Exotic Cubic Carbon Allotropes
Elemental carbon exists in various aesthetically pleasing architectures. These forms include a group of synthesized allotropes with cubic modifications that have taken controversial or even unidentified crystal structures, which makes determining their physical properties difficult. In this study, four novel cubic carbon polymorphs (fcc-C10, fcc-C12, bcc-C20, and fcc-C32) that exhibit lattice parameters within the same range as those of undetermined cubic carbon allotropes are proposed by employing a newly developed ab initio particle-swarm optimization methodology for crystal structure prediction. The four structures are all three-dimensional polymers consisting of unique, small C10, C12, C20, and C32 cages with quite low density. Investigation of their electronic and mechanical properties illustrate that the cage-like cubic carbons are all semiconductors with excellent mechanical performance, specifically superhardness and high ductility. Moreover, we readily explain a long-standing controversial experimentally synthesized cubic carbon (viz., the so-called “superdense” carbon) using the previously proposed bcc C6 based on the coincident lattice constant and electron diffraction data between the theoretical and experimental results.
First principles study of periodic size dependent band gap variation of Cu doped ZnO single-wall nanotube
In this contribution, the size dependent band gap variation of (Zn4/6Cu2/6O)L/(Zn5/6Cu1/6O)L (L is the periodic size) superlattices are investigated with the change of L. The results show that the variation tendency of band gap appears a minimum in S4 (L = 4) which is nearly a conductor. The band gap of S3 (L = 3) and S5 (L = 5) also decrease obviously compared to the other three configurations. Especially, the band gap of S3 has decreased to 2.16 eV which is in the region of narrow bandgap semiconductor. Thus, the band gap can be modulated by alloying through constructing an appropriate variation period. Our ZnCuO superlattices can absorb light in both visible and UV region. These properties make the superlattices a potential application in photocatalysis and the visible light emitter.
Structural transitions of BiI3 under pressure
High , pressure studies of BiI3 at 0 K are performed using first-principles pseudopotential calculations within the framework of density functional theory. The calculations indicate that BiI3 undergoes a structural transition from rhombohedral R-3 phase to monoclinic P21/c phase at 7 GPa which is accompanied by a 5.8% volume collapse. In addition, we find that P21/c phase prevails about 60 GPa range and transforms to cubic Fm-3m phase at 68 GPa, and finally takes the orthorhombic Pnma phase at high pressures up to 133 GPa. The structural and electronic properties of four competitive structures are also calculated. The analysis of density of states reveals that BiI3 has semiconductor-metal transition at about 61 GPa, which also demonstrates the metallic nature of both Fm-3m and Pnma phases.
Structure and mechanical properties of tantalum mononitride under high pressure: A first-principles study
The structure and mechanical properties of tantalum mononitride (TaN) are investigated at high pressure from first-principles using the plane wave pseudopotential method within the local density approximation. Three stable phases were considered, i.e., two hexagonal phases (ε and θ) and a cubic δ phase. The obtained equilibrium structure parameters and ground state mechanical properties are in excellent agreement with the experimental and other theoretical results. A full elastic tensor and crystal anisotropy of the ultra-incompressible TaN in three stable phases are determined in the wide pressure range. Results indicated that the elastic properties of TaN in three phases are strongly pressure dependent. And the hexagonal θ-TaN is the most ultraincompressible among the consider phases, which suggests that the θ phase of TaN is a potential candidate structure to be one of the ultraincompressible and hard materials. By the elastic stability criteria, it is predicted that θ-TaN is not stable above 53.9 GPa. In addition, the calculated B/G ratio indicated that the ε and δ phases possess brittle nature in the range of pressure from 0 to 100 GPa. While θ phase is brittleness at low pressure (below 8.2 GPa) and is strongly prone to ductility at high pressure (above 8.2 GPa). The calculated elastic anisotropic factors for three phases of TaN suggest that they are elastically highly anisotropic and strongly dependent on the propagation direction.
CASTEP Calculation of Surface Energy of α-Al2O3
First principles calculations were run on bulk and the （110）（001）（012）（113）surfaces of α-Al2O3 in order to examine the growth habit of α-Al2O3 crystals. The Materials Studio package was used, specifically the program CASTEP, utilizing Perdew Burke Ernzerhof exchange-correlation pseudo-potentials. The calculation results shows that the ranking of the face energy on different crystal face are E(001)‹ E(113) ‹ E(012) ‹E(110) which is in good agreement with experiment phenomenon observed that the ranking of the growth rates of different crystal faces are V(001) ‹ V(113) ‹ V(012) ‹ V(110).
