Showing papers on "Valence (chemistry) published in 2014"
TL;DR: The electronic structure and chemical composition of efficient CH3NH3PbI3 perovskite solar cell materials deposited onto mesoporous TiO2 were studied using photoelectron spectroscopy with hard X-rays and results indicating similar electronic structures are shown.
Abstract: The electronic structure and chemical composition of efficient CH3NH3PbI3 perovskite solar cell materials deposited onto mesoporous TiO2 were studied using photoelectron spectroscopy with hard X-rays. With this technique, it is possible to directly measure the occupied energy levels of the perovskite as well as the TiO2 buried beneath and thereby determine the energy level matching of the interface. The measurements of the valence levels were in good agreement with simulated density of states, and the investigation gives information on the character of the valence levels. We also show that two different deposition techniques give results indicating similar electronic structures.
428 citations
TL;DR: Some of the most relevant properties of these solids such as their chemical or thermal stability as well as their catalytic, redox- and photo-activities are reported.
Abstract: This article focuses on high valence 3p and transition metal based metal organic frameworks. In the first part we will discuss the complex solution chemistry of these metals which makes this sub-class of MOFs more of a challenge than the traditional low valence metal based MOFs. This is followed by a short review of the different classes of solids based on phosphonates, carboxylates and other linkers. Finally, we report some of the most relevant properties of these solids such as their chemical or thermal stability as well as their catalytic, redox- and photo-activities.
399 citations
TL;DR: In this paper, a combination of synchrotron X-ray diffraction, total scattering measurements, density functional theory calculations, and low-temperature heat capacity measurements, in conjunction with detailed temperature and time-resolved studies of luminescence properties, was employed to understand the origins of the improved luminecence properties.
Abstract: The orthosilicate phosphors SrxBa2–xSiO4:Eu2+ have now been known for over four decades and have found extensive recent use in solid-state white lighting. It is well-recognized in the literature and in practice that intermediate compositions in the solid-solutions between the orthosilicates Sr2SiO4 and Ba2SiO4 yield the best phosphor hosts when the thermal stability of luminescence is considered. We employ a combination of synchrotron X-ray diffraction, total scattering measurements, density functional theory calculations, and low-temperature heat capacity measurements, in conjunction with detailed temperature- and time-resolved studies of luminescence properties to understand the origins of the improved luminescence properties. We observe that in the intermediate compositions, the two cation sites in the crystal structure are optimally bonded as determined from bond valence sum calculations. Optimal bonding results in a more rigid lattice, as established by the intermediate compositions possessing the hi...
310 citations
TL;DR: By using a He lattice model to compress (with minimal orbital interaction at moderate pressures between the surrounding He and the contained atoms or molecules) atoms and an interstitial space, this work is able to semiquantitatively explain and predict the propensity of various elements to form HPEs.
Abstract: ConspectusElectrides, in which electrons occupy interstitial regions in the crystal and behave as anions, appear as new phases for many elements (and compounds) under high pressure. We propose a unified theory of high pressure electrides (HPEs) by treating electrons in the interstitial sites as filling the quantized orbitals of the interstitial space enclosed by the surrounding atom cores, generating what we call an interstitial quasi-atom, ISQ.With increasing pressure, the energies of the valence orbitals of atoms increase more significantly than the ISQ levels, due to repulsion, exclusion by the atom cores, effectively giving the valence electrons less room in which to move. At a high enough pressure, which depends on the element and its orbitals, the frontier atomic electron may become higher in energy than the ISQ, resulting in electron transfer to the interstitial space and the formation of an HPE.By using a He lattice model to compress (with minimal orbital interaction at moderate pressures between ...
189 citations
TL;DR: YbB12 is revealed to be in a new novel quantum state, strongly correlated topological crystalline Kondo insulator, which is characterized by its nonzero mirror Chern number.
