Investigation of Transport Behavior in Two-Dimensional Ferroelectric Heterostructures

This dissertation summarizes an investigation of the polarization-related electronic transport behavior in the ferroelectric thin films and two-dimensional (2D) materials heterostructures using Scanning Probe Microscopy (SPM) techniques. The polarization-related resistive switching in hafnium oxide thin films-based ferroelectric tunnel junction has been demonstrated by employing semiconducting MoS2 as a top electrode. We explored a coupling between the semiconducting properties of MoS2 and the polarization of Hf0.5Zr0.5O2 resulted in an enhanced tunneling electroresistance effect of up to 3 orders of magnitude. These results provide a possible pathway for the fabrication of high-density non-volatile memory devices. These results are presented in Chapter 3. Resistive switching control using conducting domain walls as functional elements has been investigated using graphene/LiNbO3 heterostructures. One approach involves the modulation of resistance through the manipulation of domain wall density using super-coercive voltage. This approach requires higher energy to switch the polarization and can induce high leakage current that makes it deleterious. To overcome this drawback, we have developed a new approach that involves tuning of domain wall conductivity by a sub-coercive voltage without altering the domain configuration. These results are presented in Chapter 4. Chapter 5 describes modulation of the transport behavior of 2D MoS2 junctions by mechanical stress induced by the sharp probe of atomic force microscope (AFM). We show that the junction resistance can be reversibly tuned by up to 4 orders of magnitude by altering the mechanical force applied via AFM tip. Additionally, we show that AFM tip generates strain gradient inducing flexoelectric effect that leads to an enhancement of photovoltaic effect. Finally, we have discovered stable room temperature ferroelectricity with out-of-plane polarization in trigonally distorted 1T”-MoS2. Here, the polarization switching has been realized by the mechanical load applied via AFM probe. The piezoelectric and the electrical properties of MoS2 flakes are probed. Moreover, we show that flipped flakes of 1T”-MoS2 samples consist of monolayers of randomly oriented polarization, showing the possibility of head-head or tail-tail configuration. These results are presented in Chapter 6. Advisor: Alexei Gruverma

Numerical and Experimental Study on the Friction of Complex Surfaces

Whenever two bodies are in contact due to a normal load and one is sliding against the other, a tangential force arises, as opposed to the motion. This force is called friction force and involves different mechanisms, such as asperity interactions, energy dissipation, chemical and physical alterations of the surface topography and wear. The friction coefficient is defined as the ratio between the friction force and the applied normal load. Despite this apparently simple definition, friction appears to be a very complex phenomenon, which also involves several aspects at both the micro- and nano-scale, including adhesion and phase transformation. Moreover, it plays a key role in a variety of systems, and must be either enhanced (e.g. for locomotion) or minimized (e.g. in bearings), depending on the application. Considering friction as a multiscale problem, an analytical model has been proposed, starting from the literature, to describe friction in the presence of anisotropy, adhesion and wear between surfaces with hierarchical structures, e.g. self-similar. This model has been implemented in a MATLAB code for the design of the tribological properties of hierarchical surfaces and has been applied to study the ice friction, comparing analytical predictions with experimental tests. Furthermore, particular isotropic or anisotropic surface morphologies (e.g., microholes of different shapes and sizes) has been investigated for their influence to the static and dynamic friction coefficients with respect to a flat counterpart. In particular, it has been proved that the presence of grooves on surfaces could decrease the friction coefficients and thus reduce wear and energy dissipation. Experimental tests were performed with a setup realized ad hoc and the results were compared with full numerical simulations. If patterned surfaces showed that they can reduce sliding friction, other applications could require an increase in energy dissipation, e.g. to enhance the toughness of microfibers. Specifically, the applied method consists of introducing sliding frictional elements (sliding knots) in biological (silkworm silk, natural or degummed) and synthetic fibres, reproducing the concept of molecules, where the sacrificial bonds provide higher toughness to the molecular backbone, with a hidden length, which occurs after their breakage. A variety of slip knot topologies with different unfastening mechanisms have been investigated, including even complex knots usually adopted in the textile industry. The knots were made by manipulation of fibres with tweezers and the resulting knotted fibres were characterized through nanotensile tests to obtain their stress-strain curve until failure. The presence of sliding knots strongly increases the dissipated energy per unit mass, without compromising the structural integrity of the fibre itself

