Graphene: An impermeable or selectively permeable membrane for atomic species?

Graphene is generally thought to be a perfect membrane that can block completely the penetration of impurities and molecules. Here we use density-functional theory calculations to examine this property with respect to prototype atomic species. We find that hydrogen and oxygen atoms have, indeed, prohibitively large barriers (4.2 eV and 5.5 eV) for permeation through a defect-free graphene layer. We also find, however, that boron permeation occurs by an intricate bond switching synergistic process with an activation energy of only 1.3 eV, indicating easy B penetration upon moderate annealing. Nitrogen permeation has an intermediate activation energy of 3.2 eV. The results show that by controlling annealing conditions, pristine graphene could allow the selective passage of atoms.     

Impermeability of graphene and its applications

This review discusses the genesis of impermeability in graphene and its extraordinary applications in fluid-encasement for wet electron-microscopy, selective gas-permeation, nanopore-bio-diffusion, and barrier coating against rusting and environmental hazards. As the thinnest material, graphene is composed of sp² hybridized carbon atoms linked to one another in a 2D honeycomb lattice with high electron-density in its aromatic rings, which blocks-off all molecules. This phenomena, in combination with its strong structure (C–C bond energy = 4.9 eV and intrinsic strength = 43 N/m) makes graphene the most impermeable membrane (thinnest membrane that is impermeable). Apart from the applications mentioned above, graphene coatings have enabled fundamental studies on chemical processes and fluid structures. For example, graphene can allow electron imaging of nanocrystal nucleation process and water-lattice-structure due to its impermeability. Along with being the strongest, most conductive, and optically-absorbing material (∼2.3% optical absorbance), graphene’s impermeability opens a wide range of exciting opportunities.     

First-principles prediction of the transition from graphdiyne to a superlattice of carbon nanotubes and graphene nanoribbons

Graphdiyne is a recently-synthesized carbon allotrope with a framework of sp- and sp²-hybridized carbon atoms. From first-principles calculations, we propose a possible transition of graphdiyne to a novel carbon allotrope (h-carbon) whose structure is a superlattice of carbon nanotubes and graphene nanoribbons. The energy barrier of this endothermic transition was estimated to be 4.30 kcal/mol at zero pressure, which is much lower than that of the graphite–diamond transition at high pressure. First-principles calculations on the phonon spectrum and the elastic constants of the h-carbon revealed that it is kinetically and mechanically stable. This unique framework of sp²- and sp³-hybridized carbon atoms is energetically neutral versus diamond. The hardness of the h-carbon (35.52 GPa) is 1/3 that of diamond and very close to β-SiC crystal. Accurate electronic structure calculations based on the Heyd, Scuseria, and Ernzerhof approach and GW approximation indicate that the h-carbon is a semiconducting material with a band gap of 2.20–2.56 eV.     

Identifying atomic sites in N-doped pristine and defective graphene from ab initio core level binding energies

Theoretical calculations have been carried out to predict N(1s) binding energy values in N-doped graphene which take into account initial and final state effects. A simple way to carry out ΔSCF Hartree–Fock calculations is proposed, validated against experiment for a series of N-containing organic molecules and applied to realistic N-doped nanosized pristine and defective graphene models. Final state effects appear to be important and strongly suggest that only two types of N are likely to be detected on N-doped pristine graphene by X-ray Photoelectron Spectroscopy with binding energy values of 398.6 and 400.5 eV, respectively and relative to C(1s) at 285 eV in agreement to recent experiments for quasi free standing N-doped graphene. Two cases of N-doping in defective graphene have also been considered and calculated results compared with recent experimental measurements. Calculated values for C(1s) including final state effects strongly suggests that values for core level binding energy of N and other dopants will be close to their absolute values if referred to C(1s) at 290.2 eV. The proposed approach is general enough to be successfully applied to other cases of interest.     

