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The role of guest–guest and guest–host interactions has been studied in the case of butane-based hydrates. It has been shown that these interactions are different for n-butane and iso-butane isomers. First-principles calculations reveal the weaker interaction of the linear trans-n-butane molecules with the water framework and the contribution of guest–guest repulsive interactions. The strongest interaction is found for the iso-butane hydrate and depends on both host–guest and guest–guest interactions. Calculations of the thermodynamic properties are in agreement with available experimental data. It is shown that the pure trans-n-butane hydrate is thermodynamically unstable. Moreover, the thermodynamic stability can be controlled by adding methane, which can occupy small cavities. The experimental observation of binary n-butane + methane hydrate is confirmed by stabilization of the gauche isomer in large cavities. This indicates the importance of accurate evaluation of weak intermolecular interactions for prediction of the phase diagram and the amount of guest storage.

Using first-principles calculations combined with a semiempirical van der Waals dispersion correction, we have investigated structural parameters, mixing enthalpies, and band gaps of buckled and planar few-layer InxGa1–xN alloys. We predict that the band gaps of buckled InxGa1–xN alloys with hydrogen passivation can be tuned from 5.6 to 0.7 eV with preservation of direct band gap and well-defined Bloch character, making them promising candidate materials for future light-emitting applications. Unlike that in their bulk counterparts, the phase separation could be suppressed in these two-dimensional systems because of reduced geometrical constraints. The disordered planar thin films undergo severe lattice distortion, nearly losing the Bloch character for valence bands, whereas the ordered planar ones maintain the Bloch character yet with the highest mixing enthalpies.

Using the first-principles approach, we investigated the electronic and chemical properties of wurtzite silicon carbide (2H-SiC) monolayer and thin film structures and substantiated their catalytic activity toward CO oxidation. 2H-SiC monolayer, being planar, is quite stable and has moderate binding with O2, while CO interacts physically; thus, the Eley–Rideal (ER) mechanism prevails over the Langmuir–Hinshelwood (LH) mechanism with an easily cleared activation barrier. Contrarily, 2H-SiC thin film, which exhibits a nonplanar structure, allows moderate binding of both CO and O2on its surface, thus favoring the LH mechanism over the ER one. Comprehending these results leads to a better understanding of the reaction mechanisms involving structural contrast. Weak overlapping between the 2pz(C) and 3pz(Si) orbitals of the SiC monolayer system has been found to be the primary reason to revert the active site toward sp3hybridization, during interaction with the molecules. In addition, the influences of graphite and Ag(111) substrates on the CO oxidation mechanism were also studied, and it is observed that the ER mechanism is preserved on SiC/G system, while CO oxidation on the SiC/Ag(111) system follows the LH mechanism. The calculated Sabatier activities of the SiC catalysts show that the catalysts are very efficient in catalyzing CO oxidation.

A model of lead phosphate which describes its rhombohedral-monoclinic improper ferroelastic phase transition is proposed. It contains a reduced number of degrees of freedom but it is constructed consistently with symmetry changes at the phase transition. Potential parameters of the model are derived from available experimental data. The crystallites of 25×25×25 and 121×121 ×25 unit cells have been simulated by the molecular-dynamics technique. The results determine the phase transition at the L point of reciprocal space, the order parameter, and the temperature behavior of monoclinic lattice parameters. In the rhombohedral phase the calculated dynamical structure factor shows inelastic peaks from which a soft branch of underdamped phonons has been established. The model has been used to calculate a diffuse scattering function which shows above Tca maximum at an incommensurate wave vector located along the L – F line of the Brillouin zone. The mentioned line is parallel to the ternary symmetry axis. On the basis of the above results we were able to vizualize the nature of the dynamical monoclinic microdomains persisting in the high-temperature rhombohedral phase. It has been shown that above Tcthe fluctuations can be treated as temporary orientational monoclinic microdomains. Each type of microdomains always contains an irregular sequence of antiphase domains.

The local magnetic properties of Ag12TM and Pd12TM clusters with Ihsymmetry (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Zr, Nb, Mo, Tc, Ru, Rh, and Pd) have been systematically studied with the density functional formalism, and the Kohn-Sham equation is solved self-consistently with the discrete variational method (DVM). A special attention is paid to the comparisons of local magnetism for impurities in Ag and Pd clusters with those in the bulk and on the (001) surface of Ag (Pd), it is found that the behavior of local moments in the Ag12cluster is more complex than the one of a single impurity in bulk Ag and on the Ag (001) surface, whereas the local moments in Pd12display very similar features as the ones of a single impurity in bulk Pd and on the Pd (001) surface. In order to better understand these results, the roles of interactions between impurity and host atoms on the local moment of impurities are explored: the interactions of impurity-d with Ag-d orbitals have important contributions to the local magnetic moments for impurities with less than half-filled d shell, such as Sc, Ti, V, Y, Zr and Nb, but have minor roles on the local magnetic moments for impurities Cr, Mn, Fe, Co Ni, Tc, Ru and Rh. However, in the Pd12cluster, d-d interactions between impurity and host have major roles on all the 3d and 4d impurities. Based on the interaction point of view, explanations are presented for the similarities and differences of moment behavior in the cluster, in the bulk and on the surface. The comparison of the obtained results with those in the Cu12cluster is also made. This study would provide more comprehensive understandings on the local magnetism of 3d and 4d impurities in different environments.

