Cluster Model Calculations Research Articles

Producing Reactive Species on Si(100), Ge(100), and Si(111) Surfaces by Attachments of Diacetylenes

Formation of a bilayer or a multilayer organic thin film on semiconductor surfaces is promising for the manufacture of new-generation microelectronic/nanoelectronic materials. A prerequisite for realization of such a goal is the formation of monolayer organic thin films with reactive functional groups exposed as a template for further chemical manipulations. We report herein a theoretical prediction that attachments of diacetylenes on Si(100)-2 × 1, Ge(100)-2 × 1, and Si(111)-7 × 7 surfaces could produce monolayers of reactive [3]cumulenic or/and enynic surface species. Our density-functional cluster-model calculations revealed the following: (i) Diacetylene can undergo either stepwise [4 + 2]-like or stepwise [2 + 2]-like cycloadditions onto a XX dimer of X(100)-2 × 1 surfaces (X = Si and Ge) via a common singlet−diradical intermediate. (ii) On Si(100), the [2 + 2]-like pathway is kinetically favored over the [4 + 2]-like one, preferentially giving rise to enynic adspecies. (iii) On Ge(100), both pathways are competitive, resulting concomitantly in [3]cumulenic and enynic adspecies. (iv) On Si(111)-7 × 7, diacetylene can undergo either 1,4 addition or 1,2 addition onto a rest atom−adatom pair with the former process being favored over the latter both thermodynamically and kinetically and leading preferentially to the formation of [3]cumulenic adspecies on the surface. It is noteworthy that the [4 + 2]-like cycloaddition of diacetylene on Ge(100) 2 × 1 is among the few examples of [4 + 2] cycloaddition with a 1,3-diyne acting as a four-electron component affording six-membered cyclic [3]cumulenes.

NO and NO2 Adsorption on Terrace, Step, and Corner Sites of the BaO Surface from DFT Calculations

The adsorption of NO and NO2 molecules at the BaO surface has been investigated by means of density functional theory (DFT) cluster model calculations. Regular adsorption sites at flat terraces have been compared with those on steps and corners. The properties of the adsorbed molecules have been monitored by computing various observable properties, such as core level binding energies, hyperfine coupling constants, and vibrational frequencies. NO binds strongly at the oxide anions at terraces, steps, and corners of the BaO surface. The bonding has a substantial polarity due to delocalization of charge from the surface to the adsorbate, but cannot be described as a full charge transfer interaction. The spin is almost entirely localized on the NO adsorbed molecule. NO2 binds to the BaO surface in two different ways, N-down and O-down. In both orientations the oxide anion on the surface is oxidized with formation of an NO2- species which interacts electrostatically with the neighboring Ba cations. The spin is localized on the surface anions by the effect of the creation of a hole in the O(2p) valence shell. The O-down mode is more stable on terrace sites, while on low-coordinated sites the two orientations have similar stabilities. For both NO and NO2 the low-coordinated sites exhibit a much larger reactivity than the flat terraces. The formation of O- ions in the case of NO2 adsorption can be very important for the further reactivity of the surface.

Electronic structure studies ofV6O13by soft x-ray emission spectroscopy: Band-like and excitonic vanadium states

Resonant soft x-ray emission (SXE) spectra of the mixed valence vanadium oxide ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ have been recorded for a series of excitation energies across the V $L$-absorption band. Resonant excitation allows one to distinguish between charge neutral low-energy excitations and continuum-excited, more band-like V $3d$ valence band states in the spectra. We find that the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ consist of two distinct components that can be assigned to nearly pure V $3d$ states, and to V $3d$ states that are strongly hybridized with O $2p$ states, respectively. Band structure calculations of the density functional theory support the assignment of these features. At threshold excitation the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ show strong signatures from excitonic states, the energy dependence of which shows Raman-like behavior. We compare these spectral features in the resonant SXE spectra with cluster model calculations and assign them to $\mathrm{dd}$ excitations and charge-transfer excitations, respectively. Finally, we discuss changes in the V $L$-absorption and emission spectra that take place when changing the sample temperature from 295 K to 120 K. We relate the changes to redistributions in the V $3d$ partial density of states, which occur at the transition temperature for the metal-semiconductor-transition ${T}_{\mathrm{MST}}=150\mathrm{K}$ and find support in our temperature dependent band structure calculations.

