Systematic Ab Initio Investigation Research Articles

Strain‐Engineered Ultrahigh Mobility in Phosphorene for Terahertz Transistors

AbstractCarrier mobility is a key parameter for the operation of electronic devices as it determines the ON state current and switching speed/frequency response of transistors. 2D phosphorene is considered as a potential candidate for field‐effect transistors due to its high mobility. Here it is proposed to further enhance the carrier mobility of phosphorene and device performance via strain engineering. A systematic ab initio investigation on the anisotropic electronic structure of few‐layer phosphorene reveals that the monolayer under 7.5–10% strain along zigzag direction shows an exceptional carrier mobility of ≈106 cm2 V−1 s−1, which is 10 times higher than the strain‐free case. The simulated device performance shows that strain‐engineered phosphorene–based field‐effect transistors demonstrate a cut‐off frequency of ≈1.14 THz with a gate length of 1.0 micron and 112 THz with a sub‐10 nm gate length.

High-temperature and high-pressure physical properties of CuI with zinc-blende phase by a systematic ab initio investigation

A systematic ab initio investigation has been performed to determine the elastic stability, the structural and finite-temperature thermodynamic properties of copper iodide (CuI) in the zinc-blende phase under pressure. All calculations are carried out based on the pseudopotential density functional method, in which we employ the generalized gradient approximation of the revised Perdew-Burke-Ernzerhof form and local density approximation of Ceperly and Adler parameterized by Perdew and Zunger together with plane-wave basis sets for expanding the periodic electron density. The obtained normalized volume dependence of the pressure and the equation-of-state parameters including equilibrium volume, isothermal bulk modulus and its pressure derivatives are in excellent agreement with the experimental data and other theoretical results. The elastic constants are calculated and the investigation of the mechanical stability from the elastic constants under pressure indicates that the zinc-blende CuI is stable up to 10GPa. Through the careful evaluation with the quasi-harmonic Debye model in which the phononic effects are considered, a complete set of thermodynamic data up to 800K, including the bulk modulus, volume thermal expansion coefficient, heat capacity, entropy, Debye temperature, and Grüneisen parameter of CuI with zinc-blende structure is achieved. This set of data is considered as the useful information to understand the high-temperature and high-pressure properties of CuI.

Longitudinal and transverse spin relaxation times of magnetic single adatoms: An ab initio analysis

We present a systematic ab initio investigation of the longitudinal and transverse spin relaxation times of magnetic single adatoms deposited on metallic substrates. Our analysis based on time-dependent density functional theory shows that the longitudinal time, $T_{\parallel}$, is of order femtosecond while the transverse time, $T_{\perp}$, is of order picosecond, i.e. $T_{\perp}\gg T_{\parallel}$. This comes as a consequence of the different energy scales of the corresponding processes: $T_{\parallel}$ involves spin-density excitations of order eV, while $T_{\perp}$ is governed by atomic spin-excitations of order meV. Comparison to available inelastic scanning tunneling spectroscopy $\mathrm{d}I/\mathrm{d}V$ experimental curves shows that the order of magnitude of $T_{\perp}$ agrees well with our results. Regarding $T_{\parallel}$, the time scale calculated here is several orders of magnitude faster than what has been measured up to now; we therefore propose that an ultrafast laser pulse measuring technique is required in order to access the ultrafast spin-dynamics described in this work.

Alloyed surfaces: New substrates for graphene growth

We report a systematic ab-initio density functional theory investigation of Ni(111) surface alloyed with elements of group IV (Si, Ge and Sn), demonstrating the possibility to use it to grow high quality graphene. Ni(111) surface represents an ideal substrate for graphene, due to its catalytic properties and perfect matching with the graphene lattice constant. However, Dirac bands of graphene growth on Ni(111) are completely destroyed due to the strong hybridization between carbon pz and Ni d orbitals. Group IV atoms, namely Si, Ge and Sn, once deposited on Ni(111) surface, form an ordered alloyed surface with 3×3-R30° reconstruction. We demonstrate that, at variance with the pure Ni(111) surface, alloyed surfaces effectively decouple graphene from the substrate, resulting unstrained due to the nearly perfect lattice matching and preserves linear Dirac bands without the strong hybridization with Ni d states. The proposed surfaces can be prepared before graphene growth without resorting on post-growth processes which necessarily alter the electronic and structural properties of graphene.

