Theoretical Study of α‐V2O5‐Based Double‐Wall Nanotubes

First‐principles calculations of the atomic and electronic structure of double‐wall nanotubes (DWNTs) of α‐V₂O₅ are performed. Relaxation of the DWNT structure leads to the formation of two types of local regions: 1) bulk‐type regions and 2) puckering regions. Calculated total density of states (DOS) of DWNTs considerably differ from that of single‐wall nanotubes and the single layer, as well as from the DOS of the bulk and double layer. Small shoulders that appear on edges of valence and conduction bands result in a considerable decrease in the band gaps of the DWNTs (up to 1 eV relative to the single‐layer gaps). The main reason for this effect is the shift of the inner‐ and outer‐wall DOS in opposite directions on the energetic scale. The electron density corresponding to shoulders at the conduction‐band edges is localized on vanadium atoms of the bulk‐type regions, whereas the electron density corresponding to shoulders at the valence‐band edges belongs to oxygen atoms of both regions.     

Computational Design of One‐Dimensional Ferromagnetic Semiconductors in Transition Metal Embedded Stannaspherene Nanowires

Developing low dimensional semiconductors with moderate band gaps, intrinsic ferromagnetism and large magnetic anisotropy energies (MAEs) is very desirable for high‐speed nano‐spintronic devices, which, however, still remains a big challenge. Here, via first principles calculations, a potential route to realize such materials is proposed based on a new class of one‐dimensional transition metal (TM) embedded stannaspherene (Sn₁₂^2–) nanowires [TM₂(Sn₁₂)]_∞ (TM = Ti‐Ni). Three semiconductors with robust ferromagnetism are achieved with TM = V, Cr and Fe, which all exhibit direct or quasi‐direct band gaps around 1.0 eV, rendering their great potentials for visible light optoelectronic applications. Interestingly, [Cr₂(Sn₁₂)]_∞ and [Fe₂(Sn₁₂)]_∞ are both identified as bipolar magnetic semiconductors (BMS) with valence and conduction band edges spin polarized in the opposite directions, which are promising for realizing switch of carriers’ spin orientation by electrical gating, while [V₂(Sn₁₂)]_∞ exhibits a half semiconductor (HSC) property with valence and conduction band edges spin polarized in the same direction and can be used for spin‐polarized carriers generation. Moreover, sizable MAEs are discovered in these nanowires, which are at least two orders of magnitude larger than those of Fe, Co and Ni bulks and also significantly larger than current ferromagnetic semiconductors.     

Resonant X-Ray Diffraction: Forbidden Bragg Reflections Induced by Atomic Displacements

The displacements of atoms from their equilibrium sites with high symmetry are accompanied by the appearance of the additional components of the resonant susceptibility tensor and become a source of additional anisotropy of resonant X-ray scattering. As a result, forbidden Bragg reflections can appear near absorption edges, which are absent far from the edges in a regular crystal. These reflections can be induced by dynamic thermal atomic vibrations or by static displacements because of modulation and point defects. A theoretical approach is suggested that allows the calculation of the susceptibility tensor and a set of forbidden reflections, in the case of small atomic displacements. This approach is based on the calculation of the displacements using the mechanic representation of space groups. The example of a space group Pnma with resonant atoms in 4(c) position is discussed in detail.     

Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal

We report on circularly polarized light emitted from a chiral nematic liquid crystal doped with a luminescent organolanthanide dye. The organolanthanide emission displays an extremely narrow spectral bandwidth of Δ λ_E≈ 8 nm. This is considerably narrower than the CNLC selective reflection bandwidth Δ λ_R≈60 nm. When conventional dyes with broader emission bandwidths are dissolved into CNLCs, the average degree of circular polarization g of emitted light is reduced from the maximum degree g _MAX ; this is due to the overlap of the emission band with the reflection band edges, and spectral regions outside the reflection band. Here, however, we can place the entire emission band inside the reflection band and achieve g ≈ g _MAX=1.27. Furthermore, a high degree of circular polarization is maintained under off-axis viewing up to a viewing angle of ≈ 30° to the normal.     

