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Our analysis based on the results of hybrid and semilocal density-functional calculations with and without Hubbard U correction for on-site Coulomb interactions reveals the true magnetic ground states of three transition-metal dichalcogenide monolayers, viz., FeTe2,VS2, and NiTe2, which comprise inhomogeneous magnetic moment configurations. In contrast to earlier studies considering only the magnetic moments of transition-metal atoms, the chalcogen atoms by themselves have significant, antiparallel magnetic moments owing to the spin polarization through p−d hybridization. The latter is found to be true for both H and T phases of FeTe2,VS2, and NiTe2 monolayers. Our predictions show that the FeTe2 monolayer in its lowest-energy structure is a half metal, which prevails under both compressive and tensile strains. Half metallicity occurs also in the FeTe2 bilayer but disappears in thicker multilayers. The VS2 monolayer is a magnetic semiconductor; it has two different band gaps of different character and widths for different spin polarization. The NiTe2 monolayer, which used to be known as a nonmagnetic metal, is indeed a magnetic metal with a small magnetic moment. These monolayers with intriguing electronic and magnetic properties can attain new functionalities for spintronic applications.

Soon after the synthesis of two-dimensional (2D) ultrathin black phosphorus and fabrication of field effect transistors thereof, theoretical studies have predicted that other group-VA elements (or pnictogens), N, As, Sb, and Bi can also form stable, single-layer (SL) structures. These were nitrogene in a buckled honeycomb structure, arsenene, antimonene, and bismuthene in a buckled honeycomb, as well as washboard and square-octagon structures with unusual mechanical, electronic, and optical properties. Subsequently, theoretical studies are followed by experimental efforts that aim at synthesizing these novel 2D materials. Currently, research on 2D pnictogens has been a rapidly growing field revealing exciting properties, which offers diverse applications in flexible electronics, spintronics, thermoelectrics, and sensors. This review presents an evaluation of the previous experimental and theoretical studies until 2019, in order to provide input for further research attempts in this field. To this end, we first reviewed 2D, SL structures of group-VA elements predicted by theoretical studies with an emphasis placed on their dynamical and thermal stabilities, which are crucial for their use in a device. The mechanical, electronic, magnetic, and optical properties of the stable structures and their nanoribbons are analyzed by examining the effect of external factors, such as strain, electric field, and substrates. The effect of vacancy defects and functionalization by chemical doping through adatom adsorption on the fundamental properties of pnictogens has been a critical subject. Interlayer interactions in bilayer and multilayer structures, their stability, and tuning their physical properties by vertical stacking geometries are also discussed. Finally, our review is concluded by highlighting new research directions and future perspectives on the challenges in this emerging field.

Interaction between two surfaces in relative motion can give rise to energy dissipation and hence sliding friction. A significant portion of the energy is dissipated through the creation of non-equilibrium phonons. Recent advances in material synthesis have made the production of specific single layer honeycomb structures and their multilayer phases, such as graphene, graphane, fluorographene, MoS2 and WO2. When coated to the moving surfaces, the attractive interaction between these layers is normally very weak and becomes repulsive at large separation under loading force. Providing a rigorous quantum mechanical treatment for the 3D sliding motion under a constant loading force within Prandtl-Tomlinson model, we derive the critical stiffness required to avoid stick-slip motion. Also these nanostructures acquire low critical stiffness even under high loading force due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby thematerials avoid stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, layered WO2 a much better performance as compared to others and promises a potential superlubricant nanocoating. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers. Graphene coated metal surfaces also attain superlubricity and hence nearly frictionless sliding through a charge exchange mechanism with metal surface. © Springer International Publishing Switzerland 2015.

The growth of the 3√×3√ reconstructed silicene on Ag substrate has been frequently observed in experiments while its atomic structure and formation mechanism is poorly understood. Here, by first-principles calculations, we show that 3√×3√ reconstructed silicene is constituted by dumbbell units of Si atoms arranged in a honeycomb pattern. Our model shows excellent agreement with the experimentally reported lattice constant and STM image. We propose a new mechanism for explaining the spontaneous and consequential formation of 3√×3√ structures from 3×3 structures on Ag substrate. We show that the 3√×3√ reconstruction is mainly determined by the interaction between Si atoms and have weak influence from Ag substrate. The proposed mechanism opens the path to understanding of multilayer silicon. ©2014 American Physical Society

With the synthesis of a single atomic plane of graphite, namely, graphene honeycomb structure, a new perspective for carbon-based electronics is opened. The one-dimensional graphene nanoribbons (GNRs) have different band-gap values depending on their edge shape and width. In this contribution, we report our results showing that repeated heterostructures of GNRs of different widths form multiple quantum-well structures. The widths of the constituent parts as well as the bandgap, and also the magnetic ground state of the superlattices are modulated in direct space. We provide detailed analysis of these structures and show that superlattices with armchair edge shapes can be used as resonant tunneling devices and those with zigzag edge shape have unique features for spintronic applications. We also discuss another route of functionalizing 2D graphene, 1D GNR, and superlattices with 3d-transition metal (TM) atom adsorption. © Springer-Verlag Berlin Heidelberg 2013.

