수소에 노출된 단겹 탄소나노튜브의 전기적 특성

학위논문 (박사) -- 서울대학교 대학원 : 자연과학대학 물리·천문학부(물리학전공), 2020. 8. 박영우.Carbon nanotube (CNT) is one of the most promising materials for the next generation electronics. However, there still have been many challenges for the realization of CNT-based electronics. Especially, the hole-favored electronic property of CNT in ambient condition has been the huge obstacle. To form an integrated circuit for logic computation, both n-type and p-type transistors are necessary. Hence, reliably working n-type CNT transistors have been desired. We report changes in the electrical property of single-walled carbon nanotube (SWNT) caused by exposure to high-pressure hydrogen gas. First, we report the electron doping effect of SWNT. In-situ 3-terminal electrical measurements with hexagonal boron nitride (hBN) substrate were used to observe the intrinsic electronic properties of hydrogen-exposed SWNT. Comparing to the planar graphene sheet device, we observed the curved structure of SWNT is advantages on the reaction with hydrogen molecules. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy verified the formation of C-H bond on SWNT surface after the hydrogen exposure process. The covalent bond yielded short-range scattering in the graphene device and band gap widening in the SWNT devices. Second, the metal work function-dependent doping effect on SWNT field effect transistor (FET) is investigated. To obtain the SWNT FET with higher Schottky barrier (SB) for the hole injection, we adapted the different contact metals that have lower work functions such as Cr and Ti replacing the Au/Ti electrodes. Comparing to the Au/Ti-contacted SWNT FET case, we observed noticeable hole carrier reduction after the hydrogen exposure in Cr- and Ti-contacted devices. It suggest the doping induced SB thickness change relies on the SB height. In other words, the changes in thickness (via electron doping) of higher SB (via low work function metal contact) causes more significant effect on the Ids-Vgs curve. Consequently, we achieved the SWNT FET that nearly perfectly working as an n-type semiconductor by hydrogen exposure to SWNT FET with Ti contacts. Since the electron doping effect was also reproduced in SWNT network devices, we expect the useful electrical properties and the stable nature of C-H bond observed in the hydrogen-exposed SWNT can leads a step forward to the application of SWNT for the next generation electronics.탄소나노튜브 (CNT)는 차세대 전자 소자로 가장 유망한 물질 중 하나로 손 꼽힌다. 하지만 CNT 기반 전자공학을 실현하기 위한 몇 가지 장애물들이 존재한다. 특히, 공기 중에서 p형 반도체 성향을 강하게 띠는 것이 문제이다. 집적회로를 구성하기 위해서는 n형 도 필요하다. 따라서 안정적으로 작동하는 n형 CNT 트랜지스터의 개발이 요구되고 있다. 이 논문에서는 단겹 탄소나노튜브 (SWNT) 를 고온 고압 수소에 노출 시킨 뒤 변화하는 전기적 특성을 소개 한다. 