First Principles Investigation of Electronic Structure, Chemical Bonding, Elastic and Optical Properties of Novel Rhenium Nitrides
we investigate the electronic structure, chemical bonding, optical and elastic properties of the novel rhenium nitrides, hexagonal phase re3n and re2n by using density-functional theory (dft) within generalized gradient approximation (gga). the calculated equilibrium lattice constants of both re3n and re2n are in reasonable agreement with the experimental results. the band structure along the higher symmetry axes in the Brillouin zone, the density of states (dos) and the partial density of states (pdos) are presented. the calculated energy band structures and dos show that re3n and re2n are metal compounds. The dos and pdos show that the dos at the fermi level (ef) is located at the bottom of a valley and originate mainly from the 5d electrons of re. population analyses suggest that the chemical bonding in re3n and re2n has predominantly covalent character with mixed covalent and ionic characteristics. the dielectric function, reflectivity, absorption coefficient, refractive index, electron energy-loss function and optical conductivity are presented in an energy range for discussing the optical properties of re3n and re2n. basic mechanical properties, such as elastic constants cij, bulk modulus b and shear modulus g are calculated. The young’s modulus e, poisson's ratio ν and bh/ghare also predicted. results conclude that the hexagonal phase re3n and re2n are mechanical stable and behaves in a ductile manner. polycrystalline elastic anisotropy is also derived from polycrystalline bulk modulus b and shear modulus g.
Anisotropic Mechanical and Thermal Properties of Nd2SrAl2O7
For a long time since the anisotropy basically confined to a single crystal, used as a polycrystalline ceramic materials generally considered to be isotropic. In this paper, the anisotropic mechanical, thermal expansion coefficient and thermal conductivity of a double perovskite slab-rocksalt layer Nd2SrAl2O7 was studied by first principles as an example. The method is using density functional theory (DFT) and crystal parameters, which has been used to calculate the modulus of elasticity of anisotropic in a three-dimensional space. While combined with traditional thermal conductivity theory, we have obtained the tensor of thermal diffusion, thermal conductivity in 3D space for the first time in the world within no. The results are in good agreement with the experiment. The advantage of method is avoiding the difficulty of experimental measurement, reducing the time and obtaining relatively accurate results.
Density-Functional Studies of Cr Adsorbed on Polar ZnO Surfaces
The atomic structure and electronic properties of Cr adsorbed on polar ZnO surfaces is studied using first-principles calculations based on density functional theory. It is found that the Cr atom at the on top of O forms a strong ionic bond, the electrons transfer from Cr to O atoms. The Cr atom is adsorbed at the hcp-hollow site on Zn-terminated surface forms metallic bonding with the surface Zn atom, shows a free-electron-like behavior. The adsorbed atoms could not effect more atoms in ZnO due to a strong screening of ZnO to the outside metal, the character of adsorption surfaces is only decided by the atoms near the surface.
Study on Electronic Properties of ZnO Doped with Cr, Mn and Co by First Principles
A method using first principles and pseudopotentials based on density functional theory is applied to calculate the electronic structure and the density of states of ZnO doped with Cr, Mn and Co. Portion of Zn atoms in ZnO crystal randomly substituted by Mn, Cr or Co elements, the electronic structure of Cr 2+ , Mn 2+ and Co 2+ change into 3d4, 3d5 and 3d7, which result in giving rise to localized magnetic moments in ZnO. It was concluded that electronic property of ZnO is not only related with levels of electrons, but also associated with spin, spin-dependent scattering and spin-dependent hopping conductivity are maybe two important mechanism.
