Controlled Catalytic Properties of Platinum Clusters on Strained Graphene

We employed graphene under isotropic strain as the supporting material for Pt clusters (Pt_<i>x</i>, <i>x</i> = 1, 4, or 6) and studied the site-resolved molecular adsorption behaviors of H₂, CO, and OH on the clusters using ab initio calculations. It was shown that the applied strain enhances the binding of the Pt atom or clusters on the graphene, which lowers the average energy of the <i>d</i> electrons (<i>d</i>-band center). However, for the Pt₄ and Pt₆ clusters that form two Pt atomic layers on the graphene, only the <i>d</i>-band center of the bottommost Pt layer can be readily tuned by the external strain on graphene. However, the site-resolved calculations of molecular binding demonstrate that controlling the <i>d</i>-band center of the bottommost Pt atoms can be a substantial factor for all the catalytic activities of the Pt cluster. We also found that the stability of the Pt/graphene system was enhanced by applying strain to the graphene support

Polymerization of Tetracyanoethylene under Pressure

Based on first-principles calculations, possible phase transformations in the molecular crystals of tetracyanoethyelene (TCNE) were investigated at high pressures. Six new carbon nitride systems with one-, two-, and three-dimensional structures were found. The predicted polymeric and graphitic structures are semiconductors with energy gaps of 0.3–1.7 eV. The predicted three-dimensional solids are also semiconductors with larger energy gaps between 1.8 and 2.2 eV with large bulk moduli >188 GPa. The calculation results suggest two polymerization mechanisms. Furthermore, inspired by the chemical vapor deposition experimentally obtained results, the possibility of folding of the predicted one-dimensional polymeric system into cylindrical molecular structures is considered. Results show that TCNE polymers longer than 18.0 Å can form TCNE-cylinders. Upon hydrogenation of the predicted TCNE-cylinders, they form very stable TCNE-cucurbituril-like structures with energy gaps larger than 2.67 eV

Three Dimensional Metallic Carbon from Distorting <i>sp</i>³‑Bond

Owing to the outstanding properties of metallic carbon as well as their great potential applications, design and synthesis of metallic carbon have long attracted considerable attention. In this work, a new three-dimensional metallic carbon (dubbed as Tri-C₉) has been built by distorting the <i>sp</i>³ hybridization bond. Our first-principles calculations results indicate that Tri-C₉ is a metastable metallic carbon, and that the metallic behavior of Tri-C₉ originates from the π bonds near Fermi level. This study offers a new way to design all-<i>sp</i>³ hybridized metallic carbon via distorting the <i>sp</i>³-bond. In addition, a feasible synthesis route for Tri-C₉ has been proposed by compressing graphite

Three-Dimensional Metallic Boron Nitride

Boron nitride (BN) and carbon are chemical analogues of each other and share similar structures such as one-dimensional nanotubes, two-dimensional nanosheets characterized by sp² bonding, and three-dimensional diamond structures characterized by sp³ bonding. However, unlike carbon which can be metallic in one, two, and three dimensions, BN is an insulator, irrespective of its structure and dimensionality. On the basis of state-of-the-art theoretical calculations, we propose a tetragonal phase of BN which is both dynamically <i>stable</i> and <i>metallic</i>. Analysis of its band structure, density of states, and electron localization function confirms the origin of the metallic behavior to be due to the delocalized B 2p electrons. The metallicity exhibited in the studied three-dimensional BN structures can lead to materials beyond conventional ceramics as well as to materials with potential for applications in electronic devices

Nanostructures of C₆₀MetalGraphene (Metal = Ti, Cr, Mn, Fe, or Ni): A Spin-Polarized Density Functional Theory Study

We used plane-wave density functional theory (DFT) to investigate the properties of C₆₀Mgraphene (C₆₀MG) nanostructures (M = Ti, Cr, Mn, Fe, or Ni). The calculated binding energies suggested that C₆₀ could be mounted on a metal–graphene surface with good bonding stability. The high-spin C₆₀CrG nanostructure was found to be more stable than the previously reported low-spin configuration. Also, C₆₀Ti was found to stand symmetrically upright on the graphene surface, while in the remaining four cases, the orientation of C₆₀M in the C₆₀MG nanostructures were bent, and the geometry of each structure is somewhat different, depending on the identity of the bridging metal atom. The large geometric distortion of C₆₀M in the tilted C₆₀MG nanostructures (with Cr, Fe, Mn, and Ni) is attributed to the spin polarization in the 3<i>d</i> orbitals and dispersion interactions between graphene and C₆₀. Additional DFT calculations on smaller C₆₀Mbenzene complexes with atomic-orbital (AO) basis sets provided consistent results on structural geometry and numbers of unpaired electrons. The DFT calculations using AO basis sets suggested that the C₆₀–M unit was flexible with respect to the bending motion. The knowledge of metal-dependent geometric differences derived in this study may be useful in designing nanostructures for spintronic and electronic applications

Effect of Elasticity of the MoS₂ Surface on Li Atom Bouncing and Migration: Mechanism from Ab Initio Molecular Dynamic Investigations

Born–Oppenheimer molecular dynamics (BOMD) has been carried out to investigate the evolution of Li atom trapping on the MoS₂ surface. A single Li atom is fired with an initial kinetic energy level (0.2 or 2.0 eV) and various targeting factors <i>x</i>, which determines the collision angle. After getting trapped, Li is observed to bounce elastically and glide on the MoS₂ surface thanks to the “breathing” vibration of MoS₂. Both firing energy and targeting factor <i>x</i> are shown to have a significant effect on the trapping and gliding processes. It is found that a higher value of targeting factor <i>x</i> (≥0.6) and initial firing energy (2.0 eV) enhances Li migration on the MoS₂ surface. Also, analysis from electronic structure calculations of six representative Li–MoS₂ interacting configurations suggests that there is ionic interaction and partial charge transfer between the absorbed Li atom and MoS₂ monolayer during the bouncing and migration process. The HSE calculations for those structures unveils the metallization of MoS₂ due to Li attachment

