Low-energy electrochemical deposition for large-area electronics via remote-controllable lateral growth

국문 초록 (Abstract)

More-than-Moore(MtM) 패러다임은 전도성 고분자(CPs)와 같은 기능성 유연 소재를 필요로 한다. 그러나 기존의 CP 제작 방식은 높은 공정 비용(진공 기반) 또는 재료의 성능 저하(용액 기반)라는 근본적인 한계에 직면해 있다. 이러한 문제의 대안으로, 전기중합(EP)은 고품질 CP를 합성하는 이상적인 방법이지만, 일반적인 EP는 전기가 통하는 유선 기판에만 제한된다. 양극성 전기중합은 양극성 전극(Bipolar electrode, BPE)을 사용하여 절연체 위에도 증착함으로써 이 문제를 해결하지만, 이전 연구는 마이크로 입자나 1D 섬유 네트워크 합성에 국한되었다. 거시적 전자소자에 필수적인 '균일한 대면적 CP 필름'을 제작하는 방법론에는 명확한 학술적 공백이 존재했다. 본 논문은 절연 기판 위에 고품질 CP 필름을 균일하게 증착하고 패터닝하는 새로운 양극성 전기중합 플랫폼을 개발하고 시연함으로써 이 공백을 해결하고자 하였다.
첫째, 본 연구는 AC/DC 전압과 새로운 이중 BPE 구조를 결합하여 다양한 절연체 위에 단일의 균일한 대면적(최대 2인치) PEDOT 필름을 증착하는 ADC-bipolar EP 시스템을 확립한다 (Chapter 2). 전해질이 필름 특성에 미치는 영향을 체계적으로 분석하였다. 이 방법의 잠재력은 필름의 넓은 표면적과 독특한 분자 수준의 상호작용에 기인하여 매우 낮은 검출 한계(LOD) 0.726 ppb를 나타내는 초고감도 NO₂ 가스 센서를 통해 입증되었다 (Chapter 3).
둘째, 이 방법론은 트랜지스터의 소스/드레인 전극을 BPE로 혁신적으로 통합하여 마스크 없이 패터닝하는 OSIEP (One-Shot Integrable EP) 시스템으로 발전되었다 (Chapter 4). 이를 통해 초유연 기판 위에 6 × 5 PEDOT 어레이를 10% 미만의 높은 소자 균일도로 제작할 수 있었다. 그 결과물인 유기 전기화학 시냅틱 트랜지스터(OEST)는 100초 이상의 장기 가소성을 보였으며, 이를 MNIST기반 인공신경망 시뮬레이션 학습시95.2%의 높은 인식 정확도를 보였다. 더 나아가 다중 게이트 회로를 통해 조작적 조건화를 시연함으로써 OSIEP 회로의 한단계 위의 가능성을 가늠하였다. 마지막으로, 이종 혼합 전해질 (TBABF₄/EMIM:TFSI) 시스템을 도입하여 결정성을 제어함으로써 OEST 선형성을 향상시켰고, ANN 정확도를 95.58%로 높였다 (Chapter 5).
결론적으로, 본 논문은 마이크로 입자/섬유 합성에 머물렀던 양극성 전기중합 기술을 균일한 대면적 필름 증착과 직접 패터닝이 모두 가능한 플랫폼(ADC-bipolar EP 및 OSIEP)으로 독창적으로 확장시켰다. 본 연구는 절연 기판 위에 고품질 CP를 저비용으로 제작할 수 있는 새로운 플랫폼을 제공하며, 센서부터 고밀도 시냅틱 어레이에 이르는 다양한 MtM 소자를 위한 기술적 기반을 확립하고, 차세대 유연, 신축성, 생체 적합성 전자 시스템을 위한 디딤돌 역할을 할 것이다.

