Showing papers on "Valence (chemistry) published in 2013"
TL;DR: Results on crystal structures, cohesive energies, and solid‐state reaction enthalpies with the modified basis sets, denoted as pob‐TZVP, are compared with selected standard basis sets available from the CRYSTAL basis set database.
Abstract: Consistent basis sets of triple-zeta valence with polarization quality for main group elements and transition metals from row one to three have been derived for periodic quantum-chemical solid-state calculations with the crystalline-orbital program CRYSTAL. They are based on the def2-TZVP basis sets developed for molecules by the Ahlrichs group. Orbital exponents and contraction coefficients have been modified and reoptimized, to provide robust and stable self-consistant field (SCF) convergence for a wide range of different compounds. We compare results on crystal structures, cohesive energies, and solid-state reaction enthalpies with the modified basis sets, denoted as pob-TZVP, with selected standard basis sets available from the CRYSTAL basis set database. The average deviation of calculated lattice parameters obtained with a selected density functional, the hybrid method PW1PW, from experimental reference is smaller with pob-TZVP than with standard basis sets, in particular for metallic systems. The effects of basis set expansion by diffuse and polarization functions were investigated for selected systems.
832 citations
TL;DR: In this paper, the authors reported a high ZT of ∼2.0 at 823 K for 2% Na-doped PbTe with 6% MgTe with excellent thermal stability.
Abstract: We report a high ZT of ∼2.0 at 823 K for 2% Na-doped PbTe with 6% MgTe with excellent thermal stability. We attribute the high thermoelectric performance to a synergistic combination of enhanced power factor, reduction of the lattice thermal conductivity and simultaneous suppression of bipolar thermal conductivity. MgTe inclusion in PbTe owns triple functions: the Mg alloying within the solubility limit in PbTe modifies the valence band structure by pushing the two valence bands (L and Σ bands) closer in energy, thereby facilitating charge carrier injection. When the solubility limit of Mg is exceeded, ubiquitous endotaxial nanostructures form, which when coupled with mesoscale microstructuring results in a very low (lattice) thermal conductivity through all-scaled length phonon scattering. Meanwhile, most significantly, the Mg alloying enlarges the energy gap of conduction band (C band) and light valence band (L band), thereby suppresses the bipolar thermal conductivity through an increase in band gap.
611 citations
TL;DR: An exceptionally simple algebraic construction allows for defining atomic core and valence orbitals, polarized by the molecular environment, which can exactly represent self-consistent field wave functions, providing an unbiased and direct connection between quantum chemistry and empirical chemical concepts.
Abstract: Modern quantum chemistry can make quantitative predictions on an immense array of chemical systems. However, the interpretation of those predictions is often complicated by the complex wave function expansions used. Here we show that an exceptionally simple algebraic construction allows for defining atomic core and valence orbitals, polarized by the molecular environment, which can exactly represent self-consistent field wave functions. This construction provides an unbiased and direct connection between quantum chemistry and empirical chemical concepts, and can be used, for example, to calculate the nature of bonding in molecules, in chemical terms, from first principles. In particular, we find consistency with electronegativities (χ), C 1s core-level shifts, resonance substituent parameters (σR), Lewis structures, and oxidation states of transition-metal complexes.
598 citations
TL;DR: In this article, the authors present an experimentally validated predictive tool that shows how the high deformability of lone-pair electron charge density can limit thermal conductivity in crystalline materials.
Abstract: As over 93% of the world's energy comes from thermal processes, new materials that maximize heat transfer or minimize heat waste are crucial to improving efficiency. Here we focus on fully dense electrical insulators at the low end of the spectrum of lattice thermal conductivity κL. We present an experimentally validated predictive tool that shows how the high deformability of lone-pair electron charge density can limit κL in crystalline materials. Using first-principles density-functional theory (DFT) calculations, we predict that several ABX2 (groups I–V–VI2) compounds based on the rocksalt structure develop soft phonon modes due to the strong hybridization and repulsion between the lone-pair electrons of the group V cations and the valence p orbitals of group VI anions. In many cases, this creates lattice instabilities and the compounds either do not exist or crystallize in a different structure. Marginally stable ABX2 compounds have anharmonic bonds that result in strong phonon–phonon interactions. We show experimentally how these can reduce κL to the amorphous limit.
