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Register a new userA van der Waals density functional study of adenine on graphite: Single molecular adsorption and overlayer binding
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The adsorption of an adenine molecule on graphene is studied using a first-principles van der Waals functional (vdW-DF) [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. The cohesive energy of an ordered adenine overlayer is also estimated. For the adsorption of a single molecule, we determine the optimal binding configuration and adsorption energy by translating and rotating the molecule. The adsorption energy for a single molecule of adenine is found to be 711 meV, which is close to the calculated adsorption energy of the similar-sized naphthalene. Based on the single molecular binding configuration, we estimate the cohesive energy of a two-dimensional ordered overlayer. We find a significantly stronger binding energy for the ordered overlayer than for single-molecule adsorption.
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Potassium intercalation in graphite is investigated by first-principles theory. The bonding in the potassium-graphite compound is reasonably well accounted for by traditional semilocal density functional theory (DFT) calculations. However, to investi
gate the intercalate formation energy from pure potassium atoms and graphite requires use of a description of the graphite interlayer binding and thus a consistent account of the nonlocal dispersive interactions. This is included seamlessly with ordinary DFT by a van der Waals density functional (vdW-DF) approach [Phys. Rev. Lett. 92, 246401 (2004)]. The use of the vdW-DF is found to stabilize the graphite crystal, with crystal parameters in fair agreement with experiments. For graphite and potassium-intercalated graphite structural parameters such as binding separation, layer binding energy, formation energy, and bulk modulus are reported. Also the adsorption and sub-surface potassium absorption energies are reported. The vdW-DF description, compared with the traditional semilocal approach, is found to weakly soften the elastic response.
The adsorption of fluorine, chlorine, bromine, and iodine diatomic molecules on graphene has been investigated using density functional theory with taking into account nonlocal correlation effects by means of vdW-DF approach. It is shown that the van der Waals interaction plays a crucial role in the formation of chemical bonding between graphene and halogen molecules, and is therefore important for a proper description of adsorption in this system. In-plane orientation of the molecules has been found to be more stable than the orientation perpendicular to the graphene layer. In the cases of F$_2$, Br$_2$ and I$_2$ we also found an ionic contribution to the binding energy, slowly vanishing with distance. Analysis of the electronic structure shows that ionic interaction arises due to the charge transfer from graphene to the molecules. Furthermore, we found that the increase of impurity concentration leads to the conduction band formation in graphene due to interaction between halogen molecules. In addition, graphite intercalation by halogen molecules has been investigated. In the presence of halogen molecules the binding between graphite layers becomes significantly weaker, which is in accordance with the results of recent experiments on sonochemical exfoliation of intercalated graphite.
Modeling layered intercalation compounds from first principles poses a problem, as many of their properties are determined by a subtle balance between van der Waals interactions and chemical or Madelung terms, and a good description of van der Waals interactions is often lacking. Using van der Waals density functionals we study the structures, phonons and energetics of the archetype layered intercalation compound Li-graphite. Intercalation of Li in graphite leads to stable systems with calculated intercalation energies of $-0.2$ to $-0.3$~eV/Li atom, (referred to bulk graphite and Li metal). The fully loaded stage 1 and stage 2 compounds LiC$_6$ and Li$_{1/2}$C$_6$ are stable, corresponding to two-dimensional $sqrt3timessqrt3$ lattices of Li atoms intercalated between two graphene planes. Stage $N>2$ structures are unstable compared to dilute stage 2 compounds with the same concentration. At elevated temperatures dilute stage 2 compounds easily become disordered, but the structure of Li$_{3/16}$C$_6$ is relatively stable, corresponding to a $sqrt7timessqrt7$ in-plane packing of Li atoms. First-principles calculations, along with a Bethe-Peierls model of finite temperature effects, allow for a microscopic description of the observed voltage profiles.
It is shown that it is now possible to include van der Waals interactions via a nonempirical implementation of density functional theory to describe the correlation energy in electronic structure calculations on infinite systems of no particular symmetry. The vdW-DF functional [Phys. Rev. Lett. 92, 246401 (2004)] is applied to the adsorption of benzene and naphthalene on an infinite sheet of graphite, as well as the binding between two graphite sheets. Comparison with recent thermal desorption data [Phys. Rev. B 69, 535406 (2004)] shows great promise for the vdW-DF method.
A calculational study of the trihalomethanes chloroform (CHCl_3) and bromoform (CHBr_3) adsorbed on graphene is presented. The study uses the van der Waals density functional method vdW-DF to obtain adsorption energies and adsorption structures for these molecules of environmental concern. In this study chloroform is found to adsorb with the H atom pointing away from graphene, with adsorption energy 357 meV (34.4 kJ/mol). For bromoform the calculated adsorption energy is 404 meV (39.0 kJ/mol). The corrugation of graphene as seen by chloroform is small, the difference in adsorption energy along the graphene plane is less than 6 meV.
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