We study the graphene band-gap engineering by introducing different defects, namely the defects breaking the inversion symmetry and the ones periodically patterning graphene into superlattice such as the regularly arranged antidots, etc. Comparing to the primitive unit cell of graphene, the pseudo-graphene superlattice, referred to the pristine graphene supercell, modulates the boundary condition accordingly. According to the energy band-folding picture, these superlattices can be categorized into two groups on the basis of the Dirac cone position. In some cases, the Dirac points K and K′ in primitive cell are folded to the Γ point of pseudo-superlattices. The coincidence of Dirac points with Γ point results in the fourfold degeneracy. In these systems, a band gap at Γ point can be opened by introducing periodically arranged defects such as the antidots, which could be easily utilized in experiment, for example, by making the graphene nanomesh through lithography technique. In the other cases, the twofold degenerate Dirac points remain nonequivalent with Γ point in pseudo-superlattice, and the method to break the inversion symmetry could open their band gaps.

Using the density functional theory with generalized gradient approximation, we have studied in detail the enhanced hydrogen interaction with a transition-metal-doped Al(100) stepped surface. Judging from the calculated total energies, the early transition metals prefer to dope at the lower edge sites of a surface step. The Sc, Ti, and V donate electrons, whereas the Cr and Mn gain electrons. The low energy costs for activating both the H2splitting and the H atomic diffusion show improved catalytic performances. In the transition states, hydrogen would bond to both transition metal and Al atoms for H2splitting on Sc- and Ti-doped surfaces, whereas it would only develop a rather weak interaction with the metals in the other studied materials. The charge transfer results in a 0.8 e charge gain and 0.4 Å increase in bond length of H2, facilitating H2dissociation on Sc- and Ti-doped surfaces. However, in the other studied materials, the presence of hydrogen only induces charge redistribution, resulting in a rather small charge gain of H2(<0.1 e). The insights into the catalytic mechanism, on the basis of our detailed analysis on structural and electronic properties, could benefit the experimental investigations in pursuing a moderate hydrogen storage medium.

As an inorganic cousin of graphene, MoS2monolayer has attracted considerable attention. However, a full understanding of its structure and stability is still lacking due to its dependence on experimental synthesis conditions. Using first-principle calculations combined with Boltzmann transport equation, we have extensively studied the geometry, energetics, electronic structure, optical absorption, and carrier mobility of various phases of MoS2. We have not only identified the stable phases of a MoS2monolayer, but also clarified the specific conditions under which different phases are formed. The possible pathways for transitions among different phases are also discussed.

On the basis of detailed studies of structural and electronic properties with first-principles calculations, we have carefully analyzed enhanced H2splitting catalyzed by the early transition metals that substitutionally doped in the top layer and the subsurface of an ideal flat Al surface and that at the edge site of a stepped surface. The 3d orbitals facilitating Kubas interaction significantly reduce the activation energy of H2splitting catalyzed by a transition metal doped in the top surface. The catalyst doped in the subsurface could not develop Kubas interaction with H2because of the screening from the charge distributed on the top surface, whose role could be understood by combining the structural deformation induced by the doping, the attraction of the dopant to the electrons distributed around Al atoms in the top layer, and the d orbital attendance in the reaction. For the sake of recycling perspectives of the doped catalyst, the diffusion of the dissociated H atoms has also been studied. Thus, the Sc and Ti doping at the lower edge site of the stepped surface are better for their low activation energies. The atomic size and electronegativity could be used to aid new catalyst design for enhancing the hydrogen recharge properties of metal alanate hydrides. Accordingly, the near-surface alloying of Sc, Ti, Zr, Nb, Hf, and Ta in the aluminum surface could be expected to have superior catalytic properties.