CASTEP + DMol3
Optical absorption and DFT calculations in L-aspartic acid anhydrous crystals: Charge carrier effective masses point to semiconducting behaviour
Density functional theory (DFT) computations within the local-density approximation and generalized gradient approximation in pure form and with dispersion correction (GGA+D) were carried out to investigate the structural, electronic, and optical properties of L-aspartic acid anhydrous crystals. The electronic (band structure and density of states) and optical absorption properties were used to interpret the light absorption measurements we have performed in L-aspartic acid anhydrous crystalline powder at room temperature. We show the important role of the layered spatial disposition of L-aspartic acid molecules in anhydrous L-aspartic crystals to explain the observed electronic and optical properties. There is good agreement between the GGA+D calculated and experimental lattice parameters, with (Δa, Δb,Δc) deviations of (0.029,−0.023,−0.024) (units in Å). Mulliken [ J. Chem. Phys. 231833 (1955)] and Hirshfeld [ Theor. Chim. Acta 44 129 (1977)] population analyses were also performed to assess the degree of charge polarization in the zwitterion state of the L-aspartic acid molecules in the DFT converged crystal. The lowest-energy optical absorption peaks related to transitions between the top of the valence band and the bottom of the conduction band involve O 2p valence states and C 1p and O 2p conduction states, with the carboxyl and COOH lateral chain group contributing significantly to the energy band gap. Among the calculated band gaps, the lowest GGA+D (4.49-eV) gap is smaller than the experimental estimate of 5.02 eV, as obtained by optical absorption. Such a wide-band-gap energy together with the small carrier effective masses estimated from band curvatures allows us to suggest that an L-aspartic acid anhydrous crystal can behave as a wide-gap semiconductor. A comparison of effective masses among directions parallel and perpendicular to the L-aspartic molecules layers reveals that charge transport must be favored in the former case. Finally, we also show that there is a strong optical anisotropy in the dielectric function of L-aspartic acid anhydrous crystals.
Probing graphene grain boundaries with optical microscopy
Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance. Although information on the atomic rearrangement at graphene grain boundaries can be obtained using transmission electron microscopy and scanning tunnelling microscopy, large-scale information regarding the distribution of graphene grain boundaries is not easily accessible. Here we use optical microscopy to observe the grain boundaries of large-area graphene (grown on copper foil) directly, without transfer of the graphene. This imaging technique was realized by selectively oxidizing the underlying copper foil through graphene grain boundaries functionalized with O and OH radicals generated by ultraviolet irradiation under moisture-rich ambient conditions: selective diffusion of oxygen radicals through OH-functionalized defect sites was demonstrated by density functional calculations. The sheet resistance of large-area graphene decreased as the graphene grain sizes increased, but no strong correlation with the grain size of the copper was revealed, in contrast to a previous report. Furthermore, the influence of graphene grain boundaries on crack propagation (initialized by bending) and termination was clearly visualized using our technique. Our approach can be used as a simple protocol for evaluating the grain boundaries of other two-dimensional layered structures, such as boron nitride and exfoliated clays.
C60-mediated hydrogen desorption in Li–N–H systems
Hydrogen desorption from a LiH + NH3 mixture is very difficult due to the formation of the stable LiNH4 compound. Using cluster models and first-principles theory, we demonstrate that the C60 molecule can in fact significantly improve the thermodynamics of ammonia-mediated hydrogen desorption from LiH due to the stabilization of the intermediate state, LiNH4. The hydrogen desorption following the path of LiNH4–C60 → LiNH3– C60 + ½H2 is exothermic. Molecular dynamic simulations show that this reaction can take place even at room temperature (300 K). In contrast, the stable LiNH4 compound cannot desorb hydrogen at room temperature in the absence of C60. The introduction of C60 also helps to restrain the NH3 gas which is poisonous in proton exchange membrane fuel cell applications.
Insight into shock-induced chemical reaction from the perspective of ring strain and rotation of chemical bonds
Density functional theory BLYP/DNP and hyperhomodesmotic equations were employed to calculate ring strain energy, the bond dissociation energy of X–NO2(X=C, N) and the charges on the nitro groups of several four-membered and six-membered heterocycle compounds. BLYP/DNP and LST/QST + CG method were also applied to calculate bond rotational energy of X–NO2 (X=C, N) of above mentioned compounds. It indicated that ring strain energy of four-membered heterocycle nitro compounds is apparently higher than that of six-membered heterocycle nitro compounds. Predictably, ring-opening reactions may preferentially occur for those compounds containing higher ring strain energy under shock. In addition, C–NO2bonds in these compounds may rotate easier than N–NO2 bonds in response to the external shock. As for N–NO2 bonds in these compounds, they also respond to the external shock by the rotation of N–NO2 bonds, once to the saddle point of the rotational energy barrier, the whole molecule will become relaxed, N–NO2 bond becomes weaker and eventually leads to the breakage. When one −C=O, −C=NH or −NH2 group is introduced to the six-membered heterocycle, the charges on the nitro groups of the new compound decrease drastically, and ring strains increase remarkably. It can be predicted that the new compounds will be more sensitive to shock, and the viewpoint is confirmed by the experimental results of shock sensitivity (small scale gap test) of several explosives.
