanical and interfacial tests.</div></div><div><br></div><div><div>Subsequently, verification is done with other tests. Finally, we analyze the effect of interface properties on the mechanical performance of CNC-based materials including, bending of a CNC bundle, tensile load and fracture in bioinspired structure of CNCs such as staggered brick-and-mortar and Bouligand structures of interest.</div></div>

<p dir="ltr">Deoxyribonucleic acid or DNA is an essential component in cells and organisms for genetic information storage and transduction. The base paring chemistry offers excellent programmability and structural predictability. This gives rise to the field of DNA nanotechnology, which uses DNA to design nanostructures and nanomachines with unprecedented designability and controllability. With the development of DNA nanotechnology, numerous chemical tools have been introduced for designing complex molecular mechanisms with DNA molecules. Various nanostructures of arbitrary shapes have been demonstrated, which shows the immense potential of DNA-based engineering. Dynamic nanodevices and their programmable actuations have also been successfully realized using DNA strand displacement and/or enzymatic reactions.</p><p dir="ltr">With controllable interactions with various biomolecules, it is possible to implement DNA in synthetic biological systems to program their behaviors. Two systems with programmed behaviors are introduced in this dissertation. The first system is a lipid-based protocell that can perform programmed migration with DNA-based mechanisms. This model system extracts chemical energy from fuel strands via enzymatic reaction and converts it into autonomous translocation on a surface. A mechanistic model is proposed to understand the migration dynamics. Furthermore, a path-tracking behavior between synthetic vesicles is demonstrated, which mimics cellular chemotaxis for the first time.</p><p dir="ltr">The second synthetic biological system explored is DNA origami structures capable of programmable auxetic reconfiguration. Auxetic materials are artificial systems with a negative Poisson’s ratio, which show great promise in various applications including space engineering and flexible/wearable electronics. With DNA-based sliding mechanisms, the proposed auxetic architecture can switch between two conformations with different Poisson’s ratios. The proposed strategy may be applied to designing adaptive materials or biochemical sensors with mechanical responses. The DNA-programmed behaviors demonstrated in this dissertation show unprecedented versatility and programmability, thus opening new opportunities for using molecular mechanisms to control synthetic biological systems with complex functions in diverse areas including biology, biomedicine, and material sciences.</p>

<p>Novel computational paradigms based on non-von Neumann architectures are being extensively explored for modern data-intensive applications and big-data problems. One direction in this context is to harness the intrinsic physics of spintronics devices for the implementation of nanoscale and low-power building blocks of such emerging computational systems. For example, a Probabilistic Spin Logic (PSL) that consists of networks of p-bits has been proposed for neuromorphic computing, Bayesian networks, and for solving optimization problems. In my work, I will discuss two types of device-components required for PSL: (i) p-bits mimicking binary stochastic neurons (BSN) and (ii) compound synapses for implementing weighted interconnects between p-bits. Furthermore, I will also show how the integration of recently discovered van der Waals ferromagnets in spintronics devices can reduce the current densities required by orders of magnitude, paving the way for future low-power spintronics devices.</p> <p>First, a spin-device with input-output isolation and stable magnets capable of generating tunable random numbers, similar to a BSN, was demonstrated. In this device, spin-orbit torque pulses are used to initialize a nano-magnet with perpendicular magnetic anisotropy (PMA) along its hard axis. After removal of each pulse, the nano-magnet can relax back to either of its two stable states, generating a stream of binary random numbers. By applying a small Oersted field using the input terminal of the device, the probability of obtaining 0 or 1 in binary random numbers (P) can be tuned electrically. Furthermore, our work shows that in the case when two stochastic devices are connected in series, “P” of the second device is a function of “P” of the first p-bit and the weight of the interconnection between them. Such control over correlated probabilities of stochastic devices using interconnecting weights is the working principle of PSL.