487 citations
TL;DR: In this paper, the authors studied the conduction band spin splitting that arises in transition metal dichalcogenide (TMD) semiconductor monolayers such as MoS, MoSe, WS, and WSe, due to the combination of spin-orbit coupling and lack of inversion symmetry.
Abstract: We study the conduction band spin splitting that arises in transition metal dichalcogenide (TMD) semiconductor monolayers such as MoS${}_{2}$, MoSe${}_{2}$, WS${}_{2}$, and WSe${}_{2}$ due to the combination of spin-orbit coupling and lack of inversion symmetry. Two types of calculation are done. First, density functional theory (DFT) calculations based on plane waves that yield large splittings, between 3 and 30 meV. Second, we derive a tight-binding model that permits to address the atomic origin of the splitting. The basis set of the model is provided by the maximally localized Wannier orbitals, obtained from the DFT calculation, and formed by 11 atomiclike orbitals corresponding to $d$ and $p$ orbitals of the transition metal (W, Mo) and chalcogenide (S, Se) atoms respectively. In the resulting Hamiltonian, we can independently change the atomic spin-orbit coupling constant of the two atomic species at the unit cell, which permits to analyze their contribution to the spin splitting at the high symmetry points. We find that---in contrast to the valence band---both atoms give comparable contributions to the conduction band splittings. Given that these materials are most often $n$-doped, our findings are important for developments in TMD spintronics.
380 citations
TL;DR: A design strategy is demonstrated which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ~ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide.
Abstract: Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we demonstrate a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ∼ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering, both electronic structure and band alignment across nanostructure–matrix interface. Utilizing density functional theory for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS, and ZnSe, we infer favorable valence band alignments between PbSe and compositionally alloyed nanostructures of CdS1–xSex/ZnS1–xSex. Then by alloying Cd on the cation sublattice of PbSe, we tailor the electronic structure of its two valence bands (...
333 citations
TL;DR: It is suggested that care must be taken when using quantum chemistry to assess metal-ligand covalency in this part of the periodic table and also topological analysis of the electron density via the quantum theory of atoms-in-molecules.
Abstract: A covalent chemical bond carries the connotation of overlap of atomic orbitals between bonded atoms, leading to a buildup of the electron density in the internuclear region. Stabilization of the valence 5f orbitals as the actinide series is crossed leads, in compounds of the minor actinides americium and curium, to their becoming approximately degenerate with the highest occupied ligand levels and hence to the unusual situation in which the resultant valence molecular orbitals have significant contributions from both actinide and the ligand yet in which there is little atomic orbital overlap. In such cases, the traditional quantum-chemical tools for assessing the covalency, e.g., population analysis and spin densities, predict significant metal–ligand covalency, although whether this orbital mixing is really covalency in the generally accepted chemical view is an interesting question. This review discusses our recent analyses of the bonding in AnCp3 and AnCp4 (An = Th–Cm; Cp = η5-C5H5) using both the trad...
266 citations
TL;DR: The energy of the 4f-5d transitions of divalent and trivalent lanthanide impurities in compounds depends strongly on the type of the impurity, its valence, and its type of compound as mentioned in this paper.
Abstract: The energy of the 4f-5d transitions of divalent and trivalent lanthanide impurities in compounds depends strongly on the type of lanthanide, its valence, and the type of compound. Despite this large variability there is much systematic in 4f-5d transition energy. Once it is known for one lanthanide that for all others when in the same compound can be predicted. The same applies for the energy of electron transfer from the valence band to the 4f-shell of lanthanides which also behaves in a systematic fashion with type of lanthanide and type of compound. This work reviews my studies during the past fifteen years that are based on an analysis of data on all divalent and all trivalent lanthanides in more than 1000 different inorganic compounds collected from the archival literature. The established redshift and charge transfer models that form the basis to construct binding energy schemes showing all lanthanide levels with respect to the host bands are reviewed and the latest developments are addressed.
264 citations
TL;DR: In this article, the scalar strange-quark matrix element of the nucleon is computed with lattice QCD and a mixed-action scheme is used with domain-wall valence fermions computed on the staggered MILC seaquark configurations.
