abstract: With the advancements in technology, it is now possible to synthesize new materials with specific microstructures, and enhanced mechanical and physical properties. One of the new class of materials are nanoscale metallic multilayers, often referred to as nanolaminates. Nanolaminates are composed of alternating, nanometer-thick layers of multiple materials (typically metals or ceramics), and exhibit very high strength, wear resistance and radiation tolerance. This thesis is focused on the fabrication and mechanical characterization of nanolaminates composed of Copper and Cobalt, two metals which are nearly immiscible across the entire composition range. The synthesis of these Cu-Co nanolaminates is performed using sputtering, a well-known and technologically relevant physical vapor deposition process. X-ray diffraction is used to characterize the microstructure of the nanolaminates. Cu-Co nanolaminates with different layer thicknesses are tested using microelectromechanical systems (MEMS) based tensile testing devices fabricated using photolithography and etching processes. The stress-strain behavior of nanolaminates with varying layer thicknesses are analysed and correlated to their microstructure. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2019

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19305号 / 工博第4102号 / 新制||工||1632(附属図書館) / 32307 / 京都大学大学院工学研究科マイクロエンジニアリング専攻 / (主査)教授 小寺 秀俊, 教授 田畑 修, 教授 中部 主敬 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24611号 / 工博第5117号 / 新制||工||1978(附属図書館) / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 大和田 拓, 教授 髙田 滋, 講師 杉元 宏 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

2D materials like graphene and MoS_2 are atomically thin, extremely strong and flexible, making them attractive for integration into strain engineered devices. Strain on these materials can change physical properties, as well as induce exotic physics, not typically seen in solid-state systems. Here, we probe the novel physics arising from distorted lattices of 2D materials, strained by nanopillars indentation and microelectromechanical systems (MEMS), using Raman and photoluminescence (PL) spectroscopy. From nanopillars strained multilayer MoS_2, we observe exciton and charge carrier funneling due to strain, inducing dissociation of excitons in to free electron-hole pairs in the indirect material. Using MEMS devices, we were able to dynamically strain monolayer and multilayer graphene. Multilayer graphene under MEMS strain showed signatures of loss in Bernal stacking due to shear of the individual layers, indicating that MEMS can be used to tune the layer commensuration with tensile strain. We further explore simulation of pseudo-magnetic fields (PMFs) generated in monolayer graphene strained by MEMS, using machine learning, to accelerate and optimize the strength and uniformity of the PMF in new graphene geometries. Nanopillars provide non-uniform, centrally biaxial strain to multilayer MoS_2 transferred on top. Raman E^1_2g and PL redshift across the pillar confirms 1-2% strain in the material. We also observe a softening in the A_1g Raman mode and an enhancement in the overall PL with an increase in radiative trions, under strain. The changes in these charge-dependent features indicates funneling of charge carriers and neutral excitons to the apex of the pillar, as strain locally deforms the band structure of the conduction and valence bands. DFT calculations of the band structure in bilayer MoS_2 under biaxial strain shows the conduction band is lowered, further increasing the indirectness of multilayer MoS_2. This should cause the PL intensity to decrease, whereas we observe an increase in MoS_2 PL intensity under strain. We theorize that this is due to a dissociation of excitons into free electron-hole pairs. The increase in charge carrier densities due to strain leads to a renormalization of the local band structure and increased dielectric screening, supporting free electron-hole recombination at the K-point without momentum restrictions. In turn, electron-hole recombination occurs around the K-point inducing a high intensity PL, which opens attractive opportunities for utilization in optoelectronic devices. MEMS chevron actuators can dynamically strain 2D materials, which we demonstrate through uniaxial strain in CVD and exfoliated graphene. We use a novel microstructure assisted transfer technique which can deterministically place materials on non-planar surfaces like MEMS devices. Building on previously reported 1.3% in monolayer MoS2 from our group, we report tunable 0.3% strain in CVD monolayer graphene and 1.2% strain in multilayer exfoliated graphene using MEMS chevron actuators, detected by Raman spectroscopy. The asymmetric-to-symmetric strain evolution of the 2D phonon line shape in multilayer graphene is evidence of changes in interlayer interactions, caused by shearing between layers. This demonstrates that MEMS can be used to tune the commensuration in few layer 2D materials, which is a promising avenue towards Moiré engineering. Using machine learning, we also simulate optimal monolayer graphene geometries for generating strong, uniform pseudo-magnetic fields by MEMS strain. The coupled use of finite-element methods, variational auto-encoder, and auxiliary neural network accelerates the search for PMFs in strained graphene, while optimizing the graphene shape for fabrication through electron-beam lithography. Our experimental and simulated work creates a road-map for rapid advancement in zero-field quantum Hall effect devices using graphene-integrated MEMS actuators.