Based on first-principles density functional theory and finite temperature molecular dynamics calculations, we predict that H 2O can be dissociated into its constituents O and H at specific vacancy defects of single-layer MoS 2 honeycomb structure, which subsequently are bound to fourfolded Mo and twofolded S atoms surrounding the vacancy, respectively. This exothermic and spontaneous process occurs, since the electronegativity and ionization energy of Mo are smaller than those of H. Once desorbed from twofolded S atoms, H atoms migrate readily on the MoS 2 surface and eventually form free H 2 molecules to be released from the surface. Present results are critical for acquiring clean and sustainable energy from hydrogen. © 2012 American Physical Society.

We investigate the hydrogen absorption capacity of two tubular structures, namely, B Be -substituted single-wall carbon nanotube (SWNT) and Ti covered single-wall boron nitride nanotube (SWBNT) using first-principles plane wave method. The interaction of H2 molecules with the outer surface of bare SWBNT, which is normally very weak, can be significantly enhanced upon functionalization by Ti atoms. Each Ti atom adsorbed on SWBNT can bind up to four H2 molecules with an average binding energy suitable for room temperature storage. While the substitution process of Be atom on SWNT is endothermic, the substituted Be strengthens the interaction between tube surface and H2 to hold one H2 molecule. © 2007 The American Physical Society.

Mechanical properties, atomic and energy band structures of bare and hydrogen-passivated Sin Gen nanowire superlattices have been investigated by using first-principles pseudopotential plane-wave method. Undoped, tetrahedral Si and Ge nanowire segments join pseudomorphically and can form superlattice with atomically sharp interface. We found that Sin nanowires are stiffer than Gen nanowires. Hydrogen passivation makes these nanowires and Sin Gen nanowire superlattice even more stiff. Upon heterostructure formation, superlattice electronic states form subbands in momentum space. Band lineups of Si and Ge zones result in multiple quantum wells, where specific states at the band edges and in band continua are confined. The electronic structure of the nanowire superlattice depends on the length and cross section geometry of constituent Si and Ge segments. Since bare Si and Ge nanowires are metallic and the band gaps of hydrogenated ones vary with the diameter, Sin Gen superlattices offer numerous alternatives for multiple quantum well devices with their leads made from the constituent metallic nanowires.

Gallium displays physical properties which can make it a potential element to produce metallic nanowires and high-conducting interconnects in nanoelectronics. Using first-principles pseudopotential plane method we showed that Ga can form stable metallic linear and zigzag monatomic chain structures. The interaction between individual Ga atom and single-wall carbon nanotube (SWNT) leads to a chemisorption bond involving charge transfer. Doping of SWNT with Ga atom gives rise to donor states. Owing to a significant interaction between individual Ga atom and SWNT, continuous Ga coverage of the tube can be achieved. Ga nanowires produced by the coating of carbon nanotube templates are found to be stable and high conducting., Comment: 8 pages, 8 figures

Electronic and atomic structures of the clean and As- and Te-covered Si(211) surface are studied using pseudopotential density-functional method. The clean surface is found to have (2 x 1) and rebonded (1 x 1) reconstructions as stable surface structures, but no π-bonded chain reconstruction. Binding energies of As and Te adatoms at a number of symmetry sites on the ideal and (2 x 1) reconstructed surfaces have been calculated because of their importance in the epitaxial growth of CdTe and other materials on the Si(211) surface. The special symmetry sites on these surfaces having the highest binding energies for isolated As and Te adatoms are identified. But more significantly, several sites are found to be nearly degenerate in binding-energy values. This has important consequences for epitaxial growth processes. Optimal structures calculated for 0.5 monolayer of As and Te coverage reveal that the As adatoms dimerize on the surface while the Te adatoms do not. However, both As- and Te-covered surfaces are found to be metallic in nature.

During epitaxial growth of semiconducting materials using either molecular beam epitaxy or organometallic vapour deposition, the addition of a surfactant can enhance two-dimensional layer-by-layer growth. This modified growth process is now called the surfactant-mediated growth (SMG) method. It has had an important impact on the development of technologically important materials in device applications, such as heterostructures used for laser applications. Recent developments that use surfactants to improve doping profiles in semiconducting systems and antisurfactants (ASMG) to grow quantum dots further ensure that SMG/ASMG will play a major role in the future development of optoelectronic materials and nanoparticles. In this paper, we review important earlier experimental work involving the SMG method as well as some recent developments. Theoretical work involving first-principles methods and kinetic Monte Carlo simulations are discussed but confined only to the surfactant effect.

