Redistribution Of Charge Density Research Articles

First-principles investigation of strain effects on the stacking fault energies, dislocation core structure, and Peierls stress of magnesium and its alloys

Taking pure Mg, Mg-Al, and Mg-Zn as prototypes, the effects of strain on the stacking fault energies (SFEs), dislocation core structure, and Peierls stress were systematically investigated by means of density functional theory and the semidiscrete variational Peierls-Nabarro model. Our results suggest that volumetric strain may significantly influence the values of SFEs of both pure Mg and its alloys, which will eventually modify the dislocation core structure, Peierls stress, and preferred slip system, in agreement with recent experimental results. The so-called ``strain factor'' that was previously proposed for the solute strengthening could be justified as a major contribution to the strain effect on SFEs. Based on multivariate regression analysis, we proposed universal exponential relationships between the dislocation core structure, the Peierls stress, and the stable or unstable SFEs. Electronic structure calculations suggest that the variations of these critical parameters controlling strength and ductility under strain can be attributed to the strain-induced electronic polarization and redistribution of valence charge density at hollow sites. These findings provide a fundamental basis for tuning the strain effect to design novel Mg alloys with both high strength and ductility.

Size-Dependent Structure Relations between Nanotubes and Encapsulated Nanocrystals.

The structural organization of compounds in a confined space of nanometer-scale cavities is of fundamental importance for understanding the basic principles for atomic structure design at the nanolevel. Here, we explore size-dependent structure relations between one-dimensional PbTe nanocrystals and carbon nanotube containers in the diameter range of 2.0-1.25 nm using high-resolution transmission electron microscopy and ab initio calculations. Upon decrease of the confining volume, one-dimensional crystals reveal gradual thinning, with the structure being cut from the bulk in either a <110> or a <100> growth direction until a certain limit of ∼1.3 nm. This corresponds to the situation when a stoichiometric (uncharged) crystal does not fit into the cavity dimensions. As a result of the in-tube charge compensation, one-dimensional superstructures with nanometer-scale atomic density modulations are formed by a periodic addition of peripheral extra atoms to the main motif. Structural changes in the crystallographic configuration of the composites entail the redistribution of charge density on single-walled carbon nanotube walls and the possible appearance of the electron density wave. The variation of the potential attains 0.4 eV, corresponding to charge density fluctuations of 0.14 e/atom.

Ab initio insight into graphene nanofibers to destabilize hydrazine borane for hydrogen release

We report the potential destabilizing effects of graphene nanofibers on the hydrogen release property of hydrazine borane via state-of-the-art ab initio calculations for the first time. Interactions of a hydrazine borane cluster with two types of graphene patch edges which exist abundantly in our synthesized graphene nanofibers have been investigated. It is found that both zigzag and armchair edges can greatly weaken the H-host bonds (especially the middle NH bond) of hydrazine borane. The dramatic decrease in hydrogen removal energy is caused by the strong interaction between hydrazine borane and the graphene patch edges concerning the electronic charge density redistribution.

First Principle DFT Study of Electric Field Effects on the Characteristics of Bilayer Graphene

Abstract First principle density functional theory methods, local density and Perdew-Burke-Ernzerhof generalized gradient approximations with Goedecker pseudopotential (LDA-G &amp; PBE-G), are used to study the electric field effects on the binding energy and atomic charges of bilayer graphene (BLG) at the Γ point of the Brillouin zone based on two types of unit cells (α and β) containing n C=8–32 carbon atoms. Results show that application of electric fields of 4–24 V/nm strengths reduces the binding energies and induces charge transfer between the two layers. The transferred charge increases almost linearly with the strength of the electric field for all sizes of the two types of unit cells. Furthermore, the charge transfer calculated with the α-type unit cells is more sensitive to the electric field strength. The calculated field-dependent contour plots of the differential charge densities of the two layers show details of charge density redistribution under the influence of the electric field.

