Graphene under hydrostatic pressure

In-situ high pressure Raman spectroscopy is used to study monolayer, bilayer and few-layer graphene samples supported on silicon in a diamond anvil cell to 3.5 GPa. The results show that monolayer graphene adheres to the silicon substrate under compressive stress. A clear trend in this behaviour as a function of graphene sample thickness is observed. We also study unsupported graphene samples in a diamond anvil cell to 8 GPa, and show that the properties of graphene under compression are intrinsically similar to graphite. Our results demonstrate the differing effects of uniaxial and biaxial strain on the electronic bandstructure.Comment: Accepted in Physical Review B with minor change

Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids

To progress from the laboratory to commercial applications, it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene. Here we show that high-shear mixing of graphite in suitable stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. X-ray photoelectron spectroscopy and Raman spectroscopy show the exfoliated flakes to be unoxidized and free of basal-plane defects. We have developed a simple model that shows exfoliation to occur once the local shear rate exceeds 10(4) s(-1). By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from hundreds of millilitres up to hundreds of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals

Exfoliation and dispersion of layered materials through liquid phase processing

THESIS 9565Graphene is a nanomaterial that has been the focus of intense research efforts in recent times. Production methods that yield high quality graphene without the need for detrimental chemical processing are needed in order to exploit this novel material in real-world applications. Our group makes use of ultrasound-assisted exfoliation in a liquid-phase production process that can meet this challenge. In this work, the energetics governing the dispersion of graphene in a wide range of solvents has been studied. 40 solvents were tested to show that good graphene solvents are characterised by surface tensions close to 40 mJ/m2 and Hildebrand parameter close to 23 MPa1/2. Hansen solubility parameters for graphene itself have been derived as (?D) = 18.0 MPa1/2, (?p) = 9.3 MPa1/2 and (?H) = 7.6MPa1/2. The resultant calculation of the Flory-Huggins parameter has shown that the energetic cost of exfoliation is a key parameter in governing the dispersibility of pristine graphene in solvents. Graphene dispersions in aqueous media have also been prepared with the aid of surfactant stabilisers. The dispersions were composed of largely few-layer graphene with significant quantities of mono and bilayer material observed. The dispersions were analysed using colloidal theory and the graphene flakes shown to be stabilised against re-aggregation by an electrostatic potential barrier. The graphene flakes were shown to be of extremely high quality by chemical analyses, demonstrating that oxidative treatments or other functionalisation routines are not required to produce graphene in water-based systems

The importance of repulsive potential barriers for the dispersion of graphene using surfactants

We have dispersed graphene in water, stabilized by a range of 12 ionic and non-ionic surfactants. In all cases, the degree of exfoliation, as characterized by flake length and thickness, was similar. The dispersed flakes were typically 750 nm long and, on average, four layers thick. However, the dispersed concentration varied from solvent to solvent. For the ionic surfactants, the concentration scaled with the square of the zeta potential of the surfactant-coated flakes. This suggests that the concentration is proportional to the magnitude of the electrostatic potential barrier, which stabilizes surfactant-coated flakes against aggregation. For the non-ionic surfactants, the dispersed graphene concentration scaled linearly with the magnitude of the steric potential barrier stabilizing the flakes. However, the data suggested that other contributions are also important

Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds

We have studied the dispersion and exfoliation of four inorganic layered compounds, WS2, MoS2, MoSe2 and MoTe2 in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersability. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However the dispersed concentration varied greatly from solvent to solvent. For all four materials the concentration peaked for solvents with surface energy close to 70 mJ/m2, implying that all four layered compounds have surface energy close to this value. Inverse gas chromatography measurements showed MoS2 and MoSe2 to have surface energies of ~75 mJ/m2, in good agreement with dispersability measurements. However, this method suggested MoTe2 to have a considerably larger value of surface energy (~120 mJ/m2). While surface energy based solubility parameters are perhaps more intuitive for 2-dimensional materials, Hansen solubility parameters are probably more useful. Our analysis shows the dispersed concentration of all four layered materials to show well-defined peaks when plotted as a function of Hansen?s dispersive, polar and H-bonding solubility parameters. This suggests that we can associate Hansen solubility parameters of ~ P ? 18 MPa1/2, ~ D ? 8.5 MPa1/2 and ~ H ? 7 MPa1/2 with all four types of layered material. Knowledge of these properties allows the estimation of the Flory-Huggins parameter, ?, for each combination of nanosheets and solvent. We found that the dispersed concentration of each material falls exponentially with ? as predicted by solution thermodynamics. This work shows that solution thermodynamics and specifically solubility parameter analysis can be used as a framework to understand the dispersion of 2-dimensional materials. Finally, we note that in good solvents such as cyclohexylpyrrolidone, the dispersions are temporally stable with >90% of material remaining dispersed after 100 h