Abstract: The electronic structures of two mixed valence insulators YbB6 and YbB12 are studied by using the local density approximation supplemented with the Gutzwiller method and dynamic mean field theory YbB6 is found to be a moderately correlated Z2 topological insulator, similar to SmB6 but having much larger bulk band gap Notably, YbB12 is revealed to be in a new novel quantum state, strongly correlated topological crystalline Kondo insulator, which is characterized by its nonzero mirror Chern number The surface calculations find an odd (three) and an even (four) number of Dirac cones for YbB6 and YbB12, respectively
148 citations
TL;DR: This work reports high-resolution angle-resolved photoemission spectroscopy on MoSe2 single crystals and monolayer films of MoS2 grown on highly ordered pyrolytic graphite substrate, and systematically image the formation of quantum well states on the surfaces of these materials.
Abstract: Transition metal dichalcogenides [corrected] have attracted much attention recently due to their potential applications in spintronics and photonics because of the indirect to direct band gap transition and the emergence of the spin-valley coupling phenomenon upon moving from the bulk to monolayer limit. Here, we report high-resolution angle-resolved photoemission spectroscopy on MoSe2 single crystals and monolayer films of MoS2 grown on highly ordered pyrolytic graphite substrate. Our experimental results resolve the Fermi surface trigonal warping of bulk MoSe2, and provide evidence for the critically important spin-orbit split valence bands of monolayer MoS2. Moreover, we systematically image the formation of quantum well states on the surfaces of these materials, and present a theoretical model to account for these experimental observations. Our findings provide important insights into future applications of transition metal dichalcogenides in nanoelectronics, spintronics and photonics devices as they critically depend on the spin-orbit physics of these materials.
131 citations
TL;DR: In this paper, a layered europium bismuth sulfofluoride (EuBiS) was found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of a possible dynamic CDW.
Abstract: Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states, which have received sustained interest for decades. Here, we report that, in a layered europium bismuth sulfofluoride, ${\mathrm{EuBiS}}_{2}\mathrm{F}$, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of a possible dynamic CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane $\mathrm{Bi}6p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by $\ensuremath{\sim}50$ times, owing to the significant hybridizations between $\mathrm{Eu}4f$ and $\mathrm{Bi}6p$ electrons, as verified by band-structure calculations. Thus ${\mathrm{EuBiS}}_{2}\mathrm{F}$ manifests itself as an unprecedented material that simultaneously accommodates SC, CDW, and $f$-electron valence instability.
115 citations
TL;DR: This is the first time that stable carriers are present in the quantum state of strongly confined quantum dot in ambient conditions, attributed to the rigid shifts of the valence and conduction band with respect to the environment, similar to the sensitivity of the work function of surfaces to adsorbates.
Abstract: HgS nanocrystals show a strong mid-infrared absorption and a bleach of the near-infrared band edge, both tunable in energy and reversibly controlled by exposure to solution ions under ambient conditions. The same effects are obtained by applying a reducing electrochemical potential, confirming that the mid-infrared absorption is the intraband transition of the quantum dot. This is the first time that stable carriers are present in the quantum state of strongly confined quantum dot in ambient conditions. The mechanism by which doping is achieved is attributed to the rigid shifts of the valence and conduction band with respect to the environment, similar to the sensitivity of the work function of surfaces to adsorbates.
113 citations
TL;DR: A gas-solid chromatography study of the adsorption of Fl on a Au surface points to the formation of a metal-metal bond of Fl with Au, the least reactive element in the group, but still a metal.
Abstract: The electron shell structure of superheavy elements, i.e., elements with atomic number Z ≥ 104, is influenced by strong relativistic effects caused by the high Z. Early atomic calculations on element 112 (copernicium, Cn) and element 114 (flerovium, Fl) having closed and quasi-closed electron shell configurations of 6d107s2 and 6d107s27p1/22, respectively, predicted them to be noble-gas-like due to very strong relativistic effects on the 7s and 7p1/2 valence orbitals. Recent fully relativistic calculations studying Cn and Fl in different environments suggest them to be less reactive compared to their lighter homologues in the groups, but still exhibiting a metallic character. Experimental gas–solid chromatography studies on Cn have, indeed, revealed a metal–metal bond formation with Au. In contrast to this, for Fl, the formation of a weak bond upon physisorption on a Au surface was inferred from first experiments. Here, we report on a gas–solid chromatography study of the adsorption of Fl on a Au surface....
107 citations
TL;DR: The specification and improved understanding of scattering parameters using the SPB model are important and instructive for further optimization of the thermoelectric performance of n-type Mg2Si0.3Sn0.7.