Substrate matters: Magnetic tuning of the Fe monolayer

The effect of substrate on the magnetism of the Fe monolayer (ML) is investigated using the total energy DFT calculations with the local spin density approximation (LSDA). The results show an in plane ferromagnetic coupling (FM) and a magnetic moment of 1.78µB for the relaxed Fe ML in the presence of the vanadium substrate. In comparison, the surface Fe(001) magnetic moment ranges between 2.97–3.01µB. This difference in the Fe surface moment of more than 1µB in the presence or absence of Vanadium allows tuning of the Fe magnetic moment and has great potential as a magnetic switch and in spintronic devices. The surface magnetic quenching of Fe with V is much more pronounced than with other transition metal substrates like Molybdenum or Tungsten. We have a reduction of 40.5% of the Fe (001) surface moment which is more than double the reduction obtained with the Fe/Mo(001) or the Fe/W(001) systems. The magnetic quenching is due to the strong hybridization between the Fe and V d bands. This is supported by the observed charge density redistribution and large inward relaxation of 18.37% for the Fe surface upon structural relaxation. The Fe ML is antiferromagnetically (AF) coupled with the V interfacial layer, which has an appreciable induced magnetic moment of 0.48µB.

Effect of Ce addition on the electronic structure and bonding in BaTi1−xCexO3 ceramics

In this work, we report the influences of Ce doping on structural, morphological, optical and charge derived properties of BaTi1−xCexO3 (x = 0.02, 0.04, 0.06 & 0.08) ceramic synthesized by high temperature solid state reaction method. This ceramic has been characterized by powder X-ray diffraction (PXRD), UV–visible spectrometry, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Lattice parameters were determined through the profile refinement technique using PXRD data. Charge density redistribution and bonding nature of the grown systems are analyzed by maximum entropy method (MEM). Incorporation of rare earth ion Ce into the BaTiO3 lattice enhances the ionic nature between Ba and O ions and decreases the ionic nature between Ti and O ions, revealed from the qualitative and quantitative analysis by MEM. Optical band gap values are estimated for the prepared samples from UV–Vis data. Formation of aggregated particles with low porosity is observed from SEM micrographs. The chemical compositions of the prepared samples are further confirmed by EDS spectral analysis.

Experimental and computational studies on the electronic excited states of nitrobenzene

The gas phase electronic absorption spectrum of nitrobenzene (C6H5NO2) in the 4.5–11.2eV region is recorded using synchrotron radiation with a view to comprehend the nature of the excited states. Electronic excited states of nitrobenzene are mainly classified as local excitations within the benzene ring or nitro group and charge transfer excitations between the benzene and nitro group, with some transitions showing percentage from both. The nature of molecular orbitals, their orderings and energies are obtained from density functional theory calculations which help in assigning partially assigned/unassigned features in earlier photoelectron spectroscopy studies. Optimized geometry of ionic nitrobenzene predicts redistribution of charge density in the benzene ring rather than the nitro group resulting in stabilization of the benzene ring π orbitals in comparison to the neutral molecule. Time dependent density functional theory computations are found to describe the experimental spectra well with respect to energies, relative intensities and nature of the observed transitions in terms of valence, Rydberg or charge transfer type. New insights into the interpretation of 1B2u←1A1g and 1B1u←1A1g shifted benzene transitions in light of the present computational calculations are presented. The first few members of the ns, np and nd type Rydberg series in nitrobenzene, converging to the first six ionization potentials, identified in the spectra as weak but sharp peaks are reported for the first time. In general, transitions to the lowest three unoccupied molecular orbitals 4b1, 3a2 and 5b1 are valence or charge transfer in nature, while excitations to higher orbitals are predominantly Rydberg in nature. This work presents a consolidated experimental study and theoretical interpretation of the electronic absorption spectrum of nitrobenzene.