Bottom-up reconstitution using giant unilamellar vesicles as membrane compartments

One of the most basic defining features of a living biological cell is its ability to confine its molecular components within an isolating boundary. In all living cells known to date, phospholipid membranes play this central role by separating cellular machinery from the outside environment. Compartmentalization by membranes is a key principle of life, not only on the scale of the cell as a whole, but, in many cases, within cells as well. This unification and isolation of essential processes within a single, definable unit has made life as we know it possible. As such, compartmentalization represents an essential task that had to be achieved in the emergence of life, as only a confining barrier to the environment allows for distinction of individual units of life and therefore for Darwinian evolution. Another core task of life, is the ability to reproduce. Modern cells are able to split into two daughter cells by deforming their compartmentalizing membrane barrier, all with a division machinery which is situated within this barrier itself – a quite remarkable feat. In this thesis, I present my doctoral work, which aimed to investigate these key features of life through bottom-up reconstitution approaches. Bottom-up cell biology uses isolated well-characterized components, like purified proteins, and tries to recreate cellular processes from these simple building blocks. One of the core challenges of classical cell biology is how to tackle the intrinsic complexity of biological systems with an analytical approach. Complexity is one of the most intrinsic features of life on earth, owing to billions of years of evolution. Biological cells are not designed to be understood and in fact, complexity itself contributes to the adaptability and robustness of biological function. However, by reconstitution of biological structure and function from the bottom-up with minimal components, scientists can observe and try to understand the emergence of biological complexity. Membranes, of course, play a great role in many cellular processes, and contribute significantly to this complexity. However, they can be difficult to incorporate into bottom-up reconstitution approaches, limiting our ability to get a complete view of such processes from in vitro experiments. In this thesis, I present a number of projects, which use giant unilamellar vesicles (GUVs) as mimicries of the compartmentalizing membranes of cells to investigate basic biological functions. I’ve encapsulated a diverse array of biological components in GUVs to achieve biomimetic behavior and function. In each case, I made use of the unique properties of the GUVs to achieve behaviors or make observations that would have not been possible with other approaches, including alternative model membrane systems. Overall, my work presents a step forward toward the reconstitution of complex and dynamic cellular processes under cell-like conditions.Eines der grundlegendsten Definitionsmerkmale einer lebenden biologischen Zelle ist ihre Fähigkeit, ihre molekularen Komponenten innerhalb einer isolierenden Barriere einzuschließen. In allen bis dato bekannten lebenden Zellen spielen Phospholipidmembranen diese zentrale Rolle, indem sie die zelluläre Maschinerie von der äußeren Umgebung abtrennen. Die Kompartmentalisierung durch Membranen ist ein Schlüsselprinzip des Lebens, nicht nur auf der Ebene der kompletten Zelle, sondern oft auch innerhalb von Zellen. Diese Einschließung und Isolierung wesentlicher Prozesse innerhalb einer einzigen, definierbaren Einheit hat Leben, wie wir es kennen, möglich gemacht. Damit stellt die Kompartimentierung eine wesentliche Aufgabe dar, die bei der Entstehung des Lebens realisiert werden musste, denn nur eine abgrenzende Barriere zur Umwelt ermöglicht die Unterscheidung einzelner Lebenseinheiten und damit die darwinistische Evolution. Eine weitere Kernaufgabe des Lebens, ist die Fähigkeit zur Vermehrung. Moderne Zellen sind in der Lage, sich durch Verformung ihrer kompartimentierenden Membranbarriere in zwei Tochterzellen zu teilen, und zwar mit einer Teilungsmaschinerie, die sich innerhalb dieser Barriere befindet - eine bemerkenswerte Leistung. In dieser Dissertation stelle ich meine Forschungsergebnisse vor, deren Ziel es war, diese Schlüsselmerkmale des Lebens durch Bottom-up-Rekonstruktionsansätze zu untersuchen. Die Bottom-up-Zellbiologie verwendet isolierte, gut charakterisierte Komponenten, wie gereinigte Proteine, und versucht, zelluläre Prozesse aus diesen einfachen Bausteinen zu rekonstruieren. Eine der Kernherausforderungen der klassischen Zellbiologie ist, wie man die intrinsische Komplexität biologischer Systeme mit einem analytischen Ansatz angehen kann. Komplexität ist eines der intrinsischen Merkmale des Lebens auf der Erde, das auf Milliarden von Jahren der Evolution zurückzuführen ist. Biologische Zellen sind nicht darauf ausgelegt, verstanden zu werden, und vielmehr trägt Komplexität zur Anpassungsfähigkeit und Robustheit biologischer Funktionsweisen bei. Durch die Rekonstruktion von biologischen Strukturen und Funktionen mit minimalen Komponenten können Wissenschaftler jedoch die Entstehung biologischer Komplexität beobachten und zu verstehen versuchen. Membranen spielen eine große Rolle in vielen zellulären Prozessen und tragen wesentlich zu dieser Komplexität bei. Sie können allerdings schwer in Bottom-up- Rekonstitutionsansätze zu integrieren sein, was die Möglichkeiten einschränken kann, einen vollständigen Blick auf solche Prozesse durch in vitro-Experimenten zu erhalten. In dieser Arbeit stelle ich eine Reihe von Projekten vor, die riesige unilamellare Vesikel (GUVs) als Nachahmung der kompartimentierenden Membranen von Zellen verwenden, um dabei grundlegende biologische Funktionen zu untersuchen. Dafür habe ich eine Vielzahl von biologischen Komponenten in GUVs eingekapselt, um biomimetisches Verhalten und Funktion zu erzielen. Dabei habe ich jeweils die einzigartigen Eigenschaften der GUVs genutzt, um Verhaltensweisen zu erreichen oder Beobachtungen zu machen, die mit anderen Ansätzen, einschließlich alternativer Modellmembransystemen, nicht möglich gewesen wären. Insgesamt stellt meine Arbeit einen Schritt vorwärts in Richtung der Rekonstruktion komplexer und dynamischer zellulärer Prozesse unter zellähnlichen Bedingungen dar

