588 research outputs found

    Development of Material Characterization Techniques using Novel Nanoindentation Approaches on Hard and Soft Materials used in MEMS

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    Investigating and modeling the mechanical properties of materials is important for many applications. The most common technique used for mechanical characterization of materials is called nanoindentation. The currently available tools utilized in order to perform nanoindentation have their limitations in terms of sensitivities in force and displacement for a broad range of material properties. When it comes to investigation of soft materials, these limitations might be more detrimental. In this dissertation work, novel nanoindentation techniques have been developed with a multi-probe scanning force microscopy (SPM) system in order to ease the major problems encountered with standard Atomic Force Microscopy (AFM) or nanoindentation systems. Tuning forks are used as probes during nanoindentation. By using the newly developed nanoindentation techniques for quasi-static nanoindentation experiments, the force information is extracted through the displacement of the indenter probe measured by a second probe with ultraresolution. For dynamic nanoindentation, frequency modulation techniques have been used to extract force information from a single indenter tuningfork probe. Thanks to the high quality of resonance (Q factor) of tuning fork probes, force measurements can be performed with an ultra high resolution. The accurate measurements of material properties on soft materials is used in characterization of microfabricated pillar sensors which can be used in measuring nN level of cell traction forces in a biomedical application. The techniques developed in this research also enable the system as an ultra-sensitive force sensor to apply nN scale lateral and vertical loads on microfabricated structures or biological specimens

    The development of micropillars and two-dimensional nanocavities that incorporate an organic semiconductor thin film

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    Photonic crystals (PC) are periodic optical structures containing low and high refractive index layers that influence the propagation of electromagnetic waves. Photonic cavities can be created by inserting defects into a photonic crystal. Such structures have received significant attention due to their potential of confining light inside volumes (V) smaller than a cubic wavelength of light (ฮป/n)3 which can be used to enhance light-matter interaction. Cavity quality factor (Q) is useful for many applications that depend on the control of spontaneous emission from an emitter such quantum optical communication and low-threshold lasing. High Q/V values can also result in an enhancement of the radiative rates of an emitter placed on the surface of the cavity by means of the Purcell effect. This thesis concerns the fabrication and study of two types of optical cavity containing an organic-semiconductor material. The cavities explored are; (1) one-dimensional micropillar microcavities based on multilayer films of dielectric and organic materials, and (2) two-dimensional nanocavities defined into a photonic crystal slab. Firstly, light emission from a series of optical micropillar microcavities containing a thin fluorescent, red-emitting conjugated polymer film is investigated. The photoluminescence emission from the cavities is characterized using a Fourier imaging technique and it is shown that emission is quantised into a mode-structure resulting from both vertical and lateral optical confinement within the pillar. We show that optical-confinement effects result in a blue-shift of the fundamental mode as the pillar-diameter is reduced, with a model applied to describe the energy and distribution of the confined optical modes. Secondly, simulation, design, and analysis of two dimensional photonic crystal L3 nanocavities photonic crystal are presented. Nanocavities were then prepared from silicon nitride (SiN) as the cavity medium with the luminescence emitted from an organic material at red wavelengths that was coated on the cavity surface. To improve the quality factor of such structures, hole size, lattice constant and hole shift are systematically varied with their effect as cavity properties determined. Finite Difference Time Domain (FDTD) modelling is used to support the experimental work and predict the optimum design for such photonic crystal nanocavity devices. It is found that by fine-tuning the nearest neighbour air-holes close to the cavity edges, the cavity Q factor can be increased. As a result, we have obtained a single cavity mode having a Q-factor 938 at a wavelength of 652 nm. Here, the cavity Q factor then increases to 1100 at a wavelength of 687 nm as a result of coating a red-emitting conjugated polymer film onto the top surface of the nanocavity. We propose that this layer planarizes the dielectric surface and helps reduce optical losses as a result of scattering

    Fabrication and Actuation of Hierarchically-Patterned Polymer Substrates for Dynamic Surface and Optical Properties