First Principles Modeling of Pd-doped (La,Sr)(Co,Fe)O3Complex Perovskites

(La,Sr)(Co,Fe)O3 (LSCF) perovskites are well known promising materials for cathodes of solid oxide fuel cells. In order to reduce cathode operational temperature, doping on B-sublattice with different metals was suggested. Indeed, as it was shown recently experimentally, doping with low Pd content increases oxygen vacancy concentration which is one of factors controlling oxygen transport in fuel cells. In this Communication, we modeled this material using first principles DFT calculations combined with supercell model. The charge density redistribution, density of states, and local lattice distortion around palladium ions are analyzed and reduction of the vacancy formation energy confirmed.

FeAs2formation and electronic nematic ordering: Analysis in terms of structural transformations

By combining DFT-based computational analysis and symmetry constraints in terms of group-subgroup relations, we analysed the formation of the native crystalline structure of loellingite FeAs$_2$. We showed that the ground state of the material exhibits the ordered patterns of the electronic localization which are mainly associated with iron $3d_{x^2 - y^2}$ orbitals and can be characterized in terms of nematic-like ordering. The ordering is the result of the close interplay of the lattice and the electron degrees of freedom. In a structural aspect, it pursues an energy quest to select the orthorhombic crystal lattice attributed to the Pnnm space group. In a charge aspect, the ordering is connected with the valence charge density redistribution that not only provides a high electronic polarizability but also gives rise to an extra-large magnitude of the negative component of the dynamical p-d charge transfer.

Comment on the paper by T. K. Thirumalaisamy, R. Saravanan, S. Saravanakumar “The redistribution of charge density in CaF2:Yb3+”, J. Mater Sci: Mater Electron, v. 26, p. 6683 (2015)

Thirumalaisamy et al. (J Mater Sci Mater Electron 26:6683, 2015), erroneously characterized a variable chemical composition of their solid solution as Ca1−xYbxF2, whereas its chemical formula should be Ca1−xYbxF2+x. As a result, Thirumalaisamy et al. calculations do not account for the additional fluoride ions that are necessary to compensate the excessive positive charge of ytterbium cations that replace calcium.

Nitrogen and Sulfur Dual-Doped Carbon Microtubes with Enhanced Performances for Oxygen Reduction Reaction

Porous carbon microtubes were prepared by pyrolysing an inexpensive and abundant natural fungus at a high temperature, followed by activation and doping with nitrogen (N) and sulfur (S) elements at a lower temperature. The performance of the porous carbon as electrocatalysts for oxygen reduction reaction (ORR) was systematically investigated. The pristine carbon without doping shows a high electrocatalytic activity, which is strongly dependent on the degree of graphitization. By N- and S- dual-doping, the catalytic performance is further enhanced, exhibiting a four-electron oxygen reduction pathway, which is attributed to the redistribution of spin and charge densities. These results suggest that obtaining graphitic porous carbon from natural precursors combined with heteroatom-doping, provide an effective method of producing high performance carbon materials which might be suitable for various electrochemical energy applications.

A study of the viscosity of hyaluronic acid solutions for the preparation of polyelectrolyte complexes with chitosan

The viscosity of hyaluronic acid solutions kept at different pH values was found to decrease in the following order of pH: 6.5 > 3.5 > 4.3, although acid hydrolysis occurred at a higher rate in the most acidic solution. The poorest stability of the solutions with pH 4.3 was attributed to the compaction of the macromolecules as a result of charge density redistribution along the polyacid chains.

Pressure-induced anomalies in structure, charge density and transport properties of Bi2Te3: A first principles study

The crystal structure, electronic structure, conductivity, electronic thermal conductivity, Seebeck and Hall coefficient of Bi2Te3 at pressures from 0 to 6 GPa are predicted using first-principles calculation and Boltzmann transport theory. A strong redistribution of charge density is found with an increase in pressure, especially around 3 GPa. The effective charge of Bi and Te atoms also show considerable anomalies at 3 GPa. These characteristics suggest significant variations in interatomic interactions and result in the discontinuities in the changes of crystal structure under high pressure. Furthermore, the band dispersion changes considerably with increasing pressure, which leads remarkable anomalies in effective mass and transport properties around 3 GPa.

