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Various forms of periodic, lateral, and vertical heterostructures constructed from magnetic transition metal dichalcogenide monolayers, FeTe2 and NiTe2, have been investigated by using hybrid density functional calculations. The lateral heterostructures formed from the stripes of these constituents joined along their armchair edges are found to be half-metallic within both constituent stripes with integer number of total magnetic moment per cell; they are semiconductor for spin-down electrons but metallic for spin-up ones. Additionally, the indirect band gap of these half-metallic structures in the energy range of visible light as well as their normal band lineup exhibit features that can be tunable with the width of stripes. Normal band lineup can lead to the confinement of excess spin-down electrons and eventually to the change of dimensionality from two to one. Strong variations of the band gap can be attained by δ-doping. Half-metallicity spreads from the FeTe2 side of the junction to the adjacent NiTe2 side, which is metallic when stand-alone, through strong Fe–Te–Ni bonds at the boundary. In contrast, in the thin, vertical heterostructures, where the interlayer coupling is rather weak, the half-metallic character is retained only in the FeTe2 side; the NiTe2 side remains a spin-polarized metal. Hence, these vertical heterostructures form a semiconductor/metal junction with a Schottky barrier for one spin direction to function as a diode but a metal/metal junction for the opposite spin direction to function as a conductor. These lateral and vertical heterostructures have inhomogeneous magnetic moment configurations due to p–d hybridization; in both sides of the junction, chalcogen atoms have magnetic moments antiparallel to those at transition metal atoms. Functionalities revealed here are rare and can lead to crucial applications.

The realization of 2D ultrathin crystals with a ferromagnetic ground state that is sustainable at room temperature has been a real challenge now. By combining ab initio density functional theory with Monte Carlo simulations, we predicted a new 2D structure, Gd2B2 monolayer, which maintains its mechanical stability at elevated temperatures. More remarkably, it has a ferromagnetic ground state with high permanent magnetic moment, which persists far above room temperature. It exhibits high magnetocrystalline anisotropy along particular directions. We find also that both its magnetic anisotropy and Curie temperature can largely be altered by applied strain providing an excellent magnetoelastic tunability. This novel 2D crystal with high magnetic moment and Curie temperature combined with high structural and thermal stability can offer critical applications in magnetoelectronics.

In this paper, we characterize amino acids and nucleic acid bases (nucleobases), such as glutamine, histidine, tyrosine, adenine, guanine, cytosine, and thymine, and examine their interaction with bare, as well as with gold cluster and Ti adatom covered, black phosphorene (α-P) monolayers using density functional theory. The binding of these amino acids and nucleobases to the bare α-P monolayer is realized generally through weak van der Waals interaction and comprises only a small amount of charge exchange. Accordingly, the electronic energy structures of adsorbates and underlying substrate are not affected significantly. However, the electronic structure of bare α-P is significantly affected upon adsorption of a gold cluster and a single Ti adatom; depending on the size of the adsorbate and the symmetry of their coverage, the energy band gap can be tuned and permanent magnetic moments can be attained. Additionally, the adsorption of amino acids or nucleobases to these adsorbates on an α-P monolayer results in enhanced binding and hence makes their sustainable fixation on α-P monolayer possible. In particular, a semiconducting Au decorated α-P monolayer undergoes a metal–insulator transition upon the adsorption of tyrosine. This and similar effects favor the α-P monolayer in biosensor applications. In contrast to the situation with adsorbates, the binding of amino acid is not enhanced when it adsorb to patterned vacancy or divacancy sites of the α-P monolayer. Our study shows that the absorbance of the bare α-P monolayer can be enhanced by coating with amino acid and nucleobases. The absorbance spectrum can be further modified by the adsorption of these molecules to gold atoms on the α-P monolayer.

