No abstract

While the attempts currently in progress in several groups for the rigorous inclusion of dispersion interactions in density functional theory (DFT) calculations mature and evolve into practical methodology, we contribute to the debate on the applicability of current functionals to the calculation of weak interaction with a systematic investigation of a few, typical, weakly bound systems. We have used both pure DFT and a hybrid approach in which the total interaction energy is partitioned into two parts: (a) the dispersion energy which, in a first approximation is the contribution due to intermonomer correlations and (b) all other interactions. The first component is accurately obtained at all distances of interest by means of a well-known damped multipolar expansion of the dispersion energy while for the second component different approximations will be evaluated. The need to avoid double counting a fraction of the correlation energy when using the hybrid approach and the choice of the appropriate functional are also discussed. We consider four systems of increasing binding strength, namely the Ar2 and Kr2 dimers, the benzene dimer, the water dimer, and a few metal carbonyls. For pure DFT calculations we confirm the conclusion reached by others concerning (a) the strong dependence of the results on the choice of the GGA functional for dispersion-dominated interaction (noble gases and benzene) with the overall tendency to yield underbinding and (b) the relatively accurate, functional-independent, description for that DFT gives of water, which we attribute to the fact that this system is dominated by electrostatic interactions. For the carbonyls we find that DFT yields results which area again strongly dependent on the choice of the functional and show a tendency to give overbinding. Our hybrid method shows instead shortcomings only for the noble gases. The problem in this case is traceable to the well-known difficulties that all current functionals experience at medium–large intermonomer separations. The quality of the hybrid results improves markedly for benzene due to the large value of both dispersion and repulsive interactions at the equilibrium distance for this dimer, which makes the balance between the two, less delicate. Excellent results are also obtained for water (for the same reason as indicated above) and more significantly for the carbonyls where we find that dispersion contributes to the binding more than it could be guessed a priori. We do not claim to have found a general solution to this difficult problem, but we aim at providing a quantitative assessment to where the problems are pointing at directions from which a general solution may, eventually, emerge.

We present a scalable and facile technique for noncovalent functionalization of graphene with 1-pyrenecarboxylic acid that exfoliates single-, few-, and multilayered graphene flakes into stable aqueous dispersions. The exfoliation mechanism is established using stringent control experiments and detailed characterization steps. Using the exfoliated graphene, we demonstrate highly sensitive and selective conductometric sensors (whose resistance rapidly changes >10,000% in saturated ethanol vapor), and ultracapacitors with extremely high specific capacitance (∼ 120 F/g), power density (∼ 105 kW/kg), and energy density (∼ 9.2 Wh/kg).

CO adsorption on small cationic, neutral, and anionic Aun (n=1–6) clusters has been investigated using density functional theory in the generalized gradient approximation. Among various possible CO adsorption sites, the on-top (one-fold coordinated) is found to be the most favorable one, irrespective of the charge state of the cluster. In addition, planar structures are preferred by both the bare and the CO-adsorbed clusters. The adsorption energies of CO on the cationic clusters are generally greater than those on the neutral and anionic complexes, and decrease with size. The adsorption energies on the anions, instead, increase with cluster size and reach a local maximum at Au5CO−, in agreement with recent experiment. The differences in adsorption energies for the different charge states decrease with increasing cluster size.

Graphene holds great promise for post-silicon electronics, however, it faces two main challenges: opening up a bandgap and finding a suitable substrate material. In principle, graphene on hexagonal boron nitride (hBN) substrate provides potential system to overcome these challenges. Recent theoretical and experimental studies have provided conflicting results: while theoretical studies suggested a possibility of a finite bandgap of graphene on hBN, recent experimental studies find no bandgap. Using the first-principles density functional method and the many-body perturbation theory, we have studied graphene on hBN substrate. A Bernal stacked graphene on hBN has a bandgap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits bandgaps larger than 2.5 eV. While the bandgap opening in graphene/hBN is due to symmetry breaking and is vulnerable to slight perturbation such as misalignment, the gra- * To whom correspondence should be addressed † Computational Center for Nanotechnology Innovations, Rensselaer Polytechnic Institute, Troy, NY 12180, USA ‡ Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA ¶ Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA phone bandgap is due to chemical functionalization and is robust in the presence of misalignment. The bandgap of graphone reduces by about 1 eV when it is supported on hBN due to the polarization effects at the graphone/hBN interface. The band offsets at graphone/hBN interface indicate that hBN can be used not only as a substrate but also as a dielectric in the field effect devices employing graphone as a channel material. Our study could open up new way of bandgap engineering in graphene based nanostructures.