Systematic ab initio investigation of the elastic modulus in quaternary transition metal nitride alloys and their coherent multilayers

We give a comprehensive overview of the elastic properties of cubic quaternary transition metal nitride alloys and coherent nitride multilayers for design of wear resistant hard coatings. The elastic stiffness constants of the alloys are calculated using the special quasirandom structure method. For multilayers with sharp interfaces we prove the applicability of a linear-elasticity approximation and show that it can be used with success instead of performing direct computationally demanding ab initio calculations. We explore the trends and the potential of multicomponent alloying in engineering the strength and ductility of both, quaternary alloys and their multilayers. We investigate X(1−x−y)TixAlyN alloys where X is Zr, Hf, V, Nb or Ta, and present an analysis based on increasing x. We show that with increasing Ti content ductility can increase in each alloy. Elastic isotropy is observed only in (Zr,Hf,V)(1−x−y)TixAlyN alloys in the middle of the compositional triangle, otherwise a high Young's modulus is observed along [001]. We predict that coherent TiN/X(1−x−y)TixAlyN and ZrN/X(1−x−y)TixAlyN alloy multilayers with the [111] interfacial direction show increasing ductility with increasing x, while the multilayers with the [001] orientation become more brittle. We show that the Young's moduli variation in the parent bulk quaternary nitride alloy provide a reliable descriptor to screen the Young's modulus of coherent multilayers in high-throughput calculations.

Theoretical Study of the Structural, Energetic, and Electronic Properties of 55-Atom Metal Nanoclusters: A DFT Investigation within van der Waals Corrections, Spin–Orbit Coupling, and PBE+U of 42 Metal Systems

An atom-level ab initio understanding of the structural, energetic, and electronic properties of nanoclusters with diameter size from 1 to 2 nm figures as a prerequisite to foster their potential technological applications. However, because of several challenges such as the identification of ground-state structures by experimental and theoretical techniques, our understanding is still far from satisfactory, and further studies are required. We report a systematic ab initio investigation of the 55-atom metal nanoclusters, (M55), including alkaline, transitional, and post-transitional metals, that is, a total of 42 systems. Our calculations are based on all-electron density functional theory within the Perdew–Burke–Ernzerhof (PBE) functional combined with van der Waals (vdW) correction, spin–orbit coupling (SOC) for the valence states. Furthermore, we also investigated the role of the localization of the d states by using the PBE+U functional. We found a strong preference for the putative PBE global-minimum...

Dirac fermions at high-index surfaces of bismuth chalcogenide topological insulator nanostructures.

Binary bismuth chalcogenides Bi2Se3, Bi2Te3, and related materials are currently being extensively investigated as the reference topological insulators (TIs) due to their simple surface-state band dispersion (single Dirac cone) and relatively large bulk band gaps. Nanostructures of TIs are of particular interest as an increased surface-to-volume ratio enhances the contribution of surfaces states, meaning they are promising candidates for potential device applications. So far, the vast majority of research efforts have focused on the low-energy (0001) surfaces, which correspond to natural cleavage planes in these layered materials. However, the surfaces of low-dimensional nanostructures (nanoplatelets, nanowires, nanoribbons) inevitably involve higher-index facets. We perform a systematic ab initio investigation of the surfaces of bismuth chalcogenide TI nanostructures characterized by different crystallographic orientations, atomic structures and stoichiometric compositions. We find several stable terminations of high-index surfaces, which can be realized at different values of the chemical potential of one of the constituent elements. For the uniquely defined stoichiometric termination, the topological Dirac fermion states are shown to be strongly anisotropic with a clear dependence of Fermi velocities and spin polarization on the surface orientation. Self-doping effects and the presence of topologically trivial mid-gap states are found to characterize the non-stoichiometric surfaces. The results of our study pave the way towards experimental control of topologically protected surface states in bismuth chalcogenide nanostructures.

Correlation between the ionic potential and thermal stability of metal borohydrides: First-principles investigations

Metal borohydrides are intensively studied because of their potential applications as versatile hydrogen storage. The relation between the formation enthalpy and the Pauling electronegativity established for these materials [Phys. Rev. B 74, 045126 (2006)] led to the idea of developing mixed-cation compounds that may provide a route for tuning the thermodynamic stability of metal borohydrides. We report a systematic ab initio investigation of the single-metallic and bimetallic borohydrides, and via an examination of the Born effective charges we provide insight into the physical mechanism determining their stabilities. We show that the decreasing stability of metal borohydrides follows the increasing polarizing ability of the cationic bonding component, expressed as the square root of the cation's dynamical charge divided by its radius in coordinated anion polyhedra around the cation. The charge-to-size ratio thus provides a simple yet physically sound measure of the stabilities of metal borohydrides that can be obtained in relatively simple calculations.

