DIRECT PRINTING/COATING/PLATING OF KEY COMPONENTS FOR ELECTRONIC DEVICES

Miniaturization has been a technological trend for several decades for electronic devices. From the practical point of view, the successful miniaturization of fully integrated systems mainly depends on their components. This dissertation examines the inkjet printing of copper oxide inks on flexible substrates for applications in microfluidic valving systems. We expand the knowledge of low-cost and high-performance electrowetting valves and fabricate the microfluidic device for fluidic control, which is necessary to enable the next-generation microfluidic devices. In addition, we also study the electromagnetic interference (EMI) shielding material, which is a crucial part of electronic devices. The basic theory of EMI shielding is discussed in this dissertation. We explore the high-performance shielding materials with novel fabrication methods, such as spray coating, laser carbonizing and electroplating. Chapter 1 describes the fabrication of flexible inkjet-printed copper electrowetting valves. We study the effects of dielectric layer thickness as well as applied voltage to the contact angle decreasing process. Taking this one step further, an electrowetting valve with two controlled channels is described for demonstration. Chapter 2 explores the fabrication of high-performance EMI shielding material of silver-coated carbon fiber fabrics (CFFs) via spray coating. EMI shielding theory is discussed in detail. The silver/CFF composite material is introduced at the lab-scale as well as at the roll-to-roll large fabrication scale. Chapter 3 introduces the high-performance optically transparent copper mesh. The laser carbonizing and electroplating techniques are described. We study the effect of different patterns to the EMI shielding performance. Chapter 4 evaluates the EMI shielding performance of graphite/polymer composites. Microwave exfoliation and acid treatment are explained from the mechanism aspect. Furthermore, future work is described for potential improvement on their performance

Synthesis and Characteristics of Carbon Nanofibers/Silicon Composites and Application to Anode Materials of Li Secondary Batteries

Among the various synthesizing technologies of carbon nanofibers (CNFs), chemical vapor deposition (CVD) technology, which uses hydrocarbon gas or carbon monoxide as a carbon source gas and pyrolyzes it to grow CNFs on transition metal catalysts, such as Ni, Fe, and Co, has been regarded as the most inexpensive and convenient method to produce CNFs for industrial use. Experimental variables for CVD are source gas, catalyst layers, temperature, and reaction time. Since the particle size of metal catalysts has an influence on the diameter of CNFs, it is possible to control the diameter of CNFs by varying particle sizes of the metal. As such, it is possible to synthesize CNFs selectively through the selective deposition of catalyst metals. In this study, CNFs were grown by CVD on C-fiber textiles, which had catalysts deposited via electrophoretic deposition. The CNFs were coated with a silica layer via hydrolysis of TEOS (tetraethyl orthosilicate), and the CNFs were oxidized by nitric acid. Due to oxidation, a hydroxyl group was created on the CNFs, which was then able to be used as an activation site for the SiO2. CNFs and the CNFs/SiO2 composite can be used in various applications, such as a composite material, electromagnetic wave shielding material, ultrathin display devices, carbon semiconductors, and anode materials of Li secondary batteries. In particular, there is an increasing demand for lightweight, small-scale, and high-capacity batteries for portable electronic devices, such as laptop computers or smart phones, along with the escalating concern of fossil energy depletion. Accordingly, CNFs and CNFs/SiO2 composites are receiving attention for their use as anode materials of Li secondary batteries, which are eco-friendly, lightweight, and high capacity. Therefore, the physicochemical properties and electrochemical performance data of synthesized CNFs and CNFs/SiO2 composite are described in this chapter

