Fully gravure printed complementary carbon nanotube TFTs for a clock signal generator using an epoxy-imine based cross-linker as an n-dopant and encapsulant.

Printed p-type single walled carbon nanotube (SWCNT) based circuits exhibit high power dissipation owing to their thick printed dielectric layers (>2 μm) and long channels (>100 μm). In order to reduce the static power dissipation of printed SWCNT-base circuits while maintaining the same printing conditions and channel lengths, complementary metal-oxide-semiconductor (CMOS) based circuits are more ideal. These circuits, however, have not been successfully implemented in a scalable printing platform due to unstable threshold voltages of n-doped SWCNT based thin film transistors (TFTs). In this work, a thermally curable epoxy-imine-based n-doping ink is presented for achieving uniform doping and sealing of SWCNT layers by gravure printing. After printing the n-doping ink, the ink is cured to initiate a cross-linking reaction to seal the n-doped SWCNT-TFTs so that the threshold voltage of the n-doped SWCNT-TFTs is stabilized. Flexible CMOS ring oscillators using such n-doped SWCNT-TFTs combined with the intrinsically p-type SWCNT-TFTs can generate a 0.2 Hz clock signal with significantly lower power consumption compared to similarly printed p-type only TFT based ring oscillators. Moving forward, this CMOS flexible ring oscillator can be practically used to develop fully printed inexpensive wireless sensor tags

Flexible Electronics for Neurological Electronic Skin with Multiple Sensing Modalities

The evolution of electronic skin (E-skin) technology in the past decade has resulted in a great variety of flexible electronic devices that mimic the physical and chemical sensing properties of skin for applications in advanced robotics, prosthetics, and health monitoring technologies. The further advancement of E-skin technology demands closer imitation of skin receptors\u27 transduction mechanisms, simultaneous detection of multiple information from different sources, and the study of transmission, processing and memory of the signals among the neurons. Motivated by such demands, this thesis focuses on design, fabrication, characterization of novel flexible electronic devices and integration of individual devices to realize prototype biomimetic E-skin with neurological and multimodal sensing functions. More specifically, we have studied flexible carbon nanotube thin-film transistors (CNT-TFTs) as control and signal processing units of E-skin and flexible ferroelectret nanogenerator (FENG) and triboelectric nanogenerator (TENG) as skin mechanoreceptors. Multiple fabrication methods, such as low-cost printing and conventional cleanroom-based microfabrication have been implemented to fabricate flexible CNT-TFTs with different structures and functions, especially the synaptic functions. Based on the research on individual devices, we have demonstrated a prototype force-sensing flexible neurological E-skin and its sensory nerve and synapse, with FENG serving as the sensory mechanoreceptor that generates action potentials (pulsed voltages) to be processed and transmitted by the flexible synaptic CNT-TFT. It allows for instantaneous detection of force stimuli and offers biological synapse-like behavior to store the stimulus information and relay the stimulus signals to the next stage. The force-sensing neurological E-skin was further augmented with visual and auditory sensing modalities by introducing phototransistor-based optical sensor and FENG-based acoustic sensor. Successful transduction of visual, auditory and tactile stimuli and synaptic processing and memory of those signals have all been demonstrated. Thanks to the multimodal sensing capability of the neurological E-skin, psychological associative learning experiment-“Pavlov’s dog\u27s experiment”, was also successfully implemented electronically by synergizing actual visual and auditory signals in the synaptic transistor. Flexible electronics and prototype neurological E-skin system demonstrated in this thesis may offer an entry into novel multimodal, user-environment interactive soft E-skin system for soft robotic and diagnostic applications