Abstract: The scalar strange-quark matrix element of the nucleon is computed with lattice QCD. A mixed-action scheme is used with domain-wall valence fermions computed on the staggered MILC sea-quark configurations. The matrix element is determined by making use of the Feynman-Hellmann theorem which relates this strange matrix element to the change in the nucleon mass with respect to the strange-quark mass. The final result of this calculation is m_s = 49 +-10 +- 15 MeV and, correspondingly f_s = m_s / m_N = 0.051 +- 0.011 +- 0.016. Given the lack of a quantitative comparison of this phenomenologically important quantity determined from various lattice QCD calculations, we take the opportunity to present such an average. The resulting conservative determination is f_s = 0.043 +- 0.011.
253 citations
TL;DR: In this article, the electronic structure and magnetic anisotropy of six complexes of high-spin FeII with linear FeX2 (X = C, N, O) cores, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiME3)3]2 (2), Fe [N(H)Ar′]2(3), Fe•N(Ar*]2,4,6-Pri2)2, Ar# = C6H3-2,6-(C6H2-2.
Abstract: The electronic structure and magnetic anisotropy of six complexes of high-spin FeII with linear FeX2 (X = C, N, O) cores, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), Fe[O(Ar′)]2 (5), and Fe[N(t-Bu)2]2 (7) [Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2], and one bent (FeN2) complex, Fe[N(H)Ar#]2 (6), have been studied theoretically using complete active space self-consistent field (CASSCF) wavefunctions in conjunction with N-Electron Valence Perturbation Theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for the treatment of magnetic field and spin-dependent relativistic effects. Mossbauer studies on compound 2 indicate an internal magnetic field of unprecedented magnitude (151.7 T) at the FeII nucleus. This has been interpreted as arising from first order angular momentum of the 5Δ ground state of FeII center (J. Am. Chem. Soc. 2004, 126, 10206). Using geometries from X-ray structural data, ligand field parameters for the Fe-ligand bonds were extracted using a 1 : 1 mapping of the angular overlap model onto multireference wavefunctions. The results demonstrate that the metal–ligand bonding in these complexes is characterized by: (i) strong 3dz2–4s mixing (in all complexes), (ii) π-bonding anisotropy involving the strong π-donor amide ligands (in 1, 3–4, 6, and 7) and (iii) orbital mixings of the σ–π type for Fe–O bonds (misdirected valence in 5). The interplay of all three effects leads to an appreciable symmetry lowering and splitting of the 5Δ (3dxy, 3dx2−y2) ground state. The strengths of the effects increase in the order 1 < 5 < 7 ∼ 6. However, the differential bonding effects are largely overruled by first-order spin–orbit coupling, which leads to a nearly non-reduced orbital contribution of L = 1 to yield a net magnetic moment of about 6 μB. This unique spin–orbital driven magnetism is significantly modulated by geometric distortion effects: static distortions for the bent complex 6 and dynamic vibronic coupling effects of the Renner–Teller type of increasing strength for the series 1–5.Ab initio calculations based on geometries from X-ray data for 1 and 2 reproduce the magnetic data exceptionally well. Magnetic sublevels and wavefunctions were calculated employing a dynamic Renner–Teller vibronic coupling model with vibronic coupling parameters adjusted from the ab initio results on a small Fe(CH3)2 truncated model complex. The model reproduces the observed reduction of the orbital moments and quantitatively reproduces the magnetic susceptibility data of 3–5 after introduction of the vibronic coupling strength (f) as a single adjustable parameter. Its value varies in a narrow range (f = 0.142 ± 0.015) across the series. The results indicate that the systems are near the borderline of the transition from a static to a dynamic Renner–Teller effect. Renner–Teller vibronic activity is used to explain the large reduction of the spin-reversal barrier Ueff along the series from 1 to 5. Based upon the theoretical analysis, guidelines for generating new single-molecule magnets with enhanced magnetic anisotropies and longer relaxation times are formulated.
225 citations
TL;DR: Topological surface states, a new kind of electronic state of matter, have recently been observed on the cleaved surfaces of crystals of a handful of small band gap semiconductors as discussed by the authors, and the underlying chemical factors that enable these states are crystal symmetry, the presence of strong spinorbit coupling, and an inversion of the energies of the bulk electronic states that normally contribute to the valence and conduction bands.
Abstract: Topological surface states, a new kind of electronic state of matter, have recently been observed on the cleaved surfaces of crystals of a handful of small band gap semiconductors. The underlying chemical factors that enable these states are crystal symmetry, the presence of strong spin–orbit coupling, and an inversion of the energies of the bulk electronic states that normally contribute to the valence and conduction bands. The goals of this review are to briefly introduce the physics of topological insulators to a chemical audience and to describe the chemistry, defect chemistry, and crystal structures of the compounds in this emergent field.