Titanium Interconnection in Metallized Carbon Nanotube Conductors

Metallized carbon nanotube (CNT) networks aim to achieve conductivities competitive with bulk metals, while retaining the favorable temperature coefficient of resistance (TCR) for CNT materials. Cu is the predominant metal for high conductivity applications due to cost and availability. However, microscopy shows that Cu poorly wets a CNT surface and requires an adhesion metal for an improved physical and electrical interface. The present dissertation utilizes thermal evaporation as a direct method to evaluate 2-D coatings of Ti as an interfacial metal for bulk Cu-CNT hybrids. Specifically, a 10 nm Ti layer maintained a continuous and uniform coating after annealing to 400 °C for an hour, demonstrating the temperature stability of Ti on a bulk CNT network. Additionally, Ti successfully suppressed the delamination of a Cu overcoat and achieved a 12% decrease in resistance for Cu-Ti-CNT hybrids after annealing at 400 °C. The benefits observed with thermally evaporated Ti adhesion layers motivated the development of a 3-D deposition approach using a novel joule-heated driven CVD technique, which can deposit metal throughout the entire bulk volume. Specifically, an oxygen-free precursor, cyclopentadienyl(cycloheptatrienyl) titanium(II), was used in the process under an inert/reducing atmosphere (95% Ar/ 5% H2) to promote a pure Ti metal deposition. Cross-sectional EDX mapping revealed that CVD successfully achieved diffusion of Ti throughout the entirety of a ~30 μm-thick, porous CNT conductor, demonstrating the capability of CVD as a method to fabricate bulk integrated Ti-CNT conductors for the first time. CVD coating morphology is shown to be tunable via the amount of precursor used, reactor pressure, and temperature, ranging from coatings localized along the individual bundles within the network to a fully connected film formation. Additionally, modification of reactor environment provides control over metal oxidation during growth onto the CNTs, achieving oxide-free to mixed Ti-oxide depositions as validated via Raman spectroscopy. The effectiveness of pure Ti as an adhesion metal on CNTs is benefitted from its wettability, temperature stability, and low contact resistance to CNTs; which can motivate investigating other potential adhesion metals that typically produce stable oxides like tungsten. Modeling of the temperature dependent electrical characteristics indicates an increase in metallic conduction behavior for the Ti-CNT conductors, with a decrease in the tunneling barrier between CNTs after Ti deposition, demonstrating the benefits of nanometal interconnection and showcasing the utility of temperature dependent modeling as a tool to assess nanoscale interaction of metallized CNT networks. CVD deposited Ti-CNT conductors electroplated with Cu, annealed, densified and then annealed a second time, realize conductivities as high as 43.1 MS/m, which is the highest conductivity reported for a bulk metal-CNT conductor at 98% weight loading. A Ti seeded CNT conductor (~9% w/w) electroplated to 98% total metal mass was demonstrated to achieve a specific conductivity of 6257 Sm2/kg, with a TCR (from 300-600 K) of 3.49 × 10-3 K-1, which combined result in a surpassing of the specific conductivity of pure Cu at temperatures above 250 °C. Thus, the overall impact of this work is demonstration of advanced conductors with a combined high conductivity and low TCR, which can provide direct energy savings at elevated temperature operation for applications such as high efficiency motors

High Conductivity Metal–Carbon Nanotube Hybrid Conductors via Chemical Vapor Deposition and Electroplating

Metal-carbon nanotube (CNT) hybrid conductors aim to combine the high conductivity of traditional metals with the low mass and temperature coefficient of resistance (TCR) of carbon nanotubes. The high conductivity of copper makes it a promising candidate to combine with CNTs in a hybrid structure, but there is limited physical and electrical interaction between copper and CNTs. The use of an interfacial layer offers one method of improving the interconnection of a Cu-CNT hybrid conductor. In this dissertation, a joule heating-driven chemical vapor deposition (CVD) technique is developed to deposit nanometal seeds throughout a porous, low-density (0.12 g/cm3, ~9 mg/m) CNT roving template. Modification of the applied current to the CNT roving allows for the tuning of depositions towards either hot-spot site-specificity or overall uniformity. The effects of temperature, pressure, precursor mass, and the interval of applied current are investigated, demonstrating nanometal depositions ranging from less than 5 % w/w to over 85 % w/w. The versatility of CVD allows for a wide variety of metals to be deposited including copper, nickel, silver, tungsten, palladium, platinum, ruthenium, rhodium, and iridium. In particular, platinum acetylacetonate [Pt(acac)2] deposits with enhanced adhesion to the CNT roving and exhibits smaller nanometal seed diameters of 5 nm compared to 40 nm for copper. The Pt(acac)2 depositions also lead to improvements in the resistance of seeded CNTs across all mass loadings studied, with the largest improvements to the specific conductivity measured with 20-50 % w/w platinum seeds. CVD seeded CNT conductors with ~30 % w/w platinum are electroplated with copper, densified, and annealed produce Cu-CNT hybrid conductors with specific conductivities as high as 5772 S·m2/kg and TCR (from 300–600 K) as low as 2.74 × 10 3 K 1, indicating good interconnection of the metal and CNT portions. Room temperature electrical conductivities of 29.8 MS/m are achieved, comparable to traditional metallic electrical conductors like aluminum and copper. High conductivity, low TCR electrical conductors such as the nanometal interconnected Cu-CNT hybrids have numerous future applications towards high efficiency motors, generators, and transformers

Synthesis and characterization of nanostructured materials

In addition to technological motivations, nanomaterials are interesting for basic scientific investigation because their properties reside in the largely unexplored realm between molecules and bulk solids. The controlled synthesis of these materials, by methods that permit their assembly into functional nanoscale structures, lies at the core of nanoscience and nanotechnology. Here, controlled synthesis refers to a process of collective nanostructure growth where the pertinent attributes such as location, size, orientation, and composition as well as the electrical, mechanical, and chemical properties of the individual elements can be predetermined by the choice of the growth conditions and the preparation of the growth substrate. This dissertation work furthers the understanding of the mechanisms by which synthesis conditions affect the morphology, composition, and crystal structure of nanostructured materials with the objective of achieving greater control over the synthesis process. Three types of systems are investigated in depth: vertically aligned carbon nanofibers (grown by plasma-enhanced chemical vapor deposition), catalytic alloy nanoparticles (sputter-deposited, carbon-encapsulated), and tungsten nanowires (grown by electron-beam-induced deposition). The effects of growth parameters on the resulting nanostructure properties are characterized by methods including high-resolution transmission electron microscopy, electron diffraction, and chemical spectroscopy

Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH4) solutions

It is clear that in order to satisfy global energy demands whilst maintaining sustainable levels of atmospheric greenhouse gases, alternative energy sources are required. Due to its high chemical energy density and the benign by-product of its combustion reactions, hydrogen is one of the most promising of these. However, methods of hydrogen storage such as gas compression or liquefaction are not suitable for portable or automotive applications due to their low hydrogen storage densities. Accordingly, much research activity has been focused on finding higher density hydrogen storage methods. One such method is to generate hydrogen via the hydrolysis of aqueous sodium borohydride (NaBH4) solutions, and this has been heavily studied since the turn of the century due to its high theoretical hydrogen storage capacity (10.8 wt%) and relatively safe operation in comparison to other chemical hydrides. This makes it very attractive for use as a hydrogen generator, in particular for portable applications. Major factors affecting the hydrolysis reaction of aqueous NaBH4 include the performance of the catalyst, reaction temperature, NaBH4 concentration, stabilizer concentration, and the volume of the reaction solution. Catalysts based on noble metals, in particular ruthenium (Ru) and platinum (Pt), have been shown to be particularly efficient at rapid generation of hydrogen from aqueous NaBH4 solutions. However, given the scarcity and expense of such metals, a transition metal-based catalyst would be a desirable alternative, and thus much work has been conducted using cobalt (Co) and nickel (Ni)-based materials to attempt to source a practical option. “Metal free” NaBH4 hydrolysis can also be achieved by the addition of aqueous acids such as hydrochloric acid (HCl) to solid NaBH4. This review summarizes the various catalysts which have been reported in the literature for the hydrolysis of NaBH4

Two-dimensional materials synthesis, characterization, and devices : working with hexagonal boron nitride and graphene

Two dimensional materials have unique properties that are anisotropic in-plane and out-of-plane. They further exhibit unique properties when they are thinned down to an isolated monolayer or a few layers. These properties have the potential to greatly impact applications in energy, computing, construction, medicine, and other industries. Many researchers have published many reports working with two dimensional (2D) materials. This dissertation describes work which has contributed to the body of research around 2D materials synthesis, characterization, and device applications primarily with graphene and hexagonal boron nitride. Graphene is a hexagonal lattice of carbon atoms which is stable in ambient down to a single monolayer. Hexagonal boron nitride is an isomorph of graphene but with boron and nitrogen atoms on the lattice instead of carbon. Chemical vapor deposition (CVD) synthesis processes have shown to be replicable and capable for obtaining 2D materials of high quality, and experimenting with process conditions has improved the understanding about the synthesis mechanisms occurring. The objective of my 2D materials synthesis work is, broadly, to better understand the mechanisms during growth for graphene and h-BN. The growth mechanism has multiple of forces acting on it, in competition, and many of them are detailed in chapter 2. Growing the body of research and knowledge about 2D materials requires us to have techniques to characterize these materials accurately and precisely. It is important to develop and demonstrate new characterization techniques which are tailored for 2D materials. In chapter 3, the research done in characterizing 2D materials and interfaces between hetero-layers will be presented. Devices which take advantage of the dimensionality and confinement within a layer of 2D material, or multiple materials, have shown high performance in a variety of applications. The range for 2D materials device applications is continually expanding and increasing in complexity. In chapter 4, research will be presented which returns to the relatively simple system of graphene to try and apply its many unique properties for a few different photovoltaic devices.Materials Science and Engineerin

FABRICATION OF SWNTs FOR WATER DESALINATION AND MULTILAYER STRUCTURE FOR DNA SEQUENCING

0.7nm single wall carbon nanotubes have been synthesized within VPI-5 zeolite channels with sucrose as carbon precursor. VPI-5 molecular sieves are synthesized hydrothermally under conventional heating. X-ray powder diffraction, micro raman, scanning electron microscope (SEM), transmission electron microscope (TEM), Thermogravimetric analysis have been used to investigate the structure of zeolite and thermal decoposition process of carbon precursors. 0.4nm single wall carbon nanotubes have also been fabricated within AlPO4-5 nanopores. A key challenge is to produce high yield single wall carbon nanotubes with uniform diameter. In order to improve the carbon nanotube yield, different organic precursors are employed. Although the problem is still the repetition and low yield of CNTs, it is still an improvement for 0.7nm SWNTs synthesis with the new template prolysis method. The novel multilayer conductor/insulator/conductor structures have been fabricated. This structure might find potential application in DNA sequential reactions because each layer might be individually addressed with voltage. When bias is applied to the conductive layer, it can be chemically functionalized, which leads to membrane pore with multiple reaction sequences when the molecule traverses the membrane reactor. In this thesis, Carbon/polymer/carbon system and copper/polymer system will be introduced. O2 RIE was used to expose the edge of carbon/polymer/carbon structure. However, the conductivity of carbon layer is not high enough for electroplating. Copper pores etched by FeCl3 solution shows good conductivity, and can be electroplated with metal nanoparticles