Carbon nanotube networks for thin film electronic applications

Scope of the thesis -- The advent of flexible organic macroelectronics -- Current challenges -- Carbon nanotubes : powering up orgnaics with nanotechnology -- Objectives of the present work -- Outline the thesis -- Overview of carbon nanotube networks -- Fabrication of carbon nanotube networks -- Electrical properties of carbon nanotube networks -- Applications of carbon nanotube networks -- Experimental procedures -- Carbon nanotubes and their purification -- Separation and suspension of carbon nanotubes in solutions -- Optical spectroscopy -- Protocols for fabricating carbon nanotube networks -- Imaging of carbon nanotube networks -- Atomic force microscopy (AFM) imaging -- Electrical contacts to carbon nanotubes -- Patterning carbon nanotube networks -- Deposition of organic semiconductors -- Electrical characterization -- Evaluating the performance limits of conducting and semicondcuting carbon nanotube networks for thin-film applications -- The fabrication and scaling characteristics of aligned and random carbon nanotube network thin film transistors -- Self-assembly of carbon nanotube networks -- Electrical properties of carbon nanotube network thin film transistors -- Substrate vs. environment- What suppresses electron conduction in field-effect transistor? -- Carbon nanotube sheets as electrodes in organic light emitting diodes -- Carbon nanotubes as injection electrodes in organic thin film transistor

Resonant frequency of gold/polycarbonate hybrid nano resonators fabricated on plastics via nano-transfer printing

We report the fabrication of gold/polycarbonate (Au/PC) hybrid nano resonators on plastic substrates through a nano-transfer printing (nTP) technique, and the parametric studies of the resonant frequency of the resulting hybrid nano resonators. nTP is a nanofabrication technique that involves an assembly process by which a printable layer can be transferred from a transfer substrate to a device substrate. In this article, we applied nTP to fabricate Au/PC hybrid nano resonators on a PC substrate. When an AC voltage is applied, the nano resonator can be mechanically excited when the AC frequency reaches the resonant frequency of the nano resonator. We then performed systematic parametric studies to identify the parameters that govern the resonant frequency of the nano resonators, using finite element method. The quantitative results for a wide range of materials and geometries offer vital guidance to design hybrid nano resonators with a tunable resonant frequency in a range of more than three orders of magnitude (e.g., 10 KHz-100 MHz). Such nano resonators could find their potential applications in nano electromechanical devices. Fabricating hybrid nano resonators via nTP further demonstrates nTP as a potential fabrication technique to enable a low-cost and scalable roll-to-roll printing process of nanodevices

Inkjet-printed MoS2-based field-effect transistors with graphene and hexagonal boron nitride inks

This article reports the design, fabrication, and characterization of an all inkjet-printed field-effect transistor (FET)

Nanogap Junctions and Carbon Nanotube Networks for Chemical Sensing and Molecular Electronics

This thesis work may be divided into two parts. The first part (chapters 2-7) focuses on the fabrication of a particular test structure, the electromigration (EM) formed metal nanogap junction, for studying the conduction through single molecules and for hydrogen sensing. The second part (chapters 8 and 9) focuses on carbon nanotube networks as electronic devices for chemical sensing. Chapters 2-4 discuss the formation of nanogap junctions in thin gold lines fabricated via feedback controlled electromigration. Using a feedback algorithm and experimenting on thin gold lines of different cross sections, I show that the feedback controls nanogap formation via controlling the temperature of the junction. Chapters 5 and 6 discuss the background and my experimental efforts towards fabricating superconducting electrodes for single molecule electronics research. Chapter 7 discusses the application of the techniques of chapters 2-4 to form palladium nanogaps via electromigration. I show that such devices can be used as hydrogen sensors, but suffer from slow response times (on the order of minutes). The results are discussed in the context of the in-plane stress buildup between the palladium metallization and the SiO2 substrate. The use of nanotube networks as chemical sensors is discussed in the second part of the thesis (chapters 8 and 9). I show measurements of the resistance and frequency-dependent (50 Hz - 20 KHz) gate capacitance of carbon nanotube thin film transistors (CNT-TFTs) as a function of DC gate bias in ultra-high vacuum as well as low-pressure gaseous environments of water, acetone, and argon. The results are analyzed by modeling the CNT-TFT as an RC transmission line. I show that changes in the measured capacitance as a function of gate bias and analyte pressure are consistent with changes in the capacitive part of the transmission line impedance due to changes in the CNT film resistivity alone, and that the electrostatic gate capacitance of the CNT film does not depend on gate voltage or chemical analyte adsorption to within the resolution of my measurements. However, the resistance of the CNT-TFT is enormously sensitive to small partial pressure (< 10-6 Torr) of analytes, and the gate voltage dependence of the resistance changes upon analyte adsorption show analyte-dependent signatures