We perform first principles density functional calculations to investigate the adsorption of Te on the Si(0 0 1) surface from low coverage up to a monolayer coverage. At low coverage, a Te atom is adsorbed on top of the Si surface dimer bond. At higher coverages, Te atoms adsorption causes the Si-Si dimer bond to break, lifting the (2 × 1) reconstruction. We find no evidence of the Te-Te dimer bond formation as a possible source of the (2 × 1) reconstruction at a monolayer coverage. Finite temperature ab initio molecular dynamics calculations show that Te covered Si(0 0 1) surfaces do not have any definitive reconstruction. Vibrations of the bridged Te atoms in the strongly anharmonic potentials prevent the reconstruction structure from attaining any permanent, two-dimensional periodic geometry. This explains why experiments attempting to find a definite model for the reconstruction reached conflicting conclusions. © 2002 Elsevier Science B.V. All rights reserved.

We present a formalism for an atomic scale study of phononic heat transfer. The expression of thermal energy current can be cast in the Landauer form and incorporates the transmission coefficient explicitly. Calculation of the thermal conductance of a monoatomic chain of N atoms between two reservoirs shows interesting quantum features. The conductance density appears as Lorentzian type resonances at the eigenfrequencies of the chain. At low-temperature limit the discrete vibrational frequency spectrum of a "soft" chain may reflect on the thermal conductance by giving rise to a sudden increase. At room temperature, the conductance through a "stiff" chain may oscillate with the number of chain atoms. The obtained quantum features are compared with similar effects found in the quantized electrical conductance.

Nanowires, point contacts and metallic single-wall carbon nanotubes are one-dimensional nanostructures which display important size-dependent quantum effects. Quantization due to the transverse confinement and resultant finite level spacing of electronic and phononic states are responsible for some novel effects. Many studies have revealed fundamental and technologically important properties, which are being explored for fabricating future nanodevices. Various simulation studies based on the classical molecular dynamics method and combined force and current measurements have shown the relationship between atomic structure and transport properties. The atomic, electronic and transport properties of these nanostructures have been an area of active research. This brief review presents some quantum effects in the electronic and phononic transport through nanowires.

We present a first-principles study to elucidate the nature of the bonding, stability, energetics, and dynamics of individual X8H8 molecules (X = C, Si, Ge). The results obtained from both "local basis" and "pseudopotential" ab initio methods are in good agreement with the experimental data that exists for cubane (C8H8). The trends among these molecules are reminiscent of those prevailing in the bulk solids of C, Si, and Ge. High-temperature dynamics and fragmentation of X8H8 were studied by the quantum molecular dynamics method which shows that at high temperatures cubane is transformed to the 8-fold ring structure of cyclooctotetraene.

We analyzed the dynamics of xenon atoms as lubricant between two Ni(110) slabs in relative motion. Atomic simulations are carried out by using classical molecular dynamics with realistic empirical potentials, where nickel as well as xenon atoms are relaxed. The resistance of the xenon layer against the loading force is examined and critical forces are determined to destroy the lubricant layer at different coverages. The relative motion of slabs in the lateral direction is investigated under constant normal force as a function of coverage ranging from zero to the monolayer xenon. Important lubrication properties of xenon atoms are analyzed by calculating the variation of potential energy, lateral force, and local hydrodynamic pressure. It is predicted that the corrugation of the potential energy associated with the sliding has a minimum value at submonolayer coverage. A phononic energy dissipation mechanism together with the theoretical analysis is proposed.

We have developed a microscopic model of phononic energy dissipation in friction that involves the generation of a local excess phonon distribution in a nanoparticle between two sliding objects, and its damping into the objects. The conversion of the energy stored in the nanoparticle into excess phonons and their decay rates are calculated. The model can be extended to include randomly distributed nanoparticles and phonon-phonon interaction through anharmonic couplings. By using this model we present a quantitative analysis of energy dissipation in sliding friction.

We investigate conductance through contacts created by pressing a hard tip, as used in scanning tunneling microscopy, against substrates. Two different substrates are considered, one a normal metal (Cu) and another a semi-metal (graphite). Our study involves the molecular dynamics simulations for the atomic structure during the growth of the contact, and selfconsistent field electronic structure calculations of deformed bodies. We develop a theory predicting the conductance variations as the tip approaches the surface. We offer an explanation for a quasiperiodic variation of conductance of the contact on the graphite surface, a behavior which is dramatically different from contacts on normal metals.

This paper presents an atomic-scale study of contact, indentation, and subsequent pulling and dry sliding of a sharp and blunt metal tip on a metal surface. The evolution of atomic structure and the variation of perpendicular and lateral forces are calculated by molecular-dynamics methods using an empirical potential based on the embedded-atom model. The sharp tip experiences multiple jumps to contact in the attractive force range. The contact interface grows discontinuously mainly due to disorder-order transformation leading to disappearance of a layer and hence abrupt changes in the normal-force variation. Atom exchange occurs in the repulsive range. During the pulling off, the connective neck is reduced discontinuously; however, not all the abrupt changes of the pulling force are associated with the creation of a new layer in the neck. The sliding of the sharp tip (or single asperity) induces two consecutive structural transformations that occur periodically, but end with the wear of a layer. The situation for a blunt tip is, however, quite different.