Using the density functional theory with the generalized gradient approximation, we have examined the stability and the catalytic properties of Au16gold cage structure supported on graphane. The substantially improved stability is confirmed by the first-principles molecular dynamics simulation at the temperature above 500 K. The energy barrier is only 0.47 eV for the Langmuir–Hinshelwood oxidation process for a CO coadsorbed on catalyst with an O2molecule. The following Eley–Rideal oxidation process can happen almost simultaneously for its low activation barrier of ∼0.13 eV. A simple model to mimic the situation of the full coverage of CO on gold catalyst can experience oxidation by overcoming 0.63 eV energy barrier, which suggests the CO tolerant property of the complex graphane-based gold catalyst.

Abstract The local atomistic structure in bulk SixGe1−x alloys in the whole composition range 0 < x < 1 was investigated experimentally and theoretically. By extended X-ray absorption fine structure measurements in Czochralski-grown bulk SiGe crystals it is found that bulk SiGe is a random mixture and that the Ge–Ge, Ge–Si and Si–Si bond lengths maintain distinctly different lengths and vary in a linear fashion as a function of alloy composition across the entire composition range 0 < x < 1, in good agreement with expectations derived from the ab-inito electronic structure calculations. The results indicate that SiGe is a typical disorder material and that the bond lengths and bond angles are distorted with alloy composition in SiGe.

The structures of neutral and anionic hydrogen-terminated oligomers (H−Sin−H:  n = 2, 4, 6, 8) and silicon six-membered rings (c-Si6) have been investigated using quantum-chemical methods to find stable planar polysilicon chains and aromatic silicon six-membered rings. Among 18 calculated molecules, planar equilibrium structures have been obtained for seven anionic chains (Si2H2^2-, Si4H2^2-, Si4H2^4-, Si6H2^4-, Si6H2^6-, Si8H2^4-, Si8H2^6-) and three anionic six-membered rings (c-Si6^2-, c-Si6^4-, c-Si6^6-). The number of π electrons formally accommodated in the out-of-plane π orbitals is the same as the number of silicon atoms and is independent of the number of doped electrons. An anionic six-membered ring, c-Si6^2-, shows a large negative value of nucleus-independent chemical shift (NICS), which indicates that c-Si6^2- is an aromatic molecule.

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Using ab initio total energy calculations we demonstrate that the recently found metal encapsulated silicon clusters Si12Be can be assembled to form hexagonal nanotubes of silicon. This is in contrast to undoped silicon structures that are distorted and have a tendency to agglomerate. The finite nanotubes have varying HOMO−LUMO gaps depending upon the length and amount of doping. However, infinite nanotubes are metallic, symmetric, and stable, making metal encapsulation a useful route to generate metallic silicon nanowires for miniature devices.

Theoretical calculations are carried out to understand the geometric and electronic properties of the 4-nitro-2,5-bis(4-mercaptophenylethynyl)phenylamine molecule, which has been demonstrated to behave like a resonant tunneling diode under an external bias voltage. Results from density functional and Hartree−Fock theory show conflicting planar and nonplanar ground-state geometries, respectively. To underscore the effect of the molecular conformation on the electronic structure of the molecular devices, the potential energy surface of this molecule was calculated under the rigid rotor model. The results suggest that even a small perturbation can lead to a significant increase in the energy gaps between the highest occupied and lowest unoccupied energy levels. The electronic transport behavior in this molecule is analyzed from the electronic structure and spatial orientations of the molecular orbitals.

Abstract:: We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.

Zn₂SiO₄:Mn²⁺ nanocrystals were grown in an oxidized porous silicon layer using a chemical impregnation method. Apparently two classes of samples have been obtained. One is characterized by the formation of α-phase zinc silicate crystalline particles, which show green luminescence, and the other one is characterized by β-phase particles, showing yellow luminescence. It was found that in general prolonged annealing, as well as a high degree of impregnation leads to the formation of green-luminescent samples. The decay time of both yellow and green luminescence decreases with the concentration of Mn activator, while generally the decay time of yellow luminescence is considerably larger than that of green luminescence.

The geometric and electronic structures of polyphenyl-based conjugated molecules (donor-spacer-acceptor), potential candidates for molecular rectifying devices, have been investigated theoretically using ab initio quantum mechanical calculations. The individual donor (n-type) and acceptor (p-type) molecular devices have been designed by substituting one or two of the hydrogen atoms of benzene with −NH2 and −NO2 functional groups, respectively. The molecular rectifier has been modeled by combining donor and acceptor molecules with methylene or dimethylene functional groups. The electronic rectification behavior in these molecules has been analyzed from the molecular orbital energy levels and the spatial orientations of the unoccupied molecular orbitals. The results suggest that in such donor−acceptor molecular complexes, while the lowest unoccupied orbital is localized on the acceptor ring, the highest occupied molecular orbital is localized on the donor ring. The approximate potential differences for mono substituted donor−acceptor complexes have been estimated to be 1.56 and 2.05 eV for the −CH2− and −CH2−CH2− spacers, respectively. It is found that for the disubstituted rectifier complex, the potential difference increases to 2.76 eV.