A graphene flake under external electric fields reconstructed from field-perturbed kernels

The energy, dipole moment, and polarizability of a finite hydrogen terminated zigzag graphene flake (C₄₆H₂₀, in 2 × 7 rings) are calculated in the absence of and in the presence of external electric fields reaching 5 × 10⁹ Vm⁻¹ [=0.01 atomic units (a.u.)]. The field intensities studied are typically found in nanoelectronics and in the tip-sample gap of a scanning tunneling microscope (STM). The change in the total energy ΔE(eV) can be closely fitted to a quadratic function of the field {1.549 × 10⁴ [E (a.u.)]²} while the dipole moment μ(debye) to a linear function [2.7 × E (a.u.)]. The results obtained with three different chemical models [MP2, density functional theory (B3LYP), and Hartree–Fock calculations, all with a 6-311G(2d,2p) basis set] are consistent in both trends and in absolute values. These results obtained from direct calculations are reproduced with a remarkable accuracy from a linear scaling fragmentation scheme called the kernel energy method (KEM). The KEM reproduces all studied field-free and response properties of this graphene flake model, in relative and absolute terms, independently of the underlying chemical model. An observation consistent with the known stiffness of graphene is that geometry optimization under a field as strong as 0.01 a.u. insignificantly alters the total energy and the geometry of a (smaller) zigzag C₂₈H₁₄ flake, the difference in the field-induced stabilization energy (ΔΔE) being only 0.006 eV (less than 1% of ΔE) and the average field-induced displacement of nuclear positions ∼0.0046 Å [B3LYP/6-31G(d,p)].     

Electron-beam engineering of single-walled carbon nanotubes from bilayer graphene

Bilayer graphene nanoribbons (BGNRs) with a predefined width have been produced directly from bilayer graphene using a transmission electron microscope (TEM) in scanning mode operated at 300 kV. The BGNRs have been subsequently imaged in high-resolution TEM mode at 80 kV. During imaging, the interaction of the electrons with the sample induces structural transformations in the BGNR, such as closure of the edges and thinning, leading to the formation of a single-walled carbon nanotube (SWCNT). We demonstrate using molecular dynamics simulations that the produced SWCNT is, in fact, a flattened SWCNT with elliptical circumference. Density functional theory calculations show that the band gap of the flattened semiconducting SWCNTs is significantly smaller than that of the undeformed semiconducting SWCNTs, and this effect is particularly profound in narrow SWCNTs.     

New insights into the properties and interactions of carbon chains as revealed by HRTEM and DFT analysis

Atomic carbon chains have raised interest for their possible applications as graphene interconnectors as the thinnest nanowires; however, they are hard to synthesize and subsequently to study. We present here a reproducible method to synthesize carbon chains in situ TEM. Moreover, we present a direct observation of the bond length alternation in a pure carbon chain by aberration corrected TEM. Also, cross bonding between two carbon chains, 5 nm long, is observed experimentally and confirmed by DFT calculations. Finally, while free standing carbon chains were observed to be straight due to tensile loading, a carbon chain inside the walls of a carbon nanotube showed high flexibility.     

The role of SiC as a diffusion barrier in the formation of graphene on Si(111)

In this paper, we investigate the role of SiC as a diffusion barrier for Si in the formation of graphene on Si(111) via direct deposition of solid-state carbon atoms in ultra-high vacuum. Therefore, various thicknesses of the SiC layer preformed on the Si substrates were produced in order to evaluate its influence on the quality of graphene formation at different substrate temperatures from 900 °C to 1100 °C. At a given temperature of 1100 °C, we found that a thicker SiC layer can suppress silicon-out diffusion from the substrate and improve the structural quality of the graphene layer. The samples were analyzed by low energy electron diffraction, Auger electron spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, and scanning tunneling microscopy.     

Growth of single and multi-layer graphene on Ir(1 0 0)

We report on the high temperature chemical vapor deposition of ethylene on Ir(1 0 0) and the resulting development of single and multi-layer graphene films. By employing X-ray photoemission electron spectromicroscopy, low energy electron microscopy and related microprobe methods, we investigate nucleation and growth of graphene as a function of the concentration of the chemisorbed carbon lattice gas. Further, we characterize the morphology and crystal structure of graphene as a function of temperature, revealing subtle changes in bonding occurring upon cooling from growth to room temperature. We also identify conditions to grow multi-layer flakes. Their thickness, unambiguously determined through the analysis of the intensity of the Ir 4f and C 1s emission, is correlated to the electron reflectivity at very low kinetic energy. The effective attenuation length of electrons in few-layer graphene is estimated to be 4.4 and 8.4 Å at kinetic energies of 116 and 338 eV, respectively.     