Two compliant bistable micro-mechanisms have been developed which can be switched in either direction using on-chip thermal actuation. The energy storage and bistable behavior of the mechanisms are achieved through the elastic deflection of compliant segments. The pseudo-rigid-body model was used for the compliant mechanism design, and for analysis of the large-deflection flexible segments. To achieve on-chip actuation, the mechanism designs were optimized to reduce their required rotation, allow them to be switched using linear-motion thermal actuators. The modeling theory and analysis are presented for several design iterations. Each iteration was successfully fabricated and tested using either the MUMPs or SUMMiT surface micromachining technology.

The electrothermomechanical characteristics of an electrically-heated polycrystallinesilicon microactuator are explored. Using finite-difference techniques, an electrothermal model based on the balance of heat dissipation and heat losses is developed. For accurate simulation, the relevant temperature dependent properties from the microactuator material are included in the model. The electrothermal model accurately predicts the steady-state power required to hold position, and the energy consumed during the thermal transient. Thermomechanical models use the predictions of temperature from the electrothermal solution to calculate displacement and force from pseudo-rigid-body approximations and commercial finite-element code. The models are verified by comparing experimental data to simulation results of a single leg-pair on a particular configuration of the device. The particular microactuator studied is called a Thermomechanical In-plane Microactuator, or TIM, and was fabricated with surface micromachining technology. A TIM requires a single releasable structural layer, is extremely flexible in design, and can operate with simple drive and control circuitry. The TIM produces linear motion of a center shuttle when slender legs on either side move the shuttle as a result of constrained thermal expansion. In a single example, when the current through a leg with dimensions 250×3×3.5 µm^3 and suspended 2 µm off the substrate is sufficient to maintain an average temperature of 615 C in air and vacuum environments, model simulated temperatures along the leg have a peak of 860 C in air and 1100 C in vacuum. The final measured and predicted displacement is 14 µm. In air, the power predicted by the model needed to maintain this average temperature profile is 95 mW while consuming 16.4 µJ in 0.22 ms to reach 90 percent of the final average temperature. In a vacuum, only 6.4 mW are required to maintain the same average temperature with 97.6 µJ consumed in 18.5 ms. Simulation results suggest that short-duration high-current pulses can improve the transient response and energy consumed in a vacuum when steady-state temperatures are not required. For a TIM leg with the dimensions above, the maximum measured force is approximately 47 µN per leg-pair when enough current is provided to move the TIM 8 µm as a result of ohmic heating and thermal expansion.