TL;DR: The molecular geometry that was determined from the X-ray analysis is consistent with a zigzag-edge-localized structure of unpaired electrons, and insight into the edge state of ZGNRs can be gained.
Abstract: The characteristics of the edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (ZGNRs) that originates from electron–electron correlation in an edge-localized π-state, are investigated by preparing and characterizing quarteranthene molecules. The molecular geometry that was determined from the X-ray analysis is consistent with a zigzag-edge-localized structure of unpaired electrons. The localized electrons are responsible for the peculiar magnetic (room-temperature ferromagnetic correlation), optical (the lowest-lying doubly excited state), and chemical (peroxide bond formation) behaviors. On the basis of these distinguishing properties and a careful consideration of the valence bonding, insight into the edge state of ZGNRs can be gained.
TL;DR: In this article, a colloidal liquid state may be observed as the system approaches zero temperature, if the bonds between particles are allowed to be flexible, and if the system can be observed with attractive patches on them.
Abstract: Patchy colloidal systems consist of particles with attractive patches on them. If the bonds between particles are allowed to be flexible, a colloidal liquid state may be observed as the system approaches zero temperature.
TL;DR: In this article, the crystal structure, cation/anion distribution, and electronic structure of the Li1.2Ni0.6O2 nanoparticle were investigated using a combination of aberration-corrected scanning transmission electron microscopy, X-ray energy-dispersive spectroscopy (XEDS), and complementary multislice image simulation.
Abstract: Li-rich layered material Li1.2Ni0.2Mn0.6O2 possesses high voltage and high specific capacity, which makes it an attractive candidate for the transportation industry and sustainable energy storage systems. The rechargeable capacity of the Li-ion battery is linked largely to the structural stability of the cathode materials during the charge–discharge cycles. However, the structure and cation distribution in pristine Li1.2Ni0.2Mn0.6O2 have not yet been fully characterized. Using a combination of aberration-corrected scanning transmission electron microscopy, X-ray energy-dispersive spectroscopy (XEDS), electron energy loss spectroscopy (EELS), and complementary multislice image simulation, we have probed the crystal structure, cation/anion distribution, and electronic structure of the Li1.2Ni0.2Mn0.6O2 nanoparticle. The electronic structure and valence state of transition-metal ions show significant variations, which have been identified to be attributed to the oxygen deficiency near certain particle surfac...
TL;DR: In this article, an explanation of the physical meaning of the electron propagator's poles and residues is followed by a discussion of its couplings to more complicated propagators and connections between Dyson orbitals and transition probabilities.
Abstract: Electron propagator theory provides a practical means of calculating electron binding energies, Dyson orbitals, and ground-state properties from first principles. This approach to ab initio electronic structure theory also facilitates the interpretation of its quantitative predictions in terms of concepts that closely resemble those of one-electron theories. An explanation of the physical meaning of the electron propagator's poles and residues is followed by a discussion of its couplings to more complicated propagators. These relationships are exploited in superoperator theory and lead to a compact form of the electron propagator that is derived by matrix partitioning. Expressions for reference-state properties, relationships to the extended Koopmans's theorem technique for evaluating electron binding energies, and connections between Dyson orbitals and transition probabilities follow from this discussion. The inverse form of the Dyson equation for the electron propagator leads to a strategy for obtaining electron binding energies and Dyson orbitals that generalizes the Hartree–Fock equations through the introduction of the self-energy operator. All relaxation and correlation effects reside in this operator, which has an energy-dependent, nonlocal form that is systematically improvable. Perturbative arguments produce several, convenient (e.g. partial third order, outer valence Green's function, and second-order, transition-operator) approximations for the evaluation of valence ionization energies, electron affinities, and core ionization energies. Renormalized approaches based on Hartree–Fock or approximate Brueckner orbitals are employed when correlation effects become qualitatively important. Reference-state total energies based on contour integrals in the complex plane and gradients of electron binding energies enable exploration of final-state potential energy surfaces. © 2012 John Wiley & Sons, Ltd.
TL;DR: In this paper, the degenerate valence and conduction bands of graphene at the Dirac point and the relationship of their orbital coefficients to the HOMO and LUMO of benzene and to the Clar structures of graphene are discussed.
