Collective Charge Fluctuations in Single-Electron Processes on Nano-Networks

Using numerical modeling we study emergence of structure and structure-related nonlinear conduction properties in the self-assembled nanoparticle films. Particularly, we show how different nanoparticle networks emerge within assembly processes with molecular bio-recognition binding. We then simulate the charge transport under voltage bias via single-electron tunnelings through the junctions between nanoparticles on such type of networks. We show how the regular nanoparticle array and topologically inhomogeneous nanonetworks affect the charge transport. We find long-range correlations in the time series of charge fluctuation at individual nanoparticles and of flow along the junctions within the network. These correlations explain the occurrence of a large nonlinearity in the simulated and experimentally measured current-voltage characteristics and non-Gaussian fluctuations of the current at the electrode.Comment: 10 pages, 7 figure

Numerical Simulation of Single-Electron Tunneling in Random Arrays of Small Tunnel Junctions Formed by Percolation of Conductive Nanoparticles

We numerically simulated electrical properties, i.e., the resistance and Coulomb blockade threshold, of randomly-placed conductive nanoparticles. In simulation, tunnel junctions were assumed to be formed between neighboring particle-particle and particle-electrode connections. On a plane of triangle 100×100 grids, three electrodes, the drain, source, and gate, were defined. After random placements of conductive particles, the connection between the drain and source electrodes were evaluated with keeping the gate electrode disconnected. The resistance was obtained by use of a SPICE-like simulator, whereas the Coulomb blockade threshold was determined from the current-voltage characteristics simulated using a Monte-Carlo simulator. Strong linear correlation between the resistance and threshold voltage was confirmed, which agreed with results for uniform one-dimensional arrays

Electrical Percolation in Metal Nanowire Networks for Bulk Polymer Nanocomposites and Transparent Conductors, and Resistive Switching in Metal/Polymer Nano-Gap Devices

This dissertation describes the electrical properties of metal nanowire-polymer hybrid systems. The first part of the thesis discusses electrical percolation of metal nanowire networks in bulk polymer nanocomposites (3D) and nanowire films (quasi-2D). Specifically, we integrate simulations of rod networks and experiments of model metal nanowire systems to establish the dependence of their electrical properties on the nanowire aspect ratio (L/D) and network structure. For bulk polymer nanocomposites, we find that narrow Gaussian distributions in filler size do not impact the electrical percolation threshold, while mixing small fractions of high-L/D rods with modest-L/D rods significantly reduces the threshold. We also generalize the widely used excluded volume model of percolation, which was originally formulated for infinite-L/D, monodisperse rod networks, to account for both finite-L/D and arbitrary distribution in the rod dimensions. Next, we adapt our 3D simulation approach to model the electrical properties quasi-2D rod networks, which are relevant to nanowire films that are being pursued for flexible, transparent conductors. We present the first quantitative predictions of the dependence of the sheet resistance in nanowire films on the aspect ratio and areal density of mono- and poly-disperse nanowires. Moreover, by combining our simulations of sheet resistance and an empirical diameter-dependent expression for the optical transmittance, we produced a fully calculated plot of optical transmittance versus sheet resistance, the primary performance criteria for transparent conductors. Further, by fitting simulation results to experimental data, we obtain an effective average contact resistance, Rc_effective, between two nanowires. Rc_effective extracted using our integrated approach enables direct comparisons between nanowires of different compositions or networks fabricated by distinct means. We also report the critical area fraction of rods required to form a percolated network in nanowire films and provide a semi-empirical analytical expression for the critical area fraction as a function of L/D for mono- and poly-disperse rods. Our simulations of electrical percolation in quasi-2D and 3D rod networks, coupled with our extensions of existing analytical models, provide critical guidance for engineering bulk conducting nanocomposites and nanowire films with well-defined properties that are optimized for specific applications. In the second part of this thesis, we demonstrate reversible resistive switching in silver/polystyrene/silver nano-gap devices comprised of Ag nano-strips separated by a nanoscale gap and encapsulated in polystyrene (PS). These devices show highly reversible switching behavior with high on-off ratios during cyclic switching tests over many cycles. We also observe evolution of the gap after extensive testing, which is consistent with metal filament formation as the switching mechanism in Ag/PS/Ag nano-gap devices. The reversible electrical bistability demonstrated here was accomplished with an electrically inactive polymer, thereby extending the range of polymers suitable for organic digital memory applications