A new technique, called nanoinjection, is being developed to insert foreign DNA into a living cell. Such DNA transfection is commonly used to create transgenic organisms vital to the study of genetics, immunology, and many other biological sciences. In nanoinjection, DNA, which has a net negative charge, is electrically attracted to a micromachined lance. The lance then pierces the cell membranes, and the voltage on the lance is reversed, repelling the DNA into the cell. It is shown that DNA motion is strongly correlated to ion transport through a process called electrophoresis. Gel electrophoresis is used to move DNA using an electric field through a gel matrix (electrolytic solution). Understanding and using electrophoretic principals, a mathematical model was created to predict the motion (trajectory) of DNA particles as they are attracted to and repulsed from the nanoinjector lance. This work describes the protocol and presents the results for DNA motion experiments using fabricated gel electrophoresis devices. Electrophoretic systems commonly use metal electrodes in their construction. This work explores and reports the differences in electrophoretic motion of DNA (decomposition voltage, electrical field, etc.) when one electrode is constructed from a semiconductor, silicon rather than metal. Experimental results are used to update and validate the mathematical model to reflect the differences in material selection. Accurately predicting DNA motion is crucial for nanoinjection. The mathematical model allows investigation of the attraction/repulsion process by varying specific parameters. Result show that the ground electrode placement, lance orientation and lance penetration significantly affect attraction or repulsion efficiency while the gap, lance direction, lance tip width, lance tip half angle and lance tip height do not. It is also shown that the electric field around the lance is sufficient to cause localized electroporation of cell membranes, which may significantly improve the efficiency of transport.

The ability to inject DNA and other foreign particles into cells, both germ cells (e.g. to produce transgenic animals) and somatic cells (e.g. for gene therapy), is a powerful tool in genetic research. Nanoinjection is a method of DNA delivery that combines mechanical and electrical methods. It has proven to have higher cell viability than traditional microinjection, resulting in higher integration per injected embryo. The nanoinjection process can be performed on thousands of cells simultaneously using an array of microneedles that is inserted into a monolayer of cells. This thesis describes the needle array design requirements and the fabrication process used to meet them. The process uses unpassivated and passivated deep reactive ion etching (DRIE) to create needles with a constant diameter shaft and a pointed tip. The needle diameter and height are about 1 µm and 8 µm, respectively. A buckling analysis and physical testing show that the needles can withstand the force required to penetrate the cells. The chip is attached to a plastic suspension with a counter electrode and electrical connections to a voltage source. The suspension's motion is defined by two compliant orthoplanar springs that have been vertically and rotationally offset for added stability. The base of the suspension is designed to exactly fit in the bottom of a cell culture dish, where the needle array can be pushed into the cell monolayer. Injection protocol was created and followed to perform tests with needle insertion only, voltage application only, and the full nanoinjection process. The average cell viability for the full injection process was 98.2% compared to an average control viability of 99.5%. Zero volt injections with a high concentration of propidium iodide, a cell impermeable dye with two positive charges, resulted in dye uptake from diffusion, proving that the needles are penetrating the cells. Tests comparing injections with and without voltage had high variability in dye uptake. Therefore, glass cover slips were placed in the culture dishes to provide more consistent injection conditions. This reduced variation in zero voltage tests. It is recommended that this procedure be followed for performing injections with voltage.

A novel method for charged macromolecule delivery, called nanoinjection, has been developed at Brigham Young University. Nanoinjection combines micro-fabrication technology, mechanism design, and nano-scale electrical phenomenon to transport exogenous DNA across cell membranes on a nano-featured lance. DNA is electrically accumulated on the lance, precision movements of microelectromechanical systems (MEMS) physically insert the lance into cell, and DNA is electrically released from the lance into the cell. Penetration into the cell is achieved through a two-phase, self-reconfiguring metamorphic mechanism. The surface-micromachined, metamorphic nanoinjector mechanism elevates the lance above the fabrication substrate, then translates in-plane at a constant height as the lance penetrates the cell membranes. In-vitro studies indicate no statistical difference in viability between nanoinjected and untreated mouse zygotes. Pronuclear nanoinjection experiments on mouse zygotes, using microinjection as a control, demonstrate integration and expression of a nanoinjected transgene, and higher rates of zygote survival and pup births than the microinjection control. A new compliant mechanism analysis method, the minimization of potential energy method (MinPE method) is presented to model the equilibrium position of compliant mechanisms with more degrees of freedom (DOF) than inputs, such as a fully-compliant nanoinjector. The MinPE method position and force predictions agree with the method of virtual work and non-linear finite element analyses of under-actuated and underconstrained compliant mechanisms. Additionally, a performance-based comparison is made between quadratic shell finite elements elements and 3-D quadratic solid elements for modeling geometrically non-linear spacial deflection of thin-film compliant mechanisms. The comparison's results suggest the more computationally efficient quadratic shell elements can be used to model spatially deforming thin-film compliant mechanisms. Finally, this dissertation presents preliminary results for a proposed method of DNA transfer called cytoplasm-to-pronucleus nanoinjection. By placing a DNA coated lance into the cytoplasm of a mouse zygote and applying a voltage pulse of sufficient magnitude and duration, pores may open in the pronuclear membranes and DNA may be electrophoretically repelled from the lance. If effective, this process could result in transgenes without having to visualize and physically penetrate into the pronucleus. While embryo survival has been demonstrated under a variety of injection conditions, further study is needed to increase the process' consistency, and to determine if cytoplasm-to-pronucleus nanoinjection can generate transgenic animals.

