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We investigate the properties of oxygen-functionalized carbides of lanthanide elements with the composition M2CO2 (M=Gd, Tb,Dy) that form two-dimensional (2D) structures. Our ab initio calculations reveal that oxygen termination turns M2C monolayers into semiconductors with two dynamically stable phases. Of these, the energetically favored alpha-phase becomes ferroelectric, whereas the beta-phase turns anti-ferroelectric. Applying in-plane biaxial strain may transform one phase into the other, changes the ferroelectric polarization of the alpha-phase in a linear fashion, and modifies the size and nature of the fundamental band gap from direct to indirect. The structure with a direct band gap exhibits in-plane isotropic electronic and optical properties. This previously unexplored class of systems also exhibits excellent photon absorption in the ultraviolet range.

We investigate tunable electric polarization and electronic structure of quasi-two-dimensional (quasi-2D) allotropes of selenium, which are formed from their constituent one-dimensional (1D) structures through an inter-chain interaction facilitated by the multi-valence nature of Se. Our em ab initio calculations reveal that different quasi-2D Se allotropes display different types of electric polarization, including ferroelectric (FE) polarization normal to the chain direction in alpha and delta allotropes, non-collinear ferrielectric (FiE) polarization along the chain axis in tau-Se, and anti-ferroelectric (AFE) polarization in eta-Se. The magnitude and direction of the polarization can be changed by a previously unexplored rotation of the constituent chains. In that case, an in-plane polarization direction may change to out-of-plane in alpha-Se and delta-Se, flip its direction, and even disappear in tau-Se. Also, the band gap may be reduced and changed from indirect to direct by rotating the constituent chains about their axes in these quasi-2D Se allotropes., Comment: Accepted by Physical Review Materials

We study the behavior of hydrated graphite oxide (GO) at high temperatures using thermally accelerated molecular dynamics simulations based on ab initio density functional theory. Our results suggest that GO, a viable candidate for water treatment and desalination membranes, is more heat resilient than currently used organic materials. The system we consider to represent important aspects of thermal processes in highly disordered GO is a hydrated GO bilayer in vacuum. Our study provides microscopic insight into reactions involving water and functional epoxy-O and OH-groups bonded to graphene layers, and also describes the swelling of the structure by water vapor pressure at elevated temperatures. We find the system to withstand simulation temperatures up to ${\approx}$2,500 K before the graphitic layers start decomposing, implying the possibility of cleaning biofouling residue from a GO-based membrane by heating in an inert gas atmosphere., Comment: 12 pages, 6 figures, 15 videos. Accepted to Physical Review Applied

We perform ab initio density functional calculations to study the flow of water, methanol and dimethyl ether through nanotubes of carbon and boron nitride with different diameters and chiralities. The liquids we choose are important solvents, with water and methanol being polar and dimethyl ether being non-polar. In terms of activation barriers for liquid transport, we find the molecular-level drag to decrease with decreasing nanotube diameter, but to be rather independent of the chiral index. We also find molecules with higher polarity to experience higher drag during the flow. Counter-intuitively, we find the drag for water in boron nitride nanotubes not to exceed that in carbon nanotubes due to frustration in competing long-range Coulomb interactions., Comment: 9 pages, 6 figures

We study the stability and electronic structure of previously unexplored two-dimensional (2D) ternary compounds BNP$_2$ and C$_2$SiS. Using $ab$ $initio$ density functional theory, we have identified four stable allotropes of each ternary compound and confirmed their stability by calculated phonon spectra and molecular dynamics simulations. Whereas all BNP$_2$ allotropes are semiconducting, we find C$_2$SiS, depending on the allotrope, to be semiconducting or semimetallic. The fundamental band gaps of the semiconducting allotropes we study range from $1.4$ eV to $2.2$ eV at the HSE06 level $0.5$ eV to $1.4$ eV at the PBE level and display carrier mobilities as high as $1.5{\times}10^5$ cm$^2$V$^{-1}$s$^{-1}$. Such high mobilities are quite uncommon in semiconductors with so wide band gaps. Structural ridges in the geometry of all allotropes cause a high anisotropy in their mechanical and transport properties, promising a wide range of applications in electronics and optoelectronics.

