D. W. Boukhvalov

Control of Graphene's Properties by Reversible Hydrogenation: Evidence for Graphane

Although graphite is known as one of the most chemically inert materials, we have found that graphene, a single atomic plane of graphite, can react with atomic hydrogen, which transforms this highly conductive zero-overlap semimetal into an insulator. Transmission electron microscopy reveals that the obtained graphene derivative (graphane) is crystalline and retains the hexagonal lattice, but its period becomes markedly shorter than that of graphene. The reaction with hydrogen is reversible, so that the original metallic state, the lattice spacing, and even the quantum Hall effect can be restored by annealing. Our work illustrates the concept of graphene as a robust atomic-scale scaffold on the basis of which new two-dimensional crystals with designed electronic and other properties can be created by attaching other atoms and molecules.

Modeling of Graphite Oxide

Based on density functional calculations, optimized structures of graphite oxide are found for various coverages by oxygen and hydroxyl groups, as well as their ratio corresponding to the minimum of total energy. The model proposed describes well-known experimental results. In particular, it explains why it is so difficult to reduce the graphite oxide up to pure graphene. Evolution of the electronic structure of graphite oxide with the coverage change is investigated.

HYDROGEN ON GRAPHENE: ELECTRONIC STRUCTURE, TOTAL ENERGY, STRUCTURAL DISTORTIONS AND MAGNETISM FROM FIRST-PRINCIPLES CALCULATIONS

Density-functional calculations of electronic structure, total energy, structural distortions, and magnetism for hydrogenated single-layer, bilayer, and multilayer graphenes are performed. It is found that hydrogen-induced magnetism can survive only at very low concentrations of hydrogen (single-atom regime) whereas hydrogen pairs with optimized structure are usually nonmagnetic. Chemisorption energy as a function of hydrogen concentration is calculated, as well as energy barriers for hydrogen binding and release. The results confirm that graphene can be perspective material for hydrogen storage. Difference between hydrogenation of graphene, nanotubes, and bulk graphite is discussed.

Chemical Functionalization of Graphene with Defects

Defects change essentially not only the electronic properties but also the chemical properties of graphene, being centers of its chemical activity. Their functionalization is a way to modify the electronic and crystal structure of graphene, which may be important for graphene-based nanoelectronics. Using hydrogen as an example, we have simulated a chemistry of imperfect graphene for a broad class of defects (Stone-Wales (SW) defects, bivacancies, nitrogen substitution impurities, and zigzag edges) by density functional calculations. We have studied also an effect of finite width of graphene nanoribbons on their chemical properties. It is shown that magnetism at graphene edges is fragile, with respect to oxidation, and, therefore, chemical protection of the graphene edges may be required for the application of graphene in spintronics. At the same time, hydrogenation of the SW defects may be a prospective way to create magnetic carbon.

Chemical functionalization of graphene

Experimental and theoretical results on chemical functionalization of graphene are reviewed. Using hydrogenated graphene as a model system, general principles of the chemical functionalization are formulated and discussed. It is shown that, as a rule, 100% coverage of graphene by complex functional groups (in contrast with hydrogen and fluorine) is unreachable. A possible destruction of graphene nanoribbons by fluorine is considered. The functionalization of infinite graphene and graphene nanoribbons by oxygen and by hydrofluoric acid is simulated step by step.

Tuning the gap in bilayer graphene using chemical functionalization : Density functional calculations

Opening, in a controllable way, the energy gap in the electronic spectrum of graphene is necessary for many potential applications, including an efficient carbon-based transistor. We have shown that this can be achieved by chemical functionalization of bilayer graphene. Using various dopants, such as H, F, Cl, Br, OH, CN, CCH, NH2, COOH, and CH3 one can vary the gap smoothly between 0.64 and 3 eV and the state with the energy gap is stable corresponding to the lowest-energy configurations. The peculiarities of the structural properties of bilayer graphene in comparison with bulk graphite are discussed.

Origin of Anomalous Water Permeation through Graphene Oxide Membrane

Water inside the low-dimensional carbon structures has been considered seriously owing to fundamental interest in its flow and structures as well as its practical impact. Recently, the anomalous perfect penetration of water through graphene oxide membrane was demonstrated although the membrane was impenetrable for other liquids and even gases. The unusual auxetic behavior of graphene oxide in the presence of water was also reported. Here, on the basis of first-principles calculations, we establish atomistic models for hybrid systems composed of water and graphene oxides revealing the anomalous water behavior inside the stacked graphene oxides. We show that formation of hexagonal ice bilayer in between the flakes as well as melting transition of ice at the edges of flakes are crucial to realize the perfect water permeation across the whole stacked structures. The distance between adjacent layers that can be controlled either by oxygen reduction process or pressure is shown to determine the water flow thus highlighting a unique water dynamics in randomly connected two-dimensional spaces.

Direct experimental evidence of metal-mediated etching of suspended graphene.

Atomic resolution high angle annular dark field imaging of suspended, single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al, and Au atoms had been deposited, was carried out in an aberration-corrected scanning transmission electron microscope. In combination with electron energy loss spectroscopy, employed to identify individual impurity atoms, it was shown that nanoscale holes were etched into graphene, initiated at sites where single atoms of all the metal species except for gold come into close contact with the graphene. The e-beam scanning process is instrumental in promoting metal atoms from clusters formed during the original metal deposition process onto the clean graphene surface, where they initiate the hole-forming process. Our observations are discussed in the light of calculations in the literature, predicting a much lowered vacancy formation in graphene when metal ad-atoms are present. The requirement and importance of oxygen atoms in this process, although not predicted by such previous calculations, is also discussed, following our observations of hole formation in pristine graphene in the presence of Si-impurity atoms, supported by new calculations which predict a dramatic decrease of the vacancy formation energy, when SiO(x) molecules are present.

A Computational Investigation of the Catalytic Properties of Graphene Oxide: Exploring Mechanisms by using DFT Methods

Herein we describe a computational study undertaken in an effort to elucidate the reaction mechanisms behind the experimentally observed oxidations and hydrations catalyzed by graphene oxide (GO). We used the oxidation of benzyl alcohol to benzaldehyde as a model reaction and DFT calculations revealed that the reaction occurred via the transfer of hydrogen atoms from the organic molecule to the GO surface. In particular, neighboring epoxide groups that decorate the GO basal plane were ring‐opened, which resulted in the formation of diols, followed by dehydration. Our calculations were consistent with the experimentally observed dependence of this chemistry on molecular oxygen, and revealed that the partially reduced catalyst was able to be recharged by molecular oxygen, which allows for catalyst turnover. Functional group‐free carbon materials, such as graphite, were calculated as having substantially higher reaction barriers, which indicates that the high chemical potential and rich functionality of GO are necessary for the observed reactivity.

Destruction of graphene by metal adatoms

The formation energies for mono- and bivacancies in graphene in the presence of adatoms of various metals and small metallic clusters have been calculated. It is shown that transition metal impurities such as iron, nickel, and, especially, cobalt reduce dramatically the vacancy formation energies whereas gold impurities have almost no effect on characteristics of the vacancies. These results highlight that special measures are required in order to protect graphene from damage by transition metal leads.