Medical ultrasound technology accounts for over half of all imaging tests performed worldwide. In comparison to other methods, ultrasonic imaging is more portable and lower cost, and is becoming more accessible to remote regions where traditionally no medical imaging can be done. However, conventional ultrasonic imaging systems still rely on expensive PZT-based ultrasound probes that limit broader applications. In addition, the resolution of PZT based transducers is low due to the limitation in hand-fabrication methods of the piezoelectric ceramics. Capacitive Micromachined Ultrasonic Transducers (CMUTs) appears as an alternative to the piezoelectric (PZT) ceramic based transducer for ultrasound medical imaging. CMUTs show better ultrasound transducer design for batch fabrication, higher axial resolution of images, lower fabrication costs of the elements, ease of fabricating large arrays of cells using MEMS fabrication, and the extremely important potential to monolithically integrate the 2D transducer arrays directly with IC circuits for real-time 3D imaging. Currently most efforts on CMUTs are silicon based. Problems with current silicon-based CMUT designs include low pressure transmission and high-temperature fabrication processes. The pressure output from the silicon based CMUTs cells during transmission are too low when compared to commercially available PZT transducers, resulting in relatively blurry ultrasound images. The fabrication of the silicon-based cells, although easier than PZT transducers, still suffers from inevitable high temperature process and require specialized and expensive equipment. Manufacturing at an elevated temperature hinders the capability of fabricating front end analog processing IC circuits, thus it is difficult to achieve true 3D/4D imaging. Therefore novel low temperature fabrication with a low cost nature is needed. A polymer (Parylene) based CMUTs transducer has been investigated recently at UCF and aims to overcome limitations posted from the silicon based counterparts. This thesis describes the numerical simulation work and proposed fabrication steps of the Parylene based CMUT. The issue of transducer cost and pressure transmission is addressed by proposing the use of low cost and low temperature Chemical Vapor Deposition (CVD) fabrication of Parylene-C as the structural membrane plus graphene for the membrane electrodes. This study focuses mainly on comparing traditional silicon-based CMUT designs against the Parylene-C/Graphene CMUT based transducer, by using MEMS modules in COMSOL. For a fair comparison, single CMUT cells are modeled and held at a constant diameter and the similar operational frequency at the structural center. The numerical CMUT model is characterized for: collapse voltage, membrane deflection profile, center frequency, peak output pressure transmission over the membrane surface, and the sensitivity to the change in electrode surface charge. This study took the unique approaches in defining sensitivity of the CMUT by calculating the membrane response and the change in the electrode surface charge due to an incoming pressure wave. Optimal design has been achieved based on the simulation results. In comparison to silicon based CMUTs, the Parylene/Graphene based CMUT transducer produces 55% more in volume displacement and more than 35% in pressure output. The thesis has also laid out the detailed fabrication processes of the Parylene/Graphene based CMUT transducers. Parylene/Graphene based ultrasonic transducers can find wide applications in both medical imaging and Non destructive evaluation (NDE).