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.
We study the configuration of atomically-thin graphene membranes on tunable microhydrogel patterns. The polyethylene oxide microhydrogel structures patterned by electron-beam lithography show increase in height, with a persistent swelling ratio up to $10, upon exposure to vapors of an organic solvent. We demonstrate that modifying the height fluctuations of the microhydrogel affects the strain and morphology of ultrathin graphene membrane over-layer. Raman spectroscopic investigations indicate that small lattice strains can be switched on in mechanically exfoliated fewlayer graphene membranes that span these microhydrogel structures. In case of chemical-vapor deposited single-layer graphene, we observe Raman signatures of local depinning of the membranes upon swelling of microhydrogel pillars. We attribute this depinning transition to the competition between membrane-substrate adhesion energy and membrane strain energy, where the latter is tuned by hydrogel swelling. V C 2014 AIP Publishing LLC. [http://dx.The morphology of a thin elastic membrane is determined by minimizing the sum of interfacial bonding energy, and system strain and bending energies. 1,2 Graphene, a monolayer of carbon atoms in honeycomb lattice structure, is a particularly interesting elastic sheet not only because it represents the truly two-dimensional (2D) limit but also because the morphology of graphene films strongly couples to its electronic degrees of freedom. Graphene membranes that are suspended across trenches show ultrahigh carrier mobility since the scattering rate from substrate impurities is suppressed. 1 The resistivity of graphene has important contributions from surface-phonon scattering from the substrate and from ripples within the graphene sheet; both these in turn depend on profile of the graphene sheet on the substrate. 3 Recently, an anomalous low-temperature transport in graphene has been related to a structural-phase transition of the underlying SrTiO 3 substrate. 3 For large uniaxial strains, a band-gap can be induced in the energy spectrum of graphene, 4,5 while giant pseudo-magnetic fields have been observed in strained graphene nanobubbles grown on metal substrates. 6,7 Graphene origami electronics has also been envisaged, with device elements derived from strain and morphology dependent electronic processes. 8 Practical realization of many theoretical propositions of strain-engineered graphene is non-trivial. Nonetheless, this has generated extensive interest in control of the nanoscale morphology of graphene. 9-11 The snap-through instability of graphene problem has also attracted recent attention, where by a partially depinned state of graphene has been proposed by appropriately engineered substrate patterns. 1,2 In this paper, we demonstrate that microhydrogel patterns with tunable height fluctuations can serve as foundation for switching-on lattice strains and morphological transitions in graphene membranes on length scales amenable to lithographic fabrication. Thin layers of hydrogels on SiO 2 /Si substrat...
We investigate temperature-dependent charge transport in reduced graphene oxide (rGO) films coated on flexible polydimethylsiloxane (PDMS) substrates which are subject to uniaxial strain. Variable strain, up to 10%, results in an anisotropic morphology comprising of quasi-periodic linear array of deformations which are oriented perpendicular to the direction of strain. The anisotropy is reflected in the charge transport measurements, when conduction in the direction parallel and perpendicular to the applied strain are compared. Temperature dependence of resistance is measured for different values of strain in the temperature interval 80-300 K. While the resistance increases significantly upon application of strain, the temperature-dependent response shows anomalous decrease in resistance ratio R /R upon application of strain. This observation of favorable conduction processes under strain is further corroborated by reduced activation energy analysis of the temperature-dependent transport data. These anomalous transport features can be reconciled based on mutually competing effects of two processes: (i) thinning of graphene at the sites of periodic deformations, which tends to enhance the overall resistance by a purely geometrical effect, and (ii) locally enhanced inter-flake coupling in these same regions which contributes to improved temperature-dependent conduction.
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.
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