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    Switchable optical materials, which possess reversible color and transparency change in response to external stimuli, are of wide interest for potential applications such as windows and skylights in architectural and vehicular settings or optical sensors for environmental monitoring. This thesis considers the tuning of optical properties by tailoring and actuating responsive materials. Specifically, we demonstrate the design and fabrication of tilted pillar arrays on wrinkled elastomeric polydimethylsiloxane (PDMS) as a reversibly switchable optical window. While the original PDMS film exhibits angle-dependent colorful reflection due to Bragg diffraction of light from the periodic pillar array, the tilted pillar film appears opaque due to random scattering. Upon re-stretching the film to the original pre-strain, the grating color is restored due to the straightened pillars and transmittance is recovered. Then, we develop a composite film, consisting of a thin layer of quasi-amorphous array of silica nanoparticles (NPs) embedded in bulk elastomeric PDMS, with initial high transparency and angle-independent coloring upon mechanical stretching. The color can be tuned by the silica NP size. The switch between transparency and colored states could be reversibly cycled at least 1000 times without losing the filmโ€™s structural and optical integrity. We then consider the micropatterning of nematic liquid crystal elastomers (NLCEs) as micro-actuator materials. Planar surface anchoring of liquid crystal (LC) monomers is achieved with a poly(2-hydroxyethyl methacrylate)-coated PDMS mold, leading to monodomains of vertically aligned LC monomers within the mold. After cross-linking, the resulting NLCE micropillars show a relatively large radial strain when heated above nematic to isotropic transition temperature, which can be recovered upon cooling. Finally, the understanding of liquid crystal surface anchoring under confined boundary conditions is applied to the self-assembly of gold nanorods (AuNRs) driven by LC defect structures and to dynamically tune the surface plasmon resonance (SPR) properties. By exploiting the confinement of the smectic liquid crystal, 4-octyl-4โ€™-cyanobiphenyl (8CB), to patterned pillars treated with homeotropic surface anchoring, topological defects are formed at precise locations around each pillar and can be tuned by varying the aspect ratio of the pillars and the temperature of the system. As a result, the AuNR assemblies and SPR properties can be altered reversibly by heating and cooling between smectic, nematic and isotropic phases

    ์›จ์–ด๋Ÿฌ๋ธ” ์„ผ์„œ ๋ฐ ์—๋„ˆ์ง€ ์†Œ์ž์˜ ๊ณต๊ฐ„ ์‹ ํ˜ธ ๋ฐ ์—ด ์ „๋‹ฌ ์ฆ์ง„์„ ์œ„ํ•œ ๋‚˜๋…ธ๋ณตํ•ฉ์ฒด๋ฅผ ์ด์šฉํ•œ ๊ธฐ๊ณ„์  ์ˆœ์‘์„ฑ ํ–ฅ์ƒ