첫 번째로는 전자 도핑 현상을 보고한다. 2차원 부도체이자 불포화 결합이 최소화 된 것으로 알려진 hexagonal boron nitride 기판을 사용한 소자를 제작하여 기판으로부터 야기되는 외부 환경의 영향을 최소화 하였다. 뿐만 아니라 이 소자의 수소 노출 전 과정을 in-situ 측정법으로 측정하여, 외부 환경에 의한 영향을 배제한 SWNT 고유의 전기적 특성을 측정 할 수 있었다. 그 결과, 수소에 노출된 SWNT에 전자가 도핑 되는 현상을 관측하였다. SWNT의 곡률이 수소와의 반응에 이점으로 작용한다는 결과를 평면 구조인 graphene 소자와의 수소노출 비교실험에서 확인 할 수 있었다. 엑스선 광전자 분광법 와 라만 분광법을 통해 고온, 고압에 노출된 SWNT에 C-H 결합이 화학적으로 생성된 것을 확인 하였다. 마지막으로 이 공유결합에 의해 graphene에서는 short-range scattering 현상이, SWNT 에서는 에너지 밴드 갭 확장 현상이 관측 되는 것을 확인하였다. 두 번째로 SWNT 전계 효과 트랜지스터 (field effect transistor, FET) 의 전기적 컨택을 변화하는 연구를 진행하였다. SWNT와 금속 컨택에서 발생하는 쇼트키 장벽의 높이가 금속과 SWNT의 일 함수 차이에 의존하기 때문에, 금 보다 낮은 일 함수를 갖는 금속인 크롬, 티타늄을 전기적인 컨택 물질로 사용하여 그 차이점을 연구한 것이다. 그 결과 일 함수가 낮은 크롬, 티타늄을 컨택으로 사용 했을 때 금을 사용한 경우보다 훨씬 적은 양의 홀(hole) 타입 전류가 흐른다는 사실이 관측되었다. 이 관측을 통해 전자 도핑으로 인해 야기된 쇼트키 장벽의 두께 변화의 영향이 쇼트키 장벽의 높이에 의해 크게 좌우 될 수 있다는 것을 알 수 있다. 즉, 크롬과 티타늄과 같이 높은 쇼트키 장벽을 갖는 금속 컨택의 경우에, 전자 도핑에 따른 쇼트키 장벽의 두께 증가가 크게 나타나 홀 타입 전류를 크게 감소시킨다는 것이다. 따라서 우리는 이 현상을 이용하여 거의 완벽하게 n형 트랜지스터로 작동 하는 SWNT FET를 제작 할 수 있었다. 수소에 노출된 SWNT에 형성된 안정적인 C-H 결합에 의한 전자 도핑 현상이 SWNT 네트워크 소자에서도 나타남을 확인 함으로써, 앞으로 이 기술이 대 면적 나노튜브 소자의 전자 도핑에 응용될 수 있음을 확인하였다.Chapter 1. Introduction 1 1.1 Introduction of carbon nanotubes 1 1.2 Electronic structure of single-walled carbon nanotubes (SWNT) 2 1.3 Electrical contact to SWNT and Schottky barrier 5 1.4 Outline of thesis 8 Chapter 2. Backgrounds on hydrogen-exposed SWNTs 11 2.1 Hydrogen storage 11 2.2 Unzipping of SWNT 15 2.2 Energy band gap widening of hydrogenated SWNT 17 Chapter 3. SWNT field-effect transistor (FET) exposed to hydrogen 25 3.1 Introduction 25 3.1.1 Electron doping effect of hydrogenated graphitic materials 25 3.1.2 Motivation of SWNT exposure to hydrogen 27 3.2 Experimental 28 3.3 Result and discussion 31 3.3.1 In-situ electrical measurements of hydrogen-exposed SWNT 31 3.3.2 Surface potential spectroscopy of hydrogen-exposed SWNTs 35 3.3.3 Exposure time-dependent electrical property 36 3.3.4 Effect of curved structure 38 3.3.5 Raman spectroscopy and XPS of hydrogen-exposed SWNT 44 3.3.6 Electrical properties with covalent bond 47 3.3.7 SWNT network FET exposed to hydrogen 53 3.4 Summary of Chapter 3 55 Chapter 4. Hydrogen-exposed SWNT FETs with low work function metal contacts 61 4.1 Introduction 61 4.1.1 Schottky barrier formation of SWNT with low work function metal contacts 61 4.1.2 Strategies for n-type SWNT FETs : Electron doping and contact engineering 64 4.2 Experimental 66 4.3 Result and discussion 68 4.3.1 Hydrogen exposure of SWNT FETs with different metal contacts 68 4.3.2 Electron doping induced significant reduction of the hole injection in Cr-contacted SWNT FET 72 4.3.3 H2 Exposure time dependence of Ids-Vgs curve shift in different metal contacts 76 4.4 Summary of Chapter 4 79 Chapter 5. Conclusion 83Docto

