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There is a demand for the manufacture of two-dimensional (2D) materials with high-quality single crystals of large size. Usually, epitaxial growth is considered the method of choice in preparing single-crystalline thin films, but it requires single-crystal substrates for deposition. Here we present a different approach and report the synthesis of single-crystal-like monolayer graphene films on polycrystalline substrates. The technological realization of the proposed method resembles the Czochralski process and is based on the evolutionary selection approach, which is now realized in 2D geometry. The method relies on 'self-selection' of the fastest-growing domain orientation, which eventually overwhelms the slower-growing domains and yields a single-crystal continuous 2D film. Here we have used it to synthesize foot-long graphene films at rates up to 2.5 cm h that possess the quality of a single crystal. We anticipate that the proposed approach could be readily adopted for the synthesis of other 2D materials and heterostructures.

In pristine graphene ribbons, disruption of the aromatic bond network results in depopulation of covalent orbitals and tends to elongate the edge, with an effective force of f e ~ 2 eV/Å (larger for armchair edges than for zigzag edges, according to calculations). This force can have quite striking macroscopic manifestations in the case of narrow ribbons, as it favors their spontaneous twisting, resulting in the parallel edges forming a double helix, resembling DNA, with a pitch t of about 15 20 lattice parameters. Through atomistic simulations, we investigate how the torsion ~1/ t decreases with the width of the ribbon, and observe its bifurcation: the twist of wider ribbons abruptly vanishes and instead the corrugation localizes near the edges. The length-scale ( e ) of the emerging sinusoidal "frill" at the edge is fully determined by the intrinsic parameters of graphene, namely its bending stiffness D=1.5 eV and the edge force f e with e ~D/f e . Analysis reveals other warping confi gurations and suggests their sensitivity to the chemical passivation of the edges, leading to possible applications in sensors.

Flash Joule heating (FJH) can convert almost any carbon-based precursor into bulk quantities of graphene. This work explores the morphologies and properties of flash graphene (FG) generated from carbon black. It is shown that FG is partially comprised of sheets of turbostratic FG (tFG) that have a rotational mismatch between neighboring layers. The remainder of the FG is wrinkled graphene sheets that resemble nongraphitizing carbon. To generate high quality tFG sheets, a FJH duration of 30−100 ms is employed. Beyond 100 ms, the turbostratic sheets have time to AB-stack and form bulk graphite. Atomistic simulations reveal that generic thermal annealing yields predominantly wrinkled graphene which displays minimal to no alignment of graphitic planes, as opposed to the high-quality tFG that might be formed under the direct influence of current conducted through the material. The tFG was easily exfoliated via shear, hence the FJH process has the potential for bulk production of tFG without the need for pre-exfoliation using chemicals or high energy mechanical shear.

The diameter dependence of the collapse of single- and double-walled carbon nanotubes to two- and four-walled graphene nanoribbons with closed edges (CE(x)GNRs) has been experimentally determined and compared to theory. TEM and AFM were used to characterize nanotubes grown from preformed 4.0 nm diameter aluminum-iron oxide particles. Experimental data indicate that the energy equivalence point (the diameter at which the energy of a round and fully collapsed nanotube is the same) is 2.6 and 4.0 nm for single- and double-walled carbon nanotubes, respectively. Molecular dynamics simulations predict similar energy equivalence diameters with the use of ε = 54 meV/pair to calculate the carbon-carbon van der Waals interaction.