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With first-principles DFT calculations, the interaction between Li and carbon in graphene-based nanostructures is investigated as Li is adsorbed on graphene. It is found that the Li/C ratio of less than 1/6 for the single-layer graphene is favorable energetically, which can explain what has been observed in Raman spectrum reported recently. In addition, it is also found that the pristine graphene cannot enhance the diffusion energetics of Li ion. However, the presence of vacancy defects can increase the ratio of Li/C largely. With double-vacancy and higher-order defects, Li ion can diffuse freely in the direction perpendicular to the graphene sheets and hence boost the diffusion energetics to some extent.

Due to the severe self-interaction errors associated with some density functional approximations{,} conventional density functionals often fail to dissociate the hemibonded structure of the water dimer radical cation (H2O)2+ into the correct fragments: H2O and H2O+. Consequently{,} the binding energy of the hemibonded structure (H2O)2+ is not well-defined. For a comprehensive comparison of different functionals for this system{,} we propose three criteria: (i) the binding energies{,} (ii) the relative energies between the conformers of the water dimer radical cation{,} and (iii) the dissociation curves predicted by different functionals. The long-range corrected (LC) double-hybrid functional{,} [small omega]B97X-2(LP) [J.-D. Chai and M. Head-Gordon{,} J. Chem. Phys.{,} 2009{,} 131{,} 174105]{,} is shown to perform reasonably well based on these three criteria. Reasons that LC hybrid functionals generally work better than conventional density functionals for hemibonded systems are also explained in this work.

To assess the accuracy of density functional theory (DFT) methods in describing hydrogen bonding in condensed phases{,} we benchmarked their performance in describing phase transitions among different phases of ice. We performed DFT calculations of ice for phases Ih{,} II{,} III{,} VI and VII using BLYP{,} PW91{,} PBE{,} PBE-D{,} PBEsol{,} B3LYP{,} PBE0{,} and PBE0-D{,} and compared the calculated phase transition pressures between Ih-II{,} Ih-III{,} II-VI{,} and VI-VII with the 0 K experimental values of Whalley [J. Chem. Phys.{,} 1984{,} 81{,} 4087]. From the geometry optimization of many different candidates{,} we found that the most stable proton orientation as well as the phase transition pressure does not show much functional dependence for the generalized gradient approximation and hybrid functionals. Although all these methods overestimated the phase transition pressure{,} the addition of van der Waals (vdW) correction using PBE-D and PBE0-D reduced the transition pressure and improved the agreement for Ih-II. On the other hand{,} energy ordering between VI and VII reversed and gave an unphysical negative transition pressure. Binding energy profiles of a few conformations of water dimers were calculated to understand the improvement for certain transitions and failures for others with the vdW correction. We conclude that vdW dispersion forces must be considered to accurately describe the hydrogen bond in many different phases of ice{,} but the simple addition of the R-6 term with a small basis set tends to over stabilize certain geometries giving unphysical ordering in the high density phases.

Boron nitride (BN) domains are easily formed in the basal plane of graphene due to phase separation. With first-principles calculations{,} it is demonstrated theoretically that the band gap of graphene can be opened effectively around K (or K[prime or minute]) points by introducing small BN domains. It is also found that random doping with boron or nitrogen is possible to open a small gap in the Dirac points{,} except for the modulation of the Fermi level. The surface charges which belong to the [small pi] states near Dirac points are found to be redistributed locally. The charge redistribution is attributed to the change of localized potential due to doping effects. The band opening induced by the doped BN domain is found to be due to the breaking of localized symmetry of the potential. Therefore{,} doping graphene with BN domains is an effective method to open a band gap for carbon-based next-generation microelectronic devices.

The effects of adsorbing simple aromatic molecules on the electronic structure of graphene were systematically examined by first-principles calculations. Adsorptions of different aromatic molecules borazine (B3N3H6), triazine (C3N3H3), and benzene (C6H6) on graphene have been investigated, and we found that molecular adsorptions often lead to band gap opening. While the magnitude of band gap depends on the adsorption site, in the case of C3N3H3, the value of the band gap is found to be up to 62.9 meV under local density approximation—which is known to underestimate the gap. A couple of general trends were noted: (1) heterocyclic molecules are more effective than moncyclic ones and (2) the most stable configuration of a given molecule always leads to the largest band gap. We further analyzed the charge redistribution patterns at different adsorption sites and found that they play an important role in controling the on/off switching of the gap—that is, the energy gap is opened if the charge redistributes to between the C–C bond when the molecule is adsorbing on graphene. These trends suggest that the different ionic ability of two atoms in heterocyclic molecules can be used to control the charge redistribution on graphene and thus to tune the gap using different adsorption conditions.

