Epitaxial Graphene Growth
The growth process of single layer graphene with and without substrate is investigated using ab initio, finite temperature molecular dynamic calculations within density functional theory. An understanding of the epitaxial graphene growth mechanisms in the atomic level is provided by exploring the transient stages which occur at the growing edges of graphene. These stages are formation and collapse of large carbon rings together with the formation and healing of Stone-Wales like pentagon-heptagon defects. The activation barriers for the healing of these growth induced defects on various substrates are calculated using the climbing image nudge elastic band method and compared with that of the Stone-Wales defect. It is found that the healing of pentagon-heptagon defects occurring near the edge in the course of growth is much easier than that of Stone-Wales defect. The role of the substrate in the epitaxial growth and in the healing of defects are also investigated in detail, along with the effects of using carbon dimers as the building blocks of graphene growth.
Chains on Graphene & BN
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 adatom and examines its migrations. Once a C2 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 adatom. 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.
Graphyne & BN-yne
We predict the stabilities of α-graphynes and their boron nitride analogues (α-BNyne), which are considered as competitors of graphene and two-dimensional hexagonal BN. On the basis of the 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 α-graphyne(n) and α-BNyne(n) are stable for even n but unstable for odd n. α-Graphyne(3) undergoes a structural transformation, where the symmetry of hexagons is broken. We present the structure-optimized cohesive energies and electronic, magnetic, and mechanical properties of stable structures. Our calculations reveal the existence of Dirac cones in the electronic structures of α-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 α-graphyne and α-BNyne structures. We expect that these layered materials can function as frameworks in various chemical and electronic applications.