Viewpoint Of Electronic Structure Research Articles

Probing the Effect of Surface Modification of MgO Filler on Charge Transport in Polyethylene/MgO Composites: From an Electronic Structure Viewpoint

In this work, with the aid of first-principles calculation, we attempt to clarify whether nanoparticles itself can trap electrons and holes. Polyethylene (PE)/MgO nanocomposite is chosen as a model system. According to the computed adsorption energies, pristine and fully terminated surfaces are stable for the MgO (100) and MgO (111) surfaces, respectively. The electronic structures of several energetically favorable surface configurations are then studied. To probe the behavior of the excess charge, we examined the change in the charge distribution upon electron and hole doping. In the case of ideal MgO, holes are localized within the MgO slab, whereas electrons reside at the PE layer. We also show that the band alignment at the MgO/PE interface and, therefore, the carrier trapping property of MgO, is mainly determined by the intrinsic band edge positions of PE and MgO and the surface dipole of MgO, which can easily be tuned by surface modification.

Does Spinel Serve As a Rigid Framework for Oxygen Redox?

A major challenge for the development of lithium-ion batteries with high energy densities is to discover a large-capacity cathode material, and an additional oxygen redox capacity has recently been regarded as a new impetus. From the electronic-structure viewpoint, the best-studied oxygen-redox cathode materials, lithium-rich layered oxides possess oxide ions with nonbonding O 2p states along the Li-O-Li coordination axis, which provide the oxygen redox activity. After oxygen oxidation, either a π-type M t 2g-O 2p interaction (M: transition metal) or an O-O bond formation stabilizes the oxidized oxide ions, allowing for reaction reversibility. However, the layered structure is generally unstable after extracting an excess amount of Li ions, leading to the degradation of both capacity and voltage during charge/discharge cycles. It is therefore particularly important to implement oxide ions with nonbonding O 2p states into a stable host framework against excess Li-ion extraction to achieve reversible oxygen-redox capacity. As spinel oxides have a stable three-dimensional framework, it is attractive to explore for spinel oxides capable of the oxygen-redox reactions. Here, we theoretically diagnose the oxygen-redox activity of spinel oxides using density functional theory (DFT) calculations. LiMg0.5Mn1.5O4 is employed as a model material, as its spinel structure is expected to have oxide ions with nonbonding O 2p state owing to the existence of ionic Mg2+ in the framework. The electronic structure change during the oxygen-redox reaction of LiMg0.5Mn1.5O4 is compared with that of a typical Li-rich layered oxide (Li2MnO3), revealing the lability of oxidized oxide ions in a spinel oxide.

Theoretical investigation of the phase stability and elastic properties of TiZrHfNb-based high entropy alloys

First principles calculations are performed to study the effects of alloying elements (X = Al, Si, Sc, V, Cr, Mn, Cu, Zn, Y, Mo, Ta, W and Re) on the phase stability and elastic properties of TiZrHfNb refractory high entropy alloys. Both equimolar and non-equimolar alloys are considered. It is shown that the calculated lattice parameters, phase stability and elastic moduli of equimolar TiZrHfNbX are consistent with the available experimental and theoretical results. The substitutions of alloying elements at Ti, Zr, and Hf sites with various contents show similar effects on the phase stability and elastic properties of the TiZrHfNb-based alloys. The substitutions on Nb site are found to generally decrease the stability of body centered cubic phase. Close connections between the charge densities at the Wigner-Seitz cell boundary and the bulk moduli of TiZrHfNb-based alloys are found. The present results provide a quantitative model for exploring the phase stability and elastic properties of TiZrHfNb-based alloys from the electronic structure viewpoint.