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    ํ•™์œ„๋…ผ๋ฌธ (๋ฐ•์‚ฌ) -- ์„œ์šธ๋Œ€ํ•™๊ต ๋Œ€ํ•™์› : ๊ณต๊ณผ๋Œ€ํ•™ ์ „๊ธฐยท์ •๋ณด๊ณตํ•™๋ถ€, 2020. 8. ํ™์šฉํƒ.Electronic skin (e-skin) that mimics mechanical and functional properties of human skin has a strong impact on the field of wearable electronics. Beyond being just wearable, e-skin seamlessly interfaces human, machine, and environment by perfectly adhering to soft and time-dynamic three-dimensional (3D) geometries of human skin and organs. Real-time and intimate access to the sources of physical and biological signals can be achieved by adopting soft or flexible electronic sensors that can detect pressure, strain, temperature, and chemical substances. Such extensions in accessible signals drastically accelerate the growth of the Internet of Things (IoT) and expand its application to health monitoring, medical implants, and novel human-machine interfaces. In wearable sensors and energy devices, which are essential building blocks for skin-like functionalities and self-power generation in e-skin, spatial signals and heat are transferred from time-dynamic 3D environments through numerous geometries and electrical devices. Therefore, the transfer of high-fidelity signals or a large amount of heat is of great importance in these devices. The mechanical conformability potentially enhances the signal/heat transfer by providing conformal geometries with the 3D sources. However, while the relation between system conformability and electrical signals has been widely investigated, studies on its effect on the transfer of other mechanical signals and heat remain in their early stages. Furthermore, because active materials and their designs for sensors and energy devices have been optimized to maximize their performances, it is challenging to develop ultrathin or soft forms of active layers without compromising their performances. Therefore, many devices in these fields suffer from poor spatial signal/heat transfer due to limited conformability. In this dissertation, to ultimately augment the functionalities of wearable sensors and energy devices, comprehensive studies on conformability enhancement via composite materials and its effect on signal/heat transfer, especially in pressure sensors and thermoelectric generators (TEGs), are conducted. A solution for each device is carefully optimized to reinforce its conformability, taking account of the structure, characteristics, and potential advantages of the device. As a result, the mechanical conformability of each device is significantly enhanced, improving signal/heat transfer and consequently augmenting its functionalities, which