Simulations of dry sliding friction between a metal asperity and an incommensurate metal surface reveal unusual atomic processes. The lateral force exhibits a quasiperiodic variation with the displacement of an asperity; each period consists of two different stick-slip processes involving structural transitions. While one layer of asperity changes and matches the substrate lattice in the first slip, two asperity layers merge into a new one through a structural transition during the second slip. This leads to wear. The lateral force decreases abruptly during these slip stages, but it increases between two consecutive slips and resists the relative motion. The analysis of the order suggests that each structural transition is associated with a first-order phase transition. Nonadiabatic atomic rearrangements during these phase transitions involve a new kind of mechanism of energy dissipation in the dry sliding friction.

This paper presents a detailed analysis of conductance and atomic structure in metal nanowires under tensile stress. We calculate the variation of conductance with the crossection of the constriction between two reservoirs, that is represented by three-dimensional circularly symmetric potentials. The absence of several observed features in the calculated conductance variations, in particular sudden jumps, suggests that the discontinuous rearrangements of atoms under stretch dominate the electron transport. To analyze the variations of atomic structure, we performed simulations based on the state of the art molecular dynamics simulations and revealed novel structural transformations. It is found that yielding and fracture mechanisms depend on the geometry, size, atomic arrangement and temperature. The elongation under uniaxial stress is realized by consecutive quasi elastic and yielding stages; the neck develops mainly by the implementation of a layer with a smaller crossection at certain stages of elongation. This causes to an abrupt decrease of the tensile force. Owing to the excessive strain at the neck, the original structure and atomic registry are modified; atoms show tendency to rearrange in closed-packed structures. In certain circumstances, a bundle of atomic chains or single atomic chain forms as a result of transition from the hallow site to the top site registry shortly before the break. The origin of the observed "giant" yield strength is explained by using results of present simulations and ab initio calculations of total energy and Young's modulus for an infinite atomic chain.

We present a theoretical study of dry sliding friction, which has a close bearing on the experiments done by using the atomic and friction force microscope. By performing atomic-scale calculations for the friction between a single atom and monoatomic infinite chain, we examined the effect of various material parameters on the stick-slip motion. We found that the perpendicular elastic deformation of the substrate that is induced by the sliding object is crucial for the energy damping in friction. In this case, the average friction force strongly depends on the perpendicular force constant of the substrate and the friction constant varies with the normal force. In particular, soft materials that continue to be elastic for a wide range of perpendicular compression may exhibit a second state. As a result, the hysteresis curve in the stick-slip motion becomes anisotropic.

In this paper we present an analysis of conduction through a single atom between two metal electrodes. Based on ab initio total-energy and electronic-structure calculations, and molecular-dynamics simulations using the embedded-atom model, we show that the conductance through an atom depends on the electronic structure of both the single atom and the metal electrodes, as well as the binding structure between the single atom and the surfaces of the metal electrodes. Our results enable us to interpret experimental results obtained by using a mechanical break junction on atomic-scale wires.

This paper presents a detailed analysis of atomic structure and force variations in metal nanowires under tensile strain. Our work is based on state of the art molecular dynamics simulations and ab initio self-consistent field calculations within the local density approximation, and predicts structural transformations. It is found that yielding and fracture mechanisms depend on the size, atomic arrangement, and temperature. The elongation under uniaxial stress is realized by consecutive quasielastic and yielding stages; the neck develops by the migration of atoms, but mainly by the sequential implementation of a new layer with a smaller cross section at certain ranges of uniaxial strain. This causes an abrupt decrease of the tensile force. Owing to the excessive strain at the neck, the original structure and atomic registry are modified; atoms show a tendency to rearrange in closed-packed structures. In certain circumstances, a bundle of atomic chains or a single atomic chain forms as a result of transition from the hollow site to the top site registry shortly before the break. The wire is represented by a linear combination of atomic pseudopotentials and the current is calculated to investigate the correlation between conductance variations and atomic rearrangements of the wire during the stretch. The origin of the observed "giant" yield strength is explained by using results of the present simulations and ab initio calculations of the total energy and Young's modulus for an infinite atomic chain.

We present the theoretical study of the controlled lateral and perpendicular motion of Xe on the Pt(111) surface. The lateral translation of Xe is manipulated by a tungsten tip of a scanning tunneling microscope. Using molecular statics and dynamics the energetics and different modes of atom translation are revealed. In the controlled and reversible transfer of Xe between two flat Pt(111) surfaces, effective charge on Xe, and the dipole moment of the Xe-Pt bond, are calculated as functions of the Xe-surface separation. The contributions of various mechanisms to the transfer rate of Xe are investigated by using the calculated quantum states of Xe under the applied bias voltage. These are tunneling and ballistic transfer, dipole excitation and excitation due to resonant tunneling of electrons, and electron wind force. We found that a single power law for the transfer rate does not exist in the whole range of applied pulse voltage. At high pulse voltage the transfer rate is dominated by the inelastic electron tunneling. At low pulse voltage the rate due to thermally assisted tunneling and ballistic transfer becomes important.

We show that an attractive interaction between two electrons confined to the surface of a sphere gives rise to a bound state, no matter how weak the interaction is. We explore the similarity between a sphere and a (two-dimensional) plane as far as pairing properties are concerned. We also discuss the relevance of the model to a recently discovered superconductor, alkali-metal-doped C60. © 1992 The American Physical Society.

