Probing the electric double-layer capacitance in a Keggin-type polyoxometalate ionic liquid gated graphene transistor.

A variety of device applications has been proposed using polyoxometalate-based ionic liquids. However, the assembly of large polyoxometalate ions on surfaces and the associated interfacial properties are not well understood, particularly since the assembly is influenced by steric effects and stronger ion-ion interactions. In this study, graphene transistors gated with a polyoxometalate-based ionic liquid were probed with in situ Raman spectroscopy and charge transport studies. The ionic liquid comprised Cu-substituted lacunary Keggin anions, [PW11O39Cu]5-, which were surrounded by tetraoctyl ammonium cations, (C32H68N)+. The application of gate voltage caused these ions to assemble at the interface with graphene, which resulted in a shift of the Fermi level of the graphene monolayer grown on a copper foil. The shift was determined by the quantum capacitance, Cq, of graphene in series with the electric-double layer capacitance. Estimates of the electric-double layer thickness, spatial density of the ions and temporal rate of the assembly of the electric double-layer were obtained. This study provides insights into the microscopic understanding of the electric double-layer formation at the graphene interface.

Mechanical tearing of graphene on an oxidizing metal surface.

Graphene, the thinnest possible anticorrosion and gas-permeation barrier, is poised to transform the protective coatings industry for a variety of surface applications. In this work, we have studied the structural changes of graphene when the underlying copper surface undergoes oxidation upon heating. Single-layer graphene directly grown on a copper surface by chemical vapour deposition was annealed under ambient atmosphere conditions up to 400 °C. The onset temperature of the surface oxidation of copper is found to be higher for graphene-coated foils. Parallel arrays of graphene nanoripples are a ubiquitous feature of pristine graphene on copper, and we demonstrate that these form crucial sites for the onset of the oxidation of copper, particularly for ∼0.3-0.4 µm ripple widths. In these regions, the oxidation proceeds along the length of the nanoripples, resulting in the formation of parallel stripes of oxidized copper regions. We demonstrate from temperature-dependent Raman spectroscopy that the primary defect formation process in graphene involves boundary-type defects rather than vacancy or sp(3)-type defects. This observation is consistent with a mechanical tearing process that splits graphene into small polycrystalline domains. The size of these is estimated to be sub-50 nm.

Estimating the thermal expansion coefficient of graphene: the role of graphene-substrate interactions.

The temperature-dependent thermal expansion coefficient of graphene is estimated for as-grown chemical vapor deposited graphene using temperature-dependent Raman spectroscopy. For as-grown graphene on copper, the extent of thermal expansion mismatch between substrate and the graphene layer is significant across the entire measured temperature interval, T = 90-300 K. This mismatch induces lattice strain in graphene. However, graphene grown on copper substrates has a unique morphology in the form of quasi-periodic nanoripples. This crucially influences the profile of the strain in the graphene membrane, which is uniaxial. An estimate of the thermal expansion coefficient of grapheme α(T) is obtained after consideration of this strain profile and after incorporating temperature-dependent Grüneisen parameter corrections. The value of α(T), is found to be negative (average value, -3.75 × 10(-6) K(-1)) for the entire temperature range and it approaches close to zero for T < 150 K. For graphene wet-transferred to three kinds of substrates: copper, poly-dimethylsiloxane, and SiO2/Si, the Raman shifts can largely be modeled with lattice expansion and anharmonic contributions, and the data suggests limited interfacial interaction with the substrate.