</p> <p>Next my work focused on compact and energy efficient implementations of p-bits and interconnecting weights using modified spin-devices. It was shown that unstable in-plane magnetic tunneling junctions (MTJs), i.e. MTJs with a low energy barrier, naturally fluctuate between two states (parallel and anti-parallel) without any external excitation, in this way generating binary random numbers. Furthermore, spin-orbit torque of tantalum is used to control the time spent by the in-plane MTJ in either of its two states i.e. “P” of the device. In this device, the READ and WRITE paths are separated since the MTJ state is read by passing a current through the MTJ (READ path) while “P” is controlled by passing a current through the tantalum bar (WRITE path). Hence, a BSN/p-bit is implemented without energy-consuming hard axis initialization of the magnet and Oersted fields. Next, probabilistic switching of stable magnets was utilized to implement a novel compound synapse, which can be used for weighted interconnects between p-bits. In this experiment, an ensemble of nano-magnets was subjected to spin-orbit torque pulses such that each nano-magnet has a finite probability of switching. Hence, when a series of pulses are applied, the total magnetization of the ensemble gradually increases with the number of pulses</p> <p>applied similar to the potentiation and depression curves of synapses. Furthermore, it was shown that a modified pulse scheme can improve the linearity of the synaptic behavior, which is desired for neuromorphic computing. By implementing both neuronal and synaptic devices using simple nano-magnets, we have shown that PSL can be realized using a modified Magnetic Random Access Memory (MRAM) technology. Note that MRAM technology exists in many current foundries.</p> <p>To further reduce the current densities required for spin-torque devices, we have fabricated heterostructures consisting of a 2-dimensional semiconducting ferromagnet (Cr₂Ge₂Te₆) and a metal with spin-orbit coupling metal (tantalum). Because of properties such as clean interfaces, perfect crystalline nanomagnet structure and sustained magnetic moments down to the mono-layer limit and low current shunting, 2D ferromagnets require orders of magnitude lower current densities for spin-orbit torque switching than conventional metallic ferromagnets such as CoFeB.</p>

Printing of 3-dimensional nanostructures with high-resolution by two-photon polymerization has gained significant attention recently. Isopropyl thioxanthone (ITX) has been studied and used as a photoinitiator because of its unique property in initiating and depleting polymerization, but to further improve the resolution of 3D structures, new photoinitiating materials are necessary to decrease the power requirements especially in industrial world. In this dissertation, different new types of thioxanthone-based photoinitiators were synthesized and our new initiators possessed a clear enhancement in terms of excitation over ITX. To clearly reveal the writing mechanism behind it, the behavior of the initiators was evaluated by several methods such as low temperature phosphorescence spectroscopy and density functional theory (DFT) calculations. The first type of new molecules with alkyne bridge will be discussed in chapter 2 and the further developed initiators with electron donating and withdrawing groups will be discussed in chapter 3. By modifying the structure of ITX, we have revealed and proposed an important pathway to guide future development of photoinitiators in direct laser writing.

<p>Laser shock imprinting (LSI) is a novel fabrication technique capable of manufacturing various membrane materials. This top-down imprinting process can fabricate membranes in high precision, high throughput, and large scalability. It reveals a variety of applications ranging from electronics to photonics, which is beneficial from its reliable and precise modulation of micro/nanostructures. </p> <p> In this thesis, we firstly proposed and developed a cost-effective LSI process to manufacture hierarchical micro/nanostructured power generators. By combining the conventional soft lithography technique, LSI is well compatible with it to fabricate metal membranes towards soft templates. It is a significant progress from the originally-developed silicon wafer template layout because it effectively reduces the process cost by replacing sophisticatedly developed silicon wafers with low-cost photocurable polymers. In addition, the use of polymer expands the boundary limit of geometrical complexity from simple patterns to hierarchical structures, as a result, we successfully conducted LSI technology to fabricate biomimic leaf structures into metallic membranes with the help of soft SU-8 templates. These fabricated metallic membraned are used as water-driven triboelectric nanogenerators. In addition to the introduction of polymer template, we further developed a successive laser shock imprinting (SLSI) process to fabricate hierarchical nanostructures in a higher resolution. Typically, grating templates are collected via recycling blank discs and used as soft templates. Then multiple times of LSI process are conducted to manufacture membranes into complex nanostructures. The use of blank disc further reduces cost and increase process resolution. The highlight of this part of work is to feature the introduction of metallic thin films on disc template, which plays a significant role during this high strain rate imprinting process. Then, the imprinting mechanism was investigated through the finite element method to validate the experimental findings. Lastly, this soft template LSI process was applied to fabricate low dimensional materials such as nanowires (1D) and nanomembranes (2D), potentially introducing homogeneous and inhomogeneous strain field. Kelvin probe force microscopy was used to directly probe strain-induced changes. This soft-template LSI process reveals a new route of precisely fabricating low dimensional membranes into nanoelectronics systems. </p>