이종구조 단일벽 탄소나노튜브를 적용한 용액공정 전계효과 트랜지스터

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 김장주.In recent years, the demand for flexible/stretchable devices for the rapid spread of mobile devices such as smartphones and wearable devices have increased. The semiconductor layer for the stretchable devices requires high mechanical properties together with excellent charge carrier mobility and a random network structure. Also a solution process is required for a large-area and low cost application. Single wall carbon nanotubes (SWNT) are one of the suitable materials for stretchable devices because they are solutions processed, have random network structure when they form films, exhibit high carrier mobility and excellent mechanical properties. However, they have both metallic and semi-conductivity properties in the synthesis, so it is necessary to sort off only the semiconducting properties from the SWNTs. There have been several ways to separate this mixture, but they usually result in low purity and yield. We tried to solve this issue. In Chapter 1, we will introduce basic transistors knowledge such as transistors history, the operating principles, characteristic parameters, and type of tfts. Also we will introduce basic SWNT knowledge include Carbon allotropes, electric band structure of SWNT, and synthesis method of single wall carbon nanotubes. In Chapter 2, we sort off high-purity semiconducting single-walled carbon nanotubes (s-SWNT) by polymer wrapping method. We demonstrate the selection of s-SWNTs in SWNTs grown by the high pressure carbon mono oxide (HiPCO) process using poly-9,9-di-n-octyl-fluorenyl-2,7-diyl (PFO) and poly (3-dodecylthiophene-2, 5-diyl) (P3DDT), the wrapping polymer was used poly (9, 9-di-n-dodecylfluorene) (PFDD) for plasma discharge process (PD).We analyzed the purity, concentration and random network surface with Ultraviolet-visible spectroscopy, Raman spectroscopy, Atomic force microscope, Field emission scanning electron microscopy, and Transmission electron microscopy. Based on the UV-vis-NIR absorption spectra, Raman spectroscopy, and electrical parameter of the resulting devices, the purity of s-SWNT in P3DDT-HiPCO is estimated to be > 99% and the purity of s-SWNT in PFDD-PD is estimated 98-99%. In Chapter 3, we will talk about method, we fabricated transistors by using high purity s-SWNTs ink. The P3DDT-HiPCO transistors show high on/off ratio, but low mobility, while the PFDD-PD exhibit high mobility with high off current level. A hetero structure system was adopted to solve these challenges. And by this way, we achieved a hole mobility of 7 cm^2/VS and on/off ratio of 1.5×10^7. And FeCl3-doping hetero structure system, we achieved a hole mobility of over than 100 cm^2/VS (Vd = -20V) and on/off ratio of 10^6 (Vd=-5V).Chapter 1. Introduction 1 Thin film transistor 3 Carbon allotropes 18 The Electronic Band Structure of SWNTs 22 Synthesis of Single Wall Carbon Nanotubes 31 Outline of this thesis 36 Chapter 2 Polymer wrapped s-SWNTs 37 Introduction 37 Experimental 41 Result and discussion 44 Summary 64 Chapter 3 Semiconducting SWNTs Transistors 65 Introduction 65 Experimental 67 Result and discussion 72 Summary 101 Chapter 4 Conclusion 103 Bibliography 105 초록 113Docto