Abstract: The well-known single parabolic band (SPB) model has been useful in providing insights into the understanding of transport properties of numerous thermoelectric materials. However, the conduction and valence bands of real semiconductors are rarely truly parabolic which limits the predictive power of the SPB model. The coincidence of the band edges of two parabolic bands, a situation arising in Mg2Si1−xSnx solid solutions when x ∼ 0.7, naturally makes the SPB approximation applicable to evaluate all transport parameters. We demonstrate this in the case of Bi-doped Mg2Si0.3Sn0.7 where the minima of the two conduction bands at the X-point of the Brillouin zone coincide. The combination of a large density-of-states effective mass m* ∼ 2.6 me arising from the enhanced valley degeneracy Nv, high mobility μd due to low deformation potential Ed (8.77–9.43 eV), and ultra-low alloy scattering parameter Ea (0.32–0.39 eV) leads to an outstanding power factor, PFmax ∝ (m*)3/2μd, of up to 4.7 mW m−1 K−2 at around 600 K. The specification and improved understanding of scattering parameters using the SPB model are important and instructive for further optimization of the thermoelectric performance of n-type Mg2Si0.3Sn0.7.
105 citations
TL;DR: In this paper, the authors reported the successful incorporation of high valence transition metals (Cr, Mo, W, V, Nb, Ti, Zr) into perovskite materials, for potential applications as symmetric electrode materials for Solid Oxide Fuel Cells.
TL;DR: In this paper, the authors used density functional theory calculations to study the adsorption of benzene on hematite (α-Fe2O3) surfaces, and the strong electron correlation effects of the Fe 3d-electrons in α-Fe 2O3 were described by a Hubbard-type on-site Coulomb repulsion (the DFT+U approach).
Abstract: The reactivity of mineral surfaces in the fundamental processes of adsorption, dissolution or growth, and electron transfer is directly tied to their atomic structure. However, unraveling the relationship between the atomic surface structure and other physical and chemical properties of complex metal oxides is challenging due to the mixed ionic and covalent bonding that can occur in these minerals. Nonetheless, with the rapid increase in computer processing speed and memory, computer simulations using different theoretical techniques can now probe the nature of matter at both the atomic and sub-atomic levels and are rapidly becoming an effective and quantitatively accurate method for successfully predicting structures, properties and processes occurring at mineral surfaces. In this study, we have used Density Functional Theory calculations to study the adsorption of benzene on hematite (α-Fe2O3) surfaces. The strong electron correlation effects of the Fe 3d-electrons in α-Fe2O3 were described by a Hubbard-type on-site Coulomb repulsion (the DFT+U approach), which was found to provide an accurate description of the electronic and magnetic properties of hematite. For the adsorption of benzene on the hematite surfaces, we show that the adsorption geometries parallel to the surface are energetically more stable than the vertical ones. The benzene molecule interacts with the hematite surfaces through π-bonding in the parallel adsorption geometries and through weak hydrogen bonds in the vertical geometries. Van der Waals interactions are found to play a significant role in stabilizing the absorbed benzene molecule. Analyses of the electronic structures reveal that upon benzene adsorption, the conduction band edge of the surface atoms is shifted towards the valence bands, thereby considerably reducing the band gap and the magnetic moments of the surface Fe atoms.
TL;DR: The chemical sensitivity of Kβ valence to core X-ray emission spectroscopy (vtc-XES) and its applications for investigating 3d-transition-metal based materials are discussed and the theoretical basis of the technique is presented using a one-electron approach.
Abstract: This Progress Report discusses the chemical sensitivity of Kβ valence to core X-ray emission spectroscopy (vtc-XES) and its applications for investigating 3d-transition-metal based materials. Vtc-XES can be used for ligand identification and for the characterization of the valence electronic levels. The technique provides information that is similar to valence band photoemission spectroscopy but the sample environment can be chosen freely and thus allows measurements in presence of gases and liquids and it can be applied for measurements under in situ/operando or extreme conditions. The theoretical basis of the technique is presented using a one-electron approach and the vtc-XES spectral features are interpreted using ground state density functional theory calculations. Some recent results obtained by vtc-XES in various scientific fields are discussed to demonstrate the potential and future applications of this technique. Resonant X-ray emission spectroscopy is briefly introduced with some applications for the study of 3d and 5d-transition-metal based systems.