We present a realistic phenomenological description of liquid transport through defective, layered membranes. We derive general expressions based on conventional models of laminar flow and extend the formalism to accommodate slip flow. We consider different types of defects including in-layer vacancies that provide an activation-free tortuous path through the membrane. Of the many factors that affect flow, the most important is the radius of in-layer vacancy defects, which enters in the fourth power in expressions for the flux density. We apply our formalism to water transport through defective multilayer graphene oxide membranes and find that the flow remains in the laminar regime. Our results show that observed high water permeability in this system can be explained quantitatively by a sufficient density of in-layer pores that shorten the effective diffusion path., Comment: 10 pages, 4 figures

Atomically thin one dimensional (1D) van der Waals wires of transition metal monochalocogenides (TMMs) have been anticipated as promising building blocks for integrated nanoelectronics. While reliable production of TMM nanowires has eluded scientists over the past few decades, we finally demonstrated a bottom up fabrication of MoTe nanowires inside carbon nanotubes (CNTs). Still, the current synthesis method is based on vacuum annealing of reactive MoTe2, and limits access to a variety of TMMs. Here we report an expanded framework for high yield synthesis of the 1D tellurides including WTe, an unprecedented family of TMMs. Experimental and theoretical analyses revealed that the choice of suitable metal oxides as a precursor provides useful yield for their characterization. These TMM nanowires exhibit a significant optical absorption in the visible light region. More important, electronic properties of CNTs can be tuned by encapsulating different TMM nanowires., Comment: accepted by Nanoscale

We introduce a computational approach to estimate the hardness and stiffness of diamond surfaces and nanoparticles by studying their elastic response to atomic nanoindentation. Results of our ab initio density functional calculations explain the observed hardness differences between different diamond surfaces and suggest bond stiffening in bare and hydrogenated fragments of cubic diamond and lonsdaleite. The increase in hardness and stiffness can be traced back to bond length reduction especially in bare nanoscale diamond clusters, a result of compression that is driven by the dominant role of the surface tension., Comment: 7 pages, 3 figures

We use ab initio spin-polarized density functional theory to study the magnetic order in a Kagom\'e-like 2D metamaterial consisting of pristine or substitutionally doped phenalenyl radicals polymerized into a nanoporous, graphene-like structure. In this and in a larger class of related structures, the constituent polyaromatic hydrocarbon molecules can be considered as quantum dots that may carry a net magnetic moment. The structure of this porous system and the coupling between the quantum dots may be changed significantly by applying moderate strain, thus allowing to control the magnetic order and the underlying electronic structure., Comment: Carbon 160 (2020) Main manuscript: 7 pages, 7 figures; Supplementary 2 pages, 2 figures

We study ballistic transport in periodically gated bilayer graphene as a candidate for a 2D electronic metamaterial. Our calculations use the equilibrium Green function formalism and take into account quantum corrections to charge density changes induced by a periodically modulated top gate voltage. Our results reveal an intriguing interference-like pattern, similar to that of a Fabry-Perot interferometer, in the resistance map as a function of the voltage $V_{BG}$ applied to the extended bottom gate and $V_{TG}$ applied to the periodic top gate., Comment: 9 pages, 7 figures, accepted in Phys. Rev. Applied

Elemental phosphorus nanostructures are notorious for a large number of allotropes, which limits their usefulness as semiconductors. To limit this structural diversity, we synthesize selectively quasi-1D phosphorus nanostructures inside carbon nanotubes (CNTs) that act both as stable templates and nanoreactors. Whereas zigzag phosphorus nanoribbons form preferably in CNTs with an inner diameter exceeding 1.4 nm, a previously unknown square columnar structure of phosphorus is observed to form inside narrower nanotubes. Our findings are supported by electron microscopy and Raman spectroscopy observations as well as ab initio density functional theory calculations. Our computational results suggest that square columnar structures form preferably in CNTs with inner diameter around 1.0 nm, whereas black phosphorus nanoribbons form preferably inside CNTs with 4.1 nm inner diameter, with zigzag nanoribbons energetically favored over armchair nanoribbons. Our theoretical predictions agree with the experimental findings.