Carbon Nanotubes:Adsorption, Point Defects and Structural Deformation from Quantum and Classical Simulations

This thesis studies carbon nanotubes using state-of-the-art computational methods. Using large-scale quantum-mechanical calculations, based on density-functional-theory, we investigate several important aspects of the physics and chemistry of single-walled-nanotubes. The focus is on the effect of defects, namely adparticles and vacancies, on the structural, electronic and dynmical properties of the nanotubes. We also present preliminary results of simulations exploring a possible route for the formation of SWNTs from graphene nanoflakes. Adparticles include atomic hydrogen, oxygen, sulfur at different concentrations as well as nitrogen--oxides. Chemisorption of hydrogen as well as oxygen and isoelectronic species results in the formation of clusters on the sidewall, with characteristic structures corresponding to characteristic signatures in the electronic spectra. Especially in the case of oxygen, we find that relatively high energy barriers separate different structures: this shows that not only thermodynamically favored configurations are relevant for the understanding of oxygen chemisorption but the presence of traps cannot be neglected. The fingerpint of these traps is confirmed by scanning-tunneling spectroscopy. Trends with size and chirality of the nanotubes and oxygen coverage are studied in detail and also explained in terms of simple chemical descriptors. The importance of large-scale atomistic models is also emphasized to obtain convergent results and thus reliable predictions, and comparison is made of results we obtain using different gradient-corrected exchange-correlation functionals and in part with hybrid functionals. The study of nitrogen-oxides faces the difficulty to correctly represent physisorption, also with empirically corrected gradient-corrected exchange-correlation and hybrid functionals. Still our calculations of vibrational frequencies of different molecules on the sidewall, once compared with experiment, are able to distinguish the specific species observed in infrared spectra. Part of our work is centered on the comparison of widely used classical potentials (reactive force fields) with DFT results. Specifically we use them to study hydrogen and oxygen chemisorption, and especially examine the validity of several different force-fields for the description of the structure and energetics of single and double vacancies. In all cases, we find rather limited agreement with DFT results, showing the intrinsic difficulty to represent the subtle and intrinsic quantum effects governing the physico-chemical behavior of carbon nanotubes. Still, the wealth of results we have obtained might be useful for an improvement of these classical schemes for a specific application or other semi-classical models

Surface Plasmon Resonance for Biosensing

The rise of photonics technologies has driven an extremely fast evolution in biosensing applications. Such rapid progress has created a gap of understanding and insight capability in the general public about advanced sensing systems that have been made progressively available by these new technologies. Thus, there is currently a clear need for moving the meaning of some keywords, such as plasmonic, into the daily vocabulary of a general audience with a reasonable degree of education. The selection of the scientific works reported in this book is carefully balanced between reviews and research papers and has the purpose of presenting a set of applications and case studies sufficiently broad enough to enlighten the reader attention toward the great potential of plasmonic biosensing and the great impact that can be expected in the near future for supporting disease screening and stratification

DEVELOPMENT OF ADSORBENTS FOR THE CAPTURE AND STORAGE OF HYDROGEN AND CARBON DIOXIDE BY MAGNETRON SPUTTERING

Concerns about climate change have rejuvenated global efforts in reducing carbon dioxide (CO2) emissions. Tactics include capture and sequestration of CO2 from point sources and the promotion of hydrogen (H2) as a “transport fuel”. Current H2 vehicles use high pressure H2 tanks which lack the convenience of their fossil fuel counterparts and present potential safety hazards. Development of adsorbent materials that reduce the energetic costs of H2 and CO2 capture, facilitating reversible storage under safer conditions, are hoped to increase the viability of these technologies for industrial application. This thesis is the first to utilise magnetron sputtering, a technique allowing fine control over nano-material synthesis, for the design of novel solid adsorbents and deposition of novel dopants for H2 storage and CO2 capture. Work includes an in-depth study of the influence of nitrogen as a sputter gas on the growth of carbonaceous films, and is the first to explore these films performance as H2 and CO2 adsorbents. Several conflicting nitrogen effects were identified, their influence on the films growth dependent upon the nitrogen fraction of the sputter gas. Performance of the deposited films as adsorbents was also dependent on the growth conditions. The H2 storage capacity at 77 K and 20 bar of an optimised adsorbent, synthesised by magnetron sputtering, was 4.7 wt.%, comparable in performance to alternatives from the literature. Further work provides the first evidence that cerium, deposited by magnetron sputtering, can function as an adsorbent catalyst and identified that sputtering is a worthwhile, yet slow process for adsorbent doping as it facilitates intimate binding between the adsorbent and the dopant. The novel synthesis of graphene by magnetron sputtering was also attempted. Whilst tests failed, results collected could provide guidance for more successful attempts in the future

Contact Metal-Dependent Electrical Transport in Carbon Nanotubes and Fabrication of Graphene Nanoribbons