탄소나노튜브/고분자 복합재료에서의 응력 완화에 따른 저항 변화

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2020. 8. 유웅열.Carbon nanotubes (CNTs) have been investigated for many structural and electronic applications due to their excellent electrical and mechanical properties. Many studies have shown that CNT can be used as a reinforcement filler for polymer composites. In addition, CNT/polymer composites show the piezoresistive behavior, which makes them potentially applicable to strain sensing applications, e.g., an adhesive for structural health monitoring purposes. This study aimed to study the resistance changing behavior of CNT/polymer composites during stress relaxation and develop a new method of characterizing the residual stress. To achieve goals, a series of research was carried out as follows. Predicting the mechanical behavior of adhesives is important, because adhesives strongly influence the strength and reliability of adhesive–adherend structures. The rate and temperature dependent mechanical behavior of an adhesive, including its failure strength was studied. We carried out a simulation of the mechanical behavior of an adhesive, including its failure strength, using Schapery's nonlinear viscoelastic model. A detailed derivation of the nonlinear viscoelastic model for 3D implementation in finite-element software was presented. Experimental procedures for obtaining the model parameters from dynamic mechanical testing of lap-joint specimens were proposed. Strain-rate dependent failure criterion was employed using the shift factor and experimental lap shear tests at different strain rates to calculate the failure strain at different temperature. Then, the mechanical behavior of the adhesive in the adhesive joint at different rates and temperatures until its failure was simulated. The simulation results were compared with the experiments, demonstrating the validity of the current approach. The electrical properties of CNT/graphene hybrids were studied. In this study, a predictive model that quantitatively describes the synergistic behavior of the CNTs and graphene to the electrical conductivity of CNT/graphene hybrids was proposed. The number of CNT-to-CNT, graphene-to-graphene and graphene-to-CNT contacts were calculated assuming random distribution of particles in the hybrids. The calculation showed optimum electrical conductivity at certain compositions. The calculation result was validated by measuring electrical conductivity of inkjet-printed CNT/graphene hybrids. Lastly, the piezoresistive behavior of CNT/polymer composites during stress relaxation was studied. In this study, the dependence of CNT aspect ratio and concentration on the resistance change during stress relaxation was studied. The resistance was measured during stress relaxation of CNT/epoxy composites. The resistance change varied