Ab Initio Study of Cooperative Effects in Complexes X:HBO:Z, with X, Z=LiH, HNC, HF, HCN, HCl, ClF, and HBO: Structures, Binding Energies, and Spin-Spin Coupling Constants across Intermolecular Bonds

Abstract A systematic ab initio investigation has been carried out to determine the structures, binding energies, and spin-spin coupling constants of ternary complexes X:HBO:Z for X, Z= LiH, HNC, HF, HCN, HCl, ClF, and HBO. All complexes X:HBO:Z are linear with C∞ v symmetry, except for HCl:HBO:Z and ClF:HBO:Z which have Cs symmetry, thereby reflecting the structures of the corresponding X:HBO and HBO:Z complexes. Cooperative effects on energies are synergistic in all ternary complexes. The enhanced binding energies of complexes X:HBO:Z correlate with the binding energies of the X:HBO and HBO:Z complexes. Coupling constants 1J(B-H) and 2hJ(B-A) across B-H···sA hydrogen bonds correlate with the B-A distance, and exhibit synergistic effects due to the presence of Z. 1hJ(H-A) indicates that these bonds have little proton-shared character. Coupling constants across D-H···sO hydrogen bonds, H-Li···sO lithium bonds, and F-Cl···sO halogen bonds are also sensitive to the synergistic effects arising from the presence of X. D-H···sO hydrogen bonds in ternary complexes are traditional (normal) hydrogen bonds.

Structural and electronic properties of bilayer and trilayer graphdiyne

Stimulated by the recent experimental synthesis of a new layered carbon allotrope-graphdiyne film, we provide the first systematic ab initio investigation of the structural and electronic properties of bilayer and trilayer graphdiyne and explore the possibility of tuning the energy gap via a homogeneous perpendicular electric field. Our results show that the most stable bilayer and trilayer graphdiyne both have their hexagonal carbon rings stacked in a Bernal way (AB and ABA style configuration, respectively). Bilayer graphdiyne with the most and the second most stable stacking arrangements have direct bandgaps of 0.35 eV and 0.14 eV, respectively; trilayer graphdiyne with stable stacking styles have bandgaps of 0.18-0.33 eV. The bandgaps of the semiconducting bilayer and trilayer graphdiyne generally decrease with increasing external vertical electric field, irrespective of the stacking style. Therefore, the possibility of tuning the electronic structure and optical absorption of bilayer and trilayer graphdiyne with an external electric field is suggested.

Electric-Field-Induced Energy Gap in Few-Layer Graphene

We provide the first systematic ab initio investigation of the possibility to create a band gap in few-layer graphene (FLG) via a perpendicular electric field. Bernal (ABA) and arbitrarily stacked FLG remain semimetallic, but rhombohedral (ABC) stacked FLG demonstrates a variable band gap. The maximum band gap in ABC stacked FLG decreases with increasing layer number and can be fitted by the relationship Δmax = 1/(2.378 + 0.521N + 0.035N2) eV. The effective masses of carriers over a wide range around the maximum band gap point in ABC stacked FLG are comparable with that in AB bilayer graphene under zero field. It is therefore possible to fabricate an effective field effect transistor operating at room temperature with high carrier mobility out of ABC stacked FLG.

An ab initio study of cooperative effects in ternary complexes X:CNH:Z with X, Z=CNH, FH, ClH, FCl, and HLi: structures, binding energies, and spin–spin coupling constants across intermolecular bonds

A systematic ab initio investigation has been carried out to determine the structures, binding energies, and spin-spin coupling constants of ternary complexes X:CNH:Z and corresponding binary complexes X:CNH and CNH:Z, for X, Z = CNH, FH, ClH, FCl, and HLi. The enhanced binding energies of ternary complexes X:CNH:Z for fixed X as a function of Z decrease in the same order as the binding energies of the binary complexes CNH:Z. In contrast, the enhanced binding energies of the ternary complexes for fixed Z as a function of X do not decrease in the same order as the binding energies of the binary complexes X:CNH, a consequence of the increased stabilities of ternary complexes FCl:CNH:Z due to very strong chlorine-shared halogen bonds. For complexes in which the X···CNH interaction is a D-H···C hydrogen bond for D-H the proton-donor group (N-H, F-H, or Cl-H), spin-spin coupling constants (1)J(D-H) and (2h)J(D-C) in ternary complexes X:CNH:Z decrease in absolute value as the binding energies of binary complexes CNH:Z and the enhanced binding energies of the ternary complexes for fixed X as a function of Z also decrease. However, (2X)J(F-C) increases as the enhanced binding energies of the ternary complexes FCl:CNH:Z decrease, a consequence of the nature of the chlorine-shared halogen bond. The one-bond coupling constants (1)J(N-H) for the CNH···Z interaction in ternary complexes vary significantly, depending on the nature of the X···CNH interaction. The largest values of (1)J(N-H) are found for ternary complexes with FCl as X. Two-bond coupling constants (2h)J(N-A) for A the proton-acceptor atom of Z, and (2d)J(N-H) decrease in absolute value in the order of decreasing enhancement energies of ternary complexes X:CNH:Z for fixed Z as a function of X.

