Deformation Charge Density Research Articles

First-principle analysis of photoelectric properties of silicon-carbon materials with graphene-like honeycomb structure

This study uses the first-principle pseudo-potential method based on density functional theory to investigate the structure stability, geometrical structure, electronic structure, deformation charge density, atomic orbital and bond population, and optical properties of graphene-like structures SixC1−x (x=0.02, 0.08, 0.2, 0.4, 0.5). Geometrical structure results show that the substituted Si atoms induce the structural deformation along the directions on the graphene plane without any buckling. The cohesion energy calculation indicates that these substituted systems are absolutely stable. The pure graphene forms a zero-band gap semiconductor, and the densities of states near the Fermi energy are composed mainly of C-2p. With Si doping, a direct band gap appears at the K point in the Brillouin zone. The band gap increases from 0.15eV to 2.60eVwith the increase in Si content. The densities of states of Si0.50C0.50 near the Fermi energy are composed mainly of C-2p and Si-3p. After substituting Si atoms for C atoms, charges transfer from Si atoms to C atoms, the covalent bond of CC weakens, and the symmetry of the covalent bond of graphene is destroyed, which is very important for the appearance of direct band gap at the K point. All curves of optical properties of SixC1−x (x=0.02, 0.08, 0.2, 0.4, 0.5) move to the high energy with the increase in Si content. Among those of all the studied SixC1−x systems, Si0.50C0.50 obtains the lowest static dielectric constant of 1.60, the lowest refractive index of 1.26, the highest absorption coefficient of 6.79×104cm−1. The results will give a guide for design and application of the single layer SiC in further experimental and the theoretical investigations.

First-principles study of the structural, elastic and electronic properties of Pt3M alloys

Abstract

Disparate Strain Dependent Thermal Conductivity of Two-dimensional Penta-Structures.

Two-dimensional (2D) carbon allotrope called penta-graphene was recently proposed from first-principles calculations and various similar penta-structures emerged. Despite significant effort having been dedicated to electronic structures and mechanical properties, little research has been focused on thermal transport in penta-structures. Motivated by this, we performed a comparative study of thermal transport properties of three representative pentagonal structures, namely penta-graphene, penta-SiC2, and penta-SiN2, by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations. Unexpectedly, the thermal conductivity of the three penta-structures exhibits diverse strain dependence, despite their very similar geometry structures. While the thermal conductivity of penta-graphene exhibits standard monotonic reduction by stretching, penta-SiC2 possesses an unusual nonmonotonic up-and-down behavior. More interestingly, the thermal conductivity of penta-SiN2 has 1 order of magnitude enhancement due to the strain induced buckled to planar structure transition. The mechanism governing the diverse strain dependence is identified as the competition between the change of phonon group velocity and phonon lifetime of acoustic phonon modes with combined effect from the unique structure transition for penta-SiN2. The disparate thermal transport behavior is further correlated to the fundamentally different bonding nature in the atomic structures with solid evidence from the distribution of deformation charge density and more in-depth molecular orbital analysis. The reported giant and robust tunability of thermal conductivity may inspire intensive research on other derivatives of penta-structures as potential materials for emerging nanoelectronic devices. The fundamental physics understood from this study also solidifies the strategy to engineer thermal transport properties of broad 2D materials by simple mechanical strain.

Redox Properties of Graphenes Functionalized with Cyclopentadiene–Transition Metal Complexes: A Potential Redox-Active Material

The redox properties of CpTM (Cp = cyclopentadienyl and TM = transition metal) on B-doped, N-doped, and pristine graphene complexes are evaluated using density functional theory in order to determine the possibility of using these complexes as novel redox-active materials for electrochemical applications. The CpFe/B-doped graphene complexes with a series of different side chains are found to have comparable redox potentials to ferrocene molecules and other ferrocene-based electrochemical sensors (in water and in acetonitrile solution), which indicates the potential application of these complexes to substitute for ferrocene-based electrochemical systems. The redox potentials of CpFe on pristine and N-doped graphene indicate that they would function as reducing agents. In addition, the charge transfer mechanism during the redox process is investigated and visualized using deformation charge densities, frontier orbitals, and natural bond orbital (NBO) population analyses. Large charge redistributions are obs...

Theoretical and experimental search for ZnSb-based thermoelectric materials

We report a combined theoretical and experimental search for thermoelectric materials based on semiconducting zinc antimony. Influence of three new doping elements (sodium, potassium and boron) on the electronic properties was investigated as well as the carrier concentration and temperature dependence of the thermoelectric coefficients obtained through density-functional calculations and Boltzmann transport theory. Distortion of the electron arrangement caused by the doping elements is displayed as a deformation charge density around the atoms. Based on the band structures, the density of states, and the transport properties, we found that the presence of Na and K in the ZnSb matrix leads to a slightly improved p-type conductivity, whereas the B substitution leads to a n-type doping. Because of the stronger need for obtaining n-type ZnSb–based material, the B0.01Zn0.99Sb structure has been transferred to the laboratory to be synthesized by direct melting. The sample was investigated using x-ray diffraction and scanning electron microscopy.