according to different CNT aspect ratio and concentrations. To explain this behavior, a simulation model that was based on a new resistor model and the number of contacts between CNTs within tunneling distance was developed. This model can also explain the dependence of CNT aspect ratio and concentration on the resistance change during tensile test. CNT composite was used as an adhesive in this work, the residual stress of which was measured during cooling. The normal stress formed in the adhesive, which was obtained from numerical simulation result using viscoelastic model, showed a good agreement with experiments, suggesting that CNT composites can be used as an adhesive that can detect the residual stress change and can monitor structural health of joints.탄소나노튜브(CNT)는 우수한 전기적 및 기계적 특성으로 인해 많은 구조적 및 전자적 응용 분야에서 연구되었다. CNT가 강화제로서 복합재의 충전제로서 사용될 수 있음이 많은 연구에서 드러났다. CNT/고분자 복합재는 압 저항 거동을 보이며 변형 감지 분야에 적용 가능하다. CNT 복합재는 구조적 건강 상태 모니터링 목적을 위한 접착제로 사용될 것으로 기대된다. 이 연구에서는 CNT 복합재의 응력 완화 동안 저항 변화 거동을 실험적, 이론적으로 나타내고 이를 활용하여 잔류응력을 나타내는 것을 목표로 하였다. 이 목적을 달성하기 위해 다음과 같은 연구들이 진행되었다. 접착제의 기계적 거동을 예측하는 것은 중요하다. 왜냐하면 접착제는 접착구조의 파괴강도 및 신뢰성에 큰 영향을 주기 때문이다. 파단 강도 등 접착제의 기계적 거동에 대한 속도 및 온도 의존성을 연구하였다. ABAQUS 소프트웨어에서 3D 구현을 위한 Schapery의 비선형 점탄성 모델을 도출하였고 이를 이용하여 접착제의 거동을 해석하였다. 모델의 파라미터들을 구하기 위해 기계적 분석(DMA)와 랩 조인트 시편의 응력완화 시험을 하였다. 변형 속도에 따라 파괴할 때까지의 변형률이 다른 특성을 해석에 적용하였다. 여기에는 쉬프트 팩터와 여러 온도, 변형 속도에서의 랩 조인트 시편의 시험 결과를 사용하였다. 다양한 변형속도, 온도에서 접착제가 파괴가 일어날 때까지의 접착제의 거동을 시뮬레이션으로 나타냈고 실험과 비교하여 이 접근 방법이 유효하다는 것을 보였다. CNT/그래핀 하이브리드의 전기적 특성이 연구되었다. 이 연구에서, CNT/그래핀 하이브리드에서 CNT와 그래핀이 시너지 효과를 내는 것을 정량적으로 예측할 수 있는 모델을 제안하였다. CNT-CNT, 그래핀-그래핀, 그래핀-CNT의 접촉점 수는 하이브리드에서 입자가 랜덤하게 분포되어 있다고 가정하고 계산하였다. 이 계산 결과는 특정 CNT/그래핀 비율에서 최적의 전기전도도가 나온다는 것을 보여주었다. 이 계산 결과는 잉크젯 프린팅으로 만든 CNT/그래핀 하이브리드 필름의 전기전도도를 측정하여 실험적으로 입증되었다. 마지막으로 응력 완화가 일어나는 동안 CNT/고분자 복합재의 전기전도도 변화에 대해 연구하였다. 이 저항 변화가 CNT 종횡비, 농도와 어떤 연관이 있는지에 대해 연구하였다. CNT/에폭시 복합재를 만들어 응력 완화 시험을 하면서 저항을 동시에 측정하였다. CNT 종횡비와 농도에 따라 저항 변화 거동이 달라지는 것을 확인하였다. 이 거동을 설명하기 위해 새로운 저항 모델과 터널링 거리 내의 CNT 사이 접촉점 수를 기반으로 한 시뮬레이션 모델을 개발하였다. 이 모델을 사용하여 인장 시험하는 동안 저항 변화가 CNT 종횡비, 농도에 따라 달라지는 현상 또한 설명할 수 있었다. 또한 CNT 복합재를 접착제로 사용하여 온도를 낮추는 동안 생기는 잔류 응력을 측정하는 데에 사용하였다. 점탄성 모델을 사용한 시뮬레이션 결과에서 나온 접착제에 발생한 응력 변화는 실험에서 측정한 저항 변화와 비슷한 경향을 보였다. 이를 통해 CNT 복합재를 사용하여 접착제의 구조 안정성 모니터링과 잔류 응력 변화를 측정하는 데에 활용할 수 있다는 것을 보였다.1. Introduction. 1 1.1. Carbon nanotube (CNT).. 1 1.2. Piezoresistive behavior of CNT composite.. 5 1.3. Research objectives. 10 2. Nonlinear viscoelastic property of adhesive 13 2.1. Introduction 13 2.2. Experimental 16 2.2.1. Materials and specimen. 16 2.2.2. Characterization.. 16 2.2.2.1. Dynamic mechanical analysis 16 2.2.2.2. Single lap shear test. 16 2.3. Model formulation and implementation.. 18 2.3.1. One-dimensional nonlinear viscoelastic model. 18 2.3.2. Three-dimensional nonlinear viscoelastic model . 