The formation of the rectifying Schottky barrier on metal-semiconductor interfaces is one of the longest standing problems of solid-state physics. We present a review of the models and theories for Schottky barrier. Two important examples of metal-semiconductor interfaces, namely those containing simple and alkali metals, are analyzed in order to evaluate these models and theories in the light of ab-initio calculations. © 1991.

The authors have studied the electronic energy structure of pseudomorphic Gem/Sin superlattices by using the empirical tight-binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. They found that GemSin superlattices grown on Ge (001) can have a direct band gap, if m+n=10 and m=6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the centre of the superlattice Brillouin zone.

We present a comparative study of the pseudomorphic (Si)6/(Ge)6 and -doped (Si)3(Sb)(Si)2/(Ge)6 superlattices using the self-consistent pseudopotential method. The strained (Si)6/(Ge)6 superlattice has the lowest conduction-band states of extended character, and the difference of energy between the direct and indirect band gap is 70 meV. Upon doping by Sb in the Si sublattice, a quasi-two-dimensional band confined to the Sb layer dips into the band gap. Furthermore, the average potential in the Ge sublattice rises relative to that of the Si side, which increases the band offset, and enhances the localization of the quantum well states. These results indicate that doping provides new means for controlling the electronic properties of strained superlattices. © 1988 The American Physical Society.

A metallic carbon monolayer in the biphenylene network (specified as C ohs) becomes an insulator upon hydrogenation (specified as CH ohs). Patterned dehydrogenation of this CH ohs can offer a variety of intriguing functionalities. Composite structures constituted by alternating stripes of C and CH ohs with different repeat periodicity and chirality display topological properties and can form heterostructures with a tunable band-lineup or Schottky barrier height. Alternating arrangements of these stripes of finite size enable one to also construct double barrier resonant tunneling structures and 2D, lateral nanocapacitors with high gravimetric capacitance for an efficient energy storage device. By controlled removal of H atom from a specific site or dehydrogenation of an extended zone, one can achieve antidoping or construct 0D quantum structures like antidots, antirings/loops, and supercrystals, the energy level spacing of which can be controlled with their geometry and size for optoelectronic applications. Conversely, all these device functions can be acquired also by controlled hydrogenation of a bare C ohs monolayer. Since all these processes are applied to a monolayer, the commensurability of electronically different materials is assured. These features pertain not only to CH ohs but also to fully hydrogenated Si ohs.

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.

We predict that specific group-IV elements and IV-IV, III-V, and II-VI compounds can form stable, freestanding two-dimensional (2D) monolayers consisting of octagon, hexagon, and square rings (ohs), in which the threefold coordination of atoms is preserved to allow sp2-type hybridization. These monolayers can also construct bilayers, multilayers, three-dimensional (3D) layered van der Waals solids, and 3D crystals with strong vertical bonds between layers as well as quasi-one-dimensional nanotubes and nanoribbons with diverse edge geometries. All these ohs structures can constitute a large class of 2D materials ranging from good metals to wide bandgap semiconductors and display physical and chemical properties rather different from those of their counterparts in the hexagonal (honeycomb) network. The metallic state of freestanding 2D C, Si, and Ge ohs monolayers and 3D C ohs bulk contrast, respectively, with graphene, silicene, germanene, and graphite.

When covered by gadolinium (Gd) atoms, silicene, a freestanding monolayer of Si atoms in a honeycomb network, remains stable above the room temperature and becomes a two-dimensional (2D) ferromagnetic semiconductor, despite the antiferromagnetic ground state of three-dimensional bulk GdSi2 crystal. In thin GdSi2 multilayers, even if magnetic moments are ordered parallel in the same Gd atomic planes, they are antiparallel between nearest Gd planes; hence they exhibit a ferrimagnetic behavior. In contrast, a freestanding Gd2Si2 monolayer constructed by covering silicene from both sides by Gd atoms is a stable antiferromagnetic metal due to the mirror symmetry. While multilayers covered by Gd from both sides having an odd number of Gd planes have a ferrimagneticlike ground state, even-numbered ones have antiferromagnetic ground state, but none of them is ferromagnetic. Silicon atoms intervening between Gd planes are responsible for these intriguing magnetic orders conforming with the recent experiments performed on Si(111) surface. Additionally, the magnetic states of these 2D gadolinium disilicide monolayers can be monitored by applied tensile strain and by the coverage/decoration of Gd. These predictions obtained by using first-principles, spin-polarized, density functional theory calculations combined with Monte Carlo simulations herald that C, B, Si, Ge, Sn, and their compounds functionalized by rare-earth atoms can lead to novel nanostructures in 2D spintronics.