We propose a previously unexplored way to form Nb nanowires from NbCl$_3$ molecules inside carbon nanotubes (CNTs). We have studied this reaction by ab initio density functional calculations and found it to be catalytically promoted in presence of graphitic carbon. Our results suggest that chemisorption of NbCl$_3$ on the CNT is accompanied by a charge transfer of ~0.5 electrons to the nanotube wall, which significantly weakens the Nb-Cl bond. We found that the bcc structure of Nb is not affected by the small diameter of the nanowire inside a CNT. We have also identified strong covalent bonds between the nanowires and the surrounding nanotube that are accompanied by a similar charge transfer of <0.5 e from surface Nb atoms to the nanotube. The large electronic density of states of bulk Nb at E$_F$ is not changed much in the confined geometry, suggesting that ultra-narrow nanowires of Nb may keep their superconducting behavior and form Josephson junctions in the quasi-1D geometry while being protected from the ambient by the enclosing CNT structure., Comment: accepted by Carbon

Band structure determines the motion of electrons in a solid, giving rise to exotic phenomena when properly engineered. Drawing an analogy between electrons and photons, artificially designed optical lattices indicate the possibility of a similar band modulation effect in graphene systems. Yet due to the fermionic nature of electrons, modulated electronic systems promise far richer categories of behaviors than those found in optical lattices. Here, we uncovered a strong modulation of electronic states in bilayer graphene subject to periodic potentials. We observed for the first time the hybridization of electron and hole sub-bands, resulting in local band gaps at both primary and secondary charge neutrality points. Such hybridization leads to the formation of flat bands, enabling the study of correlated effects in graphene systems. This work may also offer a viable platform to form and continuously tune Majorana zero modes, which is important to the realization of topological quantum computation.

We use continuum elasticity theory to revise scaling laws for radial breathing-like and shear-like vibration modes in quasi-2D nanostructures including finite-width nanoribbons and finite-size thin circular discs. Such modes can be observed spectroscopically in corresponding nanostructures of graphene and phosphorene and can be determined numerically by atomistic ab initio density functional theory and classical force-field calculation. The revised scaling laws differ from previously used expressions, some of which display an unphysical asymptotic behavior. Apart from model assumptions describing the effect of edge termination, the continuum scaling laws have no adjustable parameters and display correct asymptotic behavior. These scaling laws yield excellent agreement with experimental and numerical results for vibration frequencies in both isotropic and anisotropic structures as well as useful expressions for the frequency dependence on structure size and edge termination., Comment: accepted by Carbon (2019)

Monitoring structural integrity during and after extreme events such as an earthquake or a tsunami is a mundane yet important task that still awaits a workable solution. Currently available stress sensors are not sufficiently robust and are affected by humidity. Insufficient information about crack formation preceding structural failure increases risk during rescue operations significantly. Designing durable stress sensors that are not affected by harsh and changing environment and do not fail under catastrophic conditions is a fundamental challenge. To address this problem, we developed a stress sensor based on creased single-walled carbon nanotubes (SWCNTs) encapsulated in a non-fluorinated superhydrophobic coating. The creased SWCNT film was fabricated and integrated in polydimethylsiloxane (PDMS) to provide a highly linear response under elastic deformation. The non-fluorinated water-repellent coating was fabricated by spray-coating the film with nanosilica particles, providing water resistance during elastic deformation. The compact design and superior water resistance of the sensor, along with its appealing linearity and large stretchability, demonstrates the scalability of this approach for fabricating efficient strain sensors for applications in infrastructure and robotic safety management as well as advanced wearable sensors.

We design an all-carbon membrane for the filtration and desalination of water. A unique layered assembly of carbon nanostructures including graphite oxide (GO), buckypaper consisting of carbon nanotubes, and a strong carbon fabric provides high mechanical strength and thermal stability, resilience to harsh chemical cleaning agents and electrical conductivity, thus addressing major shortcomings of commercial reverse osmosis membranes. We use ab initio density functional theory calculations to obtain atomic-level insight into the permeation of water molecules in-between GO layers and across in-layer vacancy defects. Our calculations elucidate the reason for selective rejection of solvated Na$^+$ ions in an optimized GO membrane that is structurally stabilized in a sandwich arrangement in-between layers of buckypaper, which are protected on both sides by strong carbon fabric layers., Comment: 15 pages, 9 figures, 1 video. On-line SI linked in the references contains 37 movies

In search of an improved strategy to form low resistance contacts to MoS2 and related semiconducting transition metal dichalcogenides, we use ab initio density functional electronic structure calculations in order to determine the equilibrium geometry and electronic structure of MoO3/MoS2 and MoO2/MoS2 bilayers. Our results indicate that, besides a rigid band shift associated with charge transfer, the presence of molybdenum oxide modifies the electronic structure of MoS2 very little. We find that the charge transfer in the bilayer provides a sufficient degree of hole doping to MoS2, resulting in a highly transparent contact region., Comment: 14 pages, 9 figures