Water decomposition process was investigated by ab initio molecular dynamic simulations using a model of (H2O)2+ clusters. The proton transfer (PT) process from the cationic H-donor water to the H-acceptor water for the formation of (HO[radical dot])[middle dot]H3O+ was predicted as about 90 fs on average calculated at CCSD level of theory. The valence-electron transfer (VET) process through the formation of hemibond interaction between neutral and cationic water{,} (H2O)2+{,} was also identified in several collected trajectories. Both PT and VET processes were found to propagate along two orthogonal reaction coordinates{,} the former was through an intermolecular hydrogen bond and the latter required oxygen-oxygen hemibonding. Significant difference of the theoretical electronic transitions along the VET trajectories was also observed in comparison with the non-VET cases{,} being calculated at SAC-CI level. The strong absorption features of hemibonding (H2O)2+ may introduce an interesting consideration for experimental design to monitor the water decomposition process.

By using time-resolved Fourier-transform infrared emission spectroscopy, the fragments of HCN(v 1, 2) and CO(v 1-3) are detected in one-photon dissociation of acetyl cyanide (CH 3COCN) at 308 nm. The S 1(A ″), 1(n O, π CO) state at 308 nm has a radiative lifetime of 0.46 ± 0.01 μs, long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of Ar collision-induced internal conversion is estimated to be (1-7) × 10 -12 cm 3 molecule -1 s -1. The measurements of O 2 dependence exclude the production possibility of these fragments via intersystem crossing. The high-resolution spectra of HCN and CO are analyzed to determine the ro-vibrational energy deposition of 81 ± 7 and 32 ± 3 kJmol, respectively. With the aid of ab initio calculations, a two-body dissociation on the energetic ground state is favored leading to HCN CH 2CO, in which the CH 2CO moiety may further undergo secondary dissociation to release CO. The production of CO 2 in the reaction with O 2 confirms existence of CH 2 and a secondary reaction product of CO. The HNC fragment is identified but cannot be assigned, as restricted to a poor signal-to-noise ratio. Because of insufficient excitation energy at 308 nm, the CN and CH 3 fragments that dominate the dissociation products at 193 nm are not detected. © 2012 American Institute of Physics.

Evanescent wave-cavity ring-down spectroscopy (EW-CRDS) is employed to characterize micellization of anionic surfactants and the related capability of removing cationic substance off the silica surface. Crystal violet (CV +) cationic dye is used as a molecular probe to effectively determine critical hemimicelle concentration (HMC) of surfactants on the surface. The HMC results are 1×10 -2, 4×10 -3, 8×10 -4, and 2.5×10 -4mol/L for sodium sulfate salts with a carbon-chain length of C-10, C-12, C-14, and C-16, respectively. A stronger hydrophobic interaction results in a less concentration required to undergo micellization. The HMC values on the surface are about half of those in solution. When NaCl solution is added, the electrolyte helps reduce the electrostatic repulsion between the anionic sulfate heads to facilitate the surfactant aggregation, and thus, the subsequent HMC is reduced. Furthermore, the probable phase change for dye-surfactant interactions on the surface at the concentration below HMC is observed, and the desorption rates of CV + are measured as a function of concentration and carbon-chain length of surfactants above HMC. Given each surfactant concentration at its respective HMC, the corresponding desorption rates are along the order of C-12<C-14<C-16<-C-10. The trend may be realized by two competing factors of hemimicelle size and number density. The consequences help with understanding how to apply surfactant in the chromatographic separation. © 2012 Elsevier Inc.

The interaction between graphene and a SiO 2 surface has been analyzed with first-principles DFT calculations by constructing the different configurations based on α-quartz and cristobalite structures. The fact that single-layer graphene can stay stably on a SiO 2 surface is explained based on a general consideration of the configuration structures of the SiO 2 surface. It is found that the oxygen defect in a SiO 2 surface can shift the Fermi level of graphene down which opens up the mechanism of the hole-doping effect of graphene adsorbed on a SiO 2 surface observed in a lot of experiments.