The formation energy and bonding characteristics of small helium–vacancy clusters on the low-index surface of α-Fe by first principles calculations

The structure and energy of a small helium–vacancy (He–V) cluster on the low-index surface of α-Fe are investigated by an ab initio method. We report the stability characteristics of these He–V clusters. The He atom of the He–V cluster at the topmost surface atomic layers (SALs) will relax into the vacuum gap. Some of the surface He atoms at octahedral interstitial sites will relax into a more stable tetrahedral interstitial site. The deformation charge densities and electronic densities of the states explain the instability of the surface He atoms. The formation energies of the He–V clusters gradually increase from the topmost SAL to the bulk-like atomic layer. This indicates that all of the He–V defects considered tend to aggregate at the topmost SAL.

First-principle study on optical properties of N—La-codoped anatase TiO

The electronic structures, deformation charge density, dipole moment, and optical properties of N—La-codoped anatase titanium dioxide (TiO2) are studied using the plane-wave ultrasoft pseudopotential method based on the density functional theory (DFT). The optical properties of two-ion-doped TiO2 are analyzed via electronic structures, deformation charge density, and dipole moment. For the model of N—La-doped TiO2, a smaller atom fraction of N and La atoms induces better optical properties. The absorption edges of two doped TiO2 models redshift to the visible-light region.

Electron localization morphology of the stacking faults in Mg: A first-principles study

Electron localization morphologies of growth, deformation, and extrinsic faults of hcp Mg are calculated, yielding quantitative descriptions of charge transfer between atoms in and out of the stacking faults. We provide a physical interpretation of the relation between stacking fault energy and the difference of charge density and electron localization function between fault and non-fault planes and show that the stacking fault energy ascends in the order of growth, deformation, and extrinsic faults and is proportional to the square of the difference of maximum deformation charge density, the difference of maximum electron localization function, and the number of faulted layers.

Optical properties of N and transition metal R (R=V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) codoped anatase TiO2

The electronic structures, deformation charge density, dipole moment and optical properties of N and transition metal R (R=V, Cr, Mn, Fe, Co, Ni, Cu and Zn) codoped anatase TiO2 are studied using the plane-wave ultrasoft pseudopotential method of density functional theory (DFT). The results exhibit that the absorption coefficients of the N+Cr-doped TiO2 are in accordance with the experimental values in the visible-light region. The calculated results reveal that the N+R-doped TiO2 is helpful for enhancing the absorption coefficient in the visible-light region; especially, among the eight materials, N+Mn-doped TiO2 has the largest value of absorption coefficient in the visible-light region. The ionic bonding and Jahn–Teller distortion would contribute to increase the photocatalytic activity of TiO2 in the visible-light region. All in all, for doped TiO2, the large dipole moment of TiO6 octahedron can enhance the optical responses in the visible-light region; in this work, however, it is found that the small dipole moment of TiO6 octahedron would increase the absorption coefficient in the visible-light region.

The structural stability and electronic properties of monolayer BC2N

The structural stability and the electronic properties of two-dimensional monolayer BC2N are studied by employing the first principles method based on the density functional theory. 16 polymorphic structures of monolayer BC2N are calculated. Analysis of the 16 band structures suggests that the structure with the highest symmetry is of a semimetal which is the same as graphene. All the other structures are of semiconductors with different band gaps, of which the most stable structure is of semiconductor with a direct gap of 1.63 eV. Based on the deformation charge density and the Bader analysis, the bonds CC, CN, CB, and BN in the most stable monolayer BC2N are mainly covalent, however, they present also significant ionic behaviors. Exerting a stress on the monolayer BC2N sheet changes the band gap, showing that the band gap increases during the compression while decreases during the stretch, and the system keeps a direct semiconductor.

Experimental charge density study of (DBr-DCNQI) 2Cu for metallic phase by synchrotron X-ray diffraction

An accurate experimental charge density of (DBr-DCNQI) 2Cu in the metallic phase has been determined from the high resolution single-crystal diffraction data measured at one of the third generation synchrotron radiation X-ray sources, SPring-8. Two kinds of analytical method were used for determination of the experimental charge density. First, the maximum entropy method (MEM) was used for calculation of a total experimental charge density. Secondly, the multipolar refinement was used for calculation of the static deformation charge density, which represents the aspherical charge density in the molecules without thermal smearing effect. The charge densities indicating bonding electrons of DCNQI molecule were clearly revealed in the static deformation map. We estimated the valence charge for Cu ion by the Bader topological analysis. The determined value was +1.30(13)e, which is consistent with value in the previous studies.