ELECTRIC FIELD INDUCED DROPLET MANIPULATION

In this thesis, we explore several droplet manipulation concepts on different length scales for a surface cleaning application. The design evolution to transfer these techniques from laboratory conditions to a chaotic environment, such as on the road, is an evolving engineering challenge where reliability and performance are equally important. Electrowetting and liquid dielectrophoresis are techniques by which an electromechanical response from an applied electric field enables precise droplet manipulation. This thesis presents several contributions to these technologies, focusing primarily on scalability, simplicity, and reliability. The control of surface wettability using the electric methods attracts much attention due to their fast response (milliseconds), exceptional durability (hundreds of thousands of switching cycles) and low energy consumption (hundreds of microwatts). Furthermore, their superior performance and reliable nature have prompted a vast amount of literature to expand their application. They are widely used in several scientific and industrial fields, including microfluidics, optical devices, inject printing, energy harvesting, display technologies, and microfabrication. Droplet actuation using electric methods has been a long-standing interest in microfluidics, and most often, it is limited by high operating voltages. The first actuation method explored in this thesis is based on interdigitated electrodes to generate a dielectrophoretic response. In order to apply an effective electrostatic force for droplet manipulation, the geometry of the electrodes must be optimised, which similarly leads to a lower operating voltage (as low as 30 V). Furthermore, microscale electrodes can be iteratively combined to realise larger arrays to move larger droplets. The iterative approach was developed for a large-scale device to manipulate droplets of varying sizes while keeping the actuation process simple. In the second actuation method, a pair of microelectrodes separated by a variable gap distance generated an electrostatic gradient to produce a continuous droplet motion along the length of the electrode pad. The novel actuation method transported droplets of different sizes without active control. The droplet actuation was demonstrated on a larger scale using several platforms, including radial-symmetric, linear, and bilateral-symmetric droplet motion. An automated self-cleaning platform was tested in laboratory conditions and on the road. The technology has significant potential in the automotive sector to clean body parts, camera covers, and scanning sensors. The electrostatic force applied across the droplet was calculated by placing a continuously moving droplet on a tilted platform and measuring the critical angle at which the droplet’s gravity overcomes the opposing applied electric force. Several electrode designs were also considered to evaluate the effect of electrode geometry on the actuation force. The droplet actuation was also modelled using an analytical approach to estimate the critical signal frequency, maximum electrostatic energy, and maximum electrostatic force. Lastly, a tilting micromirror platform investigated the dielectrophoretic response without measuring the droplet contact angle. The mirror platform is also suitable for other optical applications as it provides three axes of movement for beam steering. The tilting platform enabled an angular coverage of up to 0.9° (± 0.02°), with a maximum displacement of 120 μm. We also explored the feasibility of using a microhydraulic actuator based on liquid dielectrophoresis for a microfluidic application. The actuation method opens new possibilities for positioning and manipulating particles and components. These could be hazardous medical materials or even radioactive substances, where direct contact should be avoided

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

Une membrane souple à base de polypyrrole renforcée et son utilisation pour délivrer des stimulations électriques aux kératinocytes de peau humaine