TL;DR: It is shown that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation, and a ligand that suits both of these desired properties is polyborate.
Abstract: The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations.
TL;DR: In this paper, a series of cobalt spinel oxides with the chemical composition: Co3O4, MgCo2O4 and MgAl 2O4 was synthesized, characterised and studied for catalytic decomposition of N2O.
Abstract: A series of cobalt spinel oxides with the chemical composition: Co3O4, MgCo2O4, MgCoAlO4, CoAl2O4, Mg0.5Co0.5Al2O4 and MgAl2O4 was synthesised, characterised and studied for catalytic decomposition of N2O. Selective replacement of cobalt ions in the tetrahedral and octahedral sites was employed to probe the specific activity of both types of centres. The combined experimental XRD, UV–vis, RS, XPS, BET, TPR, Temperature Programmed Catalytic Reaction (TPCatR) methods and DFT modelling allowed to explore the mechanistic role of the coordination (tetrahedral, octahedral) and valence (+2, +3) states of cobalt ions in the Co3O4 spinel matrix. The prime active sites of deN2O reaction were definitely identified as the octahedral Co3+ ions (Ea = 15–17 kcal mol−1), whereas the tetrahedral Co2+ ions were found to be clearly much less active (Ea = 27–28 kcal mol−1).
TL;DR: In this paper, the joint effect of strain and doping-induced band gap change in Sn1−xMnxO (0 ≤ x ≤ 0.05) nanoparticles is presented.
Abstract: This paper presents the joint effect of strain- and doping-induced band gap change in Sn1−xMnxO (0 ≤ x ≤ 0.05) nanoparticles. In addition, an effort was made to understand the effect of Mn doping on the structural and optical properties of SnO2. X-ray diffraction analysis showed a tetragonal structure and the unit cell volume decreased slightly with Mn4+ content. The Mn:SnO2 are spherical shaped particles with a size ranging from 7.7 to 13.8 nm as calculated by transmission electron microscopy, Scherrer's formula and Willamson–Hall plot. X-ray photoelectron spectroscopy showed clear evidence for tetragonal coordinated high-spin Mn4+ ions occupying the lattice sites of Sn4+ in the SnO2 host. Electron energy loss spectroscopy further confirmed composition and oxidation states of Sn4+ and Mn4+ ions. Manganese doping increased the band gap of SnO2 from 4 eV to 4.40 eV with Mn4+ concentration. Variation in band gap energy was attributed to the increasing lattice strain with Mn content and the charge transfer transitions between Mn4+ ions and conduction/valence bands of SnO2. Three photoluminescence emission bands observed at 320, 360 and 380 nm, when excited at 250 nm, proved Mn:SnO2 to exhibit good optical emission and to have potential application in nanoscale optoelectronic devices.
TL;DR: The main conclusion is that the valence of the counterion is highly relevant in determining the aggregation behavior, whereas its chemical nature is rather unimportant.
Abstract: The aggregation and charging behavior of sulfate and carboxyl latex particles in the presence of different multivalent salts was studied. Time-resolved light scattering and electrophoresis are the main experimental techniques used. In particular, the influence of the type of counterion is investigated. The main conclusion is that the valence of the counterion is highly relevant in determining the aggregation behavior, whereas its chemical nature is rather unimportant. Multivalent ions of higher valence destabilize the suspensions more effectively, in particular, by shifting the critical coagulation concentration (CCC) to lower values. This behavior reflects the classical Schulze-Hardy rule. Comparison with literature data reveals that the presently investigated systems behave similarly to the ones described earlier, but the observed dependence on valence is weaker than in some other systems. Moreover, we observe a slowdown of the aggregation at high electrolyte concentrations. This slowdown can be explained by the greater viscosity of the electrolyte solutions under these conditions.
TL;DR: In this paper, the authors use Ehrenfest dynamics and time-dependent density functional theory to calculate the electronic stopping power of high-energy ions in graphitic targets from first principles.