Density functional theory study of hydrogen adsorption on C@Al12 cluster

Density functional theory has been employed to investigate the structure and stability of C@Al12Hn (1≤n≤7) clusters. Hydrogenated C@Al12 clusters exhibit pronounced stability for even numbers of H atoms. Large HOMO-LUMO gaps，large binding energy and increased ionization potentials imply that these clusters should be physically and chemically stable. The analysis of the charge density of the HOMO plot illustrates that a pair of hydrogen atoms prefer to occupy opposing on-top sites for clusters with an even n number. Studies of deformation charge density plots demonstrate that the bonding characteristic between the H atoms and the C@Al12 moiety is mainly covalent. The total magnetic moment is 1μB for C@Al12Hn with an odd number for n.

Point-Defect Mediated Bonding of Pt Clusters on (5,5) Carbon Nanotubes

The adhesion of various sizes of Pt clusters on the metallic (5,5) carbon nanotubes (CNTs) with and without the point defect has been investigated by means of density functional theory (DFT). The calculations show that the binding energies of Ptn (n = 1−6) clusters on the defect free CNTs are more than 2.0 eV. However, the binding energies are increased more than three times on the point defective CNTs. The dramatic increase of the binding energy has been further explained by the partial density of states, deformation charge density, and two population analyses methods (Mulliken and Hirshfeld). The stronger orbital hybridization between the Pt atom and the carbon atom shows larger charge transfers on the defective CNTs than on the defect free CNTs, which allows the strong interaction between Pt clusters and CNTs. On the basis of DFT calculations, CNTs with point defect can be used as the catalyst supports for noble metal nanoparticles adhesion, which can be applied to a series of catalytic reactions, such as fuel cell, hydrogenation, etc.

The structure and stability of Si@Al12Hn (n=1−14) clusters

Density functional theory has been employed in order to investigate the structure and stability of Si@Al12Hn ( $n=1\!\!-\!\!14$ ) clusters. Hydrogenated Si@Al12 clusters exhibit pronounced stability for even numbers of H atoms. Large binding energy, HOMO-LUMO gaps and increased ionization potentials imply that these clusters should be physically and chemically stable. The analysis of the charge density of the HOMO plot illustrates that a pair of hydrogen atoms prefer to occupy opposing on-top sites for clusters with an even n number. Studies of deformation charge density plots demonstrate that significant charge transfer occurs from the Si@Al12 to the H atoms.

Electron charge distribution of CaAl 2−xZn x: Maximum entropy method combined with Rietveld analysis of high-resolution-synchrotron X-ray powder diffraction data

Using short wavelength X-rays from synchrotron radiation (SPring-8), high-resolution powder diffraction patterns were collected. In order to study both the structural relationship and the mechanism of stability in the CaAl 2− x Zn x system, among the Laves phases (MgCu 2 and MgNi 2 type) and KHg 2-type structures, the charge density distribution of CaAl 2− x Zn x as a function of x was obtained from the diffraction data by Rietveld analysis combined with the maximum entropy method (MEM). In the MEM charge density maps overlapping electron densities were clearly observed, especially in the Kagomé nets of the Laves phases. In order to clarify the charge redistribution in the system, the deformation charge densities from the densities formed by the constituent free atoms are discussed. In the ternary MgNi 2-type phase, partial ordering of Al and Zn atoms is observed, a finding that is supported by ab-initio total energy calculations.

Impurities potential effect on the charge density wave dynamics in one-dimensional systems

In this work, we present in the weak pinning case the numerical simulation results of the one-dimensional deformable charge density wave (CDW) properties considering the potential amplitude fluctuations effect generated by different impurity types randomly distributed in the lattice. When the electric field approaches threshold value E T , the static equilibrium characteristic time τ and the polarization P CDW become large and seem to diverge at critical field E cr from below E T following a power law [ 1 − ( E / E cr ) ] − α where α is an impurity dependant critical exponent. This divergence indicates that the CDW depinning can be described in terms of a dynamical critical phenomena, where the critical field E cr plays the role of a transition temperature as in ordinary phase transitions. In agreement with several experimental results, we show that the electric current density J CDW and electric conductivity σ CDW follow respectively a power law [ ( E / E T ) − 1 ] β and ( E T / E ) [ ( E / E T ) − 1 ] ν where β and ν are critical exponents. This results are analogous to these obtained in the case of one impurity type.