Hydrogen dissociation and diffusion on transition metal (= Ti, Zr, V, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H 2 molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab initio density functional theory investigation of H 2 dissociation and subsequent atomic H diffusion on TM (= Ti, V, Zr, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TMs on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TMs on the right of the periodic table do not bind H, however, they do not reduce the barrier to dissociate H 2 significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.

First-principles study of crystal structure, electronic structure, and second-harmonic generation in a polar double perovskite Bi2ZnTiO6

Within the density functional theory with the generalized gradient approximation, we present a systematic ab initio investigation of crystal structure, electronic structure, and linear and nonlinear optical responses in a polar double perovskite Bi(2)ZnTiO(6). The effect of B-site ordering is explored by comparing three possible configurations: A-type with alternative Zn and Ti layers stacking along the c axis; C-type with Zn and Ti c axis chains; and G-type with every Zn(Ti) atoms is surrounded by its nearby six Ti(Zn) atoms. It is found that the system with G-type B-site ordering is energetically favorable, which is lower in the total energies of 0.055 and 0.133 eV/formula unit than C-type and A-type, respectively. Optical calculations indicate that all the three configurations show large second-harmonic generation (SHG) coefficients, and the largest static SHG observed in the C-type system reaches 123 (10(-9) esu), the value of which is much larger than ever known polar oxides, e.g., 72 (10(-9) esu) in LiNbO(3). The predicted significant nonlinear optical properties are consistent with the calculated high tetragonality as well as the large off-center displacement of Zn, Ti, and Bi atoms. In particular, a large off-center displacement greater than 0.5 A in Zn atoms is revealed for the first time. A further microscopic picture is presented via the successful connection of the prominent feature of SHG in Bi(2)ZnTiO(6) with that of the linear optical dielectric function in terms of single-photon and double-photon resonances. Our calculations demonstrate the promising application of Bi(2)ZnTiO(6) in optoelectronics.

Structure, ordering, and bonding of half antiperovskites: PbNi3/2S and BiPd3/2S

In a model study the crystal structures of shandite (Pb2Ni3S2), parkerite (Bi2Ni3S2) and their Pd homologues are investigated in terms of ordered half antiperovskites AM3/2S (A = Pb, Bi; M = Ni, Pd). This addresses fundamental questions on the structural relations, ordering and chemical bonding. From crystal structure investigations a new cubic parkerite variant is presented for Bi2Pd3S2 that fits in an ordering model equivalently to shandite and parkerite. Type–antitype relations to ordered oxygen deficit perovskites are presented. With the relation to the superconductor Ni3MgC a model is deduced that provides the complete crystal structure and symmetry in terms of the Ni and Pd ordering in antiperovskite superstructures. Therein a systematic ab initio investigation on the relative stability of shandite and parkerite structures is carried out for the first time. From the DFT modelling results the preferences of the ordering variants and the distinct differences in the atomic coordination spheres are discussed. The bonding in the systems is investigated by site projected density of states and covalent bond energy calculations.

The Rh–ligand bond: RhX (X = C, N, O, F, P and Cl) molecules

Bond distances, vibrational frequencies and dipole moments of the RhX (X = C, N, O, F, P and Cl) molecules were studied by density functional theory (DFT) methods. For all molecules under consideration, spectroscopic properties are computed using various functionals. This study is the first systematic ab initio investigation of the RhX molecules and in those cases where comparison with experimental data can be made, good quantitative agreement is achieved when using the BP86 functional.

