<p>Graphene is a recently discovered material consisting of a two-dimensional sheet of Carbon atoms arranged in an hexagonal pattern. It is a zero-gap semimetal whose electrical properties can be tuned by controlled induction of defects such as vacancies. In this work, graphene flakes were produced with the standard method of mechanical exfoliation. Afterward, we have used light optical microscopy (LOM), atomic force microscopy (AFM), Raman spectroscopy and in-situ electrical measurements to investigate the changes in structural and electrical properties after defect introduction by ion irradiation. The ion bombardment was performed with two different systems, a focused ion beam at the Microstructure laboratory and an ion accelerator at the Tandem laboratory, both at Uppsala University. The main goal of the work was to develop and test a contacting scheme for the graphene flakes that would allow us to perform in-situ I-V measurements during defect insertion. In this respect, the project was a success. The different characterization techniques yielded different types of information. LOM is useful as a first screening to identify the graphene candidates; Raman spectroscopy can provide information on both the flake thickness (mono-layer or multi-layer) and on the defect density, although the latter only qualitatively. The AFM analysis did not give significant results as it could not unambiguously discern any sign of ion impact neither on the graphene flakes nor on the substrate.</p>

Graphene is a recently created truly two-dimensional carbon material with promising properties. It is a prospective candidate for the next generation of microelectronics. Current carriers in graphene have relativistic properties, its lattice is very strong and yet flexible, granting graphene's ballistic conductivity on the submicron scale at the room temperatures. Midgap bound state induced by a single impurity in graphene does not cause essential changes in the electronic liquid distribution at all reasonable values of the coupling strength. Thus there are no unusual screening effects predicted for the graphene with long-range Coulomb impurity. This result holds in case of multiple impurities localized in the finite area on the lattice. Exact expressions for the lattice Green functions are derived. The absence of critical screening for the short-range impurities in graphene is a main result of the work. Another outcome is the observation of the limitations on the Dirac approximation applicability. / Micro-Electro-Mechanical Systems and Nanosystems

A graphene field-effect-transistor (GFET) model calibrated with extracted device parameters and a commercial 65 nm silicon MOSFET model are compared with respect to their radio frequency behavior. GFETs slightly lag behind CMOS in terms of speed despite their higher mobility. This is counterintuitive, but can be explained by the effect of a strongly nonlinear voltage-dependent gate capacitance. GFETs achieve their maximum performance only for narrow ranges of V-DS and I-DS, which must be carefully considered for circuit design. For our parameter set, GFETs require at least mu = 3000 cm(2) V-1 s(-1) to achieve the same performance as 65 nm silicon MOSFETs. / <p>QC 20130115</p>

Graphene is a single layer of graphite with many unique mechanical, electrical, and optical properties. In addition, graphene is also known to adsorb wide range of biomolecules including single-stranded DNA. On the other hand, the adsorption of double-stranded DNA was much weaker. To properly disperse in water, graphene oxide (GO) is often used due to its oxygen-containing groups on the surface. Recently, it was discovered that it could efficiently quench the fluorescence of fluorophores that were adsorbed. With these properties, it is possible to prepare DNA-based optical sensors using GO. Majority of the DNA/GO-based fluorescent sensors reported so far were relied on the complete desorption of DNA probes. Even though all these reports demonstrated the sensitivity and selectivity of the system, the fundamentals of binding between DNA and GO were hardly addressed. Understanding and controlling binding between biomolecules and inorganic materials is very important in biosensor development. In this thesis, adsorption and desorption of DNA on the GO surface under different buffer conditions including ionic strength, pH, and temperature were systematically evaluated. For instance, adsorption is favored in a lower pH and a higher ionic strength buffer. It was found that once a DNA was adsorbed on the surface, little desorption occurred even in low salt buffers. Even with high pH or temperature, only small percentage of adsorbed DNA can be desorbed. To completely desorb the DNA, complementary DNA is required. The energies and activation energies associated with DNA adsorption/desorption were measured and molecular pictures of these processes were obtained. With the fundamental understanding of the DNA/GO interaction, we demonstrated that it is possible to achieve sensor regeneration without covalent immobilization. In addition, we also achieved the separation of double-stranded DNAs from single-stranded ones without using gel electrophoresis. We also studied the fluorescence property of DNA near the GO surface using covalently attached DNA probes. It was found that the fluorophore quantum yield and lifetime changed as a function of DNA length. This study is important for rational design of covalently linked DNA sensors. This study confirmed that fluorescence quenching by GO occurs in a distance-dependent manner. Energy transfer occurred between the fluorophore and GO to result in reduced quantum yield, shorter lifetime, and lower fluorescence intensity. Although fluorescent sensors based on covalently attached DNA probes on GO have not yet been reported, the study presented here clearly supported its feasibility.