In this paper we investigate periodic lateral and vertical heterostructures of transition metal dichalcogenides (TMDs). Lateral heterostructures are constructed by the alternating metallic and semiconducting single-layer stripes of TMDs joined commensurately along their armchair edges and attain different states depending on the widths of constituents. While these heterostructures acquire a composite character with metallic state for narrow stripes, large stripes lead to the confinement of electronic states and function as metal-semiconductor junctions with a tunable Schottky barrier. The interface or boundary between constituent stripes has finite extension and allows charge transfer between them. On the other hand, the weak van der Waals interaction between layers sets the features of vertical heterostructures. Their interfaces are sharp: metal-semiconductor junction and Schottky barrier developed thereof can be induced even within a few layers. In the absence of dopants, we find minute charge transfer across the interface with negligible band bending in vertical heterostructures. The δ-doping of the semiconducting constituent by the metallic one forms strictly two-dimensional metallic electrons in a three-dimensional layered semiconductor and leads to crucial directionality effects and quantization of conductance. Our work unveils significant differences between lateral and vertical heterostructures.

Lateral and vertical heterostructures constructed of two-dimensional (2D) single-layer h-GaN and h-AlN display novel electronic and optical properties and diverse quantum structures to be utilized in 2D device applications. Lateral heterostructures formed by periodically repeating narrow h-GaN and h-AlN stripes, which are joined commensurately along their armchair edges, behave as composite semiconducting materials. Direct-indirect characters of the fundamental band gaps and their values vary with the widths of these stripes. However, for relatively wider stripes, electronic states are confined in different stripes and make a semiconductor-semiconductor junction with normal band alignment. This way one-dimensinonal multiple quantum well structures can be generated with electrons and holes confined to h-GaN stripes. Vertical heterostructures formed by thin stacks of h-GaN and h-AlN are composite semiconductors with a tunable fundamental band gap. However, depending on the stacking sequence and number of constituent sheets in the stacks, the vertical heterostructure can transform into a junction, which displays staggered band alignment with electrons and holes separated in different stacks. The weak bonds between the cations and anions in adjacent layers distinguish these heterostructures from those fabricated using thin films of GaN and AlN thin films in wurtzite structure, as well as from van der Waals solids. Despite the complexities due to confinement effects and charge transfer across the interface, the band diagram of the heterostructures in the direct space and band lineup are conveniently revealed from the electronic structure projected to the atoms or layers. Prominent features in the optical spectra of the lateral composite structures are observed within the limits of those of 2D parent constituents; however, significant deviations from pristine 2D constituents are observed for vertical heterostructures. Important dimensionality effects are revealed in the lateral and vertical heterostructures.

Recent studies have shown that arsenic can form single-layer phases in buckled honeycomb as well as symmetric washboard structures, named as arsenene. These structures are stable even in freestanding form and are nonmagnetic semiconductors in the energy range which is suitable for various electronic applications. In this study we investigated the adsorption of selected adatoms (H, Li, B, C, N, O, Al, Si, P, Cl, Ti, Ga, Ge, As, Se, and Sb) and physisorption of molecules (H2, O2, and H2O) to these two arsene phases. Since the interaction of these adspecies with arsenene are studied using large supercells, the coupling between adspecies is minimized, and hence our results can be interpreted to mimic the effects of isolated adatom or physisorbed molecule. It is found that the adatoms form strong chemisorption bonds and hence modify the atomic structure and physical properties locally. Some of the adatoms give rise to significant local reconstruction of the atomic structure. Electronic states of some adatoms become spin polarized and attain net magnetic moments; they may even display half-metallic character at high coverage. A majority of adsorbed atoms give rise to localized states in the fundamental band gap. We showed that the interactions between H2, O2, and H2O molecules and single-layer arsenene are rather weak and do not cause any significant changes in the physical properties of these molecules, as well as those of arsenene phases. However, some of these molecules can be dissociated at the edges of the flakes of arsenene structures; their constituents are adsorbed to the edge atoms and cause local reconstructions.

Cataloged from PDF version of article. 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. © 2013 American Chemical Society.

Cataloged from PDF version of article. Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX2) might offer properties superior to those of graphene. So far, only very few MX2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA + U calculations. Our analysis of stability was extended to include in-plane stiffness, as well as ab initio, finite-temperature molecular dynamics calculations. Some of these single-layer structures are direct- or indirect-band-gap semiconductors, only one compound is half-metal, and the rest are either ferromagnetic or nonmagnetic metals. Because of their surface polarity, band gap, high in-plane stiffness, and suitability for functionalization by adatoms or vacancies, these single-layer structures can be utilized in a wide range of technological applications, especially as nanoscale coatings for surfaces contributing crucial functionalities. In particular, the manifold WX2 heralds exceptional properties promising future nanoscale applications.