La stimulation électrique (SE) semble favoriser la cicatrisation des plaies par ses effets sur les fibroblastes. Cependant, son interaction avec les kératinocytes n'a pas été bien établie. Le polypyrrole (PPy) en tant que biomatériau conducteur est un excellent candidat pour délivrer les SE aux cellules, ce qui devient plus évident avec le développement de la nouvelle membrane souple à base de PPy. Cependant, les faibles propriétés mécaniques limitent l'utilisation de cette membrane. La présente étude visait à améliorer la résistance mécanique de la membrane à base de PPy et étudier les comportements cellulaires et moléculaires des kératinocytes après exposition à des SE via cette nouvelle membrane PPy. Premièrement, la membrane souple à base de PPy a été renforcée par électrofilage, de manière synergique, avec des fibres de polyuréthane (PU) et de polylactide (PLLA). Des tests mécaniques ont confirmé que la résistance à la traction de la membrane a été considérablement augmentée. Ensuite, les kératinocytes ont été cultivés sur la membrane PPy renforcée, puis stimulés par des intensités électriques de 100 ou 200 mV mm⁻¹ pendant 6 ou 24 heures. Les cellules stimulées présentaient une capacité proliférative considérablement accrue. Les sécrétions d'IL-6, IL-1α, IL-8, GROα, FGF2 et VEGF-A ont également augmenté. Fait intéressant, l'SE de 24 heures a induit une « mémoire de stimulation » car les cellules stimulées ont montré une augmentation significative de formation de colonies (CFE) après 6 jours après l'exposition à la stimulation électrique. De plus, l'expression des kératines 5, 14 et 10/13 était significativement augmentée par la SE. La SE a augmenté l'expression de la phosphorylation des kinases ERK1/2. L'expression des protéines des kératinocytes de la peau humaine peut être activée par des stimulations électriques appropriées pour favoriser la cicatrisation des plaies cutanées. La membrane PPy souple renforcée peut servir de pansement conducteur pour faciliter l'exposition de la plaie à une stimulation électrique pour favoriser sa cicatrisation.Keratinocytes as the principal skin cell type play a major role in wound closure. In the meantime, electrical stimulation (ES) has been found effective in promoting wound healing. However, the role of ES on keratinocytes has not been well established. Polypyrrole (PPy), especially the recently developed soft PPy membrane, is an electrically conductive biomaterial and a good candidate to deliver ES to cells. However, the weak mechanical strength of the soft PPy membrane has limited its practical use. The present work was to enhance the mechanical strength of this soft PPy membrane and to investigate the cellular and molecular behaviors of the keratinocytes underwent ES via this novel PPy membrane. Firstly, the soft PPy membrane was synergically reinforced with polyurethane (PU) and poly (L-lactic acid) (PLLA) fibers through electrospinning technology. Mechanical tests confirmed the significantly increased tensile strength, which rendered the originally fragile PPy membrane strong enough to stand ordinary manipulations without compromising its electrical properties. Afterwards, HaCaT keratinocytes were cultured on the PU/PLLA reinforced PPy membranes under electrical intensities of 100 and 200 mV mm⁻¹ for 6 or 24 hr. The electrically stimulated cells exhibited a considerably increased proliferative ability. Meanwhile, secretions of the IL-6, IL-1α, IL-8, GROα, FGF2 and VEGF-A increased as well. Interestingly, the 24 hr ES induced a "stimulus memory" by showing a significant rise in colony forming efficiency (CFE) 6 days post-ES. Additionally, the expressions of keratin 5, keratin 14, keratin 10 and keratin 13 were significantly modulated by ES. Finally, the phosphorylation of ERK1/2 kinases was regulated by ES. The overall results demonstrated that the proliferation, differentiation, and protein expression of human skin keratinocytes can be activated through appropriate ES to benefit skin wound healing. Moreover, the PU/PLLA reinforced soft PPy membrane may server as a conductive wound dressing to facilitate ES to wound