Graphene growth on Pt(111) and Au(111) using a MBE carbon solid-source

In this work we present a Molecular Beam Epitaxy (MBE) growth method to obtain graphene on noble metals using evaporation of carbon atoms from a carbon solid-source in ultra-high vacuum conditions. We have synthesized graphene (G) on different metal surfaces: from a well studied substrate as platinum, to a substrate where it can only be formed using innovative methods, as is the case of gold. For the characterization of the graphene layers we have used in situ surface science techniques as low energy electron diffraction (LEED), auger electron spectroscopy (AES) and scanning tunneling microscopy (STM).One of the main advantages of our methodology is that low surface temperatures are required to form graphene. Thus, by annealing Pt(111) and Au(111) substrates up to 650 °C and 550 °C respectively during carbon evaporation, we have obtained the characteristic LEED diagrams commonly attributed to graphene on these surfaces. STM results further prove the formation of graphene. For the case of G on Pt(111), STM images show a long range ordering associated with moiré patterns that correspond to a monolayer of graphene on (111) platinum surface. On the other hand, G/Au(111) STM results reveal the formation of dendritic islands pinned to atomic step edges. This method opens up new possibilities for the formation of graphene on many different substrates with potential technological applications.     

Hydrothermal synthesis of titanate nanostructures with high UV absorption characteristics

The titanate nanostructures with high UV absorption characteristics could be fabricated by hydrothermal method within a temperature range of 90–150 °C. TEM, XRD, BET analyses, and UV–vis spectroscopy were employed to elucidate the synthesized titanate nanostructure characteristics which were microstructure, phase transformation, specific surface area, and band gap energy, respectively. With an increase in the hydrothermal treating temperature from 90 to 120 °C, the specific surface area of titanate nanostructures was increased from 83 to 258 m²/g, while the band gap energy of titanate nanostructures was increased from 3.44 to 3.84 eV and then slightly decreased to 3.81 eV at 150 °C. The fabricated titanate nanostructures could exhibit higher UV adsorption capability but lower photocatalytic activity when compared with that of commercial TiO₂ powders.     

Periodically rippled graphene on Ru(0 0 0 1): A template for site-selective adsorption of hydrogen dimers via water splitting and hydrogen-spillover at room temperature

The interaction of water with periodically rippled graphene deposited on Ru(0 0 0 1) and nearly-flat graphene/Pt(1 1 1) has been investigated by using high-resolution electron energy loss spectroscopy. Graphene samples were exposed to ambient air humidity as well as to water molecules under controlled conditions in vacuum. In both cases, loss measurements show that water molecules dosed at room temperature dissociate giving rise to C–H bonds. We suggest that water molecules intercalate under graphene and are split by the underlying metal catalyst. On the lattice-mismatched graphene/Ru(0 0 0 1) interface, the corrugation of the graphene overlayer induces site selectivity for H adsorption in ortho and para dimers. On the other hand, no dimer formation occurs for the nearly-flat, multidomain graphene/Pt(1 1 1) interface.     

Surface defect states analysis on diamond by photoelectron emission yield experiments

Total photoyield spectroscopy (TPYS) was applied on partially hydrogen-terminated IIa (001) diamond surfaces, to investigate the variation of the negative electron affinity (NEA) and formation of defects by hydrogen evaporation. According to the results, we find that the amplitude of negative electron affinity is not changed by partial hydrogen evaporation. The negative electron affinity of residual hydrogen-terminated areas is maintained at around − 1.0 eV. Surface states are generated by partial hydrogen desorption at around 1.6 ± 0.1 eV above the valence band maximum. Hydrogen-free surfaces exhibited positive electron affinity. These experiments demonstrate that TPYS is a promising technique for surface defect characterization.     

New carbon cone nanotip for use in a highly coherent cold field emission electron microscope

A new cathode for cold-field emission gun using a pyrolytic carbon-cone supported onto a carbon nanotube as the electron emitting tip has been developed. This tip was mounted in a TEM using a FIB based method, and the brightness measured under real operating conditions is five times better than obtained with a standard tungsten tip. Its use overcomes the many technical difficulties which have dogged the use of carbon nanotube-based tips as proposed replacements for tungsten tips. The resulting properties of the final CFEG exhibit a very good energy spread of 0.32 eV, a reduced brightness of 1.6 × 10⁹ A m^?2 sr^?1 V^?1 and a very good long-term stability with a current damping less than 16% per hour.     