have been considered as tough challenges in each area. The effect of the superior conformability on signal/heat transfer is systematically analyzed via a series of experiments and finite element analyses. Demonstrations of practical wearable electronics show the feasibility of the proposed strategies. For wearable pressure sensors, ultrathin piezoresistive layers are developed using cellulose/nanowire nanocomposites (CNNs). The unique nanostructured surface enables unprecedentedly high sensor performances such as ultrahigh sensitivity, wide working range, and fast response time without microstructures in sensing layers. Because the ultrathin pressure sensor perfectly conforms to 3D contact objects, it transfers pressure distribution into conductivity distribution with high spatial fidelity. When integrated with a quantum dot-based electroluminescent film, the transferred high-resolution pressure distribution is directly visualized without the need for pixel structures. The electroluminescent skin enables real-time smart touch interfaces that can identify the user as well as touch force and location. For high-performance wearable TEGs, an intrinsically soft heat transfer and electrical interconnection platform (SHEP) is developed. The SHEP comprises AgNW random networks for intrinsically stretchable electrodes and magnetically self-assembled metal particles for soft thermal conductors (STCs). The stretchable electrodes lower the flexural rigidity, and the STCs enhance the heat exchange capability of the soft platform, maintaining its softness. As a result, a compliant TEG with SHEPs forms unprecedentedly conformal contact with 3D heat sources, thereby enhancing the heat transfer to the TE legs. This results in significant improvement in thermal energy harvesting on 3D surfaces. Self-powered wearable warning systems indicating an abrupt temperature increase with light-emitting alarms are demonstrated to show the feasibility of this strategy. This study provides a systematic and comprehensive framework for enhancing mechanical conformability of e-skin and consequently improving the transfer of spatial signals and energy from time-dynamic and complex 3D surfaces. The framework can be universally applied to other fields in wearable electronics that require improvement in signal/energy transfer through conformal contact with 3D surfaces. The materials, manufacturing methods, and devices introduced in this dissertation will be actively exploited in practical and futuristic applications of wearable electronics such as skin-attachable advanced user interfaces, implantable bio-imaging systems, nervous systems in soft robotics, and self-powered artificial tactile systems.