Die Kristallstruktur von BaTeO3

Crystal Structure of BaTeO₃ Crystals of BaTeO₃ were prepared from melts. BaTeO₃ is isotropic with KClO₃ and crystallizes in the monoclinic system, space group P2₁/m. The lattice parameters are a = 4.63₃, b = 5.95₂, c = 7.30₈ Å, β = 111.2°. The formula unit is 2. The crystal structure was determined by Patterson synthesis, refined by least squares and confirmed by (F_o — F_c) synthesis. Using isotropic temperature corrections the R-value of 1072 observed reflections calculated as 0.748, and anisotropic the R-value as 0.702. In BaTeO₃ tellurium forms with three oxygen atoms pyramidal TeO₃-groups, barium has to oxygen atoms the coordination 7+2. The TeO₃ groups are not linked one with another, but with the BaO₉ polyhedrons by common corners and edges, the latter are linked one with another only by common edges.     

Electronic states in liquids studied photoelectrochemically: Photohole emission into H2O

Holes are injected from a photoexcited Au electrode into an aqueous electrolyte solution with a pH-independent threshold level of ?9.1 ± 0.2 eV versus vacuum. Referring to recent UPS data, this level is identified with the valence band edges of liquid H_2O forming hydrogen- bonded networks in the interfacial region.     

Small‐Band‐Gap Halide Double Perovskites

Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs₂AgTlX₆ (X=Cl ( 1 ) and Br ( 2 )) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in A^IB^IIX₃ perovskites, the band‐gap transition in A^I₂BB′X₆ double perovskites can show substantial metal‐to‐metal charge‐transfer character. This band‐edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B‐ and B′‐site metal frontier orbitals. Calculations reveal a shallow, symmetry‐forbidden region at the band edges for 1 , which results in long (μs) microwave conductivity lifetimes. We further describe a facile self‐doping reaction in 2 through Br₂ loss at ambient conditions.     

Electron energy levels in semiconductor electrochemistry

The significance of the flat-band potential and the energetic position of the band edges at the semiconductor/electrolyte interface in semiconductor electrochemistry and photoelectrochemistry is pointed out. Different methods for determining these parameters experimentally are discussed, such as methods based on the measurement of the photovoltage or photocurrent, as well as the method for determining the flat-band potential from interfacial capacitance measurements. The capacitance-voltage relationship of the ideal semiconductor/electrolyte Schottky barrier is described. Subsequently, possible complications of the capacitance behavior are discussed, and conditions indicated under which the determination of the flat-band potential from non-ideal capacitance results is still possible. A critical survey is then given of flat-band data for some selected semiconductor electrodes (ZnO, CdS, GaP, GaAs, TiO₂, SrTiO₃), comprising a discussion of problems encountere, factors on which the flat-band potential depends and discrepancies between different results. Attempts to predict the flat-band potential and the position of the band edges from atomic electronegativity data are reviewed. The relationship between flat-band potential or band-edge position and electrochemical behaviour is considered, i.e., as far as the magnitude of the photovoltage as well as the electrochemical and photoelectrochemical reactivity are concerned.     

The Crystal Structure of Ba2Cu2AlF11: a New Structure Type in Copper Fluoride Crystal Chemistry

Ba₂Cu₂AlF₁₁ is trigonal: a = 7.301(1) Å, c = 14.145(2) Å, γ = 120°, Z = 3. The crystal structure was solved in the space group P3₂ (n° 145), from X-ray single crystal data using 2675 unique reflections (2476 with F/σ(F) > 4). It consists in a complex tridimensional arrangement of copper-fluorine and aluminium-fluorine octahedra, with an original kind of linkage which involves simultaneously edges and vertices.     

Energetics and dynamics of the interface of RuS2 and implications for photoelectrolysis of water

RuS₂ (E_G=1.3 eV), grown from liquid Bi, has proved to be a stable and fairly efficient photoanode for a potential assisted photoelectrolysis of water using visible and near infrared light. Its valence band has a quite pure d-character. Holes generated on d-states in the valence band lead to the formation of Ru-based surface states which induce interfacial coordination bonding. The average positive charge accumulated in these complexes determines the energetic position of the energy bands with the consequence that the band edges are shifted with application of an electrode potential until their position becomes stabilized by electron injection from the electrolyte. This conclusion is derived from capacity measurements performed in the region of light intensity limited and diffusion controlled anodic photocurrents (rotating disk experiments). The energetic limitation of RuS₂ photoelectrodes has mainly to be seen in the position of the surface states which are formed too high above the edge of the valence band. For the present this compound appears to be a very attractive model system for research on low photon energy photooxidation of water.     

