Characterizing the Conductivity and Enhancing the Piezoresistivity of Carbon Nanotube-Polymeric Thin Films.

The concept of lightweight design is widely employed for designing and constructing aerospace structures that can sustain extreme loads while also being fuel-efficient. Popular lightweight materials such as aluminum alloy and fiber-reinforced polymers (FRPs) possess outstanding mechanical properties, but their structural integrity requires constant assessment to ensure structural safety. Next-generation structural health monitoring systems for aerospace structures should be lightweight and integrated with the structure itself. In this study, a multi-walled carbon nanotube (MWCNT)-based polymer paint was developed to detect distributed damage in lightweight structures. The thin film's electromechanical properties were characterized via cyclic loading tests. Moreover, the thin film's bulk conductivity was characterized by finite element modeling

Fabrication and Application of Flexible Sensors

A transfer printing method was developed to transfer carbon nanotubes (CNTs) from polyethylene terephthalate (PET) film to poly(dimethyl siloxane) (PDMS) polymer. Carbon nanotubes are composed of carbon atoms arranged in a honeycomb lattice structure, which are electrically conducting. When embedded in a nonconducting polymer, carbon nanotubes impart electrical conductivity to the nanocomposite, thus forming a nanocomposite that has potential applications in highly sensitive strain and pressure sensors. Several printing methods have been studied to deposit carbon nanotubes onto PDMS, including inkjet printing. Inkjet printing is a desirable deposition method since it is low-cost, simple, and allows the processing of aqueous-based inks. However, directly inkjet printing carbon nanotubes onto PDMS has been a challenge because the printed film becomes non-uniform due to the uneven drying of the droplets. Therefore, a method of transfer printing was developed to embed carbon nanotubes uniformly in PDMS. The transfer printing method consists of first inkjet printing patterns of carbon nanotubes onto a PET film, which quickly absorbs the aqueous ink and allows uniformity of the printed carbon nanotube patterns. The next step is spin-coating PDMS on the PET film to cover the carbon nanotube patterns, followed by curing the PDMS. The following step is thermally treating the PET film to promote the transfer of carbon nanotubes to PDMS, and finally peeling off PDMS from PET film to complete the transfer of carbon nanotube patterns. The transferred patterns had widths as small as 125 µm, while the obtained PDMS thickness was as low as 27.1 µm, which enabled the fabrication of highly sensitive force and pressure sensors. The transfer printing method was employed to fabricate a two-dimensional force sensor, which was composed of lines of carbon nanotubes in the x and y directions. The transduction mechanism lies in the generation of strain on the carbon nanotube pattern. When strain is produced, the resistance of the pattern changes due to the increase or decrease of the number of conduction paths in the carbon nanotube pattern. The practical application as a two-dimensional sensor was shown by monitoring the touch force exerted by multiple objects on the sensor. Due to the flexibility and stretchability of PDMS, fabricated air pressure sensors were capable of detecting small pressure differences. The sensors were composed of a circular diaphragm containing inkjet-printed carbon nanotube patterns. When air pressure increased on one side of the diaphragm, the deflection caused a strain on the CNT line, thus changing its resistance. Pressure sensors with a diaphragm diameter of five millimeters, diaphragm thickness of 27.1 µm showed sensitivity of 10.99 percent change in resistance per kilopascal (%/kPa) and limit of detection of 3.1 Pa. The pressure sensor has potential applications in monitoring minute air pressure differences such as those generated by the breathing pattern. The application of the highly sensitive and biocompatible pressure sensor was shown through the measurement of the pressure generated by a 3D-printed respiratory system