์ธ๊ฐ„ ํ”ผ๋ถ€์˜ ๊ธฐ๊ณ„์  ํŠน์„ฑ ๋ฐ ๊ธฐ๋Šฅ์„ ๋ชจ๋ฐฉํ•˜๋Š” ์ „์žํ”ผ๋ถ€(electronic skin, e-skin)๋Š” ์›จ์–ด๋Ÿฌ๋ธ” ์ „์ž๊ธฐ๊ธฐ ๋ถ„์•ผ์˜ ํŠธ๋ Œ๋“œ๋ฅผ ๋ฐ”๊พธ๊ณ  ์žˆ๋‹ค. ๊ธฐ์กด์˜ ์›จ์–ด๋Ÿฌ๋ธ” ์ „์ž๊ธฐ๊ธฐ๊ฐ€ ๋‹จ์ง€ ์ฐฉ์šฉํ•˜๋Š”๋ฐ ๊ทธ์ณค๋‹ค๋ฉด, ์ „์žํ”ผ๋ถ€๋Š” ์ธ๊ฐ„์˜ ํ”ผ๋ถ€์™€ ์žฅ๊ธฐ ํ‘œ๋ฉด์— ์™„๋ฒฝํ•˜๊ฒŒ ๋ถ™์–ด ๋™์ž‘ํ•จ์œผ๋กœ์จ ๊ธฐ์กด์—๋Š” ์ ‘๊ทผ ๋ถˆ๊ฐ€๋Šฅ ํ–ˆ๋˜ ๋‹ค์–‘ํ•œ ์ƒ์ฒด ์‹ ํ˜ธ๋ฅผ ๋†’์€ ์‹ ๋ขฐ๋„๋กœ ๊ฐ์ง€ํ•˜๊ณ  ์ฒ˜๋ฆฌํ•  ์ˆ˜ ์žˆ๋‹ค. ์‹ค์‹œ๊ฐ„์œผ๋กœ ๊ฐ์ง€ ๊ฐ€๋Šฅํ•œ ์ƒ์ฒด ์‹ ํ˜ธ์˜ ํ™•์žฅ์€ ์‚ฌ๋ฌผ์ธํ„ฐ๋„ท(Internet of Things, IoT)์˜ ์„ฑ์žฅ์„ ํš๊ธฐ์ ์œผ๋กœ ๊ฐ€์†ํ™”ํ•˜๊ณ  ํ—ฌ์Šค์ผ€์–ด, ์˜๋ฃŒ์šฉ ์ž„ํ”Œ๋ž€ํŠธ, ์†Œํ”„ํŠธ ๋กœ๋ด‡ ๋ฐ ์ƒˆ๋กœ์šด ํœด๋จผ ๋จธ์‹  ์ธํ„ฐํŽ˜์ด์Šค๋กœ์˜ ์‘์šฉ์„ ๊ฐ€๋Šฅํ•˜๊ฒŒ ํ•œ๋‹ค. ์ „์žํ”ผ๋ถ€์˜ ํ•„์ˆ˜์š”์†Œ์ธ ์„ผ์„œ์™€ ์—๋„ˆ์ง€ ์†Œ์ž์—์„œ๋Š” ์‚ผ์ฐจ์› ํ‘œ๋ฉด์˜ ๊ณต๊ฐ„์‹ ํ˜ธ์™€ ์—ด์—๋„ˆ์ง€๋ฅผ ์†์‹ค ์—†์ด ์ „๋‹ฌํ•˜๋Š” ๊ฒƒ์ด ๋งค์šฐ ์ค‘์š”ํ•˜๋‹ค. ์ด๋Ÿฌํ•œ ๊ณต๊ฐ„ ์‹ ํ˜ธ์™€ ์—ด์—๋„ˆ์ง€๋Š” ๋‹ค์–‘ํ•œ ๊ธฐํ•˜ ๊ตฌ์กฐ์™€ ์ „์ž์†Œ์ž๋ฅผ ๊ฑฐ์ณ ์ฒ˜๋ฆฌ ๊ฐ€๋Šฅํ•œ ์‹ ํ˜ธ๋กœ ์ „๋‹ฌ๋œ๋‹ค. ์ด ๊ณผ์ •์—์„œ 3์ฐจ์› ํ‘œ๋ฉด์— ๋นˆํ‹ˆ์—†์ด ๋ถ™๋Š” ๊ธฐ๊ณ„์  ์ˆœ์‘์„ฑ(mechanical conformability)์€ ๊ณต๊ฐ„์‹ ํ˜ธ์™€ ์—ด์—๋„ˆ์ง€๋ฅผ ์™œ๊ณก ์—†์ด ์ „๋‹ฌํ•˜๋Š” ๊ฒƒ์„ ๊ฐ€๋Šฅํ•˜๊ฒŒ ํ•œ๋‹ค. ์ „์žํ”ผ๋ถ€์˜ ๊ธฐ๊ณ„์  ์ˆœ์‘์„ฑ์„ ์ฆ๊ฐ€์‹œํ‚ค๋Š” ๋ฐฉ๋ฒ•์€ ํฌ๊ฒŒ ๋‹ค์Œ๊ณผ ๊ฐ™์ด ๋‘ ๊ฐ€์ง€๋กœ ๋‚˜๋ˆŒ ์ˆ˜ ์žˆ๋‹ค. (1) ์ „์žํ”ผ๋ถ€๋ฅผ ๋‘๊ป˜๋ฅผ ๋‚ฎ์ถ”๋Š” ์ „๋žต๊ณผ (2) ์ „์žํ”ผ๋ถ€์˜ ์˜๋ฅ (Youngs modulus)์„ ๋‚ฎ์ถ”์–ด ๊ณ ๋ฌด์™€ ๊ฐ™์ด ๋ถ€๋“œ๋Ÿฝ๊ฒŒ ๋งŒ๋“œ๋Š” ์ „๋žต์ด๋‹ค. ํ•˜์ง€๋งŒ, ๊ธฐ์กด ์„ผ์„œ ๋ฐ ์—๋„ˆ์ง€ ์†Œ์ž๋ฅผ ์œ„ํ•œ ์žฌ๋ฃŒ์™€ ๋””์ž์ธ์ด ๊ฐ ์žฅ์น˜์˜ ์„ฑ๋Šฅ์„ ํ–ฅ์ƒ์‹œํ‚ค๋Š” ๊ฒƒ์— ์ดˆ์ ์ด ๋งž์ถ”์–ด์ ธ ์žˆ๊ธฐ ๋•Œ๋ฌธ์—, ๊ณ ์„ฑ๋Šฅ์„ ์œ ์ง€ํ•˜๋ฉด์„œ ๋งค์šฐ ์–‡๊ฑฐ๋‚˜ ์—ฐ์งˆ ํ˜•ํƒœ์˜ ์†Œ์ž๋ฅผ ๊ฐœ๋ฐœํ•˜๋Š” ๊ฒƒ์€ ๋งค์šฐ ๋„์ „์ ์ด์—ˆ๋‹ค. ๋”ฐ๋ผ์„œ ๊ณ ์œ ์—ฐ์„ฑ์„ ํ™•๋ณดํ•˜์ง€ ๋ชปํ•œ ๊ธฐ์กด ์„ผ์„œ์™€ ์—๋„ˆ์ง€ ์†Œ์ž๋Š” ๊ณต๊ฐ„ ์‹ ํ˜ธ ๋ฐ ์—ด ์ „๋‹ฌ์ด ์‹ฌ๊ฐํ•˜๊ฒŒ ์ €ํ•ด๋˜๊ณ , ์ด๋กœ ์ธํ•ด ๊ณต๊ฐ„ ์••๋ ฅ์˜ ์™œ๊ณก, ์—ด์ „ ํšจ์œจ์˜ ์ €ํ•˜์™€ ๊ฐ™์€ ํ•œ๊ณ„๋ฅผ ๋ณด์—ฌ์ค€๋‹ค. ์ด ๋…ผ๋ฌธ์—์„œ๋Š” ์›จ์–ด๋Ÿฌ๋ธ” ์„ผ์„œ์™€ ์—๋„ˆ์ง€ ์†Œ์ž์˜ ๋น„์•ฝ์ ์ธ ๊ธฐ๋Šฅ ํ–ฅ์ƒ์„ ๊ถ๊ทน์ ์ธ ๋ชฉํ‘œ๋กœ, ๊ฐ ์†Œ์ž์— ์ตœ์ ํ™”๋œ ์žฌ๋ฃŒ์™€ ์ œ์ž‘๋ฐฉ์‹, ๊ตฌ์กฐ๋ฅผ ์ด์šฉํ•ด ์ด๋“ค์˜ ๊ธฐ๊ณ„์  ์ˆœ์‘์„ฑ์„ ํš๊ธฐ์ ์œผ๋กœ ๋†’์ด๊ณ , ์ด๋ฅผ ํ†ตํ•œ ๊ณต๊ฐ„ ์‹ ํ˜ธ ๋ฐ ์—ด ์ „๋‹ฌ์˜ ํ–ฅ์ƒ์„ ์‹ฌ๋„ ์žˆ๊ฒŒ ๋ถ„์„ํ•œ๋‹ค. ํŠนํžˆ, ๋‘๊ป˜๋ฅผ ๋‚ฎ์ถ”๊ฑฐ๋‚˜ ์˜๋ฅ ์„ ๋‚ฎ์ถ”๋Š” ๋‘ ๊ฐ€์ง€ ์ „๋žต ์ค‘ ๊ฐ ์†Œ์ž์— ๊ฐ€์žฅ ์ ํ•ฉํ•œ ์ „๋žต์„ ์„ ํƒํ•˜๊ณ , ์ฒด๊ณ„์ ์ธ ๋ฐฉ๋ฒ•๋ก ์„ ์ ์šฉํ•˜์—ฌ ์ด๋“ค์˜ ๊ธฐ๊ณ„์  ์ˆœ์‘์„ฑ๊ณผ ๊ณต๊ฐ„ ์‹ ํ˜ธ ๋ฐ ์—ด ์ „๋‹ฌ์„ ์ฆ์ง„์‹œํ‚จ๋‹ค. ์ด ๊ณผ์ •์—์„œ ๋‚˜๋…ธ์œต๋ณตํ•ฉ์žฌ๋ฃŒ๊ฐ€ ๊ฐ ์ „๋žต์„ ๊ตฌํ˜„ํ•˜๋Š” ํ•ต์‹ฌ ์š”์†Œ๋กœ ์ž‘์šฉํ•œ๋‹ค. ๊ฐ ์†Œ์ž์— ๋”ฐ๋ฅธ ๊ตฌ์ฒด์ ์ธ ์—ฐ๊ตฌ ๋‚ด์šฉ์€ ๋‹ค์Œ๊ณผ ๊ฐ™๋‹ค. ์ฒซ์งธ, ์••๋ ฅ ์„ผ์„œ์˜ ๊ฒฝ์šฐ ์ดˆ๋ฐ•๋ง‰ ์…€๋ฃฐ๋กœ์˜ค์Šค/๋‚˜๋…ธ์™€์ด์–ด ๋ณตํ•ฉ์ฒด๋ฅผ ์ด์šฉํ•˜์—ฌ ๊ณ ์„ฑ๋Šฅ์˜ ์ €ํ•ญ๋ฐฉ์‹ ์••๋ ฅ ์„ผ์„œ๋ฅผ ๊ฐœ๋ฐœํ•œ๋‹ค. ์ด๋Ÿฌํ•œ ๋ณตํ•ฉ์ฒด๋Š” ํ‘œ๋ฉด์— ํ˜•์„ฑ๋œ ๊ณ ์œ ํ•œ ๋‚˜๋…ธ๊ตฌ์กฐ ๋•๋ถ„์— ๋งˆ์ดํฌ๋กœ๊ตฌ์กฐ์ฒด๋ฅผ ์ด์šฉํ•œ ๊ธฐ์กด ์••๋ ฅ ์„ผ์„œ๋ณด๋‹ค ์›”๋“ฑํ•œ ์„ฑ๋Šฅ์„ ๋ณด์—ฌ์ค€๋‹ค. ํŠนํžˆ, 1 ๋งˆ์ดํฌ๋กœ ๋ฏธํ„ฐ ์ˆ˜์ค€์˜ ๋งค์šฐ ์–‡์€ ๋‘๊ป˜๋กœ ์ธํ•ด ์ ‘์ด‰ ๋ฌผ์ฒด์˜ ๋ณต์žกํ•œ ํ˜•์ƒ์— ์™„๋ฒฝํ•˜๊ฒŒ ์ˆœ์‘ํ•  ์ˆ˜ ์žˆ๊ณ , ์ด๋กœ ์ธํ•ด ๊ณ ํ•ด์ƒ๋„ ์••๋ ฅ ๋ถ„ํฌ๋ฅผ ์™œ๊ณก ์—†์ด ์ €ํ•ญ ๋ถ„ํฌ๋กœ ์ „๋‹ฌํ•  ์ˆ˜ ์žˆ๋‹ค. ์ด๋Ÿฌํ•œ ์••๋ ฅ ์„ผ์„œ๋ฅผ ์–‘์ž ์  ๋ฐœ๊ด‘์†Œ์ž์™€ ๊ฒฐํ•ฉํ•˜์—ฌ ๊ณ ํ•ด์ƒ๋„์˜ ์••๋ ฅ๋ถ„ํฌ๋ฅผ ๋†’์€ ์ •๋ฐ€๋„๋กœ ์ด๋ฏธ์ง• ๊ฐ€๋Šฅํ•œ ๋ฐœ๊ด‘ ์†Œ์ž๋ฅผ ๋ณด๊ณ ํ•œ๋‹ค. ๋‘˜์งธ, ์—ด์ „ ์†Œ์ž์˜ ๊ฒฝ์šฐ ๊ธฐ์กด์˜ ๊ธˆ์† ์ „๊ทน์œผ๋กœ ์ธํ•œ ๋‚ฎ์€ ์œ ์—ฐ์„ฑ๊ณผ ํƒ„์„ฑ์ค‘ํ•ฉ์ฒด์˜ ๋‚ฎ์€ ์—ด ์ „๋„๋„๋ฅผ ๊ทน๋ณตํ•˜๊ธฐ ์œ„ํ•ด ์—ด ์ „๋‹ฌ ๋Šฅ๋ ฅ์ด ํš๊ธฐ์ ์œผ๋กœ ํ–ฅ์ƒ๋œ ๋‚ฎ์€ ์˜๋ฅ ์˜ ์†Œํ”„ํŠธ ์ „๊ทน ํ”Œ๋žซํผ์„ ๊ฐœ๋ฐœํ•œ๋‹ค. ์†Œํ”„ํŠธ ํ”Œ๋žซํผ์€ ๋‚ด๋ถ€์— ์€ ๋‚˜๋…ธ์™€์ด์–ด ๊ธฐ๋ฐ˜์˜ ์‹ ์ถ•์„ฑ ์ „๊ทน์„ ๊ฐ–๊ณ  ์žˆ์œผ๋ฉฐ, ์ž๊ธฐ์žฅ์„ ํ†ตํ•ด ์ž๊ฐ€ ์ •๋ ฌ๋œ ๊ธˆ์† ์ž…์ž๋“ค์ด ํšจ๊ณผ์ ์œผ๋กœ ์™ธ๋ถ€ ์—ด์„ ์—ด์ „ ์žฌ๋ฃŒ์— ์ „๋‹ฌํ•œ๋‹ค. ์ด๋ฅผ ๊ธฐ๋ฐ˜์œผ๋กœ ์ œ์ž‘๋œ ๊ณ ์œ ์—ฐ์„ฑ ์—ด์ „ ์†Œ์ž๋Š” ์‚ผ์ฐจ์› ์—ด์›์— ๋นˆํ‹ˆ์—†์ด ๋ถ™์–ด ์—ด ์†์‹ค์„ ์ตœ์†Œํ™” ํ•˜๋ฉฐ, ์ด๋กœ ์ธํ•ด ๋†’์€ ์—ด์ „ ํšจ์œจ์„ ๋‹ฌ์„ฑํ•œ๋‹ค. ์ด ๋…ผ๋ฌธ์€ ๋‹ค์–‘ํ•œ ์ „์ž์†Œ์ž์˜ ์œ ์—ฐ์„ฑ์„ ์ฆ์ง„์‹œํ‚ค๊ณ  ์ด๋ฅผ ํ†ตํ•œ ๊ณต๊ฐ„ ์‹ ํ˜ธ ๋ฐ ์—ด ์ „๋‹ฌ์˜ ํ–ฅ์ƒ์„ ๋„๋ชจํ•˜๊ณ  ๋ถ„์„ํ•˜๋Š” ์ฒด๊ณ„์ ์ด๊ณ  ์ข…ํ•ฉ์ ์ธ ๋ฐฉ๋ฒ•๋ก ์„ ์ œ์‹œํ–ˆ๋‹ค๋Š” ๋ฐ ํฐ ์˜์˜๊ฐ€ ์žˆ๋‹ค. ์ œ์•ˆ๋œ ๋ฐฉ๋ฒ•๋ก ์€ ๋ถ„์•ผ์— ๊ตญํ•œ๋˜์ง€ ์•Š๊ณ  ๋‹ค์–‘ํ•œ ์†Œ์ž์˜ ๊ฐœ๋ฐœ์— ์ ์šฉํ•  ์ˆ˜ ์žˆ์–ด ์›จ์–ด๋Ÿฌ๋ธ” ๊ธฐ๊ธฐ์™€ ์ „์žํ”ผ๋ถ€ ๋ถ„์•ผ์˜ ๊ธฐ๊ณ„์ , ๊ธฐ๋Šฅ์  ๋ฐœ์ „์— ํฌ๊ฒŒ ๊ธฐ์—ฌํ•  ๊ฒƒ์œผ๋กœ ๊ธฐ๋Œ€๋œ๋‹ค. ๋ฟ๋งŒ ์•„๋‹ˆ๋ผ ์ด ์—ฐ๊ตฌ์—์„œ ์ตœ์ดˆ๋กœ ๊ฐœ๋ฐœํ•œ ์†Œ์žฌ ๋ฐ ์†Œ์ž๋“ค์€ ๋‹ค์–‘ํ•œ ์›จ์–ด๋Ÿฌ๋ธ” ์–ดํ”Œ๋ฆฌ์ผ€์ด์…˜๊ณผ ์‚ฐ์—…์— ๊ณง๋ฐ”๋กœ ์œตํ•ฉ๋˜๊ณ  ์‘์šฉ๋  ์ˆ˜ ์žˆ๋‹ค. ์ด๋ฅผ ํ†ตํ•ด ์‹ ์ฒด ๋ถ€์ฐฉ ๋ฐ ์‚ฝ์ž… ๊ฐ€๋Šฅํ•œ ์ƒ์ฒด ์ด๋ฏธ์ง• ์‹œ์Šคํ…œ, ์†Œํ”„ํŠธ ๋กœ๋ด‡์„ ์œ„ํ•œ ์‹ ๊ฒฝ ์ฒด๊ณ„, ์ž๊ฐ€ ๋ฐœ์ „์ด ๊ฐ€๋Šฅํ•œ ์ธ๊ณต ๊ฐ๊ฐ ๊ธฐ๊ด€, ๊ฐ€์ƒ ๋ฐ ์ฆ๊ฐ• ํ˜„์‹ค์„ ์œ„ํ•œ ์ƒˆ๋กœ์šด ์œ ์ € ์ธํ„ฐํŽ˜์ด์Šค์™€ ๊ฐ™์€ ๋ฏธ๋ž˜ ์ง€ํ–ฅ์  ์œตํ•ฉ ์–ดํ”Œ๋ฆฌ์ผ€์ด์…˜์˜ ์‹คํ˜„์„ ์•ž๋‹น๊ธธ ๊ฒƒ์œผ๋กœ ๊ธฐ๋Œ€๋œ๋‹ค.Chapter 1. Introduction 1 1.1 Wearable Electronics and Electronic Skin 1 1.2 Mechanical Conformability of Electronic Skin 6 1.2.1 Definition and Advantages 6 1.2.2 Thickness-Based Conformability 11 1.2.3 Softness-Based Conformability 15 1.3 Conformability for Enhanced Signal/Heat Transfer in Wearable Sensors and Energy Devices 19 1.3.1 Conformability for Spatial Signal Transfer in Pressure Sensors 20 1.3.2 Conformability for Heat Transfer in Thermoelectric Generators 22 1.4 Motivation