Tungsten disulfide is a proven material as a low friction solid coating. The material is well characterized and has proven its capabilities the last century. Graphene is this centurys most promising material with electrical and mechanical properties. With it the 2D material revolution have started. In this thesis I present a feasible way to sputter tungsten disulfide on graphene as a substrate with little damage to the graphene from energetic particles and a straight forward method to quantize the damage before and after deposition. Further I investigate compositional changes in the sputtered films depending on processing pressure and how tungsten disulfide film thickness and the amount of graphene damage affects the materials low friction capabilities. It is shown that graphene is not a viable substrate for a low friction tungsten disulfide film and that tungsten disulfide is an excellent material for low friction coatings even down too a few nanometers and that the films behavior during load in the friction testing significantly depends on the processing pressure during sputtering.

Interest in high performance portable energy devices for electronics and electric vehicles is the basis for a significant level of activity in battery research in recent history. Li-ion batteries are of particular interest due to their high energy density, decreasing cost, and adaptable form factor. A common goal of researchers is to develop new materials that will lower the cost and weight of Li-ion batteries while simultaneously improving the performance. There are several approaches to facilitate improved battery system-level performance including, but not limited to, the development of new material structures and/or chemistries, manufacturing techniques, and cell management. The performed research sought to enhance the understanding of structure-property relationships of carbon-containing composite anode materials in a Li-ion cell through extensive materials and anode performance characterization. The approach was to focus on the development of new electrode material designs to yield higher energy and power characteristics, as well as increased thermal and electrical conductivities or mechanical strength, using techniques that could be scaled for large volume manufacturing. Here, three different electrode architectures of nanomaterial composites were synthesized and characterized. Each electrode structure consisted of a carbon substrate that was conformally coated with a high Li capacity material. The dimensionality and design for each structure was unique, with each offering different advantages. The addition of an external coating to further increase the stability of high capacity materials was also investigated.

This dissertation consists of six chapters: In the first chapter, electrical and material properties and charge transport in organic semiconductors and graphene based field-effect transistors (FETs) are introduced. In the second chapter, device architectures of indenofluorene-phenanthrene copolymer based thin-film transistors (TFTs) are discussed. The combination of recessed source/drain and surface treatments on electrical contact and low-voltage-operated TFTs with solution-processed high-k dielectric are investigated. In the third chapter, device physics and charge transport of diketopyrrolopyrrole-naphthalene copolymer based TFTs are discussed. Top-gate TFTs with the polymer dielectric exhibit mobilities of ~1 cm2/V-s and charge transport measurements in steady-state and under non-quasi-static conditions reveal device physics in dual-gate configuration. In the fourth chapter, device characteristics and charge transport in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer based TFTs are focused. The ambipolar polymer TFTs possess balanced electron and hole mobilities which are both > 0.5 cm2/V-s. The trap density of states is calculated using two analytical methods developed by Lang et al. and Kalb and Batlogg. In the fifth chapter, charge transport of diketopyrrolopyrrole-thiophene copolymer based TFTs employing 4-point-probe configuration is studied. Such polymer TFTs possess the mobilities of up to 3 cm2/V-s. The activation energy as a function of carrier concentration represents multiple trapping and thermally release model or Monroe-type model of charge transport. In the sixth chapter, transformation of electrical characteristics of graphene FETs with an interacting capping layer of fluoropolymers and pi-conjugated organic semiconductors is investigated. The electrical properties of graphene by wafer-scale chemical vapor deposition can be favorably tuned by fluorocarbon capping methods. / text

Chemical vapor deposition is a process that enables the deposition of thin films material with a high degree of thickness control, composition and film quality. In an ultra-high vacuum environment (UHV), films of high purity and controlled crystal structure can be achieved. The control of the crystal structure is achieved thanks to reduced contamination, e.g. oxygen, which allows the grown film to align itself with the underlying substrate. The film purity is also ensured by the reduced amount of contaminants present in the UHV environment. This master’s thesis discusses the design and construction of a cold wall reactor using a pyrolytic graphite heater encased in a thin layer of pyrolytic boron nitride, and an Oerlikon-Leybold Turbovac 361 turbomolecular pump. This heater is shown to achieve temperatures greater than 1200°C, as well as reach pressures in the 10-10 Torr range. Graphene growth on copper is discussed as well as the ultra-high vacuum annealing of graphene devices on boron nitride substrates. The graphene growth experiments coupled with this system’s annealing capabilities demonstrate the functionality and versatility of this type of chemical vapor deposition system. / text