정전기수력학 인쇄를 활용한 단일벽 탄소나노튜브 기반 트랜지스터 및 응용

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2020. 8. 홍용택.As the demand and research for electronic devices on flexible and stretchable substrates gradually continues comparable to the conventional rigid silicon-based electronic devices, interest in new semiconducting materials capable of low-temperature processes and large-area processes is increasing. Single-walled carbon nanotube (SWCNT) is one of the representative materials satisfying the new interests thanks to its excellent electrical and mechanical properties. SWCNT can be advantageous for non-vacuum, low-temperature, and large-area processes in response to various solution processes such as dipping, inkjet printing, and gravure printing. For high-performance devices with low power consumption based on next-generation electronics, the demand for ultra-fine patterning technology based on the solution process is also increasing. In this thesis, SWCNT-based all electrohydrodynamic-jet (E-jet) printing system was established, a SWCNT-based thin-film transistor (SWCNT-TFT) with a channel length of 5 microns was implemented through the system. In addition, by developing and grafting technology to control the threshold voltage of SWCNT-TFTs based on the solution process, we have demonstrated highly integrated and high-resolution SWCNT-based applications including logic gate, pixel circuits for image detector and display. In addition to the micrometer scale fine pattern technology by the E-jet printing system, a new solution process-based vertical stacking technology is also introduced to further improve the transistor density, enabling high-resolution, highly integrated electronic applications in a continuous environment without any vacuum or high temperature process. The technology introduced in this thesis for high performance, high resolution, and high integration of SWCNT-based devices makes it possible to fabricate a 250 pixel per inch active matrix backplane utilizing only the solution process.유연 기판 및 신축성 기판상의 전자 소자에 대한 수요 및 연구가 종래의 단단한 실리콘 기반의 전자 기술만큼이나 많은 관심을 받고 있어, 이를 위한 저온 공정 및 대면적 공정이 가능한 새로운 반도체 물질 연구에 대한 관심이 증가하고 있다. 단일벽 탄소나노튜브는 뛰어난 전기적 및 기계적 특성 뿐만 아니라 비 진공, 저온, 그리고 대면적 공정이 가능한 담금 공정, 잉크젯 프린팅, 그리고 그라비아 인쇄법과 같은 용액공정에 대응하기에 이러한 요구를 충분히 충족시킨다. 마찬가지로 용액 공정 기반 소자의 고성능 및 저전력화를 위한 용액 공정기반의 초 미세 패터닝 기술에 대한 필요성도 증가하고 있다. 본 학위 논문에서는 단일벽 탄소나노튜브 기반의 전 정전기수력학 인쇄 시스템을 구축하여 5마이크론의 채널 길이를 갖는 단일벽 탄소나노튜브 기반 박막트랜지스터를 구현하였다. 또한 용액 공정기반의 단일벽 탄소나노튜브 기반 박막트랜지스터의 문턱 전압을 조절하는 기술을 개발하고 이를 접목시켜 논리소자와 영상센서 및 디스플레이를 위한 픽셀 회로를 포함한 단일벽 탄소나노튜브 기반의 고해상도, 고집적화된 응용소자를 개발하였다. 정전기수력학 인쇄 시스템을 통한 마이크론 수준의 미세 패터닝 기술 뿐만 아니라 집적도를 더욱 향상시키기 위한 용액 공정기반의 새로운 수직 적층형 기술을 도입하여 고해상도 및 고집적화된 단일벽 탄소나노튜브 기반의 전자 소자를 어떠한 진공 공정이나 고온공정 없이 연속된 환경에서 구현하였다. 본 학위논문에서 제시한 단일벽 탄소나노튜브 기반 소자의 고성능, 고해상도, 고집적화를 위한 기술은 250 ppi급의 능동형 매트릭스 백플레인의 제작을 순수 용액공정만으로 실현 가능하게 한다.1 Introduction 1 1.1 Single-Walled Carbon Nanotubes 1 1.2 Band structure of SWCNTs 8 1.2.1 Energy bandgap of SWCNTs 8 1.2.2 Density of states for SWCNTs 11 1.2.3 Detection for classifying species of SWCNTs 13 1.3 Sorting out semiconducting SWCNTs 16 1.3.1 Pre-deposition of the nanotubes and sorting later 16 1.3.2 First sorting out SWCNTs and deposition later 18 1.4 Operation of SWCNT-TFTs 21 1.4.1 SWCNT-TFTs as Schottky-barrier FETs 22 