Supercapacitors are an emerging energy storage device that have gained attention because of the large specific power, at a reasonable specific energy, that they exhibit. These energy storage devices could be used alongside of or in the place of traditional electrochemical battery technologies to power reliable electrical devices. The performance of supercapacitorsis largely determined by electrode properties including the surface area to volume ratio, the electrical conductivity, and the ion diffusivity. Nanomaterial synthesis has been proposed as a method of enhancing the performance of many macroscopic supercapacitor electrodes due to the high surface area to volume ratio and unique tunable properties that are often size or thickness dependent for many materials. Specifically, carbon materials (such as carbon nanotubes), metal organic frameworks, (such as ZIF-8), and transition metal dichalcogenides (such as molybdenum disulfide) have been of interest due to their conductivity, large surface area, and ion diffusivity that they exhibit when one or more of their characteristic lengths is on the order of several nanometers.<div><br></div><div>For the experiments, a carbon nanotube-/ZIF-8-/MoS2framework was synthesized into an electrode material. This process involved first dispersing the carbon nanotubes in DMF using ultrasonication and then modifying the structure with polydopamine to create a binding site for the ZIF-8 to attach to the carbon nanotubes. The ZIF-8 was synthesized by combining 1,2,4-Triazole-3-thiol and ZnCl under 120 degrees Celsius. Afterwards, the MoS2was associated with the carbon nanotube and ZIF-8 framework by a disulfide bond with the sulfur vacancy of the MoS2andthe sulfide group of the ZIF-8. Finally, the sample solution was filtered by vacuum filtration and then annealed at 110 degrees Celsius before being deposited on a nickel foam substrate and tested in a 3-electrode electrochemical cyclic voltammetry study.<br></div><div><br></div><div>The resulting materials were found to have a capacitance of 262.15 F/g with corresponding specific energy and specific power values of 52.4Whr/kg and 1572W/kg. Compared to other supercapacitor research materials, this electrode shows a much larger capacitance than other exclusively carbon materials, and comparable capacitance values to the ZIF-8 and MoS2materialswith the added benefits of an easier and faster manufacturing process. Overall, the electrodes developed in this study, could potentially reduce the cost per farad of the supercapacitor to be more competitive energy storage devices<br></div>

<div>Enhancing the burn rates of solid propellants and energetics is a crucial step towards improving the performance of several solid propellant based micro-propulsion systems. In addition to increasing thrust, high burn rates also help simplify the propellant grain geometry and increase the volumetric loading of the rocket motor, which in turn reduces the overall size and weight. <b><i>Thus, in this work, burn rate enhancement of solid propellants when coupled to highly conductive graphene-based micro-structures was studied using both experiments and molecular dynamic (MD) simulations.</i></b></div><div><b><i><br></i></b></div><div><div>The experiments were performed using three different types of graphene-structures i.e. graphite sheet (GS), graphene nano-pellets (GNPs) and graphene foam (GF), with nitrocellulose (NC) as the solid propellant.</div></div><div><br></div><div><div>For the NC-GS samples, propellant layers ranging from 25 µm to 170 µm were deposited on the top of a 20 µm thick graphite sheet. Self-propagating combustion waves were observed, with burn rate enhancements up to 3.3 times the bulk NC burn rate (0.7 cm/s). The burn rates were measured as a function of the ratio of fuel to graphite layer thickness and an optimum thickness ratio was found corresponding to the maximum enhancement. Moreover, the ratio of fuel to graphite layer thickness was also found to affect the period and amplitude of the combustion wave oscillations. Thus, to identify the important non-dimensional parameters that govern the burn rate enhancement and the oscillatory nature of the combustion waves, a numerical model using 1-D energy conservation equations along with simple first-order Arrhenius kinetics was also developed.</div><div><br></div><div><div>For the GNP-doped NC lms, propellant layers, 500 30 µm thick, were deposited on the top of a thermally insulating glass slide with the doping concentrations of GNPs being varied from 1-5% by mass. An optimum doping concentration of 3% was obtained for which the burn rate enhancement was 2.7 times. In addition, the effective thermal conductivities of GNP-doped NC lms were also measured experimentally using a steady state, controlled, heat flux method and a linear increase in the thermal conductivity value as a function of the doping concentration was obtained.