Engineering of Band Gap in Metal–Organic Frameworks by Functionalizing Organic Linker: A Systematic Density Functional Theory Investigation

A systematic investigation on electronic band structure of a series of isoreticular metal–organic frameworks (IRMOFs) using density functional theory has been carried out. Our results show that halogen atoms can be used as functional groups to tune not only the band gap but also the valence band maximum (VBM) in MOFs. Among halogen atoms (F, Cl, Br, I), iodine is the best candidate to reduce the band gap and increase the VBM value. In addition, it has been found that for the antiaromatic linker DHPDC (1,4-dihydropentalene-2,5-dicarboxylic acid) the energy gap is 0.95 eV, which is even lower than those calculated for other aromatic linkers, i.e., FFDC (furo­[3,2-b]­furan-2,5-dicarboxylic acid<b>)</b> and TTDC (thieno­[3,2-b]­thiophene-2,5-dicarboxylic acid). By analyzing the lowest unoccupied molecular orbital–highest occupied molecular orbital gaps calculated at the molecular level, we have highlighted the important role of the corresponding organic linkers in the MOF band gap. In particular, the change of C–C–CO dihedral angle in the organic linker can be used to analyze the difference of band gaps in MOF crystals. It is shown that a deep understanding of chemical bonding within linker molecules from electronic structure calculations plays a crucial role in designing semiconductor properties of MOF materials for engineering applications

Stability and Composition of Helium Hydrates Based on Ices I_h and II at Low Temperatures

The recently developed approach describing host lattice relaxation, guest–guest interactions and the quantum nature of guest behavior (Belosudov, R. V.; Subbotin, O. S.; Mizuseki, H.; Kawazoe, Y.; Belosludov, V. R. J. Chem. Phys. 2009, 131, 244510) has been used to derive the thermodynamic properties of helium hydrates based on ices I_h and II. The<i> p</i>–<i>T</i> phase diagrams of the helium hydrates in different ices are presented for a wide range of pressures and temperatures, and the structural transitions between pure ice I_h and ice II as well as between ice I_h-based helium hydrate and ice II-based helium hydrate have been found to be in agreement with the available experimental data. The “ice II-based helium hydrate–ice I_h-based helium hydrate” equilibrium shifts toward the higher pressures in comparison with the line of “ice II–ice I_h” equilibrium. The degrees of interstitial space filling by helium in ice I_h-based and ice II-based hydrates decrease with increasing temperature and lowering of pressure. It is demonstrated that the helium filling in ice I_h proceeds more slowly than in ice II. However, the mole fraction of helium in the hydrate based on ice I_h is significantly higher than that in the ice II-based hydrate. We predict that during the phase transition from the ice I_h-based hydrate to the ice II-based one a discharge of gaseous helium should be observed. This may serve as an indicator of the phase transition in experiment

Emergence of Different Replica Dirac Cones and Intra- and Intervalley Scatterings in Short-Wavelength Graphene Superlattices Modulated by an Atomic-Scale Sharp Potential

Here, we calculate the unfolded band structure of short-wavelength graphene superlattices modulated by the atomic-scale sharp potential of a semiconductor surface, using density functional theory. We show that in the case of the superlattice graphene Brillouin zone (SBZ) center folded into the primitive graphene Brillouin zone (GBZ) corner, the emergence of different replica Dirac cones and their scattering behaviors are driven by the substrate-induced potential or atomic-scale disorders as well as by the correlation between the rotation angle of the superlattice and the trigonal warping orientation of the Dirac cone because of the quantum interference associated with the scattering at the SBZ edge. In particular, a superlattice with a rotational angle of φ = 30° (G–N√3 × N√3–R30°, N > 1) facilitates intervalley scattering, whereas an unrotated superlattice (G–N3 × N3) is favorable to intravalley scattering. For a superlattice with a rotational angle in the range of 0 < φ < 30°, the intervalley and intravalley scatterings may be comparable to each other, leading to the emergence of mixed inequivalent replica cones at the SBZ center (or GBZ corner). Interestingly, such inequivalent replica cones facilitate a stronger intervalley scattering at the GBZ corner, compared to the G–N√3 × N√3–R30° and G–N3 × N3 superlattices, thus opening a larger energy gap at the primitive Dirac point. In addition to the inversion symmetry breaking in graphene, we show that intervalley scattering can also generate an energy gap at the secondary Dirac point

Band Gap on/off Switching of Silicene Superlattice

On the basis of density functional theory calculations with generalized gradient approximation, we have investigated in detail the cooperative effects of uniaxial strain and degenerate perturbation on manipulating the band gap in silicene. The uniaxial strain would split π bands into π_a and π_<i>z</i> bands, resulting in Dirac cone movement. Then, the hexagonal antidot would split π_a (π_<i>z</i>) bands into π_a1 and π_a2 (π_z1 and π_z2) bands, accounting for the band gap opening in the superlattices with the Dirac cone being folded to the Γ point, which is a different mechanism as compared to the sublattice equivalence breaking. The energy interval between the split π_a and π_<i>z</i> bands could be tuned to switch band gap on and off, suggesting a reversible switch between the high charge carrier velocity properties of massless Fermions attributed to the linear energy dispersion relation around the Dirac point and the high on/off properties associated with a sizable band gap. In addition, the gap width could be continuously tuned by manipulating strain, resulting in fascinating application potentials