1.4.2 Random network of SWCNTs 26 1.5 Reported SWCNT-TFTs and applications 28 1.6 Technical points for microelectronics based on SWCNT-TFTs 32 1.7 Organization 34 2 Tunable threshold voltage in single-walled carbon nanotube thin-film transistors 35 2.1 Introduction 35 2.2 Experimental details 37 2.2.1 Fabrication process for solution-processed SWCNT-TFTs 37 2.2.2 Post-treatments for tunable threshold voltage in solution-processed SWCNT-TFTs and measurement of their electrical properties 38 2.3 Results and discussion 39 2.3.1 Post-chemical encapsulation for tunable threshold voltage 39 2.3.2 Contact resistance analysis by the Y-function method in SWCNT-TFTs employing chemical encapsulation 41 2.3.3 Shift of energy band in SWCNT-TFTs 42 2.3.4 Cycling tests for post-treatments 45 2.3.5 SWCNTs-based p-type only inverter 46 2.4 Conclusion 49 3 All electrohydrodynamic-jet printing system for single-walled carbon nanotube thin-film transistors 50 3.1 Introduction 50 3.2 Experimental details 55 3.2.1 Ink manufacturing for E-jet printed metal, dielectric, and active layers 55 3.2.2 Optimized E-jet printing conditions and fabrication process for all E-jet printed SWCNT-TFTs 57 3.3 Results and discussion 60 3.3.1 Constituting of all E-jet printing system 60 3.3.2 Optimized E-jet printed metal electrode 63 3.3.3 Optimized E-jet printed polymer dielectric 67 3.3.4 E-jet printing of S/D electrodes with short channel length 74 3.3.5 Formation of SWCNT networks in E-jet printing system 76 3.3.6 Overall process for all E-jet printing and electrical characteristics of all E-jet printed SWCNT-TFTs 78 3.4 Conclusion 83 4 All electrohydrodynamic-jet printing system based circuit design for high-resolution and highly integrated applications 85 4.1 Introduction 85 4.2 Experimental details 89 4.2.1 In-situ fabrication of via-hole and diode-connected SWCNTs-TFTs in all E-jet printing system 89 4.2.2 Fabrication process of all E-jet printed inverter with vertically stacked SWCNT-TFTs 90 4.2.3 Fabrication process of all E-jet printed active pixel sensor for image sensor with vertical stacking structure 92 4.2.4 Fabrication process of all E-jet printed pixel circuit for active matrix polymer light-emitting diode with vertical stacking structure 95 4.3 Results and discussion 98 4.3.1 In-situ via-hole formation technology based on all E-jet printing system 98 4.3.2 Additional E-jet printing of PVP layer on the SWCNT-TFTs 99 4.3.3 Electrical characteristics for all E-jet printed diode-connected SWCNT-TFTs 101 4.3.4 Electrical characteristics for all E-jet printed inverter with vertically stacked SWCNT-TFTs 103 4.3.5 Structure design for active pixel sensor based on vertically stacked E-jet printed SWCNT-TFTs 107 4.3.6 All E-jet printed pixel circuit for active matrix polymer light-emitting diode with vertical stacking structure 110 4.4 Conclusion 118 5 Conclusion 119 Appendix 121 A.1 Post-treatment with DI-water on SWCNT-TFT 121 A.2 Variation of characteristics of SWCNT-TFTs by post-treatment time with NH4OH 123 A.3 Surface energy variation by a ratio between cross-liking agent and PVP 124 A.4 Analysis for surface roughness parameters 125 A.5 Electrical characteristics of E-jet printed SWCNT-TFTs according to channel structure 128 Bibliography 130 Abstract in Korean 149Docto

