Nanoscale friction and wear of a polymer coated with graphene

Friction and wear of polymers at the nanoscale is a challenging problem due to the complex viscoelastic properties and structure. Using molecular dynamics simulations, we investigate how a graphene sheet on top of the semicrystalline polymer polyvinyl alcohol affects the friction and wear. Our setup is meant to resemble an AFM experiment with a silicon tip. We have used two different graphene sheets, namely an unstrained, flat sheet, and one that has been crumpled before being deposited on the polymer. The graphene protects the top layer of the polymer from wear and reduces the friction. The unstrained flat graphene is stiffer, and we find that it constrains the polymer chains and reduces the indentation depth.

Highly Durable Graphene Monolayer Electrode on Insulating Substrate under Long-term Hydrogen Evolution Cycling

Electrochemical hydrogen evolution reaction (HER) at single graphene sheets has been investigated widely either in its pristine form or after chemical modification. One important challenge is the long-term stability of single graphene sheets on Si/SiO2 substrates under HER. Previous reports have found that due to stress developing under gas evolution, the sheets tend to break apart, with a very low lifetime limited to just a few cycles of HER. Here, we show through appropriate electrode preparation that it is possible to achieve highly durable single graphene electrodes on insulating substrates, which can survive several hundreds of HER cycles with virtually no damage to the sp2-carbon framework. Through systematic investigations including atomic force microscopy, Raman spectroscopy and electroanalysis, we show that even after so many cycles, the sheet is physically intact and the electron transfer capability of the electrodes remain unaffected. This extremely high stability of a single atomic sheet of carbon, when combined with appropriate chemical modification strategies, will pave way for the realization of novel 2D electrocatalysts.

Controllable assembly of continuous hollow graphene fibers with robust mechanical performance and multifunctionalities

Macroscopic conformation of individual graphene sheets serves as the backbone of translating their intrinsic merits towards multifunctional practical applications. However, controllable and continuous assemblies of graphene-based nanomaterials to create stable macroscopic structural components are always in face of great challenge. We have developed a scalable converging-flow assisted wet-spinning methodology for continuously fabricating hollow graphene fibers (HGFs, the newest variation of solid graphene fibers) with high quality. The degradable silk thread is selectively utilized as the continuous hollow structure former that holds the coaxially stacked graphene sheets aligned through the converging-flow modulating process. For the first time, we have created the longest freestanding HGF in length of 2.1 m. The continuous HGFs are in an average diameter of 180 μm and with 4-8 μm adjustable wall thicknesses. The optimal HGF demonstrates an average tensile strength of 300 MPa and modulus of 2.49 GPa (comparable to typical solid graphene fibers, but the highest among the reported HGFs in literature) and an exceptional failure elongation of 10.8%. Additionally, our continuous HGFs exhibit spontaneous resistive response to thermal and strain stimuli (in form of large deformations and human motions), offering great potential for developing multifunctional sensors. We envision that this work demonstrates an effective and well-controlled macroscopic assembly methodology for the scaled-up mass production of HGFs.

Hyteroatoms Si, P, S as possible factors for the formation of the structure of pyrolyzed carbon materials

The aim of the study was to investigate the effect of heteroatoms on the deformation of graphene, as well as on the formation of the Stone-Wallace defect. To date, research on processes involving nanocarbon materials is relevant. In particular, in the formation of fullerenes, nanoonions and a number of other carbon nanoforms, the five-membered carbon cycles (pentactagonis) of the hepatogenesis (pentactagon) play the most important role in the curvature of initially flat graphene sheets and the formation of fullerene-like structures in the form of closed, skeletal, macromolecular formations. It should be noted, however, that the Pentagon is not the only factor in distorting the flat structure of graphene sheets in layered carbon materials. Some other defects of the carbon lattice (in particular, seven-membered carbon cycles and heteroatoms of a number of nonmetals with covalent radii exceeding the radius of the carbon atom) may play a similar role to one degree or another. These heteroatoms (primarily Si, P, S) are usually part of the precursors of mineral or vegetable origin and can be embedded in the carbon lattice in the process of coal production. Stone-Wallace there is their mutual compensation and preservation of a flat structure. The calculations were performed using quantum chemical modeling of doped nanographs in clusters of different size, composition and morphology, using the theory of density functional (DFT) with exchange-correlation functional B3LYP, based on the extended valence-split basis 6-31G (d) with full optimism clusters using the Firefly software package. It has been found that heteroatoms of non-metals with covalent radii exceeding the radius of the C atom, which are usually present in the precursors of mineral or vegetable origin used to produce pyrolyzed carbon materials, can play a significant role in energy. a number of nanoforms of carbon, activated carbon and other pyrolyzed nanostructured carbon materials.

Correction: Foroozani Behbahani, A.; Harmandaris, V. Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets. Polymers 2021, 13, 830

The authors wish to make the following corrections to this paper: [...]

Industrial-Scale Production of High-Quality Graphene Sheets by Millstone Grinders

Graphene exhibits a variety of unprecedented innate properties and has sparked great interest in both fundamental science and regarding prospective commercial applications. To meet the ever-increasing demand for high-quality graphene sheets, an industrial-scale, reliable, environmental-friendly, low-cost production process is required. However, large-scale production high quality graphene remains elusive. Here we demonstrate a scalable mechanical cleavage method for large-quantity production of high quality large-area and few-layer graphene sheets by introducing a millstone grinding process. The average thickness of the graphene sheets is around 5 nm. This procedure is simpler than the state-of-the-art methods that allows for scalable preparation of graphene dispersion in hundreds of litres by mechanical cleavage of graphite, and the yield is 30-40%. The size of the prepared graphene sheets can be tuneable from few micrometres to tens of micrometres by varying the dimension of raw graphite, which is larger than that produced by the state-of-the-art methods. Moreover, comparing to conductive agents, the conductivity of wafers containing graphene can be increased by one order of magnitude, suggesting a high potential of the prepared graphene sheets for the application as conductive agent in lithium battery cathodes. This allows the requirements of different sizes graphene sheets for industry applications in different fields.

Geometry effects in topologically confined bilayer graphene loops

We investigate the electronic confinement in bilayer graphene by topological loops of different shapes. These loops are created by lateral gates acting via gap inversion on the two graphene sheets. For large-area loops the spectrum is well described by a quantization rule depending only on the loop perimeter. For small sizes, the spectrum depends on the loop shape. We find that zero-energy states exhibit a characteristic pattern that strongly depends on the spatial symmetry. We show this by considering loops of higher to lower symmetry (circle, square, rectangle and irregular polygon). Interestingly, magnetic field causes valley splittings of the states, an asymmetry between energy reversal states, flux periodicities and the emergence of persistent currents.

Photonic Stopband Filters Based on Graphene-Pair Arrays

We investigate the photonic bandgaps in graphene-pair arrays. Graphene sheets are installed in a bulk substrate to form periodical graphene photonic crystal. The compound system approves a photonic band structure as a light impinges on it. Multiple stopbands are induced by changing the incident frequency of light. The stopbands widths and their central frequencies could be modulated through the graphene chemical potential. The number of stopbands decreases with the increase in the spatial period of graphene pairs. Otherwise, two full passbands are realized in the parameter space composed of the incident angle and the light frequency. This investigation has potentials applied in tunable multi-stopbands filters.