20 2.3.3. Algorithm to update stress and tangent stiffness.. 21 2.3.4. Implementation of failure criteria... 27 2.3.5. Parameters for numerical simulation. 29 2.4. Experimental results. 30 2.4.1. Dynamic mechanical analysis. 30 2.4.2. Stress relaxation tests. 32 2.4.3. Lap shear test at a constant strain rate 37 2.5. Numerical simulation. 40 2.5.1. Geometry.. 40 2.5.2. Simulation results (stress relaxation test).. 41 2.5.2. Simulation results (lap shear test until failure).. 44 2.5.3. Hyperelastic model combined with Prony series. 49 2.6. Summary.. 55 3. CNT/graphene hybrids. 56 3.1. Introduction. 56 3.2. Experimental 59 3.2.1. Materials and ink formulation. 59 3.2.2. Characterization and inkjet printing 59 3.3. Predictive model for electrical conductivity of CNT/graphene hybrids. 63 3.3.1. Relationship between conductivity and the number of contacts 63 3.3.2. Estimating the number of contacts 64 3.3.2.1. Estimating the number of CNT-CNT contacts 64 3.3.2.2. Estimating the number of graphene-graphene contacts 68 3.3.2.3. Estimating the number of CNT-graphene contacts 70 3.3.2.4. Total number of contacts in CNT-graphene hybrids 72 3.3.3. Number of contacts for hybrids composed of different particle sizes. 78 3.3.4. Calculation of percolation threshold for CNT assembly. 83 3.4. Experimental results 86 3.4.1. Characterization of the CNT/graphene hybrid inks. 86 3.4.2. Morphology of the inkjet-printed CNT/graphene hybrids. 91 3.4.3. Electrical conductivity of CNT/graphene hybrids. 93 3.5. Other examples using the model 101 3.5.1. Application of the model in SWNT/MWNT hybrid 101 3.5.2. Predicted electrical conductivity of SWNT/MWNT hybrid. 101 3.5.3. Experimental result of the SWNT/MWNT hybrid 105 3.6. Summary 108 4. Piezoresistive behavior of CNT composites. 109 4.1. Introduction 109 4.2. Experimental 111 4.2.1. Materials and specimen. 111 4.2.2. Characterization of CNT dispersion.. 112 4.2.3. Resistance measurement during mechanical tests 113 4.3. Experimental results... 114 4.3.1. Dispersion of CNTs in the epoxy resin.. 114 4.3.2. Electrical conductivity of the CNT composites. 117 4.3.3. Resistance change during stress relaxation test. 119 4.3.4. Resistance change during tensile test. 122 4.4. Model. 126 4.4.1. Resistor model. 126 4.4.2. Number of contacts between CNTs . 127 4.4.3. Calculation of tunneling resistance change 128 4.4.4. Effect of aspect ratio and concentration on resistance change. 133 4.5. Application in residual stress measurement 136 4.5.1. Experimental procedure. 136 4.5.2. Experimental results. 139 4.5.2.1. Resistance change of adhesive joint during cooling. 139 4.5.2.2. Material property measured for numerical simulation 142 4.5.3. Simulation result of residual stress in the adhesive joint. 143 4.6. Summary. 149 5. Conclusion. 150 Reference. 153 Korean abstract. 168Docto