Highly reproducible, hysteresis-free, flexible strain sensors by inkjet printing of carbon nanotubes

In order to build upon the exceptional interest for flexible sensors based on carbon nanotube networks (CNNs), the field requires high device-to-device reproducibility. Inkjet printing has provided outstanding results for flexible ohmic sensors in terms of reproducibility of their resistance. However, the reproducibility of the sensitivity, the most critical parameter for sensing application, has been only marginally assessed. In the present paper, CNN based resistive strain sensors fabricated by inkjet-printing on flexible Ethylene Tetrafluoroethylene (EFTE) sheets are presented. The variability on the device initial resistance is studied for 5 different batches of sensors from 3 to 72 devices each. The variability ranges between 8.4% and 43% depending on the size of the batches, with a 20% average. An 8-device batch with 15% variability on initial resistance is further studied for variability on the strain and thermal sensitivity. Standard deviation values are found to be as low as 16% on the strain sensitivity and 8% on the temperature sensitivity. Moreover, the devices are hysteresis free, a rare achievement for CNT strain sensors on plastics

Piezoresistive Hybrid Nanocomposites for Strain and Damage Sensing: Experimental and Numerical Analysis

Carbon nanomaterials such as carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) demonstrate remarkable electrical and mechanical properties, which suggest promising structural and functional applications as fillers for polymer nanocomposites. The piezoresistive behavior of these nanocomposites makes them ideal for sensing applications. Besides, hybrid nanocomposites with multiple fillers like carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) are known to exhibit improved electrical and mechanical performance when compared to mono-filler composites. To comprehensively understand the mechanisms of electrical percolation, conductivity, and piezoresistivity in hybrid nanocomposites, the author develops a two-dimensional (2D) and a three-dimensional (3D) computational Monte Carlo percolation network models for hybrid nanocomposites with CNT and GNP fillers. In the experimental studies correlated to the computational models, the author fabricates the hybrid nanocomposites made of both fillers using resin infiltration techniques and show an improvement of their electromechanical performance when compared to CNT nanocomposites. Due to the limitations of the resin infiltration techniques, the author develops an inkjet printing procedure with a new water-based CNT ink to fabricated printed nanocomposites on both polyimide film (Kapton) and paper with high device-todevice reproducibility. The ink formulation, as well as the substrate surface treatment, have been optimized to obtain conductive and piezoresistive devices. The author shows the effectiveness of the printed devices as strain sensors and impact damage sensors respectively under mechanical strains and hypervelocity impact damages. Devices printed with the minimum number of ink deposited layers lead to the best sensing performance

Integration of conductive materials with textile structures : an overview

In the last three decades, the development of new kinds of textiles, so-called smart and interactive textiles, has continued unabated. Smart textile materials and their applications are set to drastically boom as the demand for these textiles has been increasing by the emergence of new fibers, new fabrics, and innovative processing technologies. Moreover, people are eagerly demanding washable, flexible, lightweight, and robust e-textiles. These features depend on the properties of the starting material, the post-treatment, and the integration techniques. In this work, a comprehensive review has been conducted on the integration techniques of conductive materials in and onto a textile structure. The review showed that an e-textile can be developed by applying a conductive component on the surface of a textile substrate via plating, printing, coating, and other surface techniques, or by producing a textile substrate from metals and inherently conductive polymers via the creation of fibers and construction of yarns and fabrics with these. In addition, conductive filament fibers or yarns can be also integrated into conventional textile substrates during the fabrication like braiding, weaving, and knitting or as a post-fabrication of the textile fabric via embroidering. Additionally, layer-by-layer 3D printing of the entire smart textile components is possible, and the concept of 4D could play a significant role in advancing the status of smart textiles to a new level

Printable stretchable interconnects

This article presents recent progress and a comprehensive overview of stretchable interconnects based on printable nanocomposites. Nanocomposite-based inks for printed stretchable interconnects have been categorized according to dispersed filler materials. They comprise of carbon-based fillers and metal-based fillers. Benefits in terms of excellent electrical performance and elastic properties make nanocomposites the ideal candidates for stretchable interconnect applications. Deeper analysis of nanocomposites-based stretchable interconnects includes the correlation between the size of fillers, percolation ratio, maximum electrical conductivity and mechanical elasticity. The key trends in the field have been highlighted using curve fitting methods on large data collected from the literature. Furthermore, a wide variety of applications for stretchable interconnects are presented

Challenges in Design and Fabrication of Flexible/Stretchable Carbon- and Textile-Based Wearable Sensors for Health Monitoring: A Critical Review

To demonstrate the wearable flexible/stretchable health-monitoring sensor, it is necessary to develop advanced functional materials and fabrication technologies. Among the various developed materials and fabrication processes for wearable sensors, carbon-based materials and textile-based configurations are considered as promising approaches due to their outstanding characteristics such as high conductivity, lightweight, high mechanical properties, wearability, and biocompatibility. Despite these advantages, in order to realize practical wearable applications, electrical and mechanical performances such as sensitivity, stability, and long-term use are still not satisfied. Accordingly, in this review, we describe recent advances in process technologies to fabricate advanced carbon-based materials and textile-based sensors, followed by their applications such as human activity and electrophysiological sensors. Furthermore, we discuss the remaining challenges for both carbon- and textile-based wearable sensors and then suggest effective strategies to realize the wearable sensors in health monitoring