Graphene is a recently discovered material consisting of a two-dimensional sheet of Carbon atoms arranged in an hexagonal pattern. It is a zero-gap semimetal whose electrical properties can be tuned by controlled induction of defects such as vacancies. In this work, graphene flakes were produced with the standard method of mechanical exfoliation. Afterward, we have used light optical microscopy (LOM), atomic force microscopy (AFM), Raman spectroscopy and in-situ electrical measurements to investigate the changes in structural and electrical properties after defect introduction by ion irradiation. The ion bombardment was performed with two different systems, a focused ion beam at the Microstructure laboratory and an ion accelerator at the Tandem laboratory, both at Uppsala University. The main goal of the work was to develop and test a contacting scheme for the graphene flakes that would allow us to perform in-situ I-V measurements during defect insertion. In this respect, the project was a success. The different characterization techniques yielded different types of information. LOM is useful as a first screening to identify the graphene candidates; Raman spectroscopy can provide information on both the flake thickness (mono-layer or multi-layer) and on the defect density, although the latter only qualitatively. The AFM analysis did not give significant results as it could not unambiguously discern any sign of ion impact neither on the graphene flakes nor on the substrate.

The first-principles calculation method has been used to obtain electronic and structural properties of few-layer-graphenes (FLG), the layer spacing for N = 2, 3, 4, 5, 6, 7 and 8 AB stacked FLG¡¦s are calculated. It is found that the AB stacking is more favorable than the AA stacking and the layer spacing for the two-layer FLG is only 2.725&#x00C5;, which is substantially reduced from that of the graphite. The average layer spacing for 3-, 4-, 5-, 6-, 7-, and 8-layer AB stacked FLG¡¦s are 3.389&#x00C5;, 3.331&#x00C5;, 3.317&#x00C5;, 3.192&#x00C5;, 3.220&#x00C5;, and 3.220&#x00C5;, respectively, which show that the average layer spacing approaches the bulk value when the number of layers is increased. For all 2- to 8-layer AB stacked FLG¡¦s energy bands overlap near EF and near K, which show that FLG¡¦s are semi-metallic.

Various solution-processed methods have been employed in this work. For the synthesis of graphene, a chemical exfoliation method has been used to generate large graphene flakes in the solution phase. In addition, chemical or electro polymerization has been used for synthesizing polyanthracene, which tends to form graphene nanoribbon through cyclodehydrogenation. For the device fabrication, graphene oxide (GO) thin films were deposited from solution phase on the vapor-silanzed aminosilane surface to make semiconducting active layer or conducting electrodes. Gold nanoparticles (AuNPs) were selectively self-assembled from solution phase to pattern nanowires.