Location and role of exchangeable cations in zeolite catalysis: a first principle study

The structure of Li +, Na +, K +, Mg 2+, Ca 2+, Ni 2+ and La 3+ exchanged zeolite is studied using periodic ab initio and cluster model calculation by density functional theory (DFT). Periodic calculation is performed to monitor the cation migration in the zeolite lattice. A correlation to monitor the strength of Lewis acidity of the cations is established by Fukui function based reactivity descriptors. The activity order does not depend on the size of the cations, but depends on the localized geometry of the cations. The Lewis acidity order for different cation site considered is derived and compared with the experimental trend.

Electronic structure of the Kagomé lattice compoundRb2Ni3S4

We have studied the electronic structure of the layered Kogom\'e lattice compound ${\mathrm{Rb}}_{2}{\mathrm{Ni}}_{3}{\mathrm{S}}_{4}$ by photoemission spectroscopy and comparison of the results with band-structure and configuration-interaction cluster-model calculations. The band-structure calculation reproduced the gross features of the experimental spectra but could not reproduce the weak satellite feature, and predicted the narrow peak of Ni $3d$ character just below the Fermi level ${(E}_{F})$ to be too strong. On the other hand, the cluster-model calculation assuming the low-spin $(S=0)$ ground state reproduced the satellite but failed to predict the same peak just below ${E}_{F}$ at the correct energy position. Angle-resolved photoemission measurements revealed momentum dispersions for the S $3p$--derived and Ni $3d$--S $3p$ hybridized bands, in agreement with the band-structure calculation, whereas the Ni $3d$--derived band just below ${E}_{F}$ was found to be dispersionless, unlike the result of the band-structure calculation. We conclude that ${\mathrm{Rb}}_{2}{\mathrm{Ni}}_{3}{\mathrm{S}}_{4}$ is a moderately correlated, strongly $p\ensuremath{-}d$ hybridized system.

Ir5dstate ofCuIr2S4: A cluster-model approach

The Ir $5d$ state in ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ is investigated by using ${\mathrm{IrS}}_{6}$-cluster-model calculations for the Ir $4f$ core-level and valence-band photoemission and bremsstrahlung isochromat spectra. The three experimental spectra are consistently reproduced in such a localized picture with a single parameter set of the model. The average $d\ensuremath{-}d$ Coulomb energy between Ir $5d$ electrons in ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ is 3.5 eV smaller than those for $3d$ transition-metal oxides, while the hybridization between Ir $5d$ and S $3p$ orbitals is larger. This indicates that the lattice distortion is important in the charge-order transition of ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}.$

Electronic structure and band gap of the negative charge-transfer material Sr 3Fe 2O 7

We studied the electronic structure of the Sr 3Fe 2O 7 compound using X-ray photoelectron spectroscopy (XPS). The charge-transfer satellites in the Fe 2p XPS spectra were analyzed using standard cluster model calculations. The analysis indicates that Sr 3Fe 2O 7 is in the negative charge-transfer regime, and that the ground state is dominated by the 3 d 5 L configuration (where L denotes an O 2p hole in the oxygen band). These results are similar to those found in the related SrFeO 3 and Sr 2FeO 4 compounds. The band gap of the Sr 3Fe 2O 7 compound is split off by the relatively large value of the p–d transfer integral T σ. The lowest lying excitations are 3 d 5 L +3 d 5 L →3 d 5+3 d 5 L 2 and consequently the band gap is of the p–p type. The band gap in the Sr n+1 Fe n O 3 n+1 series can be understood taking into account the trend in the O 2p bandwidths.

Resonant soft X-ray emission spectroscopy of doped and undoped vanadium oxides

Resonant soft X-ray emission (RSXE) spectra of NaV 2O 5, Mo x V 1− x O 2 and V 2O 3 have been recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. Resonant excitation allows us, firstly, to separate V 3d and O 2p projected density-of-states of the valence band and, secondly, to study charge-neutral low-energy excitations due to resonant inelastic X-ray scattering (RIXS). We found that both the V L- and the O K-emission spectra clearly show components originating from O 2p- and V 3d-states, reflecting the high degree of hybridization of the valence band in all compounds. At threshold excitation we observed that NaV 2O 5 spectra are dominated by RIXS whereas Mo x V 1− x O 2 and V 2O 3 spectra show bandlike features, which may be due to differences in the correlation effects of the compounds. We compared the RSXE spectra with cluster model calculations, which gives a good account for NaV 2O 5 whereas the RSXE spectra of the other compounds show RIXS only at certain energies well above the threshold. In fact, we interpret the trend in the RSXE spectra of the Mo x V 1− x O 2 compound system as a successive filling of the (rigid) V 3d band with increasing Mo content.