We study the effect of electron doping on the bonding character and stability of two-dimensional (2D) structures of elemental boron, called borophene, which is known to form many stable allotropes. Our {\em ab initio} calculations for the neutral system reveal previously unknown stable 2D $\epsilon$-B and $\omega$-B structures. We find that the chemical bonding characteristic in this and other boron structures is strongly affected by extra charge. Beyond a critical degree of electron doping, the most stable allotrope changes from $\epsilon$-B to a buckled honeycomb structure. Additional electron doping, mimicking a transformation of boron to carbon, causes a gradual decrease in the degree of buckling of the honeycomb lattice that can be interpreted as piezoelectric response. Net electron doping can be achieved by placing borophene in direct contact with layered electrides such as Ca$_{2}$N. We find that electron doping can be doubled by changing from the B/Ca$_{2}$N bilayer to the Ca$_{2}$N/B/Ca$_{2}$N sandwich geometry., Comment: accepted by Nano Letters

We design two-dimensional (2D) mechanical metamaterials that may be deformed substantially at little or no energy cost. Examples of such deformable structures are assemblies of rigid isosceles triangles hinged in their corners on the macro-scale, or polymerized phenanthrene molecules forming porous graphene on the nano-scale. In these and in a large class of related structures, the Poisson ratio $\nu$ diverges for particular strain values. $\nu$ also changes its magnitude and sign, and displays a shape memory effect., Comment: Accepted by Phys. Rev. Applied 10 (2018)

In twisted bilayer graphene (TBLG), extremely small deviations from the magic twist angle $\theta_m{\approx}1.08^\circ$ change its electronic structure near the Fermi level drastically, causing a meV-wide flat band to appear or disappear. In view of such sensitivity to minute structural deformations, we investigate the combined effect of shear and atomic relaxation on the electronic structure. Using precise experimental data for monolayer and bilayer graphene as input in a simplified formalism for the electronic structure and elastic energy, we find TBLG near $\theta_m$ to be unstable with respect to global shear by the angle $\alpha{\approx}0.08^\circ$. In TBLG, the effect of shear on the electronic structure is as important as that of atomic relaxation. Under optimum global shear, calculated $\theta_m$ is reduced by $0.04^\circ$ and agrees with the observed value., Comment: Phys. Rev. B 98 (2018) (in press). 8 pages, 6 figures

We introduce a minimum tight-binding model with only three parameters extracted from graphene and untwisted bilayer graphene. This model reproduces quantitatively the electronic structure of not only these two systems and bulk graphite near the Fermi level, but also that of twisted bilayer graphene including the value of the magic angle, at which bands at $E_F$ flatten without overlap and two gaps open, one above and one below $E_F$. The Hamiltonian is sufficiently transparent and flexible to be adopted to other twisted layered systems., Comment: 5 pages, 3 figures

We study the conversion of bulk Se and Te, consisting of intertwined a helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our ab initio calculations reveal that previously unknown and unusually stable \delta - and \eta-2D allotropes may form in an intriguing multi-step process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers <0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting., Comment: accepted by Nano Letters

We propose to raise the critical temperature $T_c$ for superconductivity in doped C$_{60}$ molecular crystals by increasing the electronic density of states at the Fermi level $N(E_F)$ and thus the electron-phonon coupling constant in low-dimensional C$_{60}$ nanoarrays. We consider both electron and hole doping and present numerical results for $N(E_F)$, which increases with decreasing bandwidth of the partly filled $h_u$ and $t_{1u}$ derived frontier bands with decreasing coordination number of C$_{60}$. Whereas a significant increase of $N(E_F)$ occurs in 2D arrays of doped C$_{60}$ intercalated in-between graphene layers, we propose that the highest $T_c$ values approaching room temperature may occur in bundles of nanotubes filled by 1D arrays of externally doped C$_{60}$ or La@C$_{60}$, or in diluted 3D crystals, where quasi-1D arrangements of C$_{60}$ form percolation paths., Comment: Accepted as Phys. Rev. B rapid communication