Darstellung und Struktur von α-CrPO4. Beiträge zum thermischen Verhalten von wasserfreien Phosphaten. I

Contributions on the Thermal Behaviour of Anhydrous Phosphates. I. Preparation and Structure of α-CrPO₄ By a chemical transport reaction (1100°C → 1000°C) using chlorine as transporting agent we obtained dark-green, well shaped crystals of α-CrPO₄. The compound crystallizes in the space group Imma. The lattice constants are a = 10.403(2), b = 12.898(2), c = 6.299(1) Å. The crystal structure has been determinated from single crystal data and refined to a conventional residual of R = 0.038 (1481 unique reflections, 34 variables). The structure consists of CrO₆-octahedra and PO₄-tetrahedra. Especially remarkable are pairs of edge sharing CrO₆-octahedra which are connected with two PO₄-tetrahedra at opposite edges. Parallel to the a- and b-axis are large channels extending through the whole structure.     

Dipole‐Induced Band‐Gap Reduction in an Inorganic Cage

Metal‐doped polyoxotitanium cages are a developing class of inorganic compounds which can be regarded as nano‐ and sub‐nano sized molecular relatives of metal‐doped titania nanoparticles. These species can serve as models for the ways in which dopant metal ions can be incorporated into metal‐doped titania (TiO₂), a technologically important class of photocatalytic materials with broad applications in devices and pollution control. In this study a series of cobalt(II)‐containing cages in the size range ca. 0.7–1.3 nm have been synthesized and structurally characterized, allowing a coherent study of the factors affecting the band gaps in well‐defined metal‐doped model systems. Band structure calculations are consistent with experimental UV/Vis measurements of the Ti_xO_y absorption edges in these species and reveal that molecular dipole moment can have a profound effect on the band gap. The observation of a dipole‐induced band‐gap decrease mechanism provides a potentially general design strategy for the formation of low band‐gap inorganic cages.     

Die Kristallstruktur von InTeCl; ein neuer defekttetraedrischer Strukturtyp

The Crystal Structure of InTeCl; a New Defect Tetrahedral Structure Type InTeCl is monoclinic, space group P2₁/c with a = 7.42, b = 14.06, c = 7.07 Å, β = 92.1°, and Z = 8. The crystal structure was determined from single crystal X-ray data by means of three dimensional PATTERSON and FOURIER syntheses. The parameters were refined by the least-squares method to an R value of 0.084 for 932 observed reflections. The compound represents a new ternary defect tetrahedral type. Strongly distorted InTe₃Cl-tetrahedra from layer complexes situated parallel to the (100) plane by sharing corners and edges which are occupied by Te atoms. The Cl atoms belong to one tetrahedron only and do not contribute to the linkage. The Te atoms are surrounded by three In atoms in a approximately trigonal pyramidal arrangement. The bonding is discussed.     

The photodissociation of water by doped iron oxides: The unblased p/n assembly

Mott-Schottky capacitance measurements are used to locate semiconductor band edges for a short-circuited p-type/n-type α-Fe₂O₃ assembly in aqueous solution. The thermodynamic feasibility of the catalytic photodiscussion of water without external bias is verified for this assembly from an energy level diagram obtained for the electrode/electrolyte interfaces. Photocurrent stability and Auger analysis of the electrode show no evidence of electrode dissolution. Oxygen evolution is monitored from an assembly using mass spectrometry and H₂¹⁸O enriched water.     

Expanded Analogs of Three-Dimensional Lead-Halide Hybrid Perovskites

Replacing the Pb−X octahedral building unit of A^IPbX₃ perovskites (X=halide) with a pair of edge-sharing Pb−X octahedra affords the expanded perovskite analogs: A^IIPb₂X₆. We report seven members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrazinium-based cations with low-lying π* orbitals that form the conduction band, substantially decreasing the band gap of the expanded lattice. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with A^II=pyridinium and ammonium, we can isolate the contributions of the pyrazinium-based orbitals in the band gap transition of A^IIPb₂X₆. The organic-molecule-based conduction band and the inorganic-ion-based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.     