Development of an integrated opto-electric biosensor to dynamically examine cytometric proliferation and cytotoxicity

My doctoral research has focused on the development of microscale optical techniques for examining micro/bio fluidics. Preliminary work measured the velocity field in a microchannel, by optical slicing, using Confocal Laser Scanning Microscopy (CLSM). Next, Optical Serial Sectioning Microscopy (OSSM) was applied to examine thermometry by detecting the free Brownian motion of nano-particles suspended in mediums at different temperatures. An extension of this work used objective-based Total Internal Reflection Fluorescence Microscopy (TIRFM) to examine the hindered Brownian motion of nano-particles that were very close to a solid surface (within 1 mm). An optically transparent and electrically conductive Indium Tin Oxide (ITO) biosensor and an integrated dynamic live cell imaging system were developed to dynamically examine changes in cell coverage area, cell morphology, cell-substrate adhesion, and cell-cell interaction. To our knowledge this is the first sensor capable of conducting simultaneous optical and electrical measurements. This system consists of an incubator, which keeps cells viable by providing the necessary environmental conditions (37 °C temperature and 5 % CO2), and multiple microscopy techniques, including multispectrum Interference Reflection Microscopy (MS-IRM), TIRFM, Epi-fluorescence Microscopy, Phase Contrast Microscopy (PCM), and Differential Interference Contrast Microscopy (DICM). Along with investigations of cytometric proliferation including cellular barrier functions, in vitro cytotoxicity experiments were also conducted to examine the effect of a drug (cytochalasin D, a toxic agent) on cellular motility and cellular morphology. These cytotoxicity results give us a fundamental understanding of the cellular processes induced by the drug, which will be invaluable in the search for methods of preventing metastases. In this research, MS-IRM is used to examine the focal contacts and the gap morphology between cells and substrates, DICM is used to examine the coverage area of cells, and impedance measurements are used to correlate these two parameters. Advances in the understanding of vascular bio-transport in endothelial cells will have an impact on many aspects of cell biology, tissue engineering, and pharmacology. Particularly important will be the ability to test the popular hypothesis that the cell barrier function is regulated by specific cytoskeleton elements controlling intercellular and extracellular coupling