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

High-Concentration, Surfactant-Stabilized Graphene Dispersions

A method is presented to produce graphene dispersions, stabilized in water by the surfactant sodium cholate, at concentrations up to 0.3 mg/mL. The process uses low power sonication for long times (up to 400 h) followed by centrifugation to yield stable dispersions. The dispersed concentration increases with sonication time while the best quality dispersions are obtained for centrifugation rates between 500 and 2000 rpm. Detailed TEM analysis shows the flakes to consist of 1−10 stacked monolayers with up to 20% of flakes containing just one layer. The average flake consists of ∼4 stacked monolayers and has length and width of ∼1 μm and ∼400 nm, respectively. These dimensions are surprisingly stable under prolonged sonication. However, the mean flake length falls from ∼1 μm to ∼500 nm as the centrifugation rate is increased from 500 to 5000 rpm. Raman spectroscopy shows the flake bodies to be relatively defect-free for centrifugation rates below 2000 rpm. The dispersions can be easily cast into high-quality, free-standing films. The method extends the scope for scalable liquid-phase processing of graphene for a wide range of applications

Measurement of Multicomponent Solubility Parameters for Graphene Facilitates Solvent Discovery

We have measured the dispersibility of graphene in 40 solvents, with 28 of them previously unreported. We have shown that good solvents for graphene are characterized by a Hildebrand solubility parameter of δT ∼ 23 MPa1/2 and Hansen solubility parameters of δD ∼ 18 MPa1/2, δP ∼ 9.3 MPa1/2, and δH ∼ 7.7 MPa1/2. The dispersibility is smaller for solvents with Hansen parameters further from these values. We have used transmission electron microscopy (TEM) analysis to show that the graphene is well exfoliated in all cases. Even in relatively poor solvents, >63% of observed flakes have <5 layers

Large Populations of Individual Nanotubes in Surfactant-Based Dispersions without the Need for Ultracentrifugation

Stable dispersions of single-walled carbon nanotubes have been produced using the surfactant sodium dodecylbenzene sulfonate (SDBS). Atomic force microscopy analysis shows that, on dilution of these dispersions, the nanotubes exfoliate from bundles, resulting in a concentration-dependent bundle diameter distribution which saturates at Drms ≈ 2 nm for concentrations, CNT < 0.05 mg/mL. The total bundle number density increases with concentration, saturating at ∼6 bundles per μm3 for CNT > 0.05 mg/mL. As the concentration is reduced the number fraction of individual nanotubes grows, approaching 50% at low concentration. In addition, partial concentrations of individual SWNTs approaching 0.01 mg/mL can be realized. These values are far superior to those for solvent dispersions due to repulsion stabilization of the surfactant-coated nanotubes. These methods facilitate the preparation of high-quality nanotube dispersions without the need for ultracentrifugation, thus significantly increasing the yield of dispersed nanotubes

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 dispersibility. 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 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 dispersibility measurements. However, this method suggested MoTe₂ to have a considerably larger surface energy (∼120 mJ/m²). While surface-energy-based solubility parameters are perhaps more intuitive for two-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 δ_D ∼ 18 MPa^1/2, δ_P ∼ 8.5 MPa^1/2, and δ_H ∼ 7 MPa^1/2 with all four types of layered material. Knowledge of these properties allows the estimation of the Flory–Huggins parameter, χ, for each combination of nanosheet and solvent. We found that the dispersed concentration of each material falls exponentially with χ 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 two-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

Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions

We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water−surfactant solutions. Optical characterization of these suspensions allowed the partial optimization of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with ∼3% of flakes consisting of monolayers. Atomic resolution transmission electron microscopy shows the monolayers to be generally free of defects. The dispersed graphitic flakes are stabilized against reaggregation by Coulomb repulsion due to the adsorbed surfactant. We use DLVO and Hamaker theory to describe this stabilization. However, the larger flakes tend to sediment out over ∼6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides, although X-ray photoelectron spectroscopy shows evidence of a small oxide population. Individual graphene flakes can be deposited onto mica by spray coating, allowing statistical analysis of flake size and thickness. Vacuum filtered films are reasonably conductive and are semitransparent. Further improvements may result in the development of cheap transparent conductors

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