Two-dimensional graphene, a single layer of graphite, has emerged as an excellent candidate for future electronic material due to its unique electronic structure and remarkably high carrier mobility. Even higher carrier mobility has been demonstrated in graphene devices using hexagonal boron nitride as an underlying dielectric support instead of silicon oxide. Interestingly, both graphene and boron nitride exhibit superior thermal properties, therefore may potentially offer a solution to the increasingly severe heat dissipation problem in nanoelectronics caused by increased power density. In this thesis, we focus on the investigation of the thermal properties of graphene and hexagonal boron nitride. First, scanning thermal microscopy based on a sub-micrometer thermocouple at the apex of a microfabricated tip was employed to image the temperature profiles in electrically biased graphene devices with ~ 100 nm scale spatial resolution. Non-uniform temperature distribution in the devices was observed, and the "hot spot" locations were correlated with the charge concentrations in the channel, which could be controlled by both gate and drain-source biases. Hybrid contact and lift mode scanning has enabled us to obtain the quantitative temperature profiles, which were compared with the profiles obtained from Raman-based thermometry. The temperature rise in the channel provided an important insight into the heat dissipation mechanism in Joule-heated graphene devices. Next, thermal conductivity of suspended single and few-layer graphene was measured using a micro-bridge device with built-in resistance thermometers. Polymer-assisted transfer technique was developed to suspend graphene layers on the pre-fabricated device. The room temperature thermal conductivity values of 1-7 layer graphene were measured to be lower than that of bulk graphite, and the value appeared to increase with increasing sample thickness. These observations can be explained by the impact of the phonon scattering by polymer residue remaining on the sample surfaces. Lastly, thermal conductivity of few-layer hexagonal boron nitride sample was measured by using the same device and technique used for suspended graphene. Measurements on samples with different suspended lengths but similar thickness allowed us to extract the intrinsic thermal conductivity of the samples as well as the contribution of contact thermal resistance to the overall thermal measurement. The room temperature thermal conductivity of 11 layer sample approaches the basal-plane value reported in the bulk sample. Lower thermal conductivity was measured in a 5 layer sample than an 11 layer sample, which again supports the polymer effect on the thermal transport in few-layer hexagonal boron nitride. / text

Two-dimensional materials such as self-assembled monolayers (SAMs), graphene, etc. are candidate materials for improving the performance of microelectronics components and MEMS/NEMS devices. In view of their relatively large in-plane dimensions, surface forces are likely to dominate their behavior. The purpose of the current work was to extract not only the adhesion energy (or steady state fracture toughness) but also the traction-separation relation associated with interactions between various two-dimensional materials and substrates. In particular, interactions between SAMs terminated by carboxyl and diamine (COOH/NMe2) groups, hydroxylated silicon surfaces, graphene and silicon, graphene and its seed copper and graphene and epoxy over large areas was considered. Traction-separation relations, which are a continuum description of such molecular interactions, were determined by a direct method, which makes use of measurements of crack tip opening displacements; an inverse approach where the key parameters are extracted by comparing measured global parameters with finite element solutions and a hybrid approach in which the direct method was supplemented by finite element analysis. Furthermore, the surface free energy of graphene was measured by contact angle measurements. The most striking observation across all the interactions that were considered is that the interaction ranges were much larger than those attributed to van der Waals forces. While van der Waals models might have been at play between graphene and its seed copper foil and graphene and epoxy, the adhesion energies were surprisingly high. This coupled with the long interaction range suggests that roughness effects modulated the basic force field. Interactions between graphene and silicon and hydroxylated silicon surfaces may have been due to capillary and/or electrostatic again possibly modulated by roughness. The interactions between COOH and NMe2 SAMs became stronger under vacuum, which may have induced chemical bonding, and tougher under mixed-mode loading. / text

Two dimensional electronic materials provide a vibrant area for applying basic quantum mechanics and scattering theory. In quantum corrals, multiple scattering leads to resonances closely approximating eigenstates of an equivalently shaped billiard. We extend the analogy using methods from acoustics to demonstrate that the billiard conception of quantum corrals is a useful one even in wavelength regimes close to corral size. Resonance widths can be described by a simple relationship proportional to the perimeter to area ratio of the enclosure and the average reflection of a classical path. / Physics