</div></div><div><br></div><div><div>The third type of graphene structure used was the GF - synthesized using a chemical vapor deposition (CVD) technique. The effects of both the fuel loading ratio and GF density were studied. Similar to the GNPs, there existed an optimum fuel loading ratio that maximized the burn rates. However, as a function of the GF density, a monotonic decreasing trend in the burn rate was obtained. Overall, burn rate enhancement up to 7.6 times was observed, which was attributed to the GF's unique thermal properties resulting from its 3D interconnected network, high thermal conductivity, low thermal boundary resistance and low thermal mass. Moreover, the thermal conductivity of GF strut walls as a function of the GF density was also measured experimentally.</div></div><div><br></div><div><div>Then as a next step, the GF structures were functionalized with a transition metal oxide (MnO₂). The use of GF-supported catalyst combined the physical eect of enhanced thermal transport due to the GF structure with the chemical effect of increased chemical reactivity (decomposition) due to the MnO₂ catalyst, and thus, resulted in even further burn rate enhancements (up to 9 times). The burn rates as a function of both the NC-GF and MnO₂-NC loadings were studied. An optimum MnO₂-NC loading corresponding to the maximum burn rate was obtained for each NC-GF loading. In addition, thermogravimetric (TG) and differential scanning</div><div>calorimetry (DSC) analysis were also conducted to determine the effect of NC-GF and MnO₂-NC loadings on the activation energy (E) and peak thermal decomposition (PTD) temperatures of the propellant NC.</div></div><div><br></div><div><div>In addition to the experimental work, molecular dynamics simulations were also conducted to investigate the thermal transport and the reactivity of these coupled solidpropellant/graphene-structures. A solid monopropellant, Pentaerythritol Tetranitrate (PETN), when coupled to highly conductive multi-walled carbon nanotubes (MWCNTs) was considered. The thickness of the PETN layer and the diameter of the MWCNTs were varied to determine the effect of PETN-MWCNT loading on the burn rates obtained. Burn rate enhancement up to 3 times was observed and an optimal PETN-MWCNT loading of 45% was obtained. The enhancement was attributed to the faster heat conduction in CNTs and to the layering of PETN molecules around the MWCNTs surface. Moreover, the CNTs remained unburned after the combustion process, conrming that these graphene-structures do not take part in the chemical reactions but act only as thermal conduits, transferring heat from the burned to the unburned portions of the fuel.</div></div><div><br></div><div><div>A long-pursued goal, which is also a grand challenge, in nanoscience and nanotechnology is to create nanoscale devices, machines and motors that can do useful work. However, loyal to the scaling law, combustion would be impossible at nanoscale because the heat loss would profoundly dominate the chemical reactions. <b><i>Thus, in addition to the solid propellant work, a preliminary study was also conducted to understand as how does the heat transfer and combustion couple together at nano-scales.</i></b></div></div><div><b><i><br></i></b></div><div><div>First, an experimental study was performed to understand the feasibility of combustion at nano-scales for which a nano-scale combustion device called "nanobubbles" was designed. These nanobubbles were produced from short-time (< 2000 µs) water electrolysis by applying high-frequency alternating sign square voltage pulses (1-500 kHz), which resulted in H₂ and O₂ gas production above the same electrode. Moreover, a 10 nm thick Pt thermal sensor (based on resistance thermometry) was also fabricated underneath the combustion electrodes to measure the temperature changes obtained. A signicant amount of bubble production was seen up to 30 kHz but after that the bubble production decreased drastically, although the amount of faradaic current measured remained unchanged, signifying combustion. The temperature changes measured were also found to increase above this threshold frequency of 30 kHz.</div></div><div><br></div><div><div>Next, non-reactive molecular dynamic simulations were performed to determine as how does the surface tension of water surrounding the electrodes is affected by the presence of dissolved external gases, which would in turn help to predict the pressures inside nanobubbles. Knowing the bubble pressure is a perquisite towards understanding the combustion process. The surface tension of water was found to decrease with an increase in the supersaturation ratio (or an increase in the external gas concentration), thus, the internal pressure inside a nanobubble is much smaller than what would have been predicted using the planar-interface surface tension value of water. Once the pressure behavior as a function of external gas supersaturation was understood, then as a next step, reactive molecular dynamic simulations were performed to study the effects of surface-assisted dissociation of H₂ and O₂ gases and initial system pressure on the ignition and reaction kinetics of the H₂/O₂ system at nano-scales. A signicant amount of hydrogen peroxide (H₂O₂), 6-140 times water (H₂O), was observed in the combustion products. This was attributed to the low temperature(~300 K) and high pressure (2-80 atm) conditions at which the chemical reactions were taking place. Moreover, the rate at which heat was being lost from the combustion chamber (nanobubble) was also compared to the rate at which heat was being released from the chemical reactions and only a slight rise in the reaction temperature was observed (~68 K), signifying that, at such small-scales, heat losses dominate.</div></div><div><br></div></div>