Abstract: In this Account, we discuss the chemistry of graphitic materials with particular reference to three reactions studied by our research group: (1) aryl radical addition, from diazonium precursors, (2) Diels–Alder pericyclic reactions, and (3) organometallic complexation with transition metals. We provide a unified treatment of these reactions in terms of the degenerate valence and conduction bands of graphene at the Dirac point and the relationship of their orbital coefficients to the HOMO and LUMO of benzene and to the Clar structures of graphene.In the case of the aryl radical addition and the Diels–Alder reactions, there is full rehybridization of the derivatized carbon atoms in graphene from sp2 to sp3, which removes these carbon atoms from conjugation and from the electronic band structure of graphene (referred to as destructive rehybridization). The radical addition process requires an electron transfer step followed by the formation of a σ-bond and the creation of a π-radical in the graphene lattice,...
TL;DR: In this paper, a systematic study of soft X-ray absorption spectroscopy in various manganese oxides and fluorides was performed and compared with each other, showing that the Mn-L spectra fingerprint the Mn valence and spin states through spectral lineshape and energy position consistently and evidently.
TL;DR: In this article, the significance of recombination of a photoexcited electron and a hole in conduction and valence bands, respectively, of a titania photocatalyst, has been discussed.
Abstract: This short review paper shows the significance of recombination of a photoexcited electron and a hole in conduction and valence bands, respectively, of a titania photocatalyst, since recombination has not yet been fully understood and has not been evaluated adequately during the past several decades of research on heterogeneous photocatalysis.
TL;DR: The combination of equilibrium and dynamic behavior of DNA nanostars demonstrates the potential of DNA molecules in diversifying the pathways toward collective properties and self-assembled materials, beyond the range of phenomena accessible with ordinary molecular fluids.
Abstract: Colloidal particles with directional interactions are key in the realization of new colloidal materials with possibly unconventional phase behaviors. Here we exploit DNA self-assembly to produce bulk quantities of “DNA stars” with three or four sticky terminals, mimicking molecules with controlled limited valence. Solutions of such molecules exhibit a consolution curve with an upper critical point, whose temperature and concentration decrease with the valence. Upon approaching the critical point from high temperature, the intensity of the scattered light diverges with a power law, whereas the intensity time autocorrelation functions show a surprising two-step relaxation, somehow reminiscent of glassy materials. The slow relaxation time exhibits an Arrhenius behavior with no signs of criticality, demonstrating a unique scenario where the critical slowing down of the concentration fluctuations is subordinate to the large lifetime of the DNA bonds, with relevant analogies to critical dynamics in polymer solutions. The combination of equilibrium and dynamic behavior of DNA nanostars demonstrates the potential of DNA molecules in diversifying the pathways toward collective properties and self-assembled materials, beyond the range of phenomena accessible with ordinary molecular fluids.
TL;DR: In this paper, it was shown that the oxygen reduction reaction (ORR) activities of (001)-oriented manganese perovskite films decrease from 10 to 1 nm by more than an order of magnitude, which can be attributed to the barrier associated with interfacial band bending that impedes electron transfer to the electrolyte.
Abstract: We report that the oxygen reduction reaction (ORR) activities of (001)-oriented manganese perovskite films decrease from 10 to 1 nm by more than an order of magnitude, which can be attributed to the barrier associated with interfacial band bending that impedes electron transfer to the electrolyte, and reduction of Mn3+ due to charge transfer from the Nb:SrTiO3 substrate. Furthermore, we show by substitution in La1−x(Ca,Sr)xMnO3 that Mn3+, not Mn4+, is the active valence state for ORR.
TL;DR: The Fermi levels at the bottom of conduction bands indicated the existence of Ti(3+) ions, which enhanced the separation rates of photogenerated electrons and holes, and the energies of the edges of band gaps, determining the dominant types of oxidants in the photocatalytic degradation, were discussed.