Abstract: We use Ehrenfest dynamics and time-dependent density functional theory to calculate electronic stopping power ${S}_{e}$ of energetic ions in graphitic targets from first principles. By treating core electrons as valence electrons within the projected augmented wave framework, we demonstrate that this approach provides an accurate description of ${S}_{e}$ for a wide range of ions and ion energies, even when not only valence, but also core electron excitations are essential. Our impact-parameter-dependent approach capable of describing the stopping of both low- and high-energy ions is a significant step forward in ${S}_{e}$ calculations, as it makes it possible to monitor projectile charge state during impacts, estimate contributions of core and valence electron excitations to ${S}_{e}$, and it gives a quantitative description of electronic stopping in the cross-over region for bulk solids and nanostructures from first principles.
TL;DR: In this paper, the density functional theory with repulsive potential (DFT+U) method and a nonlinear core-corrected norm-conserving Ce pseudopotential were used to investigate the native point defects in CeO2.
Abstract: We investigated the native point defects in CeO2 by the density functional theory with repulsive potential (DFT+U) method and by use of a nonlinear core-corrected norm-conserving Ce pseudopotential. We find the neutral oxygen vacancy (VO0) in CeO2 to have a very low formation energy of only 0.39 eV in the O-poor limit. It is a deep donor with negative U behavior, stable only in its neutral and doubly positive states. The anion Frenkel defect is found to be the lowest energy disorder defect, with a formation energy of only 2.08 eV/defect site. These low formation energies arise from the improved transferability of our Ce pseudopotential for its +3 and +4 valence states. The negative U behavior of VO leads to excellent photocatalytic behavior, while the low formation energy of the anion Frenkel defect leads to a superior oxygen storage-and-release capability.
TL;DR: It has been identified that the strength of secondary intramolecular heteropolar hydrogen bonding interactions, CH···O andCH···N, increases when going from ZnL toZnL3, and this result supports the findings of an increase in the local electronic CH··HC stabilization found from QTAIM, IQA, and ETS-NOCV.
Abstract: In the present account factors determining the stability of ZnL, ZnL2, and ZnL3 complexes (L = bpy, 2,2′-bipyridyl) were characterized on the basis of various techniques: the quantum theory of atoms in molecules (QTAIM), energy decomposition schemes based on interacting quantum atoms (IQA), and extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV). Finally, the noncovalent interactions (NCI) index was also applied. All methods consistently indicated that the strength of the coordination bonds, Zn–O and Zn–N, decreases from ZnL to ZnL3. Importantly, it has been identified that the strength of secondary intramolecular heteropolar hydrogen bonding interactions, CH···O and CH···N, increases when going from ZnL to ZnL3. A similar trend appeared to be valid for the π-bonding as well as electrostatic stabilization. In addition to the above leading bonding contributions, all techniques suggested the existence of very subtle, but non-negligible additional stabilization from the CH...
TL;DR: Fundamental structural differences at the atomic scale and nanoscale that are observed between NW surface facets can explain the device behavior.
Abstract: We determine the detailed differences in geometry and band structure between wurtzite (Wz) and zinc blende (Zb) InAs nanowire (NW) surfaces using scanning tunneling microscopy/spectroscopy and photoemission electron microscopy. By establishing unreconstructed and defect-free surface facets for both Wz and Zb, we can reliably measure differences between valence and conduction band edges, the local vacuum levels, and geometric relaxations to the few-millielectronvolt and few-picometer levels, respectively. Surface and bulk density functional theory calculations agree well with the experimental findings and are used to interpret the results, allowing us to obtain information on both surface and bulk electronic structure. We can thus exclude several previously proposed explanations for the observed differences in conductivity of Wz-Zb NW devices. Instead, fundamental structural differences at the atomic scale and nanoscale that we observed between NW surface facets can explain the device behavior.
TL;DR: In this article, a generic definition of oxidation state (OS) is formulated: "The OS of a bonded atom equals its charge after ionic approximation" and two principal algorithms are outlined for OS determination of an atom in a compound; one based on composition, the other on topology.
