We report selective ionic transport through controlled, high-density, subnanometer diameter pores in macroscopic single-layer graphene membranes. Isolated, reactive defects were first introduced into the graphene lattice through ion bombardment and subsequently enlarged by oxidative etching into permeable pores with diameters of 0.40 ± 0.24 nm and densities exceeding 1012 cm–2, while retaining structural integrity of the graphene. Transport measurements across ion-irradiated graphene membranes subjected to in situ etching revealed that the created pores were cation-selective at short oxidation times, consistent with electrostatic repulsion from negatively charged functional groups terminating the pore edges. At longer oxidation times, the pores allowed transport of salt but prevented the transport of a larger organic molecule, indicative of steric size exclusion. The ability to tune the selectivity of graphene through controlled generation of subnanometer pores addresses a significant challenge in the dev...

/pdf/selective-ionic-transport-through-tunable-subnanometer-pores-1jya7cyr05.pdf

Selective ionic transport through tunable subnanometer pores in single-layer graphene membranes.

The support plays an important role for supported metal catalysts by positioning itself as a macromolecular ligand, which conditions the nature of the active site and contributes indirectly but also sometimes directly to the reactivity. Metal species such as nanoparticles, clusters, or single atoms can be deposited on carbon materials for various catalytic reactions. All the carbon materials used as catalyst support constitute a large family of compounds that can vary both at textural and at structural levels. Today, the recent developments of well-controlled synthesis methodologies, advanced characterization techniques, and modeling tools allow one to correlate the relationships between metal/support/reactant at the molecular level. Based on these considerations, in this Review article, we wish to provide some answers to the question "How and why anchoring metal nanoparticles, clusters, or single atoms on carbon materials for catalysis?". To do this, we will rely on both experimental and theoretical studies. We will first analyze what sites are available on the surface of a carbon support for the anchoring of the active phase. Then, we will describe some important effects in catalysis inherent to the presence of a carbon-type support (metal-support interaction, confinement, spillover, and surface functional group effects). These effects will be commented on by putting into perspective catalytic results obtained in numerous reactions of thermal catalysis, electrocatalysis, or photocatalysis.

A Theory/Experience Description of Support Effects in Carbon-Supported Catalysts.

Molecular dynamics simulations were employed to study the transport of water and ions through pores created on the basal plane of one graphene sheet (GS). Graphene pore diameters ranged from 7.5 to 14.5 A. Different pore functionalities were considered, obtained by tethering various functional groups to the terminal carbon atoms. The ease of ion and water translocation across the pores was monitored by calculating the potential of mean force along the direction perpendicular to the GS pore. The results indicate that effective ion exclusion can be achieved only using nonfunctionalized (pristine) pores of diameter ∼7.5 A, whereas the ions can easily penetrate pristine pores of diameters ∼10.5 and 14.5 A. Carboxyl functional groups can enhance ion exclusion for all pores considered, but the effect becomes less pronounced as both the ion concentration and the pore diameter increase. When compared to a carbon nanotube of similar pore diameter, our results suggest that GS pores functionalized with COO– groups a...

Simulation Insights for Graphene-Based Water Desalination Membranes

We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically 3 graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H-SiC, and characterized by surface-science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kOhm to 225 kOhm at 4 K, with positive magnetoconductance). Low resistance samples show characteristics of weak-localization in two dimensions, from which we estimate elastic and inelastic mean free paths. At low field, the Hall resistance is linear up to 4.5 T, which is well-explained by n-type carriers of density 10^{12} cm^{-2} per graphene sheet. The most highly-ordered sample exhibits Shubnikov - de Haas oscillations which correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. We show that the high-mobility films can be patterned via conventional lithographic techniques, and we demonstrate modulation of the film conductance using a top-gate electrode. These key elements suggest electronic device applications based on nano-patterned epitaxial graphene (NPEG), with the potential for large-scale integration.

Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics

Thin films formed from small molecules are rapidly gaining importance in different technological fields. To explain their growth, methods developed for zero-dimensional atoms as the film-forming particles are applied. However, in organic thin-film growth the dimensionality of the building blocks comes into play. Using the special case of the model molecule para-Sexiphenyl, we will emphasize the challenges that arise from the anisotropic and one-dimensional nature of building blocks. Differences or common features with other rod-like molecules will be discussed. The typical morphologies encountered for this group of molecules and the relevant growth modes will be investigated. Special attention is given to the transition between a flat-lying and upright orientation of the building blocks during nucleation. We will further discuss methods to control the molecular orientation and describe the involved diffusion processes qualitatively and quantitatively.

https://iopscience.iop.org/article/10.1088/0953-8984/25/14/143202/pdf

Nucleation and growth of thin films of rod-like conjugated molecules.

In this work, effects of oxygen adsorption and diffusion on the stability, morphology, and charge transfer in single-layer graphene with structural point defects were investigated by density functional theory, specifically for the experimentally characterized monovacancy, double-vacancy, 555–777, 5555–6–7777, and Stone-Wales defects. The theoretical analysis demonstrated strengthened oxygen adsorption on defective graphene as compared to pristine graphene, resulting in trapping of the oxygen onto defects. This was accompanied by significant charge transfer of up to 3e, unlike for pristine graphene. At the same time, atomic oxygen diffuses at different rates dependent on the local environment, however with relatively low barriers (mostly <1 eV), lower than for pristine graphene, thus, revealing an interplay between diffusion and adsorption in this case. Addition of a nonempirical correction to the exchange-correlation functional to take into account London dispersion demonstrated that the vdW-DF PBE functi...

Adsorption and Diffusion of Oxygen on Single-Layer Graphene with Topological Defects

Graphene came to the forefront in the nanosciences in the early 2000s, in particular, when high-quality graphene with atomic thickness and two-dimensional extension in the micrometer range was isolated and the resulting novel electronic properties were demonstrated. Graphene has two unique features: lateral size up to tens of micrometers or larger and quantum confinement in an atomically thin sheet. It provides an excellent platform for exploring novel material properties, designing new materials, and enhancing material performance. Now, after extensive research for nearly a decade, graphene research has moved well beyond electronic applications and has begun to extend into a wide variety of disciplines. This expanded issue of MRS Bulletin is focused on graphene and consists of 20 articles and three commentaries that collectively address the major impact of graphene on materials science, highlight the newest advances, discuss challenging issues, explore applications, and reveal future directions.

/pdf/graphene-fundamentals-and-functionalities-47jlpzysdf.pdf

Graphene: Fundamentals and functionalities

The structural transition from a three-dimensional SiC lattice to a two-dimensional graphene sheet is a crucial element in the growth mechanism of graphene on SiC. An interfacial defective transition layer near the surface of the SiC substrate is believed to be an intermediate structure for graphene layer formation. The transition layer consists of SiOxCy, vacancies, and other defects in the SiC lattice, which result from Si evaporation via thermal degradation of the SiC lattice and oxidation reactions of residual oxygen and other oxygen containing molecules on the SiC surface at high temperatures. This partially oxidized and structurally degraded SiC lattice layer is formed at temperatures lower than the graphene growth temperature but then decomposes with increasing temperature, leading to graphene formation. Then, the growth mechanism for graphene on SiC (0001) in high vacuum consists of multiple steps, including Si removal by thermal decomposition and oxidation, collapsing of the near surface SiC latt...

Weijie Lu

Papers

Adsorption and Diffusion of Oxygen on Single-Layer Graphene with Topological Defects

Graphene: Fundamentals and functionalities

Interface Oxidative Structural Transitions in Graphene Growth on SiC (0001)