Route to Stable Lead-Free Double Perovskites with the Electronic Structure of CH3NH3PbI3: A Case for Mixed-Cation [Cs/CH3NH3/CH(NH2)2]2InBiBr6

During the past year, halide double perovskites attracted attention as potential lead-free alternatives to Pb-based halide perovskites. However, none of the compounds discovered so far can match the optoelectronic properties of MAPbI3 (MA = CH3NH3). Here we argue that, from the electronic structure viewpoint, the only option to make Pb-free double perovskites retaining the remarkable properties of MAPbI3 is to combine In and Bi as B+ and B3+ cations, respectively. While inorganic double perovskites such as Cs2InBiX6 were found to be unstable due to In+ oxidizing into In3+, we show that the +1 oxidation state of In becomes progressively more stable as the A-site cation changes from K to Cs. Hence, we propose the use of MA and FA [FA = CH(NH2)2] to stabilize A2InBiBr6 double perovskites. We show that the optoelectronic properties of MA2InBiBr6 are remarkably similar to those of MAPbI3 and explore the mixed-cation (Cs/MA/FA)2InBiBr6 halide double perovskites.

High-purity Ni electroless plating on screen-printed anodized Al substrates

By using an electroless plating process with a Ni source solution including dimethylamine borane (DMAB) at pH 6.5 and 65 ◦ C, we obtained a higher purity Ni film (< 1 at.% boron) without damage to the anodized Al substrate. With increasing plating time, the thickness of the film increased gradually, although the average deposition rate of the Ni films decreased steadily. We can infer that the abrupt decrease in sheet resistance (Rs) at the interface region is due to the change in the boron concentration caused by surface reactions, and the gradual decrease in Rs in the bulk region is due mainly to the effect of the saturation of boron’s concentration on the thickness. From a boron-distribution viewpoint, this result indicates that the B concentration in the Ni film increases gradually with increasing plating time for plating times ≤ 60 s as a kind of initial stage (that is, interface region), and then saturates uniformly for plating times ≥ 300 s as a kind of bulk region. On the other hand, from an electronic structure viewpoint, this result implies that Ni gains 3d electrons with respect to elemental Ni. The increase in the number of electrons gained by the Ni 3d states may result in an enhancement of the electrical conductivity.

Inherent instability by antibonding coupling in AgSbTe2

In the present paper, an inherent instability in the ternary chalcogenide compound AgSbTe2 is described from the electronic structure viewpoint. Our calculations, which are based on the cluster expansion method, suggest nine stable crystal structures involving the most stable structure with symmetry. The effective pair interactions calculated by the generalized perturbation method point out that the stability of these structures originates from the number of linear arrangements of the Ag–Te–Sb atomic bonds. Moreover, it is found that AgSbTe2 has a special electronic structure, where the dominant components of the top of the valence band are the Te-5p antibonding states. Such an antibonding contribution leads to an inherent instability, such that the system spontaneously forms various mutation phases caused by charge-compensated defect complexes. We propose that these mutation phases play an important role in the thermal conductivity and thermoelectric efficiency in AgSbTe2.

Experimental and ab initio studies on sub-lattice ordering and magnetism in Co2Fe(Ge1−xSix) alloys

Crystallographic and magnetic properties of bulk Co2Fe(Ge1−xSix) alloys with 0 ≤ x ≤ 1, synthesized by arc melting method, have been studied. Co2FeSi alloy has been found to crystallize with L21 structure, but the super-lattice peaks are absent in the X-ray diffraction patterns of alloys containing high Ge concentration. Unit cell volume of this series of alloys decreased from 185.2 to 178.5 Å3 as Si content was increased from 0 to 1.00. All alloy compositions exhibit ferromagnetic behavior with a high Curie temperature (TC). TC showed a systematic variation with x. A comparison between the values of saturation magnetization (Ms) and effective moment per magnetic atom pc estimated from the temperature dependent susceptibility data above TC, shows that the alloys have half-metallic character. The alloy with x = 0 follows Slater-Pauling (S-P) rule with Ms of 5.99μB. However, Ms for the alloy with x = 1.00 was found to be 5.42μB, which is lower than the value of 6.0μB predicted by S-P rule. Since atomic disorder is known to affect the Ms and electronic structure of these alloys, ab initio calculations were carried out to explain the deviation in observed Ms from S-P rule prediction and the half-metallic character of the alloys. Ab initio calculations reveal that alloys with L21 structure have Ms value as predicted by S-P rule. However, introduction of 12.5% DO3 disorder, which occurs due to swapping of Co and Fe atoms in the unit cell, decreases Ms of alloys with x &amp;gt; 0 from the S-P prediction to values obtained experimentally. The results analyzed from the view point of electronic structure of the alloys in different ordered states bring out the influence of disorder on the observed magnetic properties of these technologically important alloys.