and Organization of This Dissertation 24 Chapter 2. Ultrathin Cellulose Nanocomposites for High-Performance Piezoresistive Pressure Sensors 28 2.1 Introduction 28 2.2 Experimental Section 31 2.2.1 Fabrication of the CNNs and Pressure Sensors 31 2.2.2 Measurements 34 2.3 Results and Discussion 38 2.3.1 Morphology of CNNs 38 2.3.2 Piezoresistive Characteristics of CNNs 41 2.3.3 Mechanism of High Sensitivity and Great Linearity 45 2.3.4 Fast Response Time of CNN-Based Pressure Sensors 49 2.3.5 Cyclic Reliability of CNN-Based Pressure Sensors 53 2.3.6 Mechanical Reliability and Conformability 57 2.3.7 Temperature and Humidity Tolerance 63 2.4 Conclusion 66 Chapter 3. Ultraflexible Electroluminescent Skin for High-Resolution Imaging of Pressure Distribution 67 3.1 Introduction 67 3.2 Main Concept 70 3.3 Experimental Section 72 3.3.1 Fabrication of Pressure-Sensitive Photonic Skin 72 3.3.2 Characterization of Photonic Skin 74 3.4 Results and Discussion 76 3.4.1 Structure and Morphology of Photonic Skin 76 3.4.2 Pressure Response of Photonic Skin 79 3.4.3 Effect of Conformability on Spatial Resolution 85 3.4.4 Demonstration of High-Resolution Pressure Imaging 99 3.4.5 Pressure Data Acquisition 104 3.4.6 Application to Smart Touch Interfaces 106 3.5 Conclusion 109 Chapter 4. Intrinsically Soft Heat Transfer and Electrical Interconnection Platforms Using Magnetic Nanocomposites 110 4.1 Introduction 110 4.2 Experimental Section 115 4.2.1 Fabrication of SHEPs 115 4.2.2 Measurements 117 4.3 Results and Discussion 119 4.3.1 Fabrication Scheme and Morphology of SHEPs 119 4.3.2 Calculation of Particle Concentration in STCs 124 4.3.3 Enhancement of Heat Transfer Ability via Magnetic Self-Assembly 127 4.3.4 Softness of STCs 131 4.3.5 Mechanical Reliability of Stretchable Electrodes 133 4.3.6 Optimization of Magnetic Self-Assembly Process 135 4.4 Conclusion 139 Chapter 5. Highly Conformable Thermoelectric Generators with Enhanced Heat Transfer Ability 140 5.1 Introduction 140 5.2 Experimental Section 142 5.2.1 Fabrication of Compliant TEGs 142 5.2.2 Measurements 144 5.2.3 Finite Element Analysis 147 5.3 Results and Discussion 149 5.3.1 Enhancement of TE Performance via STCs 149 5.3.2 Mechanical Reliability of Compliant TEGs 157 5.3.3 Enhanced TE Performance on 3D Surfaces via Conformability 162 5.3.4 Self-Powered Wearable Applications 167 5.4 Conclusion 171 Chapter 6. Summary, Limitations, and Recommendations for Future Researches 172 6.1 Summary and Conclusion 172 6.2 Limitations and Recommendations 176 6.2.1 Pressure Sensors and Photonic Skin 176 6.2.2 Compliant TEGs 177 Bibliography 178 Publication List 186 Abstract in Korean 192Docto