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 the interaction between a pristine blue phosphorene monolayer and selected organic molecules like amino acids and nucleic acid bases. These molecules are bound to the substrate by a weak van der Waals interaction leading to their physisorption. When isolated, they tend to orient themselves parallel to the surface and are located in flat minima with very low libration frequencies; thus the electronic structures of the substrate and physisorbed molecules are not affected except for relative shifts. Even though the regular self-assembly of these molecules on the pristine blue phosphorene cannot be realized under this weak interaction, only their irregular coating of the substrate can occur due to increased intermolecular coupling. In a solvent like water, the weak binding energy is further decreased. Gold adatoms and gold clusters can form strong chemical bonds with pristine blue phosphorene and modify its electronic and magnetic state depending on the coverage. While full coverage of a blue phosphorene monolayer by gold adatoms leads to instabilities followed by clustering, relatively lower coverage can attribute very interesting magnetic and electronic states, like a spin gapless semiconductor. When bound to the gold clusters already adsorbed on the blue phosphorene monolayer, amino acid and nucleic acid base molecules form relatively strong chemical bonds and hence can be fixed to the surface; they are reoriented to gain self-assembly character and the whole system acquires new functionalities.

Ciraci, Salim/0000-0001-8023-9860; Jahangirov, Seymur/0000-0002-0548-4820; Demirci, Salih/0000-0002-1272-9603 WOS:000516533300035 PubMed: 31970352 Semiconducting selenium and tellurium in their 3D bulk trigonal structures consist of parallel and weakly interacting helical chains of atoms and display a number of peculiarities. We predict that thermal excitations, 2D compressive strain and excess charge of positive and negative polarity mediate metal-insulator transitions by transforming these semiconductors into different metallic crystal structures. When heated to high temperature, or compressed, or charged positively, they change into a simple cubic structure with metallic bands, which is very rare among elemental crystals. When charged negatively, they transform first into body-centered tetragonal and subsequently into the body-centered orthorhombic structures with increasing negative charging. These two new structures stabilized by excess electrons also have overlapping metallic bands and quasi 2D and 1D substructures of lower dimensionality. Since the external charging of crystals can be achieved through their surfaces, the effects of charging on 2D structures of selenium and tellurium are also investigated. Similar structural transformations have been mediated also in 2D nanosheets and free-standing monolayers of these elements. These phase changes assisted by phonons are dynamical, reversible and tunable; the resulting metal-insulator transitions can occur within very short time intervals and may offer important device applications. Scientific and Technological Research Council of Turkey (TuBTAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [118F097]; National Center for High Performance Computing of Turkey (UHeM)Istanbul Technical University [5004132016]; TuBA, Turkish Academy of Sciences The Academy of Science of Turkey; The Academy of Science of Turkey - Outstanding Young Scientists Award Program (TuBA-GEBIP) This work was supported by the Scientific and Technological Research Council of Turkey (TuBTAK) under Project No 118F097. The computational resources are provided by TuBITAK ULAKBM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure) and the National Center for High Performance Computing of Turkey (UHeM) under Grant No. 5004132016. S. D. thanks UNAM, National Nanotechnology Center at Bilkent University for the hospitality. S. C. thanks TuBA, Turkish Academy of Sciences The Academy of Science of Turkey for the financial Support. S. J. acknowledges support from The Academy of Science of Turkey - Outstanding Young Scientists Award Program (TuBA-GEBIP).

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.

Group-VA elements P, As, Sb, and Bi can construct free-standing, stable zigzag monatomic chain structures, which show unusual properties. They are normally semimetals with bands crossing at the Fermi level, but a very narrow gap opens due to spin-orbit coupling. They attain one quantum of conductance under a small bias potential; Bi, being an exception, attains two quanta of conductance. Finite size chains are magnetic semiconductors; their magnetic moments and the order of spin states show an even-odd disparity depending on the number of chain atoms. Variations of the HOMO-LUMO band gaps depending on the spin polarization and the size of the finite chains offer critical tunability. In the periodic, zigzag compound chains, a small band gap opens at the Fermi level. The mysterious zigzag geometry, cohesion, stability and band order of all these chains are well-explained by a simple bond model. When placed on the parent or other monolayers like graphene, h-BN and GaSe, these chains become weakly bound and construct a 1D metallic channel. The artificial grids or networks of these metallic chains on the insulating substrates can constitute metal-insulator frameworks of desired geometry. The zigzag phosphorene chain, having the highest stability, remains stable even at full coverage of adsorbates like H and OH, whereas other chains dissociate. While P-chains can be synthesized on GaSe and graphene substrates, phosphorene nanoribbons can transform into suspended chains under excessive tensile strain. Additionally, we showed that As, Sb, and Bi zigzag chains are weakly bound to their parent monolayers and remain stable. Nitrogen monatomic chains, on the other hand, are prone to instability. The diverse properties unveiled in this study based on the density functional method offer tunability through electric fields and strain.