Using {\em ab initio} density functional theory calculations, we characterize changes in the electronic structure of MoS$_{2}$ monolayers introduced by missing or additional adsorbed sulfur atoms. We furthermore identify the chemical and electronic function of substances that have been reported to reduce the adverse effect of sulfur vacancies in quenching photoluminescence and reducing electronic conductance. We find that thiol-group containing molecules adsorbed at vacancy sites may re-insert missing sulfur atoms. In presence of additional adsorbed sulfur atoms, thiols may form disulfides on the MoS$_{2}$ surface to mitigate the adverse effect of defects., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in ACS Nano \copyright American Chemical Society after peer review. To access the final edited and published work see http://pubs.acs.org/articlesonrequest/AOR-4FSKK5nZgdxksJmTEje4

Using continuum elasticity theory, we describe the elastic behavior of helical coils with an asymmetric double-helix structure and identify conditions, under which they become very rigid. Theoretical insight gained for macro-structures including a stretched telephone cord and an unsupported helical staircase is universal and of interest for the elastic behavior of helical structures on the micro- and nanometer scale., Comment: Phys. Rev. Appl. (2017)

Superconducting behavior even under harsh ambient conditions is expected to occur in La@C(60) if it could be isolated from the primary metallofullerene soot when functionalized by CF(3) radicals. We use ab initio density functional theory calculations to compare the stability and electronic structure of C(60) and the La@C(60) endohedral metallofullerene to their counterparts functionalized by CF(3). We found that CF(3) radicals favor binding to C(60) and La@C(60), and have identified the most stable isomers. Structures with an even number m of radicals are energetically preferred for C(60) and structures with odd m for La@C(60) due to the extra charge on the fullerene. This is consistent with a wide HOMO-LUMO gap in La@C(60)(CF(3))(m) with odd m, causing extra stabilization in the closed-shell electronic configuration. CF(3) radicals are both stabilizing agents and molecular separators in a metallic crystal, which could increase the critical temperature for superconductivity., Comment: Nano Letters

We propose a continuum model to predict long-wavelength vibrational modes of empty and liquid-filled tubules that are very hard to reproduce using the conventional force-constant matrix approach based on atomistic ab initio calculation. We derive simple quantitative expressions for long-wavelength longitudinal and torsional acoustic modes, flexural acoustic modes, as well as the radial breathing mode of empty or liquid-filled tubular structures that are based on continuum elasticity theory expressions for a thin elastic plate. We furthermore show that longitudinal and flexural acoustic modes of tubules are well described by those of an elastic beam resembling a nanowire. Our numerical results for biological microtubules and carbon nanotubes agree with available experimental data., Comment: The paper has been accepted by Physical Review B

We have identified an unusually stable helical coil allotrope of phosphorus. Our ab initio Density Functional Theory calculations indicate that the uncoiled, isolated straight 1D chain is equally stable as a monolayer of black phosphorus dubbed phosphorene. The coiling tendency and the attraction between adjacent coil segments add an extra stabilization energy of about 12 meV/atom to the coil allotrope, similar in value to the approximately 16 meV/atom inter-layer attraction in bulk black phosphorus. Thus, the helical coil structure is essentially as stable as black phosphorus, the most stable phosphorus allotrope known to date. With an optimum radius of 2.4 nm, the helical coil of phosphorus may fit well and even form inside wide carbon nanotubes., Comment: The paper has been accepted by Nano. Lett. (2016)

As an alternative to atomistic calculations of long-wavelength acoustic modes of atomically thin layers, which are known to converge very slowly, we propose a quantitatively predictive and physically intuitive approach based on continuum elasticity theory. We describe a layer, independent of its thickness, by a membrane, characterize its elastic behavior by a $(3{\times}3)$ elastic matrix as well as the flexural rigidity. We present simple quantitative expressions for frequencies of long-wavelength acoustic modes, which we determine using 2D elastic constants calculated by {\em ab initio} Density Functional Theory. The calculated spectra accurately reproduce observed and calculated long-wavelength phonon spectra of graphene and phosphorene, the monolayer of black phosphorus. Our approach also correctly describes the observed dependence of the radial breathing mode frequency on the diameter of carbon fullerenes and nanotubes., Comment: Accepted by Physical Review B (2016)

We study theoretically the structural and electronic response of layered bulk black phosphorus to in-layer strain. Ab initio density functional theory (DFT) calculations reveal that the strain energy and interlayer spacing display a strong anisotropy with respect to the uniaxial strain direction. To correctly describe the dependence of the fundamental band gap on strain, we used the computationally more involved GW quasiparticle approach that is free of parameters and superior to DFT studies, which are known to underestimate gap energies. We find that the band gap depends sensitively on the in-layer strain and even vanishes at compressive strain values exceeding about 2%, thus suggesting a possible application of black P in strain-controlled infrared devices., Comment: Phys. Rev. B (2016)