Tunable and sizable band gap in silicene by surface adsorption
Opening a sizable band gap without degrading its high carrier mobility is as vital for silicene as for graphene to its application as a high-performance field effect transistor (FET). Our density functional theory calculations predict that a band gap is opened in silicene by single-side adsorption of alkali atom as a result of sublattice or bond symmetry breaking. The band gap size is controllable by changing the adsorption coverage, with an impressive maximum band gap up to 0.50 eV. The abinitio quantum transport simulation of a bottom-gated FET based on a sodium-covered silicene reveals a transport gap, which is consistent with the band gap, and the resulting on/off current ratio is up to 108. Therefore, a way is paved for silicene as the channel of a high-performance FET.
Search for global minimum geometries of medium sized CdnTen clusters (n = 15, 16, 20, 24 and 28)
Following our recent work which revealed the lowest-energy structures of CdnTen(n = 1–14) clusters follow the hollow cage and the endohedral cage structural growth patterns, we extend the search for the most stable structures to some larger clusters, i.e., CdnTen (n = 15, 16, 20, 24 and 28). Within the size range studied, the endohedral cages are more stable than the hollow cages. The endohedral atoms increase as the cluster size increases. The computed dipole moments and polarizabilities show a clear dependence on the cluster geometry and symmetry. The hollow cage isomers possess smaller dipole moments and larger polarizabilities than the endohedral ones.
Origin of the anomalous magnetic behavior of the Fe13+ cluster
By using density functional theory, we show that the exceptionally low value of the total magnetic moment of Fe13+ observed in an experiment [ Phys. Rev. Lett. 108057201 (2012)] is not due to antiferromagnetic coupling between the spins of the core and surface atoms as hypothesized but is due to the symmetry-driven quenching of the local spin moments of all atoms with the largest quenching observed for the central atom. Our study of Fe12+, Fe13+, Fe14+, and their neutral parents reveals that the total magnetic moment of Fe13+ decreases by 9μB with respect to that of neutral Fe13, whereas, the changes are 1μB and 3μB for Fe12+and Fe14+, respectively.
Density functional calculations on 13-atom Pd12M (M = Sc—Ni) bimetallic clusters
The geometric structures, electronic and magnetic properties of the 3d transition metal doped clusters Pd12M (M = Sc—Ni) are studied using the semi-core pseudopots density functional theory. The groundstate geometric structure of the Pd12M cluster is probably of pseudoicosahedron. The Ih-Pd12M cluster has the most thermodynamic stability in five different symmetric isomers. The energy gap shows that Pd12M cluster is partly metallic. Both the absolutely predominant metal bond and very weak covalent bond might exist in the Pd12M cluster. The magnetic moment of Pd12M varies from 0 to 5 μB, implying that it has a potential application in new nanomaterials with tunable magnetic properties.
Nonclassical fullerene C22H22 doped with transition metal atoms (ScNi): Density functional calculations
Geometric structures, electronic properties, hydrogen storage, optical absorption spectra, and magnetic properties of the nonclassical fullerenes M@C22H22 (MScNi) have been systematically studied using the density functional theory. The energy gap (5.77 eV) of the most stable C22H22 isomer has been multiplied up almost eight times compared with that of the pristine C22 cage (0.68 eV). The M@C22H22 (MScNi) cages with one four-membered ring are calculated the most stable. The new nanomaterials based on M@C22H22 could be excellent electron acceptors for potential photonic/photovoltaic applications in consequence of the increased VIP compared with that of C22H22. The optical properties of M@C22H22 can be tuned broadly in the ultraviolet–visible region. This is important for optoelectronic applications. Doping the transition metal atoms into the C22H22cage can tune the magnetic properties.
Structures and magnetism of multinuclear vanadium-pentacene sandwich clusters and their 1D molecular wires
Two types of multinuclear sandwich clusters, (V3)nPenn+1, (V4)nPenn+1 (Pen = Pentacene; n = 1, 2), and their corresponding infinite one-dimensional (1D) molecular wires ([V3Pen]∞, [V4Pen]∞) are investigated theoretically, especially on their magnetic coupling mechanism. These sandwich clusters and molecular wires are found to be of high stability and exhibit intriguing magnetic properties. The intra-layered V atoms in (V3)nPenn+1 clusters prefer antiferromagnetic (AFM) coupling, while they can be either ferromagnetic (FM) or AFM coupling in (V4)nPenn+1 depending on the intra-layered V-V distances via direct exchange or superexchange mechanism. The inter-layered V atoms favor FM coupling in (V3)2Pen3, whereas they are AFM coupled in (V4)2Pen3. Such magnetic behaviors are the consequence of the competition between direct exchange and superexchange interactions among inter-layered V atom, s. In contrast, the 1D molecular wires, [V3Pen]∞ and [V4Pen]∞, appear to be FM metallic with ultra high magnetic moments of 6.8 and 4.0 μB per unit cell respectively, sugges, ting that they can be served as good candidates for molecular magnets.