A Theoretical Study of the Mechanism of the Water-Catalyzed HCl Elimination Reactions of CHXCl(OH) (X = H, Cl) and HClCO in the Gas Phase and in Aqueous Solution

A systematic ab initio investigation of the water-assisted decomposition of chloromethanol, dichloromethanol, and formyl chloride as a function of the number of water molecules (up to six) building up the solvation shell is presented. The decomposition reactions of the chlorinated methanols and formyl chloride are accelerated substantially as the reaction system involves additional explicit coordination of water molecules. Rate constants for the decomposition of chlorinated methanols and formyl chloride were found to be in reasonable agreement with previous experimental observations of aqueous phase decomposition reactions of dichloromethanol [CHCl(2)(OH)] and formyl chloride. For example, using the calculated activation free energies in conjunction with the stabilization free energies from the ab initio calculations, the rate constant was predicted to be 1.2-1.5 x 10(4) s(-1) for the decomposition of formyl chloride in aqueous solution. This is in good agreement with the experimental rate constant of about 10(4) s(-1) reported in the literature. The mechanism for the water catalysis of the decomposition reactions as well as probable implications for the decomposition of these chlorinated methanol compounds and formaldehydes in the natural environment and as intermediates in advanced oxidation processes are briefly discussed.

Ab initio study of curvature effects on the physical properties of the Xe-doped nanotubesand nanoropes

A systematic comprehensive ab initio investigation of the energetics, structural, electronic,and optical properties of weakly and heavily Xe-doped ultrathin 4 Å and large-diametercarbon nanotubes has been performed. The contributions of the dispersion as well as thezero-point energies have been considered. The effects of the tube diameter and the differentchiralities of the carbon nanotubes on the adsorbant-induced physical propertieshave been investigated. The favoured tubes for Xe atom adsorption are armchair(n,n) tubes, and one may not observe any appreciable surface Xe adsorption on zigzag(n,0) orchiral (m,n) tubes. The band gap of a semiconducting achiral zigzag nanotube is reduced whereasthe band gap of a chiral semiconducting tube is enhanced by the adsorption ofXe atoms. Xe adsorption in/on the metallic tubes creates extra (one or more)conductive channels which run inside/outside the nanotubes and may result in quiteenhanced conduction. The optical absorption of the achiral (chiral) 4 Å diametertube is considerably changed (unchanged) by the adsorption of the Xe atoms.The calculated peaks in large-diameter pristine nanotubes have been detectedin the experimental measurements. In the (10, 0) and (10, 10) nanotubes, thesmall adsorption of Xe does not change the optical absorption of the pristinetube. The computed binding energy and the saturation Xe concentration arein reasonable agreement with the experimental data available for the (10, 10)nanotube.

An ab initio study of optical and Raman spectra of heavily Li-doped 4 Å carbonnanotubes

An extended and systematic ab initio investigation of the energetics, structural, electronic,optical and Raman-active properties of heavily Li-doped ultrathin 4 Å diameter carbonnanotubes of different chiralities has been performed. A number of new features notdiscussed earlier are observed in the present study. Our values for the binding energies ofthe Li-doped tubes are higher than the ones reported earlier. We find the Li-doped (3, 3)tubes stable in contrast to an earlier prediction. For each type of the tube, the saturationintake of Li atoms inside the tube is 8–10% only, whereas on the surfaces of the (5, 0)and (4, 2) tubes, this intake is as high as 100%, and it forms a circular tube ofLi+ ions around the tube.Li atoms residing on the axis inside the tube do not modify the symmetry of the host tubeand thus the electronic structure (except the upward shifting of the Fermi surface,EF) in contrast to surface Li atoms which (apart from the (4, 2) tube, which possesses nosymmetry) destroy the symmetry and drastically alter the electronic structure. In general,Li doping inside or outside the tube increases the electron density of states (DOS) severaltimes except in the (5, 0) tube doped with a small concentration of Li atoms, where itdecreases. For one inner Li atom in each tube, the peak structure in the opticalabsorption remains essentially the same as that of the pristine tube. In general, theoptical absorption is different for the various nanotube–Li configurations studied.For each type of tube, in general, the RBM frequency reduces with the dilation of thediameter of the optimized tube. The high values of DOS obtained for certain Liconcentrations may lead to high values of electrical conductivity and the superconductingtransition temperatures in Li-doped tubes as has already been reported in pristine tubesand ropes.

A Preliminary Study of Host-Guest Interactions in Polymeric Clathrates – An Ab Initio Study of the Model Complexes Benzene/X2 (X = F, Cl, Br, I)

A systematic ab initio investigation of the complexes benzene/halogen molecules is presented. For the whole halogen series, the T-shaped geometry is clearly favored relative to the sandwich geometry. This preference is rationalized in terms of the electrostatic fields generated by the interacting molcules. Higher binding energies have been calculated on going from /F2 to /I2 molecular complexes.