Ab initio study of noncovalent sidewall functionalization of carbon nanotubes

We investigated noncovalent molecular adsorptions onto carbon nanotubes (CNTs), using density-functional theory methods including recently developed hybrid functionals. Planar aromatic molecules have greater binding strength than fully saturated nonaromatic molecules, and adsorption geometries are consistent with the features of intermolecular ?? -?? stacking. We show that modifications with alcohol or a thiol terminal group increase the adsorption strength consistently throughout the considered molecules. This result may be used in experiments involving noncovalent sidewall functionalizations of CNTs.open7

Formation of polybromine anions and concurrent heavy hole doping in carbon nanotubes

Using density-functional theory calculations, we investigate the atomic and electronic structure of the bromine species encapsulated in carbon nanotubes. We find that the odd-membered molecular structures (Br3 and Br5) are energetically favored than the common Br2 molecule. The transformation from bromine molecules (Br2) into Br3 or Br5 is found to be almost barrierless. A strong electron transfer from the nanotube to the adsorbates, which has been doubtful in previous studies, is accompanied by the formation of such odd-membered polybromine anions. We suggest that the tip-opened carbon nanotube samples can be heavily hole-doped after exposure to Br2 gas.open3

Effective metal dispersion in pyridinelike nitrogen doped graphenes for hydrogen storage

We study the pyridinelike nitrogen-doped graphene (PNG) with dispersed transition metal (TM) atoms as a potential hydrogen storage medium using the pseudopotential density functional method. It is found that highly localized states near the Fermi level, which are derived from the nitrogen defects, contribute to strong TM bindings and favorable hydrogen adsorption in the PNG. The strong TM binding prevents the metal aggregation and improves the material stability. The hydrogen molecular binding energy in TM+PNG complex is shown to be optimistic for room temperature storage and release.open403

Pressure-dependent Schottky barrier at the metal-nanotube contact

We carry out first-principles density-functional calculations to investigate the electronic structure of the gold-carbon nanotube contact. It is found that a pressure applied on the gold-nanotube contact shifts the Fermi level from the valence edge to the conduction edge of the carbon nanotube. This can explain the n -type transport behavior frequently observed in the nanotube field-effect transistor using the gold as electrodes. An atomistic model is proposed for a possible origin of the pressure when the nanotube is embedded in the gold electrode.open101

Extremely large perpendicular magnetic anisotropy of an Fe(001) surface capped by 5d transition metal monolayers: A density functional study

Significant enhancement of the magnetocrystalline anisotropy (MCA) of an Fe(001) surface capped by 4d and 5d transition metal monolayers is presented in this study using first principles density functional calculations. In particular, an extremely large perpendicular MCA of +10 meV/Ir was found in Ir-capped Fe(001), which originates not from the Fe but from the large spin-orbit coupling of the Ir atoms. From the spin-channel decomposition of the MCA matrix and electronic structure analyses, we find that strong 3d-5d band hybridization in the minority spin state is responsible for the sign changes of the MCA from parallel to perpendicular.open0

Effect of charge-transfer complex on the energy level alignment between graphene and organic molecules

We performed density-functional theory calculations to study the electronic structures at the interfaces between graphene and organic molecules that have been used in organic light-emitting diodes. In terms of work function, graphene itself is not favorable as either anode or cathode for commonly used electron or hole transport molecular species. However, the formation of charge transfer complex on the chemically inert sp(2) carbon surface can provide a particular advantage. Unlike metal surfaces, the graphene surface remains non-bonded to electron-accepting molecules even after electron transfer, inducing an improved Fermi-level alignment with the highest-occupied-molecular-orbital level of the hole-injecting-layer molecules.open1

Effect of vacancy defects in graphene on metal anchoring and hydrogen adsorption

The dispersion of transition and alkaline-earth metals on defective graphenes is studied using first-principles calculations. The effect of vacancy defects on binding properties of metal atoms to the graphene and with hydrogen molecules is particularly investigated. It is shown that vacancy defects enhance efficiently the metal binding energy and thus its dispersion, particularly for alkaline-earth metals. Mg on vacancy defects shows a substantial increase in its binding energy and hydrogen uptake capacity. Among metals considered, Ca-vacancy complexes are found to exhibit the most favorable hydrogen adsorption characteristics in terms of the binding energy and the capacity.open363

Conversion of multilayer graphene into continuous ultrathin sp 3-bonded carbon films on metal surfaces

The conversion of multilayer graphenes into sp 3-bonded carbon films on metal surfaces (through hydrogenation or fluorination of the outer surface of the top graphene layer) is indicated through first-principles computations. The main driving force for this conversion is the hybridization between sp 3 orbitals and metal surface d z 2 orbitals. The induced electronic gap states and spin moments in the carbon layers are confined in a region within 0.5â.nm of the metal surface. Whether the conversion occurs depend on the fraction of hydrogenated (fluorinated) C atoms at the outer surface and on the number of stacked graphene layers. In the analysis of the Eliashberg spectral functions for the sp 3 carbon films on a metal surface that is diamagnetic, the strong covalent metal-sp 3 carbon bonds induce soft phonon modes that predominantly contribute to large electron-phonon couplings, suggesting the possibility of phonon-mediated superconductivity. Our computational results suggest a route to experimental realization of large-area ultrathin sp 3-bonded carbon films on metal surfaces.open3

Magnetic ordering at the edges of graphitic fragments: Magnetic tail interactions between the edge-localized states

To understand the formation mechanism of magnetic moments at the edges of graphitic fragments, we carry out first-principles density-functional calculations for the electronic and magnetic structures of graphitic fragments with various spin and geometric configurations. We find that interedge and interlayer interactions between the localized moments can be explained in terms of interactions between the magnetic tails of the edge-localized states. In addition, the dihydrogenated edge states as well as Fe ad-atoms at the edge are studied in regard to the magnetic order and proximity effects.open28621

Crossover between multipole Coulomb and Kubas interactions in hydrogen adsorption on metal-graphene complexes

The hydrogen adsorption on alkaline-earth metal dispersed in doped graphenes was studied through ab initio calculations. Substitutional doping in graphenes is explored to control the ionic state of the metal atoms that plays a crucial role for dispersion and hydrogen adsorption. It was found that the adsorption behavior, particularly in Ca-dispersed graphene complexes, exhibits a crossover between the multipole Coulomb and Kubas-type (or orbital) interactions as the ionic state of Ca and the number of adsorbed hydrogen molecules change. The level exchange in s and d orbitals of Ca is responsible for the crossover. This finding enables the optimization of hydrogen adsorption and metal dispersion in graphitic materials, which is useful for developing solid hydrogen storage and efficient catalysts.open403