Exciton and piezoelectricity in insulating 2-dimensional boron carbide

Using first-principles density functional theory, we predict a hexagonal structure of boron carbide with two shells, which consists of the sp² hybridized boron and carbon in (001) plane and the pz–pz (σ) bonding carbon along [001] direction. The calculated results show that the structure is thermodynamically stable and possesses lower formation energy than other candidates. In addition, the quasiparticle calculations within the GW approximation reveal that the boron carbide, which is a two dimensional insulator, exhibits the indirect band gap of 2.4 eV and large exciton bonding energy of 1.35 eV. In optical absorption spectra, a bright Frenkel class bound exciton has been discovered at about 2.98 eV, which is desirable for light emitting applications. Besides, the piezoelectric coefficient (e₂₂) of −2.38×10⁻¹⁰ Cm⁻¹ is predicted for monolayer boron carbide, which indicates that the monolayer boron carbide is a potential candidate for piezoelectric applications in the nanoelectromechanical systems.     

Band-gap tuning of monolayer graphene by oxygen adsorption

We have performed high-resolution angle-resolved photoemission spectroscopy of oxygen-adsorbed monolayer graphene grown on 6H–SiC(0 0 0 1). We found that the energy gap between the π and π¹ bands gradually increases with oxygen adsorption to as high as 0.45 eV at the 2000 L oxygen exposure. A systematic shrinkage of the π¹ electron Fermi surface was also observed. The present result strongly suggests that the oxidization is a useful technique to create and control the band gap in monolayer graphene.     

Optical properties of ultrananocrystalline diamond/amorphous carbon composite films prepared by pulsed laser deposition

Ultrananocrystalline diamond (UNCD)/amorphous carbon (a-C) composite thin films were grown in ambient hydrogen by pulsed laser deposition using a graphite target, and their optical properties were determined by optical absorption spectroscopy and Raman scattering spectroscopy. Three optical bandgaps exist. Two bandgaps are indirect and their values were estimated to be 1.0 eV and 5.4 eV; these bandgaps correspond to the a-C surrounding the UNCDs and the UNCDs respectively. The third bandgap is direct and has a value of 2.2 eV, which significantly contributes to a large absorption coefficient, (10⁶ cm^− 1 at 3.0 eV). Possible origins of the third bandgaps are the grain boundaries (GBs) between the UNCDs and the a-C since they are specific to the UNCD/a-C composite films. The infrared absorption spectrum and the Raman scattering spectrum revealed the incorporation of hydrogen in the GBs. The hydrogen incorporated in the GBs might also have some influence on the appearance of the direct bandgap and its value.     

The transformation of a gold film on few-layer graphene to produce either hexagonal or triangular nanoparticles during annealing

The shape transformation of gold directly on graphene has been well studied by thermally annealing gold-deposited graphene samples at the temperature range from 600 to 800 °C. We find that few-layer graphene can be served as a platform to transform a gold film into mainly hexagonal gold nanoparticles (AuNPs) at 600 or 700 °C, or coexistence of hexagonal and triangular AuNPs at 800 °C. Especially, the size and density of these AuNPs are dependent on the number of graphene layers, indicating the strong relationship between gold shape transformation and the number of graphene layers on the substrate. We propose that annealing-induced growth of gold islands and the layer-dependent interactions among Au and n-layer graphene are the two main causes for this shape transformation. Meanwhile, Raman enhancing effects of these AuNPs are also investigated. These faceted AuNPs exhibit excellent SERS effects on Raman spectra of few-layer graphene with the enhancement factors up to several hundreds. Combined with n-layer graphenes, these faceted AuNPs can be used as graphene-based SERS substrates for increasing Raman signals of adsorbed rhodamine 6G molecules with a larger scale than those based on fresh graphenes.     

Probing graphene interfaces with secondary electrons

The empty states of graphene and graphene adsorbed on nickel (1 1 1) are studied using a plane-wave pseudo-potential method based on local density functional theory. The analysis is used to assign spectroscopic features observed in secondary electron emission experiments either to the graphene sheet or to the nickel substrate. The calculations provide insights into the key role of the graphene–substrate interaction, through the hybridization between carbon and nickel states, and indicate secondary electron emission as an efficient probe for interfaces made of two dimensional crystals.