Ground- and excited-state properties ofM-center oxygen vacancy aggregates in the bulk and surface of MgO

Aggregates of oxygen vacancies (F centers) represent a particular form of point defects in ionic crystals. In this study we have considered the combination of two oxygen vacancies, the M center, in the bulk and on the surface of MgO by means of cluster model calculations. Both neutral and charged forms of the defect M and ${M}^{+}$ have been taken into account. The ground state of the M center is characterized by the presence of two doubly occupied impurity levels in the gap of the material; in ${M}^{+}$ centers the highest level is singly occupied. For the ground-state properties we used a gradient corrected density functional theory approach. The dipole-allowed singlet-to-singlet and doublet-to-doublet electronic transitions have been determined by means of explicitly correlated multireference second-order perturbation theory calculations. These have been compared with optical transitions determined with the time-dependent density functional theory formalism. The results show that bulk M and ${M}^{+}$ centers give rise to intense absorptions at about 4.4 and 4.0 eV, respectively. Another less intense transition at 1.3 eV has also been found for the ${M}^{+}$ center. On the surface the transitions occur at 1.6 eV ${(M}^{+})$ and 2 eV (M). The results are compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

Bonding of NO on NixMg1−xO powders: an EPR and computational study

Abstract In this paper, we review the reactivity of the paramagnetic NO molecule with Ni-doped MgO based on electron paramagnetic resonance (EPR) spectra and density functional theory (DFT) cluster model calculations. The EPR spectrum of the NiO/MgO sample recorded at 77 K under NO pressure is characterized by two distinct signals, one assigned to monomeric NO physisorbed on low-coordinated Mg2+ ions and the other to NO adsorbed on terrace Ni2+ ions. The latter interaction is found to be at least three times larger than the former. The nature of NO bonding to Ni2+ ions is analyzed in detail. The EPR spectrum proves the presence of one unpaired electron localized on the Ni 3d shell. The DFT calculations indicate that the unpaired electron sits on the dx2–y2 orbital, while the Ni2+ dz2 electron is coupled with the NO π xz ∗ unpaired electron, producing a NONi2+ bond with dominant covalent character.

Spin and Pseudo Spins in Theoretical Chemistry. A Unified View for Superposed and Entangled Quantum Systems

A unified picture for magnetism, superconductivity, quantum optics and other properties of molecule-based materials has been presented on the basis of effective model Hamiltonians, where necessary parameter values have been determined by the first principle calculations of cluster models and/or band models. These properties of the matetials are qualitatively discussed on the basis of the spin and pseudo-spin Hamiltonian models, where several quantum operators are expressed by spin variables under the two level approximation. As an example, ab initio broken-symmetry DFT calculations are performed for cyclic magnetic ring constructed of 34 hydrogen atoms in order to obtain effective exchange integrals in the spin Hamiltonian model. The natural orbital analysis of the DFT solution was performed to obtain symmetry-adapted molecular orbitals and their occupation numbers. Several chemical indices such as information entropy and unpaired electron density were calculated on the basis of the occupation numbers to elucidate the spin and pair correlations, and bonding characteristic (kinetic correlation) of this mesoscopic magnetic ring. Both classical and quantum effects for spin alignments and singlet spin-pair formations are discussed on the basis of the true spin Hamiltonian model in detail. Quantum effects are also discussed in the case of superconductivity, atom optics and quantum optics based on the pseudo spin Hamiltonian models. The coherent and squeezed states of spins, atoms and quantum field are discussed to obtain a unified picture for correlation, coherence and decoherence in future materials. Implications of theoretical results are examined in relation to recent experiments on molecule-based materials and molecular design of future molecular soft materials in the intersection area between molecular and biomolecular materials.

Diradical mechanisms for the cycloaddition reactions of 1,3-butadiene, benzene, thiophene, ethylene, and acetylene on a Si(111)-7x7 surface.

The cycloaddition chemistry of several representative unsaturated hydrocarbons (1,3-butadiene, benzene, ethylene, and acetylene) and a heterocyclic aromatic (thiophene) on a Si(111)-7x7 surface has been explored by means of density functional cluster model calculations. It is shown that (i) 1,3-butadiene, benzene, and thiophene can undergo both [4+2]-like and [2+2]-like cycloadditions onto a rest atom-adatom pair, with the former process being favored over the latter both thermodynamically and kinetically; (ii) ethylene and acetylene undergo [2+2] cycloaddition-like chemisorptions onto a rest atom-adatom pair; and (iii) all of these reactions adopt diradical mechanisms. This is in contrast to the [4+2] cycloaddition-like chemisorptions of conjugated dienes on a Si(100) surface and to the prototype [4+2] cycloadditions in organic chemistry, which were believed to adopt concerted reaction pathways. Of particular interest is the [4+2]-like cycloaddition of s-trans-1,3-butadiene, whose stereochemistry is retained during its chemisorption on the Si(111) surface.