We report the prediction of a two-dimensional (2D) allotrope common to group-IV and group-V elements and III-V compounds, which consist of two nonplanar atomic layers connected by vertical bonds and form deformed octagon, hexagon, and squares (dohs) with threefold and fourfold coordinated atoms. Specifically for silicon, it is a semiconductor with cohesion stronger than silicene and can be chemically doped to have localized donor and acceptor states in the band gap. This allotrope can be functionalized to construct quasi-2D clathrates with transition metal atoms and attain spin polarized metallic, half-metallic, or semiconducting states. It is demonstrated that these properties can be maintained, when it is grown on a specific substrate. Stringent tests show that the atomic structure is dynamically stable and can sustain thermal excitation at high temperatures. Additionally, stable bilayer, as well as 3D layeredlike structures, can be constructed by the vertical stacking of single-layer dohs. Surprisingly, C, Ge, AlP, and GaAs can form also similar 2D semiconducting structures. In contrast to semiconducting black and blue phosphorene, P-dohs is a semimetal with band inversion. While the premise of using well-developed silicon technology in 2D electronics has been hampered by the semimetallic silicene, the realization of this 2D, semiconducting allotrope of silicon and compounds can constitute a productive direction in 2D nanoelectronics/spintronics.

We investigated the rotational and translational dynamics of hydrogen-passivated, black phosphorene and blue phosphorene nanoflakes of diverse size and geometry anchored to graphene, black phosphorene, blue phosphorene, and MoS2 monolayer substrates. The optimized attractive interaction energy between each nanoflake and monolayer substrates are harmonic for small angular displacements, leading to libration frequencies. We showed that the relevant dynamical parameters and resulting libration frequencies, which vary with the size/geometry of nanoflakes, as well as with the type of substrate, can be monitored by charging, external electric field, pressure, and also by a molecule anchored to the flake. The optimized energy profiles and energy barriers thereof have been calculated in translational and in large angle rotational dynamics. Owing to the weak interaction between the flakes and monolayers the energy barriers are particularly small for incommensurate systems and can renders nearly frictionless rotation and translation, which is crucial for nanoscale mechanics. Even if small for particular combined nanoflake + monolayer heterostructures, the energy band gaps exhibit variations with angular and linear displacements of nanoflakes. However, these band gaps undergo considerable reduction under pressure. With tunable dynamics, electronic structure, and low friction coefficients, individual or periodically repeating nanoflakes on a monolayer substrate constitute critical composite structures offering the design of novel detectors, nanomechanical, electromechanical, and electronic devices.

Two-dimensional (2D) antimonene, bismuthene, and their binary compound 2D BiSb possess high spin-orbit coupling (SOC) and potential topological insulator properties upon engineering their structural and chemical properties. Based on many-body first-principles calculations, we show that these materials can exhibit isotropic or anisotropic optoelectronic properties depending on their geometry, i.e. buckled (hb) or asymmetrical washboard (aw) phases. SOC significantly alters their optoelectronic properties, which is predominantly evident in 2D bismuthene. hb-antimonene absorbs light in the visible and partially in the ultraviolet regimes, while the absorption band edge for aw-antimonene, hb- and aw-bismuthene is set at the infrared region, absorption being spread as a broadband optical response through the spectral range. An exciton binding with 0.18 eV energy is detected for hb-bismuthene. Due to their broadband optical response, antimonene, bismuthene, and their binary compound offer possibilities towards applications as 2D materials in solar cells, light-emitting devices, photodetectors and light modulation.

Despite its high solar absorbance and surface coating abilities, pristine graphene as a semimetal is not promising for photovoltaic applications. In this study, we predict that Glycine (Gly), an amino acid, which is normally bound to pristine graphene by a weak van der Waals attraction, can form an organic coating durable to ambient condition when adsorbed on vacancy patterned graphene surface. Moreover, adsorbed Gly coating induces metal-insulator transition and concomitantly increases the solar absorbance of pristine graphene more than three times. This way, graphene attain critical functionalities to be used in solar energy and photovoltaic applications.Despite its high solar absorbance and surface coating abilities, pristine graphene as a semimetal is not promising for photovoltaic applications. In this study, we predict that Glycine (Gly), an amino acid, which is normally bound to pristine graphene by a weak van der Waals attraction, can form an organic coating durable to ambient condition when adsorbed on vacancy patterned graphene surface. Moreover, adsorbed Gly coating induces metal-insulator transition and concomitantly increases the solar absorbance of pristine graphene more than three times. This way, graphene attain critical functionalities to be used in solar energy and photovoltaic applications.

Further, to recently introduced metallic NaSi6 and Si-6 clathrate structures, we show that not only Si but also other group-IV elements, such as C, Ge, and Sn, can form stable and metallic clathrate structures with open channels at the corners of hexagons. These elemental clathrates of Si, Ge, and Sn can be viewed as if they are a combination of 2D metallic planes and perpendicular 1D metallic chains, the interplay of which can give rise to interesting physical effects. 'When free-standing, these atomic planes transform to 2D semiconducting, single-layer structures. The clathrate structure of C, which consists of weakly interacting, vertical hexagonal tubes situated at the corners of a 2D hexagonal lattice, is insulating in the plane but 1D metallic perpendicular to the planes. We also show that stable compound clathrate structures can form by hosting different alkali, alkaline earth, and light transition metal atoms in the open channels of elemental clathrates. These new metallic allotropes of group-IV elements predicted by first-principles calculations based on the density functional theory exhibit features that can be critical fundamentally and technologically.