탄소 나노 튜브 트랜지스터의 트랩핑 전하 및 접촉 저항 특성 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2018. 2. 이창희.Carbon nanotubes (CNTs) have received tremendous attention due to unique electrical properties because of their one-dimensional (1D) material nature. Owing to extraordinary thermal conductivity, mechanical, and electrical properties, CNTs have been considered as the candidate material for next-generation electronics. Among various applications, CNTs as semiconducting layers in transistors are the most promising, since silicon technology is expected to reach its performance limits soon, and the demand for flexible/transparent electronics is high. Although the unique 1D feature of CNTs is beneficial in terms of flexibility and tuning electrical properties for the required device operation, this 1D feature also yields some issues. (i) transistor operation could largely be influenced by charge diffusion from the CNTs to the surrounding dielectric. This causes the electrical performance of CNT transistors to be largely influenced by the trapping of charges. (ii) The effective contact area is smaller than the geometrically defined channel to the source/drain (S/D) contact region, since the diameter of a CNT is typically only 1–3 nm. Thus, in this thesis, two important interfaces, the CNT/gate insulator and the CNT/S/D contact, are discussed. First, the correlation between charge trapping and the charge transport properties in random-network single-walled carbon nanotube (SWCNT) transistors was investigated using direct current (DC) and transient analysis. DC analysis was conducted throughout the temperature-dependent forward (12 V to -12 V) and reverse (- 12 V to 12 V) gate sweep. The activated energy (Ea) extracted from the temperature-dependent mobility showed that the charge transport in our SWCNTs is not governed by the residual ions or the defects in the gate dielectric layer. Further investigation was conducted by extracting the temperature-dependent charge carrier density (n) and trap density (Nt). The charge carrier density (n) and trap density (Nt) showed similar temperature-dependent behavior, which indicates that the charge trapping and hysteric behavior in SWCNT transistors is primarily from the charge injection from the CNTs to the surrounding dielectric. Subsequently, transient measurement was carried out for further investigation. The transient measurement was performed with a small load resistor (Rload), such that the measurement circuit was small enough to extract the intrinsic RC time constant (τ) value of the SWCNTs channel. To investigate the effect of charge trapping on the transient response, an empirical equation was developed based on the theoretical trapping model. The transient mobility (μ_tr) showed similar temperature-dependent trends with the mobility (μ) extracted from the DC analysis, which further supports that the main factor of charge trapping in SWCNTs is not the residual ions, or the defects in the gate dielectric layer. Throughout the empirical equation, the charge velocity distribution in SWCNTs was successfully explained by the trapping of charges. The correlation between charge carrier density (n) and intrinsic time constant difference (Δτ) was also investigated. Throughout this analysis, we confirmed that the transient response is significantly influenced by charge trapping. We also found that the charge transport in the SWCNTs channel is largely influenced by the shallow traps rather than the deep traps. The trapping and detrapping rates were also extracted from the transient analysis. Second, the effect of the graphene S/D contact on the electrical performance of the SWCNT transistor was investigated. The contact resistance between SWCNTs and the S/D electrode can be improved by forming a large number of junctions between the SWCNTs and the S/D electrode. Thus, forming a dense SWCNT film is an effective way to increase the junctions between the SWCNT film and the S/D electrode. However, too dense an SWCNT film can cause the CNTs to form bundles and results in the decrease in the on/off current ratio (Ion/Ioff). Thus, a trade-off relationship exists between the contact resistance and the Ion/Ioff. To overcome this