Abstract:: We investigate characteristics of spin tunneling time in ZnSe/Ze1-xMnxSe multilayers under the influence of both an electric field and a magnetic field. The results indicate that the tunneling time shows complicated oscillations and significant spin separation for electrons with different spin orientations traversing semimagnetic semiconductor heterostructures. It is also shown that the tunneling time exhibits obvious asymmetry in opposite tunneling directions for electrons tunneling through asymmetric heterostructures, which mainly occurs in resonant regions. The degree of the asymmetry of the tunneling time is not only spin-polarization dependent but also external-field induced.

Computer simulations on the self- and mutual-avoiding effects of two star polymers in a good solvent are reported where the number f of the branches has been extended to the region where no experimental results are yet available. A simple and efficient Monte Carlo (MC) sampling technique was used for the lattice-model simulations. Calculations were performed for 8- to 24-arm star polymers, which complement our previous work and also Rubio et al.'s off-lattice MC simulations. The radius of gyration, the total number of configurations, and its exponent γ(f) are evaluated. The values of γ(f) obtained are consistent with the large f behavior ~ f^3/2 predicted by Ohno (Phys. Rev. 1989, A40, 1424). The pair-distribution function, the second virial coefficient, and the penetration function are also evaluated. The first order ε-expansion, which is a naive approximation of the penetration function, has been known to become increasingly inaccurate for large f. The results of the simulations give further confirmation of the inaccuracy.

Abstract:: Using the local-density approximation, calculating the Hellmann-Feynman forces, applying the direct method and deriving the phonon dispersion relations, the stability of the perovskite-like structures of MgSiO3 at T =0 have been studied. The cubic Pm$$Pm\overline 3 m$$m phase shows a dispersion-less soft phonon branch spreading from the R to M points of the cubic Brillouin zone. This soft branch persists up to high pressures of 150 GPa. The low-symmetry phases I4/mcm and Imma, P4/mbm can be considered as a result of the soft mode condensation at the M and R points, respectively. These phases prove to be unstable at T =0. The experimentally observed Pmnb phase is a consequence of the intersection of Imma and P4/mbm space groups. Thus, it can be regarded as a simultaneous condensation of two soft modes: one at the M and a second at the R high-symmetry points of the cubic Brillouin zone. The phonon dispersion relations of Pmnb show that this phase is stable and its optical phonons appear above 4.0 THz only.

In order to simulate a gelation process that occurs by condensation-polymerization reactions in solution, the cluster–cluster aggregation (CCA) model is modified by introducing four- and two-functional units as monomers. The modified CCA model shows a clear dependence of the fractal dimension of the resulting gel structure on the ratio between the numbers of the four- and two-functional units. The model should be applicable to various real systems. For example, it explains the behavior of experimentally found fractal dimension that depends on the amount of water added in the sol–gel aggregation process of SiO2systems.

The enrichment Monte Carlo algorithm is applied here, for the first time, to calculate the osmotic pressure of an athermal solution of linear chains on a lattice, in dilute and semi-dilute regimes. Complementarily to the Widom's test particle insertion method, the algorithm enables one to estimate directly the free energy and the osmotic pressure of the solution. The result shows that there is a large deviation in the free energy from the Flory–Huggins theory which may be attributed to the chain internal entropy. Moreover, the osmotic pressure shows des Cloizeaux's c9/4behavior clearly when the concentration, c, is greater than the estimated overlap concentration. We estimate also the second and third virial coefficients that are compared with experimental values.

AbstractBased on first-principles calculations, structure optimization of Pd intercalated graphite is performed and the equilibrium values for lattice constants are obtained, in agreement with experimental values. In graphite the carbon atoms within the basal plane are bound together by strong covalent σ-bonds, while atoms in adjacent layers are weakly bound by Van der Waals bonds, results in semimetal. Being intercalated with Pd, due to Pd-4d and C-2p hybridizations, partial charge transfers occur from Pd to C, which increase the density of states at Fermi level and make the compound metallic-like; the heat of formation is found to be 2.45eV for per Pd atom intercalation.

Abstract:: Using the local-density approximation, calculating the Hellmann-Feynman forces and applying the direct method, the phonon dispersion relations for the rutile-like structure of crystalline SnO2 have been derived for the first time. The phonon frequencies at the point agree very well with Raman and infrared data and other phenomenological model calculations. The LO/TO splitting is estimated by calculating phonons from an elongated supercell. The computations under pressure reveal a soft mode of B1g symmetry which leads to a ferroelastic phase transition. The pressure-dependence of the lattice constants and the Grüneisen parameters of the modes are calculated.