Abstract: Abstract A generic definition of oxidation state (OS) is formulated: “The OS of a bonded atom equals its charge after ionic approximation”. In the ionic approximation, the atom that contributes more to the bonding molecular orbital (MO) becomes negative. This sign can also be estimated by comparing Allen electronegativities of the two bonded atoms, but this simplification carries an exception when the more electronegative atom is bonded as a Lewis acid. Two principal algorithms are outlined for OS determination of an atom in a compound; one based on composition, the other on topology. Both provide the same generic OS because both the ionic approximation and structural formula obey rules of stable electron configurations. A sufficiently simple empirical formula yields OS via the algorithm of direct ionic approximation (DIA) by these rules. The topological algorithm works on a Lewis formula (for a molecule) or a bond graph (for an extended solid) and has two variants. One assigns bonding electrons to more electronegative bond partners, the other sums an atom’s formal charge with bond orders (or bond valences) of sign defined by the ionic approximation of each particular bond at the atom. A glossary of terms and auxiliary rules needed for determination of OS are provided, illustrated with examples, and the origins of ambiguous OS values are pointed out. An electrochemical OS is suggested with a nominal value equal to the average OS for atoms of the same element in a moiety that is charged or otherwise electrochemically relevant.
TL;DR: In this paper, the electronic structures of a series of donor-acceptor mixed-stack crystals have been investigated by means of density functional theory calculations, and the results highlight that a number of these crystals under consideration are characterized by wide valence and conduction bands, and as a result very low hole and electron effective masses.
Abstract: The electronic structures of a series of donor–acceptor mixed-stack crystals have been investigated by means of density functional theory calculations. The results highlight that a number of the donor–acceptor crystals under consideration are characterized by wide valence and conduction bands, large hole and electron electronic couplings, and as a result very low hole and electron effective masses. The fact that the effective masses and electronic couplings for holes and electrons are nearly equal along the stacking directions implies that the hole and electron mobilities in these systems are also similar. In addition, in several of these crystals, charge transport has a two-dimensional character. The impact on the charge transport properties of the electronic couplings between donor and acceptor frontier orbitals and of the related energy gaps is also discussed.
TL;DR: The electrochemical characteristics of the high voltage, high capacity Li-ion battery cathode material Li[Li2/12Ni3/12Mn7/12]O2 prepared using three different synthesis routes are determined, revealing differences in their surface chemistries upon cycling, which correlate with voltage fading.
Abstract: We have determined the electrochemical characteristics of the high voltage, high capacity Li-ion battery cathode material Li[Li2/12Ni3/12Mn7/12]O2 prepared using three different synthesis routes: sol–gel, hydroxide coprecipitation, and carbonate coprecipitation. Each route leads to distinct morphologies and surface areas while maintaining the same crystal structures. X-ray photoelectron spectroscopy (XPS) measurements reveal differences in their surface chemistries upon cycling, which correlate with voltage fading. Indeed, we observe the valence state of Mn on the surface to decrease upon lithiation, and this reduction is specifically correlated to discharging below 3.6 V. Furthermore, the data shows a correlation of the formation of Li2CO3 with the Mn oxidation state from the decomposition of electrolyte. These phenomena are related to each material’s electrochemistry in order to expand upon the reaction mechanisms taking place–specifically in terms of the particle morphology produced by each synthetic a...
TL;DR: The generalized two-center cluster model (GTCM), which can treat covalent, ionic and atomic configurations in general systems with two inert cores plus valence nucleons, is formulated in the basis of the microscopic cluster model.
Abstract: The generalized two-center cluster model (GTCM), which can treat covalent, ionic and atomic configurations in general systems with two inert cores plus valence nucleons, is formulated in the basis of the microscopic cluster model. In this model, the covalent configurations constructed by the molecular orbital (MO) method and the atomic (or ionic) configuration obtained by the valence bonding (VB) method can be handled in a consistent manner. The GTCM is applied to the light neutron-rich system 10,12Be = α + α + Xn (X = 2, 4). The continuous and smooth changes of the neutron orbits from the covalent MO states to the ionic VB states are clearly observed in the adiabatic energy surfaces (AESs), which are the energy curves obtained with a variation of the α–α distance. The energy levels obtained from the AESs nicely reproduce the recent observations over a wide energy region. The individual spectra are characterized in terms of chemical-bonding-like structures, such as the covalent MO or ionic VB structures, according to analysis of their intrinsic wave functions. From the analysis of AESs, the formation of the mysterious states in 10,12Be, which have anomalously small excitation energies in comparison to a naive shell-model prediction, is investigated. A large enhancement in a monopole transition from a ground MO state to an ionic α + 6,8He VB state is found, which seems to be consistent with a recent observation. In the unbound region, the structure problem, which handles the total system of α + α + Xn (X = 2, 4) as a bound or quasi-bound state, and the reaction problem, induced by the collision of an asymptotic VB state of α + 6,8He, are combined by the GTCM. The properties of unbound resonant states are discussed in close connection to the reaction mechanism, and some enhancement factors originating from the properties of the intrinsic states are predicted in the reaction observables.