La3,5Ru4O13: Un nouveau compose´a`feuillets de type pe´rovskite

The title compound formula, La_3.5Ru₄O₁₃, has been established by means of crystal structure determination. The orthorhombic unit cell has the following dimensions:a = 11.994, b = 5.609, c = 3.856A?. Least-squares refinement using single-crystal intensity data corrected for absorption reachedR = 0.029 for 777 measured independent reflections. La_3.5Ru₄O₁₃ is built up from perovskite-type slabs parallel to (100) which are running throughout the crystal; these three octahedra slabs are linked together by either La atoms or RuO₆ octahedra. The latter are also joined together by opposite edges to form one-dimensional infinite strings parallel to theb-axis. The doubling of thec parameter can be caused by the ordering of these RuO₆ chains and La atoms.     

Electronic properties of titanium dioxide nanotubes doped with 4<Emphasis Type="Italic">d</Emphasis> metals

The effect of 4d-metal dopants on the densities of states of hexagonal TiO₂ nanotubes has been calculated by the linearized augmented cylindrical wave method. It has been demonstrated that the substitution of Nb, Mo, Tc, or Pd atoms for a part of Ti atoms leads to a decrease in the band gap width of the material due to the formation of impurity levels in the band gap of TiO₂. Doping TiO₂ nanotubes with these metals is a promising way to produce materials for electrodes for electrochemical photolysis of water. Doping with Y, Rh, or Ag leads to the displacement of the absorption edge from the UV to the visible range owing to a considerable broadening of the valence and conduction band edges; Zr, Ru, and Cd have a lower disturbing effect on the electronic levels of TiO₂.     

Band Edges Engineering of 2D/2D Heterostructures: The C3N4/Phosphorene Interface

We investigate the interface between carbon nitride (C₃N₄) and phosphorene nanosheets (P-ene) by means of Density Functional Theory (DFT) calculations. C₃N₄/P-ene composites have been recently obtained experimentally showing excellent photoactivity. Our results indicate that the formation of the interface is a favorable process driven by Van der Waals forces. The thickness of P-ene nanosheets determines the band edges offsets and the charge carriers’ separation. The system is predicted to pass from a nearly type-II to a type-I junction when the thickness of P-ene increases, and the conduction band offset is particularly sensitive. Last, we apply the Transfer Matrix Method to estimate the efficiency for charge carriers’ migration as a function of the P-ene thickness.     

Photonic Modulation Enabled by Controlling the Edge Structures of Boron-Doped Molecular Carbons

Control over topological edges of molecular carbons (MCs) is of importance for achieving diverse molecular topologies and desirable physical properties. However, it remains very challenging for heteroatom-doped MCs due to the synthetic difficulty. Herein, we report control over the edge structures of boron-doped MCs (BMCs) via the sequential cyclization strategy. Three BMC molecules that feature the C₅₆B₂ or C₈₄B₂ polycyclic π-skeletons with selective cove/fjord or cove/bay edges, respectively, were synthesized through the rational combination of Mallory photoreaction and Scholl reaction. We not only obtain the largest boron-doped π-system reported so far, but also disclose that fine control of their edges and length greatly affects electronic structures and thereby photonic properties of BMCs, such as tunable aromaticity, decreased band gaps, as well as redshifted absorptions and fluorescence. Remarkably, the C₅₆B₂ molecule exhibits stimulated emission behavior and amplified spontaneous emission property, both of which have never been reported for pristine boron-doped π-systems, thus demonstrating the potential of BMCs as optical gain materials for laser cavities.     

Tetraphenylphosphonium-trichloroplumbat(II), PPh4PbCl3 · CH3CN

Tetraphenylphosphonium Trichloroplumbate(II), PPh₄PbCl₃ · CH₃CN PPh₄PbCl₃ · CH₃CN was obtained by reaction of PbCl₂ and PPh₄Cl in acetonitrile. It was also formed along with (PPh₄)₂Se₂Cl₆ when PbSe was treated with chlorine in the presence of PPh₄Cl. Its crystal structure was determined by X-ray diffraction (R = 0.029 for 4186 reflections). The triclinic crystals contain PbCl₃^– ions that are associated to polymer chains. Each Pb atom has distorted square pyramidal coordination; the pyramids share two opposite basal edges. The chloro bridges are rather asymmetrical.