First principles study of the behavior of hydrogen atoms in a W monovacancy

We report on the structural, energetic, and electronic properties which explain the behavior of atomic hydrogen in a W monovacancy. Based on the density functional theory, we calculate the most stable configurations of up to eleven hydrogen atoms in a monovacancy using the SIESTA and the VASP codes. We discuss about the deformation produced in a unit cell when a vacancy is formed and subsequently filled with different number of H atoms. We estimate the formation and the binding energies as a function of hydrogen accumulation in the monovacancy, comparing to data previously reported and highlighting the origin of the disparity of the results. Our calculations show that the maximum number of hydrogen atoms that the monovacancy can trap depends on the procedure followed for its filling, being 10 when it is filled in sequentially and larger than 11 when it is filled in via simultaneous insertion of the hydrogen atoms. Finally, we study the influence of the valence charge density and charge redistribution in the formation of the most stable structures.

The redistribution of charge density in CaF2:Yb3+

The fluorite type optical material Yb doped CaF2 was synthesized by the co-precipitation method. The synthesized materials with various Yb concentration were characterized by X-ray powder diffraction technique. The phase pure CaF2 was observed and the impurity doping effect was analyzed from the observed powder X-ray diffraction. The presence of Yb3+ in CaF2 materials was identified by energy dispersive X-ray analysis measurement and the morphological features were studied through scanning electron microscopy. The charge density distribution of Ca1−xYbxF2 (x = 0.00, 0.03, 0.06, 0.09 and 0.12) materials was analyzed using the maximum entropy method. The rearrangement of charge density distribution between Ca and F due to effect of Yb3+ impurity was analyzed.

First principles study of isostructural phase transition in Sb2Te3 under high pressure

The structural properties, electronic band structure and Bader charge of Sb2Te3 under hydrostatic pressure were simulated using density functional theory in order to study isostructural phase transitions (IPT) in Sb2Te3. The theoretical results showed that the axial ratio c /a did not exhibit any anomaly below 6 GPa. The variations of bond lengths were discontinuous at 2.5 GPa, which suggested considerable changes in interatomic interactions and provided sound support to the IPT. The effective charges of Sb and Te atoms showed significant discontinuous variations at 2.5 GPa, which revealed a strong redistribution of the electronic charge density and considerably changed interactions among bonding atoms. Thus, the IPT is originated from the considerable variation in the electronic charge density. (© 2015 WILEY‐VCH Verlag GmbH &amp;Co. KGaA, Weinheim)

Facile Synthesis and Electrocatalytic Activity of Sulfur Doped Carbon for Oxygen Reduction