다국어 초록 (Multilingual Abstract)

The More-than-Moore (MtM) paradigm necessitates the integration of functional, flexible materials, among which conducting polymers (CPs) are prominent candidates. Conventional CP fabrication methodologies, however, present fundamental limitations: vacuum-based processes are capital-intensive, whereas solution-based processes often result in compromised material performance. Electropolymerization (EP) emerges as a viable alternative for synthesizing high-quality CPs, yet conventional EP is restricted to conductive, wired substrates. Bipolar EP, which utilizes floating bipolar electrodes (BPEs), circumvents this constraint by enabling deposition on insulators. Nonetheless, prior research was predominantly limited to the synthesis of micro-particles or 1D fiber networks. Consequently, a significant academic deficiency persisted regarding a methodology capable of fabricating the uniform, large-area CP films, which is essential for macro-scale electronics. This dissertation seeks to address this deficiency by developing and demonstrating a new bipolar EP platform for the uniform deposition and patterning of high-quality CP films on insulating substrates.
Initially, this research establishes the ADC-bipolar EP system, which integrates combined AC/DC voltages with a novel dual-BPE structure to deposit singular, uniform, large-area (up to 2-inch) PEDOT films on various insulating substrates (Chapter 2). The influence of electrolyte selection on resultant film properties was systematically analyzed. The potential of this methodology was subsequently demonstrated through an ultrasensitive NO₂ gas sensor, which exhibited an exceptionally low limit of detection of 0.726 ppb. This high sensitivity was attributed to the film's substantial surface area and distinctive molecular-level interactions (Chapter 3).
Subsequently, this methodology was advanced into the one-shot integrable EP system for mask-free patterning, achieved by innovatively integrating the transistor's source/drain electrodes to function as BPEs (Chapter 4). This approach enabled the fabrication of 6×5 PEDOT arrays on ultraflexible substrates, yielding high device-to-device uniformity (<10%). The resultant organic electrochemical synaptic transistors (OESTs) exhibited long-term plasticity exceeding 100 seconds and demonstrated high recognition accuracy (95.2%) in MNIST-based artificial neural network (ANN) simulations. The advanced capabilities of this fabrication process were further substantiated through the implementation of a multi-gate circuit to demonstrate operant conditioning. Finally, a mixed binary electrolyte (TBABF₄/EMIM:TFSI) system was introduced to modulate channel crystallinity, which consequently enhanced OEST linearity and increased the ANN accuracy to 95.58% (Chapter 5).
In conclusion, this dissertation significantly expands bipolar EP technology, previously confined to micro-particle synthesis, into a robust platform (ADC-bipolar EP and OSIEP) capable of both uniform large-area film deposition and direct patterning. This research provides a novel, cost-effective methodology for fabricating high-quality CPs on insulating substrates, thereby overcoming the critical limitations of conventional fabrication routes. This work establishes a technical foundation for diverse MtM devices, from sensors to high-density synaptic arrays, and serves as a foundational stepping stone for next-generation flexible, stretchable, and biocompatible electronic systems.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. Background 1
• 1.2. Vacuum-based processes 4
• 1.3. Solution-based processes 5
• 1.4. Electropolymerization (EP) 6
• 1) Conventional EP system 7
• 2) Bipolar EP system 8
• 1.5. Issue 9
• 1.6. Motivation and objectives 10
• 1.7. Fundamental setup and working mechanism 11
• 1) Fundamental setup 11
• 2) Theoretical basis for voltage configuration 12