TL;DR: It is shown that the asymptotically correct sTDA-RSH combination yields results often superior to those based on global hybrids and that it opens up new possibilities for the computation of excited states in materials science and bio-molecular systems.
Abstract: The recently introduced sTDA methodology [S Grimme, J Chem Phys, 2013, 138, 244104] to compute excitation spectra of huge molecular systems is extended to range-separated hybrid (RSH) density functionals The three empirical parameters of the method which describe a screened two-electron interaction are obtained for some common RSH functionals (ωB97 family, CAM-B3LYP, LC-BLYP) from a fit to theoretical SCS-CC2 reference vertical excitation energies for a set of small to medium-sized chromophores The method is cross-validated on a set of inter- and intramolecular charge transfer states and a set composed of typical valence transitions Overall small deviations from reference data of only about 02–04 eV are found with best performance for CAM-B3LYP and ωB97X-D3 To demonstrate versatility and robustness of the new methodology, applications (the UV/Vis spectrum of the pyridine polymer and the ECD spectrum of (P)-[11]helicene) and frequently used charge transfer examples are discussed In one case, 11 000+ excited electronic states of a system containing 330 atoms were calculated We show that the asymptotically correct sTDA–RSH combination yields results often superior to those based on global hybrids and that it opens up new possibilities for the computation of excited states in materials science and bio-molecular systems
TL;DR: In this paper, the size and valence state effect of Pt on photocatalytic H2 evolution over platinized TiO2 photocatalyst were studied for the first time, and it was found that Pt particle size does not affect the photoreaction rate with the size range of several nanometers.
TL;DR: A cyclic C-bromo-iminium bromide reacts with lithium trimethylsilyl acetylide to afford the title compound 2+˙ in 40% yield as discussed by the authors.
Abstract: A cyclic C-bromo-iminium bromide reacts with lithium trimethylsilyl acetylide to afford the title compound 2+˙ in 40% yield. In contrast to other organic mixed valence compounds, this radical cation can be stored in air. Oxidation and reduction of 2+˙ afford the corresponding dication 2++ and neutral cumulene 2, respectively, in good yields.
TL;DR: In this article, the first direct chemical and imaging evidence of lithium-induced atomic ordering in amorphous TiO2 nanomaterials was reported and new reaction mechanisms that contradict the many works on the lithiation behavior of these materials were proposed.
Abstract: In this paper, we report the first direct chemical and imaging evidence of lithium-induced atomic ordering in amorphous TiO2 nanomaterials and propose new reaction mechanisms that contradict the many works in the published literature on the lithiation behavior of these materials. The lithiation process was conducted in situ inside an atomic resolution transmission electron microscope. Our results indicate that the lithiation started with the valence reduction of Ti4+ to Ti3+ leading to a LixTiO2 intercalation compound. The continued intercalation of Li ions in TiO2 nanotubes triggered an amorphous to crystalline phase transformation. The crystals were formed as nano-islands and identified to be Li2Ti2O4 with cubic structure (a = 8.375 A). The tendency for the formation of these crystals was verified with density functional theory (DFT) simulations. The size of the crystalline islands provides a characteristic length scale (∼5 nm) at which the atomic bonding configuration has been changed within a short ti...
TL;DR: In this article, first principles calculations are performed to study the electronic and optical properties of Cu-doped, Ndoped and (Cu+2N)-co-dope anatase TiO 2, where strong hybridization between Cu 3 d and N 2 p orbitals above the valence band leads to the formation of an isolated intermediate band (IB) deep in the band gap of pure TiO2.