Ultra-high hydrogen storage capacity of Li-decorated graphyne: A first-principles prediction
Graphyne, consisting of sp- and sp2-hybridized carbon atoms, is a new member of carbon allotropes which has a natural porous structure. Here, we report our first-principles calculations on the possibility of Li-decorated graphyne as a hydrogen storage medium. We predict that Li-doping significantly enhances the hydrogen storage ability of graphyne compared to that of pristine graphyne, which can be attributed to the polarization of H2 molecules induced by the charge transfer from Li atoms to graphyne. The favorite H2 molecules adsorption configurations on a single side and on both sides of a Li-decorated graphyne layer are determined. When Li atoms are adsorbed on one side of graphyne, each Li can bind four H2 molecules, corresponding to a hydrogen storage capacity of 9.26 wt. %. The hydrogen storage capacity can be further improved to 15.15 wt. % as graphyne is decorated by Li atoms on both sides, with an optimal average binding energy of 0.226 eV/H2. The results show that the Li-decorated graphyne can serve as a high capacity hydrogen storage medium.
Relative stability of nanosized β-C3N4 and graphitic C3N4 from first principles calculations
Relative stability of nanosized β-C3N4 and graphitic C3N4 has been studied by using first principles calculations. It has been demonstrated that the relative stability sequence changes with increasing size of the nanostructure. When the number of C3N4 molecules in the C3N4 nanostructure is less than a threshold number, the β-C3N4 nanostructure is in the stability phase. When the number of C3N4 molecules in the C3N4 nanostructure exceeds the threshold number, the graphitic C3N4 is in the stability phase. In addition, size-dependence electronic properties of β-C3N4 and graphitic C3N4 nanostructures have also been analyzed in this work.
Is Dual Morphology of Rock-Salt Crystals Possible with a Single Additive? The Answer Is Yes, with Barbituric Acid
CO Adsorption on the AunS (n=1~6) Clusters: The First-Principles Study
Density functional theory (DFT) calculations are performed to investigate CO bonded on the AunS (n=1~6) bimetallic clusters. It is found that the adsorption energies of CO on the AunS(n=1~6) clusters are greater than those on the pure Au clusters of corresponding sizes. This means that doped S atom can enhance CO adsorption on the Au clusters. Furthermore, through the Mulliken population analysis, we can see that charges transfer from the Au clusters to S atom, while charges donate to the Au clusters from the CO in CO/AunS system.
Boratabenzene-vanadium sandwich molecular wire and its properties
Different from all reported sandwich molecular wires (SMWs), a novel class of SMW including vanadium boratabenzene (HBBz) clusters and their related one-dimensional (1D) SMWs are explored by using a density functional theory approach. The uniqueness of this class of SMWs lies in the boron heterocycles, where they possess a reactive functional atom. These features may overcome the limitation of existing SMWs which are inert and are difficult to be absorbed stably on surfaces. Theoretical calculations of the novel vanadium boratabenzene clusters indicated that they are energetically stable, with the rings having a restricted degree of rotation. In addition, its metallic 1D analog shows great promise in the applications of molecular electronics and spintronics.
Molecular dynamics simulations of oxygen v, acancy diffusion in SrTiO3
A classical force-field model with partial ionic charges was applied to study the behaviour of oxygen vacancies in the perovskite oxide strontium titanate (SrTiO3). The dynamical behaviour of these point defects was investigated as a function of temperature and defect concentration by means of molecular dynamics (MD) simulations. The interaction between oxygen vacancies and an extended defect, here a Σ3(111) grain boundary, was also examined by means of MD simulations. Analysis of the vacancy distribution revealed considerable accumulation of vacancies in the envelope of the grain boundary. The possible clustering of oxygen vacancies in bulk SrTiO3 was studied by means of static lattice calculations within the Mott–Littleton approach. All binary vacancy–vacancy configurations were found to be energetically unfavourable.