4D full-vector radio frequency complex magnetic susceptibility mapping. Near-field imaging of RFID tags

Radio frequency identification (RFID) is a technology permeating both everyday life and scientific applications alike. The most prolific passive tag-based system uses inductively-powered tags with no internal power source [V. Chawla and D. S. Ha, “An overview of passive RFID,” IEEE Commun. Mag. 45(9), 11–17 (2007)]. Here we demonstrate an inductive magnetic field mapping platform on the example of passive near-field RFID tags (ID-1), operating at 13.56 MHz (HF) [Identification cards - Contactless integrated circuit(s) cards - Proximity cards - Part 1: Physical characteristics, ISO/IEC 14443-1, 2000; Part 2: Radio frequency power and signal interface, ISO/IEC 14443-2, 2010; Part 3: Initialization and anticollision, ISO/IEC 14443-3, 2011; Part 4: Transmission protocol, ISO/IEC 14443-4, 2008]. With smaller modules currently being integrated in wrist-bands, watches and items of jewelry, a possible counter-measure to the reduced size is the use of flux-concentrating magnetic material - low-permeability insulating ferrites or high-permeability metallic μ-particle systems such as sendust. Sendust is a magnetically soft iron-rich alloy of Fe, Al and Si - a higher permeability cheaper alternative to permalloy. The integration of sendust components in RFID tags creates a non-trivial multiple-parameter optimization problem, which requires a quantitative RF field imaging system to be used. The RF susceptibility mapping system is comprised of a stepper-motor-driven 4-axial table, which holds the device under test (DUT) or the RFID tag assembly, a source coil (2 turns of 0.5 mm diameter wire, of overall diameter of 21 cm), a 4-micro-coil assembly, allowing for the measurement of Hx, Hy, Hz and dHz/dz, and a 4-channel Vector Network Analyzer (VNA). Four complex transmission spectra are obtained for each spatial point of a rectangular (x, y) grid, and then repeated for a different z-cut. 4D Complex Vector field maps are thus obtained. Simultaneous fitting of the real and imaginary parts of the frequency spectra is possible, at essentially any point of space, to a model comprised of two damped harmonic oscillators. This type of 3D-spatial, full-vector, complex magnetic susceptibility imaging opens ways to the integration of magnetic materials in near-field systems, and is not limited to RFID

Inkjet Printing of Silver Nanowire Networks

The development of printed electronics will require the ability to deposit a wide range of nanomaterials using printing techniques. Here we demonstrate the controlled deposition of networks of silver nanowires in well-defined patterns by inkjet printing from an optimized isopropyl alcohol–diethylene glycol dispersion. We find that great care must be taken while producing the ink and during solvent evaporation. The resultant networks have good electrical properties, displaying sheet resistances as low as 8 Ω/□ and conductivities as high as 10⁵ S/m. Such optimized performances were achieved for line widths of 1–10 mm and network thicknesses of 0.5–2 μm deposited from ∼10–20 passes while using processing temperatures of no more than 110 °C. Thin networks are semitransparent with dc to optical conductivity ratios of ∼40

Development of MoS2-CNT composite thin film from layered MoS2 for lithium batteries

Layered MoS 2 prepared by liquid-phase exfoliation has been blended with single-walled carbon nanotubes (SWNTs) to form novel composite thin films for lithium battery applications. The films were formed by vacuum filtration of blended dispersions onto nitrocellulose membranes. The resulting composite films were transferred onto Cu foil electrodes via a facile filtration//wet transfer technique from nitrocellulose membranes. The morphology of the fi lm was characterised by fi eld emission scanning electron microscopy, which suggests that the MoS 2 -SWNT composite fi lm shows good adherence to the Cu foil substrate. The MoS 2 -SWNT composite thin fi lms show strong electrochemical performance at different charge-discharge rates. The capacity of a MoS2-SWNT composite film with thickness of 1um is approximately 992 mAh g-1 after 100 cycles. The morphology study showed that the MoS2-SWNT thin film retains structural integrity after 100 cycles, while the MoS2 thin film without SWNTs displays significant cracking. In addition, the novel composite thin film preparation and transfer protocols developed in this study could be extended to the preparation of various layered-material based composite films, with the potential for new device designs for energy applications