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.

The ultra-small sized nanomaterials are important for basic functional components of future nanoelectronics, spintronics and sensor devices. In this study, based on first-principles density functional theory, the free-standing and supported nanoflakes of bare and hydrogen saturated black and blue phosphorene of diverse size and shape have been investigated. Cohesion, formation energy, thermal stability and electronic structure of these nanoflakes have been revealed. For nanoflakes supported by specific substrates, such as phosphorene, graphene and Mos2 monolayer, the equilibrium configuration and the binding energy of the flakes, as well as the effects of substrate on the electronic structure have been investigated. While the cohesive and formation energies and HOMO-LUMO gaps of nanoflakes with their edges passivated by hydrogen display clear size, shape and edge geometry dependencies, they are rather dispersed in bare nanoflakes. The binding of phosphorene nanoflakes to two-dimensional (2D) phosphorene, graphene and MoS2 monolayers is generally weak and originate from van der Waals interaction. Accordingly, when supported by these monolayers, the electronic structure of free-standing nanoflakes can be preserved for critical applications. The ultra-small sized nanomaterials are important for basic functional components of future nanoelectronics, spintronics and sensor devices. In this study, based on first-principles density functional theory, the free-standing and supported nanoflakes of bare and hydrogen saturated black and blue phosphorene of diverse size and shape have been investigated. Cohesion, formation energy, thermal stability and electronic structure of these nanoflakes have been revealed. For nanoflakes supported by specific substrates, such as phosphorene, graphene and Mos2 monolayer, the equilibrium configuration and the binding energy of the flakes, as well as the effects of substrate on the electronic structure have been investigated. While the cohesive and formation energies and HOMO-LUMO gaps of nanoflakes with their edges passivated by hydrogen display clear size, shape and edge geometry dependencies, they are rather dispersed in bare nanoflakes. The binding of phosphorene nanoflakes to two-dimensional (2D) phosphorene, graphene and MoS2 monolayers is generally weak and originate from van der Waals interaction. Accordingly, when supported by these monolayers, the electronic structure of free-standing nanoflakes can be preserved for critical applications.

Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, delta-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. Published by AIP Publishing.

This study unveils critical physical and chemical processes taking place at the interface between an amino acid, glycine, and defected graphene. Although glycine interacts weakly through van der Waals attraction with pristine graphene, it can set rather strong bonding at the close proximity of low-coordinated C atoms of single and triple vacancies and also at the edges of nanoribbons. The adsorption of a glycine molecule first leads to a reconstruction of the defect through a concerted process, which in turn induces magnetic metal-nonmagnetic insulator transition. This way, glycine can be pinned at the defect site with well-defined atomic configuration and electronic structure. In particular, libration frequency of the adsorbed glycine is attributed to significant restoring forces, which is essential for the self-assembly of glycines. Organic overlayer produced by self-assembled glycines on defect-patterned graphene constitutes a junction (heterostructure) with bilateral electronic and optical properties and offers novel device functions from biographene interfaces. These predictions are obtained from first-principles calculations using density functional theory. Authors acknowledges useful discussion with Dr. Urartu Şeker and Yelda Kadioglu. This research was supported in part by TÜBITAK̇ (The Scientific & Technological Research Council of Turkey) through TR-Grid e-Infrastructure Project, part of the calculations have been carried out at ULAKBIḾ computer center. S.C. acknowledges the financial support from the Academy of Science of Turkey TÜBA.

We investigated the effects of chemical/substitutional doping, hydrogenation, and anti-site and vacancy defects on the atomic, optoelectronic and magnetic properties of AlN and GaN monolayers. Upon doping of selected atoms, AlN and GaN monolayers can acquire magnetic properties, and their fundamental band gaps are modified by the localized gap states. Spin-polarized gap states broaden into bands at patterned coverage of adatoms, whereby half-metallic or magnetic semiconducting properties can be attained. Specific adatoms adsorbed to Ga atoms break the nearest vertical Ga-N bonds in the GaN bilayer in the heackelite structure and result in changes in the electronic and atomic structure. While adjacent and distant pairs of anion + cation vacancies induce spin polarization with filled and empty gap states, anti-site defects remain nonmagnetic; but both defects induce dramatic changes in the band gap. Fully hydrogenated monolayers are stable only for specific buckled geometries, where one geometry can also lead to an indirect to direct band gap transition. Also, optical activity shifts to the ultra-violet region upon hydrogenation of the monolayers. While H2 and O2 molecules are readily physisorbed on the surfaces of the monolayers with weak van der Waals attraction, they can be dissociated into constituent atoms at the vacancy site of the cation. Our study performed within density functional theory shows that the electronic, magnetic and optical properties of AlN and GaN monolayers can be tuned by doping and point defect formation in order to acquire diverse functionalities. The computational resources were provided by TUBITAK ULAKBIM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure). S. C. acknowledges financial support from the Academy of Sciences of Turkey (TÜBA).