Geometric and electronic properties of endohedral Si@C74

The generalized gradient approximation based on density functional theory is used to analyze the geometric and electronic properties of Si @ C(74). It is found that among the five possible optimized geometries of Si @ C(74), the most favorable endohedral site of Si atom is under the center of a pentagon ring on the sigma(h) plane, i.e., Si @ C(74)-5, which is different from the center stable site for Si in C(74) calculated by the semiempirical molecular orbital calculations and molecular mechanics calculations, and it is also different from the stable site, i.e., under a [6, 6] bond along the C(2) axis on the sigma(h) plane in C(74) for metal atoms Ba, Ca, and Eu. The deformation charge density on the sigma(h) plane reveals that the Si-C bonds in Si @ C(74)-5 have covalent character, while the Mulliken charge analysis together with a longer Si-C bond length reveals that the Si-C bonds in Si @ C(74)-5 have ionic character. Therefore, we infer that Si-C bonds in Si @ C(74)-5 contain both covalent and ionic characters.

First-principles study on the effect of impurities at the front of cracks inα−Fe

By using two first-principles methods, discrete variational method (DVM) and DMol, within the framework of density-functional theory, the effects of nonmetal light impurities (C, N, and O) on a fracture in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$ are studied. We calculate the curve of binding energy $({E}_{b})$ vs the displacements of the nearest-neighbor Fe atoms of the impurity, and find that C and N tend to hold onto these two Fe atoms. Calculated PDOS and deformation charge density reveal that C and N prefer to form polarized bonds vertical to the propagation direction of the crack with the nearest-neighbor Fe atoms. Furthermore, interaction of the C $(\mathrm{N})\ensuremath{-}\mathrm{Fe}$ pair which is vertical to the crack plane is much larger than that of the C $(\mathrm{N})\ensuremath{-}\mathrm{Fe}$ pair which is parallel to the plane. This anisotropic bonding character indicates that doping C or N impedes the propagation of the crack, thus can improve the mechanical property of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$. On the contrary, O tends to push its nearest-neighbor Fe atoms away, and exhibits weak interaction with its nearest-neighbor atoms. O also weakens the interaction of $\mathrm{Fe}\ensuremath{-}\mathrm{Fe}$ pair in bulk. Thus O has detrimental effects on the ductility of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$.

A Combination Method of Charge Density Measurement in Hard Materials Using Accurate, Quantitative Electron and X-ray Diffraction: The α-Al2O3 Case

Current X-ray diffraction techniques intended for "ideally imperfect" specimens provide structure factors only on a relative scale and ever-present multiple scattering in strong low-angle Bragg reflections is difficult to correct. Multiple scattering is implicit in the quantitative convergent beam electron diffraction (QCBED) method, which provides absolutely scaled structure factors. Conventional single crystal X-ray diffraction has proved adequate in softer materials where crystal perfection is limited. In hard materials, the highly perfect nature of the crystals is often a difficulty, due to the inadequacy of the conventional corrections for multiple scattering (extinction corrections). The present study on alpha-Al2O3 exploits the complementarity of synchrotron X-ray measurements for weak and medium intensities and QCBED measurement of the strong low-angle reflections. Two-dimensional near zone axis QCBED data from different crystals at various accelerating voltages, thicknesses, and orientations have been matched using Bloch-wave and multislice methods. The reproducibility of QCBED data is better than 0.5%. The low-angle strong QCBED structure factors were combined with middle and high-angle extinction-free data from synchrotron X-ray diffraction measurements. Static deformation charge density maps for alpha-Al2O3 have been calculated from a multipole expansion model refined using the combined QCBED and X-ray data.

Structure and bonding in alpha-copper phthalocyanine by electron diffraction.

Energy-filtered quantitative electron diffraction at liquid nitrogen temperature has been used to examine the atomic structure and bonding of metastable alpha-Cu phthalocyanine crystals. Three theoretical methods (kinematic, kinematic with excitation errors and Bloch wave) were employed for the intensity calculations. The Bloch-wave method was found to account for dynamical effects by greatly reducing the residual factor between experimental and simulated results. A new method for calculating electron scattering factors for partially charged ions is proposed and the sensitivity of electron diffraction to charge transfer is discussed. The atomic charge states were analyzed for alpha-Cu phthalocyanine using a charge cloud model in which the Gaussian bond charge is positioned along the bonds. Spot patterns were collected in the Kohler mode at two beam energies to reduce error. Using the best-fitting model, a deformation charge-density map is produced and compared to the neutral-atom model. From this, the main features of atomic charge transfer in the alpha-Cu phthalocyanine structure can be seen in the (010) plane.