Abstract: The geometry structures, formation energies and electronic properties of the B-, C- and B/C-doped anatase TiO2 were investigated by the density functional theory (DFT) calculations of first-principles. The results indicated that the visible-light absorption and photocatalytic activities of the B-, C- and B/C-doped anatase TiO2 were not only influenced by the energy gaps (Eg) and the distributions of impurity states, but also affected by the locations of Fermi levels (EF) and the energies of the edges of band gaps (Ev for the top of valence bands and Ec for the bottom of conduction bands). However, the above four factors changed with the doped models of TiO2. The impurity states in the band gaps reduced the maximum energy gaps in the band gaps, which is responsible for the absorption of visible light. The Fermi levels at the bottom of conduction bands indicated the existence of Ti3+ ions, which enhanced the separation rates of photogenerated electrons and holes. Further, the energies of the edges of band gaps, determining the dominant types of oxidants (O2−, hole, ˙OH) in the photocatalytic degradation, were discussed. Moreover, the stability of the doped TiO2 depended on its growth conditions (O-rich or Ti-rich environment). The O-rich growth condition is beneficial to the substitutional B and C atoms to Ti atoms, while the Ti-rich growth condition is favorable to the other doped TiO2 including the most stable co-doped TiO2 with the interstitial B atom and the substitutional C atom to O atom. In addition, our results also showed that the B/C-doped TiO2 inherited the partial electronic properties of single-doped TiO2, but also exhibited many new electronic properties, implying that the electronic properties of co-doped systems are not a mechanical mixture of those of both single-doped systems.
TL;DR: Results of theoretical calculation indicated that a higher formation energy and lower solubility of impurity ions led to serious SnO(2) segregation on the surface of the Sn(4+)-doped BiVO(4) thin film, which was the main reason for the poor performance of Sn-doped biVO( 4).
Abstract: Photoelectrochemical water splitting is an attractive method to produce H2 fuel from solar energy and water. Ion doping with higher valence states was used widely to enhance the photocurrent of an n-type oxide semiconductor. In this study, the different doping sites and the photoelectrochemical properties of Mo6+, W6+ and Sn4+-doped BiVO4 were studied systematically. The results suggested that Mo6+ or W6+-doped BiVO4 had a much higher photocurrent while the photocurrent of Sn4+-doped BiVO4 did not change obviously. Raman and XPS were used to identify the doping sites in the BiVO4 crystal lattice. It was found that Mo or W substituted V sites but Sn did not substitute Bi sites. Results of theoretical calculation indicated that a higher formation energy and lower solubility of impurity ions led to serious SnO2 segregation on the surface of the Sn4+-doped BiVO4 thin film, which was the main reason for the poor performance of Sn-doped BiVO4. The higher formation energy of Sn4+ came from the large mismatch of ion radius and different outer shell electron distribution. These results can offer guidance in choosing suitable doping ions for other semiconductor photoelectrodes.
TL;DR: The role of the oxygen vacancies on the phase transition behavior of this VO2 film is discussed in the framework of the hybridization theory and the valence state of vanadium in this paper.
Abstract: VO2 epitaxial film with large size has been prepared by oxide-molecular beam epitaxy method on Al2O3 (0001) substrate. The VO2 film shows a perfect crystal orientation, uniformity, and distinct metal-insulator phase transition (MIT) characteristics. It is observed that the MIT character is closely associated with the crystal defects such as oxygen vacancies. By controlling the growth condition, the MIT temperature can be tuned through modifying the content of oxygen vacancies. The role of the oxygen vacancies on the phase transition behavior of this VO2 film is discussed in the framework of the hybridization theory and the valence state of vanadium.
TL;DR: In-depth analysis of operando X-ray pair distribution function (PDF) data is combined with Li NMR spectroscopy to gain comprehensive insights into the electrochemical reaction mechanism of high-performance iron oxyfluoride electrodes.
Abstract: In-depth analysis of operando X-ray pair distribution function (PDF) data is combined with Li NMR spectroscopy to gain comprehensive insights into the electrochemical reaction mechanism of high-performance iron oxyfluoride electrodes. While the full discharge capacity could be recovered upon charge, implying reversibility of the electrochemical reaction, the atomic structure of the electrode formed after cycling (discharge–charge) differs from the pristine uncycled electrode material. Instead, the “active” electrode that forms upon cycling is a nanocomposite of an amorphous rutile phase and a nanoscale rock salt phase. Bond valence sum analysis, based on the precise structural parameters (bond lengths and coordination number) extracted from the in situ PDF data, suggests that anion partitioning occurs during the electrochemical reaction, with the rutile phase being F-rich and the rock salt phase being O-rich. The F- and O-rich phases react sequentially; Fe in a F-rich environment reacts preferentially dur...