trade-off relationship, we employed graphene as the S/D electrode for the SWCNT transistor. The bottom-gate bottom-contact (BGBC) geometry was selected for the graphene S/D contact SWCNTs (Gr-SWCNTs) transistor such that the drain current (ID) could additionally be modulated by the graphene layer. A palladium (Pd) S/D contact SWCNTs (Pd-SWCNTs) transistor with the same device geometry was also fabricated for comparison. To determine whether graphene formed by chemical vapor deposition (CVD) is suitable for the S/D electrode, our graphene film was characterized thoroughly by the optical microscopic image, atomic forced microscopic (AFM) image, scanning electron microscope (SEM), and the Raman spectra. Throughout the graphene film analysis, we found that our monolayer graphene sheet consists of the combination of large-sized grains (diameter of grain ~100 μm) and small-sized grains (~1 μm). The transmission line method (TLM) results show that the resistivity of graphene was small enough to be used as the S/D electrode in a single transistor device. The effective work function investigation result indicates that the work function of graphene is well aligned to the work function of Pd. Since the surface energy of the underlying films could influence the deposition of the SWCNTs film, contact angle measurements were performed to acquire the surface energy of each layer. The resulting surface energy was then systematically compared with the AFM image of SWNCTs in the channel. The results showed that the graphene S/D contact is favorable for the SWCNTs to be densely formed in the channel, presumably due to the selective wetting properties that led the SWCNTs to be well confined in the channel. The transfer characteristics of the Gr-SWCNT transistor showed that high mobility (μ) with good Ion/Ioff could be achieved by employing the graphene S/D contact. The conducting behavior and the influence of the contact resistance to the electrical performance of the transistor were further investigated using the stick percolating system and the TLM method. Both Gr-SWCNT and Pd-SWCNT transistors showed the exponent of the stick percolating system near 1 (m = 1). The TLM results showed that the contact resistance of Gr-SWCNTs was lower than that of Pd-SWCNTs.Chapter 1 1 1.1 Charge Trapping Sites in Carbon Nanotube Transistors 6 1.2 Contact Property in Carbon Nanotube Transistors 8 1.3 Outline of Thesis 9 Chapter 2 11 2.1 Materials 11 2.1.1 Structure and Electrical Property of Carbon Nanotubes 11 2.1.2 Carbon Nanotube Film Deposition 15 2.1.3 Raman Spectroscopy of Carbon Nanotubes 15 2.1.4 Synthesis of Graphene and Transfer Process 17 2.2 Device Fabrication Methods 18 2.2.1 Carbon Nanotube Transistors for Transient Measurement 18 2.2.2 Graphene S/D contact Carbon Nanotube Transistors 19 2.3 Device Characterization Methods 20 2.3.1 Measurement Setup for Carbon Nanotube Transistors 20 2.3.2 Direct Current (DC) Sweep Characterization 21 2.3.3 Transient Response Characterization 22 2.3.4 Transmission Line Method 24 2.3.5 Other Characterization Methods 25 Appendix 1 26 Appendix 2 28 Chapter 3 31 3.1 I-V Characteristics of SWCNT Transistors 34 3.1.1 SWCNT Film Characterization 34 3.1.2 Analytical Equation 35 3.1.3 Charge Transport and Trapping Analysis in SWCNT Transistors 37 3.2 Transient Response of SWCNT Transistors 41 3.2.1 Intrinsic RC in SWCNT Transistors 41 3.2.2 Trapping/detrapping Model 42 3.2.3 Temperature-dependent Transient Mobility 47 3.2.4 Temperature-dependent Charge Velocity Distribution 48 3.2.5 Charge Trapping/detrapping Analysis 49 3.3 Summary 53 Chapter 4 55 4.1 Advantages of Proposed Device Geometry 58 4.2 Graphene Characterization 60 4.2.1 Graphene Film Characterization 60 4.2.2 I–V Characteristics of Graphene 62 4.2.3 Effective Work Function of Graphene 64 4.3 Characteristics of Graphene Contact SWCNT Transistors 66 4.3.1 Investigation of Surface Energy for SWCNT Deposition 66 4.3.2 SWCNT Film Characterization 68 4.3.3 I-V Characterization in Graphene S/D Contact SWCNT Transistors 70 4.3.4 Contact Resistance of Graphene S/D Contact SWCNT Transistors 75 4.4 Transparent SWCNT Transistors 78 4.5 Summary 80 Chapter 5 81 Bibliography 85 Publication 93 한글 초록 99Docto