We propose previously unknown allotropes of phosphorus carbide (PC) in the stable shape of an atomically thin layer. Different stable geometries, which result from the competition between sp2 bonding found in graphitic C and sp3 bonding found in black P, may be mapped onto 2D tiling patterns that simplify categorizing of the structures. Depending on the category, we identify 2D-PC structures that can be metallic, semi-metallic with an anisotropic Dirac cone, or direct-gap semiconductors with their gap tunable by in-layer strain.

We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~ 0.3 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-1 s-1 at room temperature, which increases to >2x103 cm2 V-1 s-1 at cryogenic temperatures. We observe a similar performance also in MoS2 and MoSe2 FETs with 2D/2D drain and source contacts. The 2D/2D low-resistance ohmic contacts presented here represent a new device paradigm that overcomes a significant bottleneck in the performance of TMDs and a wide variety of other 2D materials as the channel materials in post-silicon electronics., Comment: 23 pages, 4 figures, Nano Lett. Accepted

An outstanding challenge of theoretical electronic structure is the description of van der Waals (vdW) interactions in molecules and solids. Renewed interest in resolving this is in part motivated by the technological promise of layered systems including graphite, transition metal dichalcogenides, and more recently, black phosphorus, in which the interlayer interaction is widely believed to be dominated by these types of forces. We report a series of quantum Monte Carlo (QMC) calculations for bulk black phosphorus and related few-layer phosphorene, which elucidate the nature of the forces that bind these systems and provide benchmark data for the energetics of these systems. We find a significant charge redistribution due to the interaction between electrons on adjacent layers. Comparison to density functional theory (DFT) calculations indicate not only wide variability even among different vdW corrected functionals, but the failure of these functionals to capture the trend of reorganization predicted by QMC. The delicate interplay of steric and dispersive forces between layers indicate that few-layer phosphorene presents an unexpected challenge for the development of vdW corrected DFT., Comment: 8 pages, 6 figures

As a way to further improve the electronic properties of group V layered semiconductors, we propose to form in-layer 2D heterostructures of black phosphorus and grey arsenic. We use \textit{ab initio} density functional theory to optimize the geometry, determine the electronic structure, and identify the most stable allotropes as a function of composition. Since pure black phosphorus and pure grey arsenic monolayers differ in their equilibrium structure, we predict a structural transition and a change in frontier states, including a change from a direct-gap to an indirect-gap semiconductor, with changing composition., Comment: 6 pages, 4 figures

Electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. The intricate interplay between the two give rises to a peculiar charge density wave (CDW) state, which is also known as a Mott insulator, as the ground state of layered compound 1T-TaS2. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses from the tip of a scanning tunneling microscope (STM), and find that the new phase exhibit electronic structures that are entirely different from the Mott ground state of 1T-TaS2 at low temperatures. The mosaic phase consists of nanometer-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layer underneath. We discover that the nature of the new phases is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in this layered compound.

In analogy to III-V compounds, which have significantly broadened the scope of group IV semiconductors, we propose IV-VI compounds as isoelectronic counterparts to layered group V semiconductors. Using {\em ab initio} density functional theory, we study yet unrealized structural phases of silicon mono-sulfide (SiS). We find the black-phosphorus-like $\alpha$-SiS to be almost equally stable as the blue-phosphorus-like $\beta$-SiS. Both $\alpha$-SiS and $\beta$-SiS monolayers display a significant, indirect band gap that depends sensitively on the in-layer strain. Unlike 2D semiconductors of group V elements with the corresponding nonplanar structure, different SiS allotropes show a strong polarization either within or normal to the layers. We find that SiS may form both lateral and vertical heterostructures with phosphorene at a very small energy penalty, offering an unprecedented tunability in structural and electronic properties of SiS-P compounds., Comment: 7 pages, 5 figures

We introduce a concise formalism to characterize nanometer-sized tori based on carbon nanotubes and to determine their stability by combining {\em ab initio} density functional calculations with a continuum elasticity theory approach that requires only shape information. We find that the high strain energy in nanotori containing only hexagonal rings is significantly reduced in nanotori containing also other polygons. Our approach allows to determine local curvature and link it to local strain energy, which is correlated with local stability and chemical reactivity.

