Self-assembly mechanisms of short atomic chains on single layer graphene and boron nitride

Nucleation and growth mechanisms of short chains of carbon atoms on single-layer, hexagonal boron nitride (h-BN), and short BN chains on graphene are investigated using first-principles plane wave calculations. Our analysis starts with the adsorption of a single carbon ad-atom and examines its migrations. Once a C$_2$ nucleates on h-BN, the insertion of each additional carbon at its close proximity causes a short segment of carbon atomic chain to grow by one atom at at a time in a quaint way: The existing chain leaves its initial position and subsequently is attached from its bottom end to the top of the carbon ad-atom. The electronic, magnetic and structural properties of these chains vertically adsorbed to h-BN depend on the number of carbon atoms in the chain, such that they exhibit an even-odd disparity. An individual carbon chain can also modify the electronic structure with localized states in the wide band gap of h-BN. As a reverse situation we examined the growth of short BN atomic chains on graphene, which attribute diverse properties depending on whether B or N is the atom bound to the substrate. These results together with ab-initio molecular dynamics simulations of the growth process reveal the interesting self-assembly behavior of the grown chains. Furthermore, we find that these atomic chains enhance the chemical activity of h-BN and graphene sheets by creating active sites for the bonding of various ad-atoms and can act as pillars between two and multiple sheets of these honeycomb structures leaving wider spacing between them to achieve high capacity storage of specific molecules.Comment: Accepted for Physical Review

Nanoscale Dielectric Capacitors Composed of Graphene and Boron Nitride Layers: A First Principles Study of High-Capacitance at Nanoscale

We investigate a nanoscale dielectric capacitor model consisting of two-dimensional, hexagonal h-BN layers placed between two commensurate and metallic graphene layers using self-consistent field density functional theory. The separation of equal amounts of electric charge of different sign in different graphene layers is achieved by applying electric field perpendicular to the layers. The stored charge, energy, and the electric potential difference generated between the metallic layers are calculated from the first-principles for the relaxed structures. Predicted high-capacitance values exhibit the characteristics of supercapacitors. The capacitive behavior of the present nanoscale model is compared with that of the classical Helmholtz model, which reveals crucial quantum size effects at small separations, which in turn recede as the separation between metallic planes increases.Comment: Published version in The Journal of Physical Chemistry: http://pubs.acs.org/doi/abs/10.1021/jp403706

Local Reconstructions of Silicene Induced by Adatoms

The interaction of silicene with Si, C, H, O, Ti atoms along with H$_2$, H$_2$O and O$_2$ molecules are investigated and the induced functionalities thereof are analyzed using first principles density functional theory. Si adatom initially adsorbed at the top site of silicene pushes down the Si atom underneath to form a dumbbell like structure with 3+1 coordination. This prediction is important for silicene research and reveal new physical phenomena related with the formation of multilayer Si, which is apparently the precursor state for missing layered structure of silicon. We found that dumbbell structure attributes coverage dependent electronic and magnetic properties to nonmagnetic bare silicene. Even more interesting is that silicene with dumbbells is energetically more favorable than the pristine silicene: The more dense the dumbbell coverage, the stronger is the cohesion. Incidentally, these structures appear to be intermediate between between silicene and silicon. Carbon adatom, which is initially adsorbed to the bridge position, substitutes one Si atom, if it overcomes a small energy barrier. Oxygen molecule can dissociate on silicene surface, whereby constituent oxygen atoms oxidize silicene by forming strong bonds. By varying the concentration and decoration of carbon, hydrogen and oxygen atoms one can tune the band gap of silicene. Through the adsorption of hydrogen or titanium adatom, silicene acquires spin polarized state. A half metallic ferromagnetic behavior is attained at specific uniform coverage of Ti adatom, which may function as a spin valve.Comment: Accepted for publication in The Journal of Physical Chemistry http://pubs.acs.org/doi/abs/10.1021/jp408647

Size dependence in the stabilities and electronic properties of \alpha -graphyne and its BN analogue

We predict the stabilities of \alpha-graphynes and their boron nitride analogues(\alpha-BNyne), which are considered as competitors of graphene and two-dimensional hexagonal BN. Based on first-principles plane wave method, we investigated the stability and structural transformations of these materials at different sizes using phonon dispersion calculations and ab-initio finite temperature, molecular dynamics simulations. Depending on the number of additional atoms in the edges between the corner atoms of the hexagons, n, both \alpha-graphyne(n) and \alpha-BNyne(n) are stable for even n, but unstable for odd n. \alpha-graphyne(3) undergoes a structural transformation, where the symmetry of hexagons is broken. We present the structure optimized cohesive energies, electronic, magnetic and mechanical properties of stable structures. Our calculations reveal the existence of Dirac cones in the electronic structures of \alpha-graphynes of all sizes, where the Fermi velocities decrease with increasing n. The electronic and magnetic properties of these structures are modified by hydrogenation. A single hydrogen vacancy renders a magnetic moment of one Bohr magneton. We finally present the properties of the bilayer \alpha-graphyne and \alpha-BNyne structures. We expect that these layered materials can function as frameworks in various chemical and electronic applications.Comment: Published version in The Journal of Physical Chemistr

Nanoscale Charge Balancing Mechanism in Alkali Substituted Calcium-Silicate-Hydrate Gels

Alkali-activated materials and related alternative cementitious systems are sustainable material technologies that have the potential to substantially lower CO$_2$ emissions associated with the construction industry. However, the impact of augmenting the chemical composition of the material on the main binder phase, calcium-silicate-hydrate gel, is far from understood, particularly since this binder phase is disordered at the nanoscale. Here, we reveal the presence of a charge balancing mechanism at the molecular level, which leads to stable structures when alkalis (i.e., Na or K) are incorporated into a calcium-silicate-hydrate gel, as modeled using crystalline 14{\AA} tobermorite. These alkali containing charge balanced structures possess superior mechanical properties compared to their charge unbalanced counterparts. Our results, which are based on first-principles simulations using density functional theory, include the impact of charge balancing on the optimized geometries of the new model phases, formation energies, local bonding environments, bulk moduli and diffusion barriers of the alkali atoms within the crystals

Stable single-layer honeycomb like structure of silica

Silica or SiO$_2$, the main constituent of earth's rocks has several 3D complex crystalline and amorphous phases, but it does not have a graphite like layered structure in 3D. Our theoretical analysis and numerical calculations from the first-principles predict a single-layer honeycomb like allotrope, h$\alpha$-silica, which can be viewed to be derived from the oxidation of silicene and it has intriguing atomic structure with re-entrant bond angles in hexagons. It is a wide band gap semiconductor, which attains remarkable electromechanical properties showing geometrical changes under external electric field. In particular, it is an auxetic metamaterial with negative Poisson's ratio and has a high piezoelectric coefficient. While it can form stable bilayer and multilayer structures, its nanoribbons can show metallic or semiconducting behavior depending on their chirality. Coverage of dangling Si orbitals by foreign adatoms can attribute new functionalities to h$\alpha$-silica. In particular, Si$_2$O$_5$, where Si atoms are saturated by oxygen atoms from top and bottom sides alternatingly can undergo a structural transformation to make silicatene, another stable, single layer structure of silica.Comment: Accepted for publication in Physical Review Letter