    TRIBOELECTRIC DEVICES FOR POWER GENERATION AND SELF-POWERED SENSING APPLICATIONS

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    Ph.DDOCTOR OF PHILOSOPH

    Bioinspired PDMS-graphene cantilever flow sensors using 3D printing and replica moulding

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    Flow sensors found in animals often feature soft and slender structures (e.g. fish neuromasts, insect hairs, mammalian stereociliary bundles, etc.) that bend in response to the slightest flow disturbances in their surroundings and heighten the animal's vigilance with respect to prey and/or predators. However, fabrication of bioinspired flow sensors that mimic the material properties (e.g. low elastic modulus) and geometries (e.g. high-aspect ratio structures) of their biological counterparts remains a challenge. In this work, we develop a facile and low-cost method of fabricating high-aspect ratio (HAR) cantilever flow sensors inspired by the mechanotransductory flow sensing principles found in nature. The proposed workflow entails high-resolution 3D printing to fabricate the master mould, replica moulding to create HAR polydimethylsiloxane (PDMS) cantilevers (thickness = 0.5 โ€“ 1 mm, width = 3 mm, aspect ratio = 20) with microfluidic channel (150 ยตm wide ร— 90 ยตm deep) imprints, and finally graphene nanoplatelet ink drop-casting into the microfluidic channels to create a piezoresistive strain gauge near the cantilever's fixed end. The piezoresistive flow sensors were tested in controlled airflow (0 โ€“ 9 m/s) inside a wind tunnel where they displayed high sensitivities of up to 5.8 kฮฉ/ms-1, low hysteresis (11% of full-scale deflection), and good repeatability. The sensor output showed a second order dependence on airflow velocity and agreed well with analytical and finite element model predictions. Further, the sensor was also excited inside a water tank using an oscillating dipole where it was able to sense oscillatory flow velocities as low as 16 โ€“ 30 ยตm/s at an excitation frequency of 15 Hz. The methods presented in this work can enable facile and rapid prototyping of flexible HAR structures that can find applications as functional biomimetic flow sensors and/or physical models which can be used to explain biological phenomena

    Sensitivity of cavity optomechanical field sensors

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    This article presents a technique for modeling cavity optomechanical field sensors. A magnetic or electric field induces a spatially varying strain across the sensor. The effect of this strain is accounted for by separating the mechanical motion of the sensor into eigenmodes, each modeled by a simple harmonic oscillator. The force induced on each oscillator can then be determined from an overlap integral between strain and the corresponding eigenmode, with the optomechanical coupling strength determining the ultimate resolution with which this force can be detected

    Capillary Force in High Aspect-Ratio Micropillar Arrays

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    High aspect-ratio (HAR) micropillar arrays are important for many applications including, mechanical sensors and actuators, tunable wetting surfaces and substrates for living cell studies. However, due to their mechanical compliance and large surface area, the micropillars are susceptible to deformation due to surface forces, such as adhesive force and capillary force. In this thesis we have explored the capillary force driven mechanical instability of HAR micropillar arrays. We have shown that when a liquid is evaporated off the micropillar arrays, the pillars bend and cluster together due to a much smaller capillary meniscus interaction force while still surrounded by a continuous liquid body, rather than due to often reported Laplace pressure difference because of isolated capillary bridges. We have studied both theoretically and experimentally, the capillary force induced clustering behavior of micropillar arrays as a function of their elastic modulus. To this end, we have developed a modified replica molding process to fabricate a wide range of hydrogel micropillar arrays, whose elastic modulus in the wet state could be tuned by simply varying the hydrogel monomer composition. By minimizing the sum of capillary meniscus interaction energy and bending energy of the pillars in a cluster, we have derived a critical micropillar cluster size, which is inversely proportional to elastic modulus of micropillars. The estimated cluster size as a function of elastic modulus agrees well with our experimental observation. We have also explored the utility of the clustered micropillar arrays as ultrathin whitening layers mimicking the structural whitening mechanism found in some insects in nature. Finally, we have theoretically studied the capillary force induced imbibition of a liquid droplet on a model rough surface consisting of micropillar arrays. Our theoretical model suggests that due to shrinking liquid droplet, the imbibition dynamics does not follow the diffusive Washburn dynamics but progressively becomes slower with time
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