Nanoparticle Necklace Network Arrays Exhibiting Room Temperature Single-Electron Switching

A single nanoparticle is one of the most sensitive electronic devices for sensing chemicals in a gas or liquid. The conductivity of a single Au nanoparticle is significantly modulated by the binding of a molecule that alters charge by just one electron. However, the single-electron sensitivity requires cryogenic temperatures and interconnection is not easy. A patterned two-dimensional network of one-dimensional nanoparticle necklaces made up of 10 nm Au particles are fabricated and shown to exhibit similar single-electron effect at room temperature. Furthermore, the long range conductivity of over 10’s of microns makes the structure easy to self-assemble onto conventional microelectronics circuitry. A device exhibiting single-electron effect is characterized by highly non-linear current-bias behavior where at bias, V \u3e VT current rises rapidly and scales as (V/VT – 1)ζ, where ζ ≥ 1 is the critical exponent and VT is the threshold voltage. Below VT, current does not flow. Thus, VT is the switching voltage and larger ζ signifies sharper switching characteristics. While arrays of one and two dimension are well known to exhibit appreciable VT at cryogenic temperatures, at ambient temperatures the blockade effect vanishes. The unique architecture of the necklace network results in a weak dependence of VT on temperature which leads to room temperature single-electron effect. The high sensitivity of the nanoparticle necklace network array at room temperature allows coupled live cells to electronically switch, or gate, the device through cellular metabolic activity. Additionally, the critical exponent, ζ, which is a measure of how current will rise during switching, can be significantly enhanced by cementing the necklaces with the dielectric material CdS, thereby greatly increasing the switching gain and sensitivity of the device. Given robust room temperature single-electron switching, enhanced ζ values, cellular coupling capability, and natural integrability with microelectronics circuitry, nanoparticle necklace network arrays have the potential to be implemented in a wide range of applications, such as, chemical sensors, biofuel cells, biomedical devices, and data storage devices. Adviser: Ravi F. Sara

Enhanced Metal Contacts to Carbon Nanotube Networks through Chemical and Physical Modification

Carbon nanotubes (CNTs) are an emerging class of nano-structured carbon materials which are currently being studied for applications which would benefit from their desirable electrical and mechanical properties. Potential benefits such as improved current density, flexure tolerance, weight savings, and even radiation tolerance have led to their implementation into numerous devices and structures, many of which are slated for use in space environments. The role of CNTs can be quite diverse, with varied CNT electronic-types and morphologies dictated by the specific application. Despite numerous CNT types and morphologies employed by these technologies, a common link between nearly all of these devices and structures is metal contact to CNTs, where the metal components often provide the link between the carbon nanotubes and the external system. In this work, a variety of CNT-metal systems were characterized in terms of metal morphology analysis and CNT-metal electrical and mechanical interactions, in response to chemical and structural modifications. A large portion of the work additionally focuses on ion irradiation environments. A diverse number of experiments related to CNT-metal interactions will be discussed. For instance, electrochemical interactions between ion-irradiated single-wall CNTs (SWCNTs) and metal salt solutions were utilized to selectively deposit Au nanoparticles (Au-NPs) onto the SWCNTs. A direct correlation was established between defect density and Au-NP areal density, resulting in a method for rapid spatial profiling of ion-irradiation induced defects in SWCNTs. The effect of ion irradiation on the CNT-metal interface was also investigated and it was found that the contact resistance of Ag-SWCNT structures increases, while the specific contact resistance decreases. The increase in overall contact resistance was attributed to increased series resistance in the system due to damage of the bulk SWCNT films, while the decrease in specific contact resistance was attributed to Ag atoms being forward-scattered into the top 5 nm of SWCNT film, as revealed by computational simulations. Additionally, development of Ag-CNT metal matrix composite (MMC) thin films for advanced space solar cell electrodes is discussed. SWCNTs and multi-walled CNTs (MWCNTs) were utilized as reinforcement material in Ag electrodes to address problems related to micro-cracks causing electrode fracture and loss of power in solar cells. A method for creating free standing films was developed to enable mechanical property characterization of the MMCs, and it was found that SWCNTs significantly increase the toughness of Ag thin films, due to the SWCNT tensile strength and strain capabilities. CNT-MMC grid-finger structures were also fabricated by solar cell process-compatible techniques and subjected to electrical testing under mechanical stress. The results showed that CNTs are capable of spanning gaps in Ag electrodes upon fracture, both electrically and mechanically