Adsorption of hydrogen atoms on a single graphite sheet ͑graphene͒ has been investigated by first-principles electronic structure means, employing plane-wave based periodic density functional theory. A 5 ϫ 5 surface unit cell has been employed to study single and multiple adsorptions of H atoms. Binding and barrier energies for sequential sticking have been computed for a number of configurations involving adsorption on top of carbon atoms. We find that binding energies per atom range from ϳ0.8 to ϳ1.9 eV, with barriers to sticking in the range 0.0-0.15 eV. In addition, depending on the number and location of adsorbed hydrogen atoms, we find that magnetic structures may form in which spin density localizes on a ͱ 3 ϫ ͱ 3R30°sublattice and that binding ͑barrier͒ energies for sequential adsorption increase ͑decrease͒ linearly with the site-integrated magnetization. These results can be rationalized with the help of the valence-bond resonance theory of planar conjugated systems and suggest that preferential sticking due to barrierless adsorption is limited to formation of hydrogen pairs.

We report on the local atomic and electronic structures of a nitrogen-doped graphite surface by scanning tunneling microscopy, scanning tunneling spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. The nitrogen-doped graphite was prepared by nitrogen ion bombardment followed by thermal annealing. Two types of nitrogen species were identified at the atomic level: pyridinic-N (N bonded to two C nearest neighbors) and graphitic-N (N bonded to three C nearest neighbors). Distinct electronic states of localized π states were found to appear in the occupied and unoccupied regions near the Fermi level at the carbon atoms around pyridinic-N and graphitic-N species, respectively. The origin of these states is discussed based on experimental results and theoretical simulations.

We study n ϫ n honeycomb superlattices of defects in graphene. The considered defects are missing p z orbitals and can be realized by either introducing C atom vacancies or chemically binding simple atomic species at the given sites. Using symmetry arguments and electronic-structure calculations we show that it is possible to open a band gap without breaking graphene point symmetry. This has the advantage that new Dirac cones appear right close to the gapped region. We find that the induced gaps have an approximate square-root dependence on the defect concentration x =1/ n 2 and compare favorably with those found in nanoribbons at the same length scale.

We investigate graphene superlattices of nitrogen and boron substitutional defects and by using symmetry arguments and electronic structure calculations we show how such superlattices can be used to modify graphene band structure. Specifically, depending on the superlattice symmetry, the structures considered here can either preserve the Dirac cones (D 6h superlattices) or open a band gap (D 3h ). Relevant band parameters (carriers effective masses, group velocities and gaps, when present) are found to depend on the superlattice constant n as 1/n p where p is in the range 1 − 2, depending on the case considered. Overall, the results presented here show how one can tune the graphene band structure to a great extent by modifying few superlattice parameters.

We investigate the details of the electronic structure in the neighborhoods of a carbon atom vacancy in graphene by employing magnetization-constrained density-functional theory on periodic slabs, and spin-exact, multi-reference, second-order perturbation theory on a finite cluster. The picture that emerges is that of two local magnetic moments (one π-like and one σ-like) decoupled from the π band and coupled to each other. We find that the ground state is a triplet with a planar equilibrium geometry where an apical C atom opposes a pentagonal ring. This state lies ∼0.2 eV lower in energy than the open-shell singlet with one spin flipped, which is a bistable system with two equivalent equilibrium lattice configurations (for the apical C atom above or below the lattice plane) and a barrier ∼0.1 eV high separating them. Accordingly, a bare carbon-atom vacancy is predicted to be a spin-one paramagnetic species, but spin-half paramagnetism can be accommodated if binding to foreign species, ripples, coupling to a substrate, or doping are taken into account.