Almost every form of alternative energy and energy system being implemented today, e.g., wind, solar, hybrid electric and hydrogen fuel cell vehicles, depends on electrical energy storage (EES). At the DOE basic energy sciences workshop on Basic Research Needs for Electrical Energy Storage held April, 2007, the conclusion was reached that "revolutionary breakthroughs in EES are perhaps the most crucial need for this nation’s secure energy future." The workshop, chaired by John Goodenough, University of Texas-Austin, focused on the two primary methods of EES - batteries and electrochemical double-layer capacitors (also referred to as ‘ultracapacitors’ and ‘supercapacitors’). As stated in the report from this DOE workshop, “The performance of current EES technologies falls well short of requirements for using electrical energy efficiently in transportation, commercial, and residential applications.” In this dissertation, increasing the energy storage capacity of ultracapacitors through the use of graphene electrode materials is investigated. Chapter 1 is a basic overview of EES applications and ultracapacitor technology. In Chapter 2, best practice experimental procedures to accurately evaluate a material’s performance are described. Because current measurement methods for determining a material’s performance for use as an ultracapacitor electrode are not well standardized, the different techniques currently being employed lead to wide variations in reported results. Reliable methods that would accurately test a large number of samples involving minute quantities of material were required. In Chapter 3, the performance of graphene-derived materials is investigated. Chemically modified graphene materials gave values competitive with current activated carbons and an ultracapacitor based on activated graphene electrodes yielded the highest specific capacitance values reported to date. Chapter 4 describes a lithium ion hybrid supercapacitor using this novel material that gave energy densities greater than lead acid batteries. The exceptional performance of these graphene derived materials will likely result in their rapid adoption as well as an expanded range of applications utilizing ultracapacitors. As increasingly higher surface area graphene materials are developed, a fundamental understanding of the components that affect interfacial capacitance is critical for further capacity increases. In the last chapter, the first direct measurement of the interfacial capacitance for one and two sides of single layer graphene is presented. The results show that the quantum capacitance increasingly becomes a factor with the result being a reduced increase in capacitance, not the linear increase with surface area as would be expected for bulk conductive materials. These results indicate that the development of higher surface area graphene materials alone is not sufficient for additional increases in performance; the modification of the electronic properties will also be required. / text

Graphene is a two dimensional honeycomb lattice of carbon atoms which has recently attracted considerable attention because of rapid experimental progress, and because of its novel physical properties. In this work, we will discuss recent theoretical work in which we have proposed new types of ordered electronic states in graphene bilayers, including pseudospin magnets which show spontaneous charge transfer between two layers, and excitonic superfluids which could have remarkably high transition temperatures. This work will conclude with some speculations on the possibility of radically new types of electronic devices in these systems whose operation is based on collective electronic behavior. / text

The overall objective of this work was to develop processing, structure, and property relationships in graphene/polymer nanocomposites. To this end, different types of graphene platelets were produced from graphite oxide, dispersed into various thermoplastics and elastomers, and the morphology and properties of the resulting nanocomposites were evaluated. A range of tests were carried out on the nanocomposites to assess property improvements, including stress-strain testing, dynamic mechanical analysis, and thermal and electrical conductivity testing. Extensive morphological characterization, primarily through transmission electron microscopy (TEM) analysis, was performed to gain insight into the mechanisms behind the observed property improvements. The processing method used to disperse graphene platelets into a given polymer was found to exert significant influence over the nanocomposite morphology and properties. In both thermoplastics and elastomers, liquid-based dispersion methods were typically found to yield a better dispersion of graphene platelets compared with melt processing; the effectiveness of melt processing appeared to depend in part upon the method used to produce the graphene platelets. Latex compounding of graphene platelets and natural rubber generated nanocomposites with a network morphology with properties that were sensitive to further processing. The effect of graphene platelet intrinsic structure on nanocomposite properties was studied and property improvements with other nanofillers were compared to graphene platelets. The impact of platelet oxidation on nanocomposite properties was explored in two different systems and produced varying results depending on the polarity of the polymer matrix. An increased average aspect ratio of graphene platelets was not found to improve mechanical properties or a lower percolation threshold when dispersed in natural rubber. Graphene platelets produced superior reinforcement to multi-walled carbon nanotubes and exfoliated montmorillonite when dispersed in natural rubber; however, the carbon nanotubes produced the largest thermal and electrical conductivity enhancements. Qualitative observation of platelet dispersion by TEM was found to provide excellent correlation with nanocomposite properties when comparing different processing methods or filler materials. The average platelet aspect ratio of three different nanocomposite systems was determined by quantitative TEM analysis and used as a parameter in composite models to generate modulus predictions. Good agreement was found between model predictions and the experimental data. / text