When energy relaxation between electrons and the lattice is slow, an elevated electronic temperature different from that of the lattice persists. In this regime, hot charge carriers control the energy transport in a material. In this thesis, I show how hot carriers can dominate graphene's response enabling it to exhibit novel properties. First, I examine how light is converted to electrical currents in graphene and show that hot carriers play an integral role in this multi-stage process. I show that photocurrent in graphene p-n junctions is dominated by a Photo-thermoelectric effect in which a light-induced elevated hot carrier temperature drives a thermoelectric current. Furthermore, I show that the generation and cooling of hot carriers in graphene during photoexcitation proceeds in an unusual way. In the former, carrier-carrier scattering dominates the initial photoexcitation cascade enabling efficient hot carrier generation. In the latter, a new cooling mechanism - disorder-assisted scattering (supercollisions) - dominates electron-lattice cooling over a wide range of temperatures (including room temperature). Second, I examine the transport characteristics of double layer graphene heterostructures (specifically, G/h-BN/G heterostructures). I show that Coulomb coupling results in vertical (out-of-plane) energy transfer between electrons in proximal (but electrically insulated) graphene layers. This couples lateral (in-plane) charge and energy transport of electrons in the two layers to give rise to a new energy-driven Coulomb drag (inter-layer transresistance) that dominates when the two layers are at charge neutrality. Third, I examine energy transport in charge neutral graphene. I show that the combination of fast carrier-carrier scattering, high electronic quality, and slow electron-lattice cooling (hot carriers) gives rise to a regime of ballistic heat transport. This manifest as electronic energy waves with velocity on the order of graphene's Fermi velocity. The new phenomena enabled by hot carriers and the ideas/approaches described in this thesis provide a basis with which to exploit hot carrier effects in graphene and opens new vistas for controlling and harnessing energy flows on the nanoscale. / Engineering and Applied Sciences

<p>Energy storage systems are critically important for many areas in modern society including consumer electronics, transportation and renewable energy production. This dissertation summarizes our efforts on improving the performance metrics of energy storage and conversion devices through rational design and fabrication of hybrid nanomaterial systems. </p><p>This dissertation is divided into five sections. The first section (chapter 2) describes comparison of graphene and carbon nanotubes (CNTs) on improving the specific capacitance of MnO2. We show that CNTs provided better performance when used as ultrathin electrodes but they both show similar performance with rapid MnO2 specific capacitance decrease as electrodes become thicker. We further designed ternary composite electrodes consisting of CNTs, graphene and MnO2 to improve thick electrode performance (chapter 3). We demonstrate that these electrodes were flexible and mechanically strong, had high electrical conductivity and delivered much higher capacity than electrodes made without CNTs. </p><p>Chapter 4 describes assembly of flexible asymmetric supercapacitors using a graphene/MnO2/CNTs flexible film as the positive electrode and an activated carbon/CNTs flexible film as the negative electrode. The devices were assembled using roll-up approach and can operate safely with 2 V in aqueous electrolytes. The major advantage of these devices is that they can deliver much higher energy under high power conditions compared with those designed by previous studies, reaching a specific energy of 24 Wh/kg at a power density of 7.8 kW/kg. </p><p>Chapter 5 describes our approach to improve the energy and power densities of nickel hydroxides for supercapacitors. This was done by assembling CNTs with Co-Ni hydroxides/graphene nanohybrids as freestanding electrodes. The assembled electrodes have dramatically improved performance metrics under practically relevant mass loading densities (~6 mg/cm2), reaching a specific capacitance of 2360 F/g at 0.5 A/g and 2030 F/g even at 20 A/g (~86% retention). </p><p>Finally, we discuss our efforts on designing highly active electrocatalysts based on winged nanotubes for oxygen reduction reactions (ORR). The winged nanotubes were prepared through controlled oxidization and exfoliation of stacked-cup nanotubes. When doped with nitrogen, they exhibited strong activity toward catalyzing ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance owning to their unique carbon structure.</p> / Dissertation

This thesis provides the study of graphene’s electrostatic interaction with the substrate surrounding it. Mathematical models based on current experimental configurations of graphene field-effect transistors (FET) are developed and analyzed. The conductivity and mobility of charge carriers in graphene are examined in the presence of impurities trapped in the substrate near graphene. The impurities encompass a wide range of possible structures and parameters, including different types of impurities, their distance from graphene, and the spatial correlation between them. Furthermore, we extend our models to analyze the influence of impurities on the fluctuations of the electrostatic potential and the charge carrier density in the plane of graphene. The results of our mathematical models are compared with current experimental results in the literature.