<div>In this work, we investigate the trade-off between scalability and reliability for next generation logic-transistors i.e. Gate-All-Around (GAA)-FET, Multi-Bridge-Channel (MBC)-FET. First, we analyze the electronic properties (i.e. bandgap and</div><div>quantum conductance) of ultra-thin silicon (Si) channel i.e. nano-wire and nano-sheet based on first principle simulation. In addition, we study the influence of interface</div><div>states (or dangling bonds) at Si-SiO₂ interface. Second, we investigate the impact of bandgap change and interface states on GAA-FETs and MBC-FETs characteristics by</div><div>employing Non-equilibrium Green's Function based device simulation. In addition to that, we calculate the activation energy of Si-H bond dissociation at Si-SiO₂ interface for different Si nano-wire/sheet thickness and different oxide electric-field. Utilizing these thickness dependent activation energies for corresponding oxide electric-field, in conjunction with reaction-diffusion model, we compute the characteristics shift and analyze the negative bias temperature instability in GAA-FET and MBC-FET. Based on our analysis, we estimate the operational voltage of these transistors for a life-time of 10 years and the ON current of the device at iso-OFF-current condition. For example, for channel length of 5 nm and thickness < 5 nm the safe operating voltage needs to be < 0.55V. Furthermore, our analysis suggests that the benefit of Si thickness scaling can potentially be suppressed for obtaining a desired life-time of GAA-FET and MBC-FET.</div>

<p>This investigation delineates the optimal synthesis and characterization of innovative epoxy-carbon nanotube (CNT) nanocomposite filaments via electrospinning. Electrospinning thermosetting materials such as epoxy resins presents significant challenges due to the polycationic behavior arising from intermolecular noncovalent interactions between epoxide and hydroxyl groups, resulting in a substantial increase in solution surface tension. In this study, electrospinning submicron epoxy filaments was achieved through partial curing of epoxy via a thermal treatment process in an organic polar solvent, circumventing the necessity for plasticizers or thermoplastic binders. The filament diameter can be modulated to as low as 100 nm by adjusting electrospinning parameters.</p> <p><br></p> <p>Integrating a minimal amount of CNT into the epoxy matrix yielded enhanced structural, electrical, and thermal stability. The CNTs were aligned within the epoxy filaments due to the electrostatic field present during electrospinning. The modulus of the epoxy and epoxy-CNT filaments were determined to be 3.24 and 4.84 GPa, respectively, resulting in a 49% improvement. Epoxy-CNT nanofibers were directly deposited onto carbon fiber reinforced polymer (CFRP) prepreg layers, yielding augmented adhesion, interfacial bonding, and significant mechanical property enhancements. The interlaminar shear strength (ILSS) and fatigue resistance demonstrated a 29% and 27% increase, respectively, under intense stress conditions. Up to 45% of the Barely Visible Impact Damage (BVID) energy absorption was increased. In addition, the strategic incorporation of CNT (multi-walled) networks between the layers of CFRP resulted in a significant increase in thermal and electrical conductivities.</p> <p>This study also introduces a scalable fabrication procedure to address large volume processing, reproducibility, accuracy, and electrospinning safety. Electric fields of the experimental multi-nozzle setups were simulated to elucidate the induced surface charges responsible for the Taylor cone formation of the epoxy-CNT solution droplet on the nozzle tips. Electrospinning parameters were subsequently optimized for the multi-nozzle system and analyzed alongside simulated data to improve stability and synthesize fibers with smaller diameters.</p> <p><br></p> <p>Smaller diameter epoxy-CNT nanofibers proved critical as CNTs maintained alignment within the nanofibers when compared to larger diameter nanofibers. This research examines the impact of effective parameters on the diameter of electrospun epoxy-CNT nanofibers using artificial neural networks (ANNs). Consequently, employing a genetic algorithm (GA) and Bayesian optimization (BO) methods enable accurate prediction of epoxy-CNT nanofiber diameters prior to electrospinning. The presented models could aid researchers in fabricating electrospun thermosetting and thermoplastic scaffolds with specified fiber diameters, thereby tailoring these scaffolds for specific applications.</p>