In this thesis, we fabricate and characterize carbon nanotube (CNT) and graphene-based field effect transistor devices. The CNT-based work centers on the physics of metal contacts to CNT, particularly relating the work function of contact metals to carrier transport across the junction. The graphene work is motivated by the desire to utilize the high carrier mobility of graphene in field effect transistors. We introduce a surface-inversion channel (SIC) model based on low temperature and electrical measurements of a distinct single-walled semiconducting CNT contacted by Hf, Cr, Ti and Pd electrodes. Anomalous barrier heights and metal-contact dependent band-to-band tunneling phenomena are utilized to show that dependent upon contact work function and gate field, transport occurs either directly between the metal and CNT channel or indirectly via injection of carriers from the metal-covered CNT region to the CNT channel. The model is consistent with previously contradictory experimental results, and the methodology is simple enough to apply in other contact-dominant systems. We further develop a model explain Isd-Vsd tendencies in CNT FETs. Using experimental and analytical analysis, we demonstrate a relationship between the contact metal work function and electrical transport properties saturation current (Isat) and differential conductance in ambient exposed CNT. A single chemical vapor deposition (CVD)-grown 6 millimeter long semiconducting single-walled CNT is electrically contacted with a statistically significant number of Hf, Cr, Ti, Pd, and Ti, Au electrodes, respectively. The observed exponentially increasing relationship of Isat and with metal-contact work function that is explained by a theoretical model derived from thermionic field emission. Next, a performance analysis on CNT Schottky diodes using source-drain current anisotropy is explored. An analytical model is derived based on thermionic field emission and used to correlate experimental data from Pd-Hf, Ti-Hf, Cr-Hf, Ti-Cr, and Pd-Au mixed metal devices fabricated on one single 6 mm-long CNT. Results suggest that the difference in work functions of the two contact-metals, and not a dominant Schottky contact, determines diode performance. Results are further applied and demonstrated in a reversible polarity diode. Lastly, we investigate the effect of UV irradiation of graphene, CNT, and graphene/CNT hybrids in an oxygen environment. Samples were irradiated by 254/185 nm UV light in an oxygen environment for up to two hours. Results suggest a unique method to generate graphene nanoribbons using aligned carbon nanotubes (CNT) as a graphene etch mask. Ambient and cryogenic Gsd-Vg measurements of resulting ultra-thin graphene nanoribbons show p-type character and field effect GOn/GOff > 10^4

A novel transparent and flexible pressure sensor for the human machine interface

The movement towards flexible and transparent electronics for use in displays, electronic skins, musical instruments and automotive industries, demands electrical components such as pressure sensors to evolve alongside circuitry and electrodes to ensure a fully flexible and transparent system. In the past, piezoresistive pressure sensors made with flexible electrodes have been fabricated, however, many of these systems are opaque. For the first time, we present a technology that exploits the natural self-assembly of polystyrene nanospheres to reproducibly create nanostructured materials to be used in optically transparent pressure sensors with sensing performance comparable to opaque industry standards. The performance of the piezoresistive pressure sensor relies on uniform elastic nano-dome arrays. A thin and homogeneous lining of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) renders the domes conductive and retains the transparent and flexible qualities of the underlying polymer. The film transparency is primarily dependant on PEDOT:PSS film thickness where transparencies as high as 79.3 \% are achieved for films of less than 100 nm in thickness. The sensors demonstrate a resistance response across the force range appropriate for all human machine interface interactions, which correspond here to 0.07 to 26 N. The fabrication process involves the creation of an electroactive mould which is used to create nanostructred polymer layers. To enable mould reuse and enhance process efficiency, an anti-adhesive treatment in the form of a self-assembled monolayer of alkanethiols has been developed. Three chain lengths for the alkanethiol of chemical structure H$_{3}$C-(CH$_{2}$)$_{n}$-SH where n = 3, 5, and 11 are investigated and SAM functionalisation is confirmed with XPS. Peel tests prove that all three are effective at preventing adhesion between the mould and PEDOT:PSS and the treatment is shown not to be detrimental to the polymer electrodeposition process. An adapted fabrication procedure with custom designed electrode housing enables larger samples to be created for prototype devices. A simple functional prototype in the form of a multi-pixel force sensor atop of an LED display is successfully designed and fabricated to highlight the technology for use at the human machine interface.Open Acces