Saturated N,X-Heterocyclic Carbenes (X=N, O, S, P, Si, C, and B): Stability, Nucleophilicity, and Basicity

Various saturated five-membered N,X-heterocyclic carbenes (X = N, O, S, P, Si, C, and B) have been studied by ab initio and density functional theory (DFT) methods. The substitutions alter the properties of the reference carbene from the viewpoint of electronic structure, stability, nucleophilicity, and basicity. Our study shows that the oxygen containing carbene (X = O) induces the highest HOMO–LUMO energy gap (ΔEHOMO–LUMO), while carbene with X = N has the widest singlet–triplet energy difference (ΔEs–t). The nucleophilicity of the carbene derivatives increased upon replacement of C, Si, and B, with the effect of the boron substituent being more pronounced. In addition, the basicity of the structure increased for the carbene derivatives with X = C and B with the latter substitution imposing a remarkably higher effect. Moreover, the substitution of boron at the α-position of the carbene increased the nucleophilicity and basicity, while inducing a reduction in the values of ΔEs–t and ΔEHOMO–LUMO.

Electronic-structure origin of the glass-forming ability and magnetic properties in Fe–RE–B–Nb bulk metallic glasses

(Fe0.71RE0.05B0.24)96Nb4 (RE=Gd, Tb, Ho, Er, Tm) bulk metallic glasses (BMGs) were found exhibiting excellent glass-forming ability (GFA) with critical diameters ranging from 3.5 to 6.5mm, and high compressive fracture strength larger than 4300MPa. Moreover, they displayed good soft-magnetic properties with saturation magnetic flux density of 0.71–0.87T, coercive force of 1.23–39.76A/m and effective permeability of 1500–12,740 at 1kHz. X-ray photoelectron spectroscopy was performed to clarify the origin of the excellent GFA from the viewpoint of electronic structure. It was found that the Tm doped alloy displayed unique electronic structure including the deepest core-level binding energy, the most numerous RE–B bonds and the minimum density of states near the Fermi level, making this alloy the best glass former. The various trends noticed in the magnetic properties were ascribed mainly to the differences in the magnetic anisotropy and magnetic moment of RE elements.

Catalytic behavior of supported Ru nanoparticles on the (101) and (001) facets of anatase TiO2

Ru/TiO2 heterogeneous catalysts were prepared by immobilizing Ru nanoparticles onto the (101) and (001) facets of anatase TiO2 substrate, and the influence of metal–support interactions on the catalytic behavior of Ru/TiO2 towards CO2 methanation was studied from the viewpoint of electronic structure. Structural investigations based on temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) indicate that a stronger metal–support interaction occurs between Ru and (101) facet in contrast to the Ru and (001) one. This gives rise to an enhancement in CO2 adsorption as well as spill-over hydrogen at the interface of Ru/TiO2(101), accounting for its largely enhanced catalytic activity towards CO2 methanation. In addition, a theoretical study based on density functional theory (DFT) calculations reveals that the Ru nanoparticles supported on the (101) plane have a relatively lower activation energy for CO dissociation (the rate-determining step), which results in their high activity toward CO2 methanation reaction.

Ideal band shape in the potential thermoelectric material CuAlO2: Comparison to NaxCoO2

A potential thermoelectric material CuAlO${}_{2}$ is theoretically studied. We first construct a model Hamiltonian of CuAlO${}_{2}$ based on the first principles band calculation, and calculate the Seebeck coefficient. Then, we compare the model with that of a well-known thermoelectric material Na${}_{x}$CoO${}_{2}$, and discuss the similarities and the differences. It is found that the two materials are similar from an electronic structure viewpoint in that they have a peculiar pudding-mold type band shape, which is advantageous for thermoelectric materials. There are, however, some differences, and we analyze the origin of the difference from a microscopic viewpoint. The band shape (a very flat band top but with an overall wide bandwidth) of CuAlO${}_{2}$ is found to be even more ideal than that of Na${}_{x}$CoO${}_{2}$, and we predict that once a significant amount of holes is doped in CuAlO${}_{2}$, thermoelectric properties (especially the power factor) even better than those of Na${}_{x}$CoO${}_{2}$ can be expected.