• 3) Working mechanism 13
• Chapter 2. ADC-bipolar electropolymerization: Toward continuous large-sized films 20
• 2.1. Introduction 21
• 2.2. Experimental parts 23
• 1) Materials 23
• 2) BPE-patterned substrate preparation 23
• 3) Fabricating PEDOT films via ADC-bipolar EP 23
• 4) Soft neural thermocouple fabrication and evaluation 24
• 5) Chemiresistive NO2 gas sensor fabrication 25
• 6) Characterization 25
• 7) Electric field calculation 26
• 2.3. Development of ADC-bipolar EP 26
• 1) Optimization of process parameters 26
• 2) Effects of the BPE designs 34
• 3) Effects of supporting electrolytes 35
• 4) Chemical and electronic properties 38
• 5) Crystallographic properties 41
• 6) Anisotropic properties 46
• 2.4. Demonstrations of electronics applications 48
• 1) Evaluation as thermoelectric materials 48
• 2) Evaluation of gas-sensing properties 50
• 3) Demonstration as soft thermometers for neural interface applications 51
• 2.5. Conclusion 52
• Chapter 3. Demonstration of Ultrasensitive NO2 Sensors on Plastics with ADC-Bipolar Electropolymerization 64
• 3.1. Introduction 65
• 3.2. Experimental parts 67
• 1) Materials 67
• 2) Preparation of BPE-patterned substrate 67
• 3) Fabrication of PEDOT films via the ADC-bipolar EP method 68
• 4) Gas sensor fabrication 68
• 5) Characterizations 68
• 6) Interaction energy calculations 69
• 3.3. Characterization of ADC-bipolar EPed Films for gas-sensing 69
• 1) Fundamentals of the ADC-bipolar EP system 69
• 2) Molecular-level analysis: Calculation of interaction energies 73
• 3) Supramolecular-level analysis: Topographical characterization 74
• 4) Crystallographic information 76
• 3.4. Evaluation of gas-sensing performance 78
• 1) Characterization of gas-sensing properties 78
• 2) Sensitivity and limit of detection (LOD) for NO2 gas 82
• 3) Investigation of chemical reactions during sensing via Raman spectroscopy 84
• 3.5. Elucidation of the sensing mechanism 86
• 3.6. Conclusion 87
• 3.7. Acknowledgements 88
• Chapter 4. One-shot Integrable Electropolymerization (OSIEP): Toward Mask-Free Patternability 97
• 4.1. Introduction 98
• 4.2. Experimental parts 100
• 1) Materials 100
• 2) BPE array patterned substrates preparation 101
• 3) Polymer channel fabrication via OSIEP method 101
• 4) Synaptic transistor fabrication and measurement 102
• 5) Characterization 102
• 4.3. Development of mask-free patternable OSIEP system 103
• 4.4. Characterization of OSIEP-fabricated PEDOT:BF4 films 108
• 1) Spectroscopic analysis on electronic and vibrational properties 108
• 2) Crystallographic information 110
• 3) Uniformity of PEDOT:BF4 channel array 113
• 4.5. Evaluation of organic electrochemical synaptic transistors 119
• 1) Emulation of artificial neuromorphic functions 119
• 2) Artificial neural networks training simulation 122
• 3) Demonstration of operant conditioning via multi-gate synaptic circuits 125
• 4.6. Conclusion 130
• 4.7. Acknowledgements 131
• Chapter 5. Mixed Binary Electrolyte System to Control Morphologies in OSIEP 139
• 5.1. Introduction 140
• 5.2. Experimental parts 142
• 1) BPE array-patterned substrate 142
• 2) Reactive solution preparation 143
• 3) OSIEP process 143
• 4) Fabrication of organic electrochemical synaptic transistors 143
• 5) Computational method 143
• 6) Characterization 144
• 5.3. Strategy to modulate synaptic characteristics 144
• 5.4. Characterization of OSIEP-fabricated PEDOT copolymer films 146
• 1) Morphological and topographical analysis 146
• 2) Electronic structure analysis via XPS 149
• 3) Electrochemical de-doing efficiency: Interaction energy calculations 151
• 4) Crystallographic information 154
• 5.5. Evaluation of organic electrochemical synaptic transistor 156
• 1) Emulation of artificial neuromorphic functions 156
• 2) Artificial neural networks training simulation 161
• 5.6. Conclusion 164
• 5.7. Acknowledgements 165
• Chapter 6. Conclusion and Future Insights 171