Novel Wastewater Treatment Applications Using Polymeric Materials

This reprint focuses on the effective removal of organic and inorganic pollutants from water and wastewater using unique freestanding polymeric-based methods. It also examines innovative green methods for efficiently reusing various solid waste products. In addition, several new eco-friendly hybrid materials are being reviewed in order to alleviate the problem of water pollution in novel, rapid, and efficient methods

Integration of thrombin-binding aptamers in point-of-care devices for continuous monitoring of thrombin in plasma

La thrombine est l'enzyme principale dans le processus d'hémostase. Les dérèglements de la concentration de thrombine clinique prédisposent les patients à des complications hémorragiques ou thromboemboliques. Le suivi en temps réel de la thrombine dans le sang est donc nécessaire pour améliorer le traitement de patients en état critique. Les aptamères, qui sont de courts nucléotides monobrins semblent constituer des candidats prometteurs pour la reconnaissance moléculaire dans les biocapteurs. L'objectif de ces travaux est l'étude de différentes solutions d'intégration des aptamères dans des dispositifs de diagnostic de type "point of care" pour le suivi en continu de la thrombine dans le plasma. La cinétique d'interaction des aptamères avec la thrombine et leur spécificité vis-à-vis de la prothrombine et des inhibiteurs de la thrombine ont été étudiés par résonance par plasmons de surface. Ces travaux ont démontré la faible spécificité de l'aptamère HD1 vis-à-vis de la thrombine, et la présence d'interactions non-spécifiques avec la prothrombine, les inhibiteurs naturels de la thrombine et l'albumine. Inversement, nous avons observé une bonne affinité de l'aptamère HD22 avec la même liste de cible. Parallèlement, nous avons évalué des stratégies d'intégration d'aptamères dans des dispositifs d'analyse. Le principe de reconnaissance a ensuite été validé et la possibilité de détecter la thrombine dans des gammes de concentration de 5 à 500nM a été démontrée. Enfin, afin d'augmenter la spécificité de la détection de la thrombine, nous avons proposé une nouvelle approche basée sur l'ingénierie de structures dimères interconnectant HD1 et HD22.Thrombin is the central enzyme in the process of hemostasis. Normally, in vivo concentration of thrombin is rigorously regulated; however, clinically impaired or unregulated thrombin generation predisposes patients either to hemorrhagic or thromboembolic complications. Monitoring thrombin in real-time is therefore needed to enable rapid and accurate determination of drug administration strategy for patients under vital threat. Aptamers, short single-stranded oligonucleotide ligands represent promising candidates as biorecognition elements for new-generation biosensors. The aim of this PhD work therefore is to investigate different solutions for the integration of thrombin-binding aptamers in point-of-care devices for continuous monitoring of thrombin in plasma. The kinetics of aptamer interaction with thrombin and specificity towards prothrombin and thrombin - inhibitor complexes was rigorously investigated using Surface Plasmon Resonance. These experiments unveiled the complex character of interaction of the HD1 with thrombin, confirming nonspecific interactions with prothrombin, natural inhibitors of thrombin, serum albumin whereas another 29-bp aptamer HD22 proved to be highly affine and specific towards thrombin. On the other hand we explored aptamer integration options. We validated the principle and at the same managed to detect different concentrations of thrombin (5-500 nM). We finally proposed a novel approach to increase sensitivity and specificity for thrombin detection based on the engineering of aptadimer structures bearing aptamers HD1and HD22 interconnected with a nucleic acid spacer

Final Report - Advanced Cathode Catalysts and Supports for PEM Fuel Cells

The principal objectives of the program were development of a durable, low cost, high performance cathode electrode (catalyst and support), that is fully integrated into a fuel cell membrane electrode assembly with gas diffusion media, fabricated by high volume capable processes, and is able to meet or exceed the 2015 DOE targets. Work completed in this contract was an extension of the developments under three preceding cooperative agreements/grants Nos. DE-FC-02-97EE50473, DE-FC-99EE50582 and DE-FC36- 02AL67621 which investigated catalyzed membrane electrode assemblies for PEM fuel cells based on a fundamentally new, nanostructured thin film catalyst and support system, and demonstrated the feasibility for high volume manufacturability