Fuel cells and metal-air batteries have attracted much attention over the past few years due to their high energy density and the potential to reduce the negative impact of climate change [1,2]. However, the sluggish oxygen reduction reaction (ORR) at the cathode, which requires highly efficient and low cost electrocatalysts, is one of the important factors that restrict the wide application of fuel cells [3-5]. Exploring low-cost and sustainable electrocatalysts with high efficiency to alternate the platinum-based noble electrocatalysts for the ORR is critical for the development of fuel cells and metal-air batteries [6-8]. Heteroatom-doped carbon materials have emerged as promising alternative electrocatalyts for the ORR due to its low price, high catalytic activity and environmental friendliness [6-13]. The heteroatom doped in carbon is usually N, S, B, F and P et al., which can modify the electronic structure (like charge and/or spin density redistribution) of carbon and create the active sites favorable for the adsorption of O2 molecule and facilitate the ORR process [9-13]. Considerable efforts have been devoted to the study of N-doped carbon catalysts for the ORR[14-17]. Sulfur doping is also convinced to be a valid method in promoting the catalytic activity of carbon towards ORR. Sulfur has an electronegativity of 2.58, which is similar to carbon’s (2.55) [12]. In contrast, the electronegativity of N(3.04) is higher than that of carbon. Therefore the modification of structure and the catalytic activity of carbon resulting from N doping and S doping is different. S-doped graphene has been reported to show improved electrocatalytic activity for ORR [11,18]. Nevertheless, there have been few reports about the ORR activity of S-doped porous carbon. It’s known that porous carbon can provide effective triple phase (solid-liquid-gas) region for the mass transfer of reactants and products during the ORR. To better understand the effect of S-doping on the structure and ORR activity of carbon, we developed a facile method to synthesize the S-doped porous carbon via a hydrothermal method with sucrose and benzyl disulfide (BDS) as the carbon and sulfur sources, respectively. The influence of S doping on the electrocatalytic activity of the carbon for the ORR in alkaline media was investigated. The pyrolysis temperature has a significant impact on the structure and texture of the S-doped carbon, which further affects the ORR activity and durability of S-doped carbon as shown in Figure 1. Acknowledgements This work is supported by National Natural Science Foundation of China (Nos.51272167 and 21206101 ).

Spatially periodic modulation of optical conductivity in doped graphene by two-dimensional diffraction grating

We show theoretically that the optical conductivity of doped graphene can exhibit a spatially periodic modulation by a diffraction grating. Doped graphene placed above the grating exhibits a periodic electrostatic potential distribution, resulting in a periodic charge-density redistribution inside the graphene. The optical conductivity of doped graphene depends linearly on the Fermi energy, which is proportional to the square root of the charge density. Therefore, the optical conductivity exhibits a spatial-periodic modulation. The periodicity implies a band structure formation of the graphene plasmon polariton. Band engineering of the graphene plasmon polariton is also discussed.

A first principles study of electronic and optical properties of the polar quaternary chalcogenides β-A2Hg3Ge2S8(A=K and Rb)

The β-A2Hg3Ge2S8 (A=K and Rb) materials have a unique structure, possessing the high infrared transmission. More studies on β-A2Hg3Ge2S8 (A=K and Rb) are significant to investigate the probability of using these materials for optoelectronic devices. This work presents the results dealing with electronic and optical properties of β-A2Hg3Ge2S8 (A=K and Rb) obtained from first-principles calculations. We used the full potential linear augmented plane wave (FPLAPW) scheme, in the framework of DFT with modified Becke Johnson approximation (mBJ). We present the band structure, density of states (DOS), and electronic charge density. In addition, the band structure calculation suggests that the β-A2Hg3Ge2S8 (A=K and Rb) are semiconductors with indirect band gaps of 2.497 and 2.481eV for β-K2Hg3Ge2S8 and β-Rb2Hg3Ge2S8 compounds, which is in excellent agreement with the estimated value of 2.7eV for β-K2Hg3Ge2S8. An exhaustive study of the electronic density of states and the electronic charge density redistribution reveals the covalent bonding characteristics between Hg, Ge and S atoms.To get the fundamental characteristics of these two compounds, we have probe their linear optical properties such as the dynamic dielectric function, energy loss function, reflectivity, refractive index and absorption coefficients, In the energy range of 0–15eV. From the dynamic dielectric constant, the structural anisotropy is clearly observed.Optical response study recommends that the imaginary part of dielectric function spectra is appropriated for to be the interband transition.

Features of Valence Electron Density Distribution in Zr–H and Zr–He

Ab initio calculations of electronic structures of Zr–H and Zr–He systems have been done. The influence of hydrogen or helium impurities on the electron density distribution of the host metal has been considered. Extremely inhomogeneous redistribution of the metal valence charge density within the first coordination sphere of the impurity was found. The character of the observed anisotropy depends on the impurity type.