The Role of Atomic Vacancy on Water Dissociation over Titanium Dioxide Nanosheet: A Density Functional Theory Study

The interaction of water with titanium dioxide (TiO2) nanosheets has been studied under the framework of density functional theory plus Hubbard model. Particularly, the effect of an oxygen vacancy and a titanium vacancy on the dissociation of water has been investigated. It is found that molecular adsorption is favored on perfect TiO2 nanosheets and a titanium vacancy, while over an oxygen vacancy water prefers to dissociate spontaneously due to the strong bonding between water and unsaturated titanium and oxygen. With the formation of water–water hydrogen bonds, dissociated water can be further stabilized. The role of titanium and oxygen vacancies is further discussed from the viewpoint of electronic structure.

Electronic structure of Pd 42.5Ni 7.5Cu 30P 20, an excellent bulk metallic glass former: Comparison to the Pd 40Ni 40P 20 reference glass

In-house photoemission and inverse-photoemission spectra (PES and IPES) were measured on Pd 42.5Ni 7.5Cu 30P 20 and Pd 40Ni 40P 20 bulk metallic glasses in order to clarify the origin of excellent glass-forming ability from the viewpoint of electronic structure. Minima are observed for both the metallic glasses at a slightly higher energy than the Fermi level. Incident photon-energy dependent PES spectra were obtained using synchrotron radiation and the Pd 4d partial density of states (DOS) was estimated from the PES data. Soft X-ray emission spectra were also measured near the Ni and Cu 2p 3/2 absorption edges to evaluate, respectively, the Ni and Cu 3d partial DOS in the valence band. The Pd 4d and the Ni and Cu 3d partials in the conduction band were obtained from X-ray absorption spectra around the Pd 3p 3/2 and Ni and Cu 2p 3/2 absorption edges, respectively. It was found that the Pd 4d partial DOS near the Fermi energy largely decreases and becomes localized by replacing the Ni atoms with the Cu atoms, which may be closely related to the excellent glass-forming ability of the Pd 42.5Ni 7.5Cu 30P 20 bulk metallic glass due to a selective formation of Pd–P covalent bonds.

Covalent bonding and the nature of band gaps in some half-Heusler compounds

Half-Heusler compounds XYZ, also called semi-Heusler compounds, crystallize in the C1b MgAgAs structure, in the space group . We report a systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18-electron half-Heusler compounds through first-principles density functional calculations. We find that the most appropriate description of these compounds from the viewpoint of electronic structures is one of a YZ zinc blende lattice stuffed by the X ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN can be written Li+ + (MgN)− and (MgN)−, which is isoelectronic with (SiSi), forms a zinc blende lattice. The 18-electron compounds can similarly be considered as obeying valence rules. A semiconductor such as TiCoSb can be written Ti4+ + (CoSb)4−; the latter unit is isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and the 18-electron compounds, when X is fixed as some electropositive cation, the computed band gap varies approximately as the difference in Pauling electronegativities of Y and Z. What is particularly exciting is that this simple idea of a covalently bonded YZ lattice can also be extended to the very important magnetic half-Heusler phases; we describe these as valence compounds, i.e. possessing a band gap at the Fermi energy albeit only in one spin direction. The local moment in these magnetic compounds resides on the X site.

Effects of Various Types of Doping on the Electronic Structure of Organic Interfaces

AbstractIn various organic electronic devices, interfaces formed by organic layers can play important roles. We have been studying various organic interfaces for clarifying their structure and electronic structure. In this talk, we will report our recent study of the effect of various types of doping for a variety of dopants – residual impurity, atmospheric gases, and metallic and organic intentional dopants. In particular, detailed and quantitative information about the effect of oxygen from the viewpoint of electronic structure was obtained for titanyl phthalocyanine (TiOPc), and the results corresponded well with the recent report of atmospheric effect on orga nic field effect transistor.