We combine {\em ab initio} density functional theory (DFT) structural studies with DFT-based nonequilibrium Green function calculations to investigate how the presence of non-hexagonal rings affects electronic transport in graphitic structures. We find that infinite monolayers, finite-width nanoribbons and nanotubes formed of 5-8 haeckelite with only 5- and 8-membered rings are generally more conductive than their graphene-based counterparts. Presence of haeckelite defect lines in the perfect graphitic structure, a model of grain boundaries in CVD-grown graphene, increases the electronic conductivity and renders it highly anisotropic., Comment: 6 figures

Elemental phosphorous is believed to have several stable allotropes that are energetically nearly degenerate, but chemically reactive. To prevent chemical degradation under ambient conditions, these structures may be capped by monolayers of hexagonal boron nitride ({\em h}-BN) or graphene. We perform {\em ab initio} density functional calculations to simulate scanning tunneling microscopy (STM) images of different layered allotropes of phosphorus and study the effect of capping layers on these images. We find that protective monolayers of insulating {\em h}-BN allow to distinguish between the different structural phases of phosphorus underneath, even though the images are filtered through only nitrogen atoms that appear transparent. No such distinction is possible for phosphorus films capped by semimetallic graphene that masks the underlying structure. Our results suggest that the real-space imaging capability of STM is not hindered by selected capping layers that protect phosphorus surfaces., Comment: Published version, replaces previous one

We present a scheme to categorize the structure of different layered phosphorene allotropes by mapping their non-planar atomic structure onto a two-color 2D triangular tiling pattern. In the buckled structure of a phosphorene monolayer, we assign atoms in "top" positions to dark tiles and atoms in "bottom" positions to light tiles. Optimum $sp^3$ bonding is maintained throughout the structure when each triangular tile is surrounded by the same number $N$ of like-colored tiles, with $0{\le}N{\le}2$. Our ab initio density functional calculations indicate that both the relative stability and electronic properties depend primarily on the structural index $N$. The proposed mapping approach may also be applied to phosphorene structures with non-hexagonal rings and 2D quasicrystals with no translational symmetry, which we predict to be nearly as stable as the hexagonal network.

We present a paradigm in constructing very stable, faceted nanotube and fullerene structures by laterally joining nanoribbons or patches of different planar phosphorene phases. Our ab initio density functional calculations indicate that these phases may form very stable, non-planar joints. Unlike fullerenes and nanotubes obtained by deforming a single-phase planar monolayer at substantial energy penalty, we find faceted fullerenes and nanotubes to be nearly as stable as the planar single-phase monolayers. The resulting rich variety of polymorphs allows to tune the electronic properties of phosphorene nanotubes (PNTs) and fullerenes not only by the chiral index, but also by the combination of different phosphorene phases. In selected PNTs, a metal-insulator transition may be induced by strain or changing the number of walls.

We use ab initio density functional theory to study the equilibrium geometry and electronic structure of few-layer grey arsenic. In contrast to the bulk structure that is semimetallic, few-layer grey As displays a significant band gap that depends sensitively on the number of layers, in-layer strain, layer stacking and inter-layer spacing. A metal-semiconductor transition can be introduced by changing the number of layers or the in-layer strain. We interpret this transition by an abrupt change in the spatial distribution of electronic states near the top of the valence band., Comment: 5 pages, 3 figures

Based on {\em ab initio} density functional calculations, we propose $\gamma$-P and $\delta$-P as two additional stable structural phases of layered phosphorus besides the layered $\alpha$-P (black) and $\beta$-P (blue) phosphorus allotropes. Monolayers of some of these allotropes have a wide band gap, whereas others, including $\gamma$-P, show a metal-insulator transition caused by in-layer strain or changing the number of layers. An unforeseen benefit is the possibility to connect different structural phases at no energy cost. This becomes particularly valuable in assembling heterostructures with well-defined metallic and semiconducting regions in one contiguous layer., Comment: 5 pages, 4 figures, accepted by PRL

Results based on {\em ab initio} density functional calculations indicate a general graphitization tendency in ultrathin slabs of cubic diamond, boron nitride, and many other cubic structures including rocksalt. Whereas such compounds often show an energy preference for cubic rather than layered atomic arrangements in the bulk, the surface energy of layered systems is commonly lower than that of their cubic counterparts. We determine the critical slab thickness for a range of systems, below which a spontaneous conversion from a cubic to a layered graphitic structure occurs, driven by surface energy reduction in surface-dominated structures., Comment: 5 pages, 3 figures

