The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

http://max.materials.drexel.edu/wp-content/uploads/Halim_XPS_of_MXenes-2016.pdf

X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes)

Highly stretchable graphene-nanocellulose composite nanopaper is fabricated for strain-sensor applications. Three-dimensional macroporous nanopaper from crumpled graphene and nanocellulose is embedded in elastomer matrix to achieve stretchability up to 100%. The stretchable graphene nanopaper is demonstrated for efficient human-motion detection applications.

Highly Stretchable Piezoresistive Graphene–Nanocellulose Nanopaper for Strain Sensors

Well-separated RGO sheets decorated with MnO2 nanoparticles facilitate easy access of the electrolyte ions to the high surface area of the paper electrode, enabling the fabrication of a thicker electrode with heavier areal mass and higher areal capacitance (up to 897 mF cm(-2) ). The electrochemical performance of the bent asymmetric device with a total active mass of 15 mg remains similar to the one in the flat configuration, demonstrating good mechanical robustness of the device.

Large Areal Mass, Flexible and Free-Standing Reduced Graphene Oxide/Manganese Dioxide Paper for Asymmetric Supercapacitor Device

A comprehensive overview and description of graphene-based nanomaterials explored in recent years for catalyst supports and metal-free catalysts for polymer electrolyte membrane (PEM) fuel cell oxygen reduction reactions (ORR) is presented. The catalyst material structures/morphologies, material selection, and design for synthesis, catalytic performance, catalytic mechanisms, and theoretical approaches for catalyst down-selection and catalyzed ORR mechanisms are emphasized with respect to the performance of ORR catalysts in terms of both activity and stability. When graphene-based materials, including graphene and doped graphene, are used as the supporting materials for both Pt/Pt alloy catalysts and non-precious metal catalyst, the resulting ORR catalysts can give superior catalyst activity and stability compared to those of conventional carbon-supported catalysts; when they are used as metal-free ORR catalysts, significant catalytic activity and stability are observed. The nitrogen-doped graphene materials even show superior performance compared to supported metal catalysts. Challenges including the lack of material mass production, unoptimized catalyst structure/morphology, insufficient fundamental understanding, and testing tools/protocols for performance optimization and validation are identified, and approaches to address these challenges are suggested.

A Review of Graphene‐Based Nanostructural Materials for Both Catalyst Supports and Metal‐Free Catalysts in PEM Fuel Cell Oxygen Reduction Reactions

The dispersion and the damping of the sheet plasmon in a graphene monolayer grown on Pt(111) have been studied by using angle-resolved electron energy loss spectroscopy. We found that the dispersion relation of the plasmon mode confined in the graphene sheet is linear, as a consequence of the screening by the metal substrate. Present results demonstrate that the presence of an underlying metal substrate could have striking consequences on the plasmon propagation even in the case of a system which exhibits a weak graphene-substrate interaction. Moreover, we found that Landau damping essentially occurs via interband excitations starting above the Fermi wave vector. On the contrary, intraband transitions do not have a significant influence on the collective mode.

/pdf/evidence-for-acoustic-like-plasmons-on-epitaxial-graphene-on-4725p3kk2f.pdf

Evidence for acoustic-like plasmons on epitaxial graphene on Pt(111)

Elastic properties of a macroscopic graphene sample from phonon dispersion measurements

High-resolution electron energy loss spectroscopy has been used to study the influence of a humid environment on the collective electronic excitations of a quasifreestanding graphene monolayer supported on Pt(111). We found that the adsorption of water molecules at room temperature on graphene/Pt(111) is dissociative and it gives rise to adsorbate fragments. Our results indicate that these adsorbed species change the energy, the dispersion, and the lifetime of the graphene sheet plasmon. Moreover, the phase velocity of this low-energy collective mode decreases by 11% even if its lifetime notably increases. By contrast, humidity has a negligible effect on the energy of the π plasmon at 6 eV and the unique effect induced by humidity is a significant attenuation of its intensity. Such information is essential for tailoring graphene-based plasmonic devices which should operate in ambient air humidity.

Effects of a humid environment on the sheet plasmon resonance in epitaxial graphene

Water interaction with quasi-freestanding graphene deposited on Pt(111) has been investigated by using vibrational spectroscopy. Loss measurements show that water molecules dosed at room temperature can dissociate giving rise to C-H bonds. The formation of the C-H bonds strongly attenuates the optical phonons of the graphene sheet. On the other hand, at 100 K water has been found to adsorb only in molecular state. Present findings should be taken into account in engineering graphene-based devices which should work at atmospheric pressure and at room temperature.

/pdf/water-adsorption-on-graphene-pt-111-at-room-temperature-a-3w3n1q3sxd.pdf

Water adsorption on graphene/Pt(111) at room temperature: A vibrational investigation

High-resolution electron energy loss spectroscopy has been used to probe phonon dispersion in quasi-freestanding graphene epitaxially grown on Pt(111). Loss spectra clearly show different dispersing features related to both acoustic and optical phonons. The present results have been compared with graphene systems which strongly interact with the substrate, i.e. the nearly-flat monolayer graphene (MLG)/Ni(111) and the corrugated MLG/Ru(0001). We found that the phonon dispersion of graphene/Pt(111) reproduces well the behavior of pristine graphite. This could be taken as an indication of the negligible interaction between the graphene sheet and the underlying Pt substrate. The softening of out-of-plane modes observed for interacting graphene/metal interfaces does not occur for the nearly-free-standing graphene/Pt(111).

https://iopscience.iop.org/article/10.1088/0953-8984/24/10/104025/pdf

A. R. Marino

Papers

Evidence for acoustic-like plasmons on epitaxial graphene on Pt(111)

Elastic properties of a macroscopic graphene sample from phonon dispersion measurements

Effects of a humid environment on the sheet plasmon resonance in epitaxial graphene

Water adsorption on graphene/Pt(111) at room temperature: A vibrational investigation

Phonon dispersion of quasi-freestanding graphene on Pt(111).