TL;DR: In this article, all-electron contracted double zeta valence plus polarization function (DZP) basis sets for the elements Cs, Ba, La, from Hf to Ir, and from Au to Rn are constructed for using in conjunction with the non-relativistic and Douglas-Kroll-Hess (DKH) Hamiltonians.
TL;DR: In this paper, the analysis of 2p XAS, XMCD and 2p EELS is discussed, including isolated iron atoms, solids and coordination compounds, including binary oxides, perovskites and spinel systems.
TL;DR: Based on the recently proposed super valence bond model, the 23c-14e bi-icosahedral Au(23)((+9)) core of Au(38)(SR)(24) is proved to be a superatomic molecule.
Abstract: Based on the recently proposed super valence bond model, in which superatoms can compose superatomic molecules by sharing valence pairs and nuclei for shell closure, the 23c-14e bi-icosahedral Au(23)((+9)) core of Au(38)(SR)(24) is proved to be a superatomic molecule. Molecular orbital analysis reveals that the Au(23)((+9)) core is an exact analogue of the F(2) molecule in electronic configuration. Chemical bonding analysis by the adaptive natural density partitioning method confirms the superatomic molecule bonding framework of Au(38)(SR)(24) in a straightforward manner.
TL;DR: Analysis has shown that deficiencies in the zeroth-order wave functions, in particular for the subset of π → π* singlet excitations, are responsible for the largest deviations from CC3.
Abstract: The multireference n-electron Valence State Perturbation Theory is applied to a benchmark set of 28 organic molecules compiled by Schreiber et al. J. Chem. Phys. (2008) 128, 13. Different types of low-lying vertical excitation energies are computed using the same geometries and TZVP basis set as in the original work. The previously published coupled cluster CC3 results are used as a reference. The complete active space second order perturbation theory (CASPT2) results, as well as the results of second order N-electron valence perturbation theory (NEVPT2) (both in their single-state variants) are evaluated against this reference set, which includes 153 singlet and 72 triplet vertical transition energies. NEVPT2 calculations are carried out in two variants: the partially contracted (PC) and the strongly contracted (SC) scheme. The statistical evaluation with respect to CC3 is found to be similar for both: the mean unsigned deviations is 0.28 eV for singlets and 0.16 eV for triplets for PC-NEVPT2, while it is 0.23 and 0.17 eV for SC-NEVPT2, respectively. Further analysis has shown that deficiencies in the zeroth-order wave functions, in particular for the subset of π → π* singlet excitations, are responsible for the largest deviations from CC3. Those states have either a charge transfer or an ionic character. For the remaining singlet and all triplet excitations the general trend was established that NEVPT2 tends to slightly overestimate excitation energies while CASPT2 slightly underestimates them. However, overall, both methods are of very similar accuracy provided that the IPEA shift is used in the CASPT2 method. Interestingly, the conclusions reached in this study are independent of the orbital canonicalization scheme used in the NEVPT2 calculation.
TL;DR: In this paper, the authors used tabletop high-harmonic extreme ultraviolet spectroscopy to measure ultrafast charge transfer processes in condensed-phase systems, and established the ability of time-resolved extreme ultraviolet (EV) spectroglobalization to measure the charge transfer process in condensed phase systems.
Abstract: Oxidation-state-specific dynamics at the Fe M2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe2O3 thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe3+ and photoinduced Fe2+ state confirm this assignment and exclude the alternative d–d excitation. The Fe2+ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.
TL;DR: In this article, the band gap of pure and nickel-doped zinc oxide thin films synthesized using the low-cost spray pyrolysis deposition method was analyzed using optical and synchrotron X-ray techniques.
Abstract: The tuning of semiconductor band gaps can often provide significant performance increases and new applications for electronic, optoelectronic, and photocatalytic devices. Here, we study the band gaps of pure and nickel-doped zinc oxide thin films synthesized using the low-cost spray pyrolysis deposition method. Nickel concentration is varied from 0 to 15%, and the effects that this doping has on the electronic structure are analyzed. Using optical and synchrotron X-ray techniques, two regimes of band gap reduction via Ni doping are uncovered. For doping up to 4% Ni, there is a strong reduction in the gap, while continued doping up to 15% further reduces the gap, but to a lesser extent. The results are explained using X-ray spectroscopy and an Anderson impurity model. These tools show that the low doping case is driven by the interaction of the Ni 3d and O 2p states in both the valence and conduction bands. At high doping, the removal of Zn 3d states from the valence band and the change in Ni coordination ...