We report the fabrication of both n-type and p-type WSe2 field effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e2/h, a high ON/OFF ratio of >107 at 170 K, and large electron and hole mobility of ~200 cm2V-1s-1 at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ~330 cm2V-1s-1 and that of holes to ~270 cm2V-1s-1. We attribute our ability to observe the intrinsic, phonon limited conduction in both the electron and hole channels to the drastic reduction of the Schottky barriers between the channel and the graphene contact electrodes using IL gating. We elucidate this process by studying a Schottky diode consisting of a single graphene/WSe2 Schottky junction. Our results indicate the possibility to utilize chemically or electrostatically highly doped graphene for versatile, flexible and transparent low-resistance Ohmic contacts to a wide range of quasi-2D semiconductors. KEYWORDS: MoS2, WSe2, field-effect transistors, graphene, Schottky barrier, ionic-liquid gate, Comment: 28 pages, 6 figures, accepted for publication in Nano Lett. 2014

We propose a fast method to determine the local curvature in two-dimensional (2D) systems with arbitrary shape. The curvature information, combined with elastic constants obtained for a planar system, provides an accurate estimate of the local stability in the framework of continuum elasticity theory. Relative stabilities of graphitic structures including fullerenes, nanotubes and schwarzites, as well as phosphorene nanotubes, calculated using this approach, agree closely with ab initio density functional calculations. The continuum elasticity approach can be applied to all 2D structures and is particularly attractive in complex systems with known structure, where the quality of parameterized force fields has not been established.

We investigate a previously unknown phase of phosphorus that shares its layered structure and high stability with the black phosphorus allotrope. We find the in-plane hexagonal structure and bulk layer stacking of this structure, which we call `blue phosphorus', to be related to graphite. Unlike graphite and black phosphorus, blue phosphorus displays a wide fundamental band gap and should exfoliate easily to form quasi-2D structures suitable for electronic applications. We study a likely transformation pathway from black to blue phosphorus and discuss possible ways to synthesize the new structure., Comment: 5 pages, 4 figures; accepted by Phys. Rev. Lett

We report results of \textit{ab initio} electronic structure and quantum conductance calculations indicating the emergence of conduction at the surface of semiconducting carbon foams. Occurrence of new conduction states is intimately linked to the topology of the surface and not limited to foams of elemental carbon. Our interpretation based on rehybridization theory indicates that conduction in the foam derives from first- and second-neighbor interactions between $p_\|$ orbitals lying in the surface plane, which are related to $p_\perp$ orbitals of graphene. The topologically protected conducting state occurs on bare and hydrogen-terminated foam surfaces and is thus unrelated to dangling bonds. Our results for carbon foam indicate that the conductance behavior may be further significantly modified by surface patterning., Comment: 5 pages, 3 figures

Preceding the current interest in layered materials for electronic applications, research in the 1960's found that black phosphorus combines high carrier mobility with a fundamental band gap. We introduce its counterpart, dubbed few-layer phosphorene, as a new 2D p-type material. Same as graphene and MoS2, phosphorene is flexible and can be mechanically exfoliated. We find phosphorene to be stable and, unlike graphene, to have an inherent, direct and appreciable band-gap that depends on the number of layers. Our transport studies indicate a carrier mobility that reflects its structural anisotropy and is superior to MoS2. At room temperature, our phosphorene field-effect transistors with 1.0 um channel length display a high on-current of 194 mA/mm, a high hole field-effect mobility of 286 cm2/Vs, and an on/off ratio up to 1E4. We demonstrate the possibility of phosphorene integration by constructing the first 2D CMOS inverter of phosphorene PMOS and MoS2 NMOS transistors.

We report the production of covalently bonded selenium double-helices within the narrow cavity inside double-wall carbon nanotubes. The double-helix structure, characterized by high-resolution transmission electron microscopy and X-ray diffraction, is completely different from the bulk atomic arrangement and may be considered a new structural phase of Se. Supporting ab initio calculations indicate that the observed encapsulated Se double-helices are radially compressed and have formed from free Se atoms or short chains contained inside carbon nanotubes. The calculated electronic structure of Se double-helices is very different from the bulk system, indicating the possibility to develop a new branch of Se chemistry., Comment: 25 pages, 7 figures, ACS Nano in Press (2013)