V. Ongun Özçelik

Size Dependence in the Stabilities and Electronic Properties of α-Graphyne and Its Boron Nitride Analogue

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 magn...

Local Reconstructions of Silicene Induced by Adatoms

The interaction of silicene with Si, C, H, O, and Ti atoms along with H2, H2O, and O2 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 reveals new physical phenomena related to the formation of multilayer Si, which is apparently the precursor state for the 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. The carbon adatom, which is initi...

Self-healing of vacancy defects in single-layer graphene and silicene

(Received 24 May 2013; published 29 July 2013)Self-healing mechanisms of vacancy defects in graphene and silicene are studied using first-principlescalculations. We investigated host adatom adsorption, diffusion, vacancy formation, and revealed atomisticmechanisms in the healing of single, double, and triple vacancies of single-layer graphene and silicene. Siliconadatom, which is adsorbed to silicene at the top site forms a dumbbell-like structure by pushing one Si atomunderneath. The asymmetric reconstruction of the single vacancy in graphene is induced by the magnetizationthrough the rebonding of two dangling bonds and acquiring a significant magnetic moment through the remainingunsaturated dangling bond. In silicene, three twofold coordinated atoms surrounding the single vacancy becomefourfold coordinated and nonmagnetic through rebonding. The energy gained through new bond formationbecomes the driving force for the reconstruction. Under the external supply of host atoms, while the vacancydefects of graphene heal perfectly, the Stone-Wales defect can form in the course of healing of silicene vacancy.The electronic and magnetic properties of suspended, single-layer graphene and silicene are modified byreconstructed vacancy defects.DOI: 10.1103/PhysRevB.88.045440 PACS number(s): 61

Band Alignment of 2D Semiconductors for Designing Heterostructures with Momentum Space Matching

We present a comprehensive study of the band alignments of two-dimensional (2D) semiconducting materials and highlight the possibilities of forming momentum-matched type I, II, and III heterostructures, an enticing possibility being atomic heterostructures where the constituent monolayers have band edges at the zone center, i.e.,

Infrared fingerprints of few-layer black phosphorus

\mathrm{\ensuremath{\Gamma}}

Effects of charging and perpendicular electric field on the properties of silicene and germanene

valley. Our study, which includes the group IV and III-V compound monolayer materials, group V elemental monolayer materials, transition-metal dichalcogenides, and transition-metal trichalcogenides, reveals that almost half of these materials have conduction and/or valence band edges residing at the zone center. Using first-principles density functional calculations, we present the type of the heterostructure for 903 different possible combinations of these 2D materials which establishes a periodic table of heterostructures.

New Phases of Germanene

Black phosphorus is an infrared layered material. Its bandgap complements other widely studied two-dimensional materials: zero-gap graphene and visible/near-infrared gap transition metal dichalcogenides. Although highly desirable, a comprehensive infrared characterization is still lacking. Here we report a systematic infrared study of mechanically exfoliated few-layer black phosphorus, with thickness ranging from 2 to 15 layers and photon energy spanning from 0.25 to 1.36 eV. Each few-layer black phosphorus exhibits a thickness-dependent unique infrared spectrum with a series of absorption resonances, which reveals the underlying electronic structure evolution and serves as its infrared fingerprints. Surprisingly, unexpected absorption features, which are associated with the forbidden optical transitions, have been observed. Furthermore, we unambiguously demonstrate that controllable uniaxial strain can be used as a convenient and effective approach to tune the electronic structure of few-layer black phosphorus. Our study paves the way for black phosphorus applications in infrared photonics and optoelectronics.

Dissociative Adsorption of Molecules on Graphene and Silicene

Using first-principles density functional theory calculations, we showed that electronic and magnetic properties of bare and Ti adatom adsorbed single-layer silicene and germanene, which are charged or subjected to a perpendicular electric field, can be modified to attain new functionalities. In particular, when subjected to a perpendicular electric field, buckled atoms have the symmetry between their planes broken, opening a gap at the Dirac points. The occupation of 3d orbitals of the adsorbed Ti atom changes with charging or applied electric field, inducing significant changes in magnetic moment. We predict neutral silicene uniformly covered by Ti atoms to become a half-metal at a specific value of coverage and hence allow the transport of electrons in one spin direction, but block the opposite direction. These calculated properties, however, exhibit a dependence on the size of the vacuum spacing between periodically repeating silicene and germanene layers, if they are treated using a plane wave basis set within periodic boundary conditions. We clarified the cause of this spurious dependence and show that it can be eliminated by the use of a local orbital basis set.

Silicite: The layered allotrope of silicon

Germanene, a graphene-like single-layer structure of Ge, has been shown to be stable and recently grown on Pt and Au substrates. We show that a Ge adatom adsorbed on germanene pushes down the host Ge atom underneath and forms a dumbbell structure. This exothermic process occurs spontaneously. The attractive dumbbell-dumbbell interaction favors high coverage of dumbbells. This Letter heralds stable new phases of germanene, which are constructed from periodically repeating coverage of dumbbell structures and display diversity of electronic and magnetic properties.

Epitaxial growth mechanisms of graphene and effects of substrates

We study the interaction of H2, O2, CO, H2O, and OH molecules with the vacancy defects of graphene and silicene. Atoms around the bare vacancy reconstruct and specific chemically active sites are created. Although H2, O2 and CO remain intact on both pristine graphene and silicene, these molecules can dissociate when they are placed at the close proximity of these chemically active sites and nucleate centers for the hydrogenation and oxygenation. Saturation of the dangling bonds at the defect sites by constituent atoms of dissociated molecules gives rise to significant modification of electronic and magnetic properties. We analyzed the mechanism of the dissociation and revealed a concerted action of surrounding host atoms together with dissociated molecules to lower the energy barrier needed for dissociation. The dissociations of H2O and OH are hindered by high energy barriers. Our study suggests that graphene and silicene can be functionalized by creating meshes of single vacancy, where specific molecules...