61.2: Invited Paper: Structure and Electronic Structure of Organic Interfaces

In many organic electronic devices, interfaces formed by organic layers can play crucial roles. We have been studying various organic interfaces for clarifying the molecular arrangement and electronic structure. In this talk, we will report a few topics from our recent efforts for understanding these interfaces, namely (1) Theoretical calculations on interfaces formed by depositing long-chain alkanes on metals, and (2) Study of the effect of various types of doping for a variety of dopants — residual impurity, atmospheric gases, and metallic and organic intentional dopants. In particular, detailed and quantitative information about the effect of oxygen from the viewpoint of electronic structure was obtained for titanyl phthalocyanine (TiOPc), and the results corresponded well with the recent report of atmospheric effect on organic field effect transistor.

Destabilization and enhanced dehydriding reaction of LiNH2: an electronic structure viewpoint

First-principles calculations have been applied to lithium amide, LiNH2, to characterize its electronic structure. Based on the theoretical study, we predict that an effective method for destabilizing LiNH2 is to partially substitute Li by other elements with larger electronegativity, such as Mg. Experimental results on dehydriding reactions of LiNH2 with/without the partial Mg substitutions suggest the destabilization of the samples with increasing Mg concentrations, which is in good agreement with our prediction. The dehydriding reactions of LiNH2 with partial Mg substitutions are useful as hydrogen-storage materials for fuel-cell applications.

Absorption edge in silica glass

Vacuum ultraviolet absorption measurement was carried out in silica glass over a wide temperature range from 4 to 1900 K, and structure of the absorption edge and its origin have been elucidated. The main factor that determines the Urbach tail of silica glass is the thermal vibration rather than the structural disorder frozen-in at the glass transition temperature, resulting in a strong temperature dependence of the absorption edge far below the glass transition temperature. Furthermore, freezing process of the disorder from the viewpoint of electronic structure was observed.

A Study of Negative Force Constants: A Method to Obtain Force Constants by Electronic Structures

The potential of negative force constants appearing in the valence-force model is discussed from the viewpoint of electronic structures. Prior to discussing this, a general procedure for obtaining force constants by the first-principles calculations is given, while simpler examples are worked out. For the strongest bond-stretching force, it is almost always calculable without ambiguity by using the breathing type deformation. For non-central forces, such as angle-bending forces, there is a practical difficulty in estimation. The expansion of the adiabatic potential obtained by the first-principles calculations involves so many force constants other than the bond-stretching and angle-bending forces that the values of angle-bending force constants so obtained usually exhibit large errors. Only for more simple molecules is it possible to evaluate the noncentral forces without ambiguity. In such a case, negative values of the angle-bending force constants appear, depending on the electronic configuration. How the Jahn–Teller effect participates in negative force constants has been investigated. The Jahn–Teller effect can contribute, but not all negative forces are ascribed to this effect.

Martensitic Transformation in a Ti50ni48fe2 Alloy Studied by Eels

Abstract Martensitic transformation is the first-order diffusionless phase transformation in solids, and is responsible for unique phenomena such as shape memory effect and superelasticity. Recently, many researchers tried to explain the origin of martensitic transformations from a viewpoint of electronic structure. It is strongly required to investigate electronic state changes associated with a martensitic transformation by an accurate experiment. EELS (Electron Energy-Loss Spectroscopy) was successfully applied to study electronic state in several alloys and oxides, e.g. a composition dependence of density-of-state (DOS) in Cu-3d band was clearly observed for Cu1-xAlx alloys by a core-loss measurement. However, this method has been less applied to studies of phase transformations to date. The purpose of the present work is to investigate electronic state changes associated with a martensitic transformation in a Ti50Ni48Fe2 alloy by EELS. A Ti50Ni48Fe2 alloy was prepared by induction-melting method. Thin-foiled specimens were solution-treated in Ar atmosphere at 1173K for 1hr.,