The chemisorption of NO on Si(1 1 1)-7 × 7 surface: a DFT study

The adsorption of NO on the Si(1 1 1)-7 × 7 surface has been investigated by means of hybrid density functional (B3LYP) cluster model calculations. It is shown that both monomeric and dimeric adspecies can be formed upon NO adsorption. The monomeric adspecies adopt bent configurations with the N-end of the admolecule forming a covalent N–Si bond with a surface atom (adatom or restatom). Three dimeric adspecies have been found either adopting cis-N 2O 2 configuration or trans-N 2O 2 configurations with rather short NN bond length. These dimeric adspecies should be subject to further decomposition to eliminate N 2 or N 2O.

FTIR spectroscopic and ab initio evidence for an amphipathic character of CO bonding with silanol groups

FTIR spectroscopy provides evidence that carbon monoxide interacts with surface silanol groups of silica and silicalite to form both SiOH⋯CO and SiOH⋯OC hydrogen-bonded complexes. The C-bonded adduct shows a characteristic IR absorption band which is blue-shifted as compared to free CO (2143 cm −1), while the O-bonded adduct is characterized by a red-shifted band. Variable temperature IR spectroscopy has shown that these two hydrogen-bonded adducts are in a temperature-dependent equilibrium which involves an enthalpy change of about 2–3 kJ mol −1. Spectroscopic data are supported by ab initio cluster model calculations of different complexity levels.

Molecular orbital study for Na, Mg, and Al adsorption on the Si (111) surface

We investigated the interactions between the Si(111) surface and the Na, Mg, and Al atoms using cluster model calculations. Calculations were performed at levels of complete-active-space self-consistent-field (CASSCF) and multi-reference singly and doubly excited configuration interaction (MRSDCI) calculations using the model core potential method. Our calculations revealed that the most favorable sites of Na, Mg, and Al adsorption on Si(111) are on top (T 1), bridge (B 2), and 3-fold filled (T 4) sites, respectively. The nature of chemical bonds between these metal atoms and the dangling bonds of the surface Si atoms are found to be essentially covalent.

Electronic structure of the wrong-bond states in amorphous germanium sulphides

First-principles calculations of the electronic structures of the wrong bonds were performed for amorphous germanium sulphides in order to explain their compositional dependence. Model cluster calculations of the density of states using a set of geometry similar to the crystalline-GeS 2 coordination can reproduce the peak structure of the experimental valence band photoemission spectra. The bonding and anti-bonding states of the covalent Ge–S bonds form valence and conduction band respectively, and the top of the valence band is occupied with S 3p lone-pair states. The bonding states are modified by S–S bonds and the anti-bonding states are modified by Ge–Ge bonds, mainly through their hybridization with the wrong-bond states between p-orbitals. The lone-pair states do not interact either of them to form a different band, and obscure the modification induced by the S–S wrong bonds. Therefore, we can conclude that the narrowing of the bandgap with increasing Ge content from GeS 2 composition is due to that of the conduction-band bottom with increasing germanium wrong bonds, though the narrowing with increasing S content is moderate due to the presence of the lone-pair states at the valence-band top.

Diradical Mechanism for the [2 + 2] Cycloaddition of Ethylene on Si(100) Surface

Density functional cluster model calculations have been performed to explore the reaction mechanism for the adsorption of ethylene on Si(100). It is shown that the [2 + 2] cycloaddition of ethylene on a Si=Si dimer of Si (100) surface follows a diradical mechanism, via a pi-complex precursor and a singlet diradical intermediate, and the rate-determining step for the overall reaction is the formation of the diradical intermediate.

The chemistry of HN 3 on Ge(1 0 0)-2×1: a theoretical study

The chemistry of HN 3 on the Ge(1 0 0)-2×1 surface has been investigated by means of first-principle density functional cluster model calculations. The calculations revealed three possible reaction pathways for HN 3 reaction with the surface dimer. Based on the predicted energetics, a conclusion can be made that at low temperatures HN 3 can either undergo barrierless 1,3-dioplar cycloaddition onto the surface dimer, resulting in a 5-member-ring metallocyclic surface species, or adsorb molecularly via its HN-end, forming a dative N→Ge bond with the buckled-down Ge atom of the surface dimer.

The influence of doping on the electronic structure of R 1− xCa xBa 2Cu 3O 6+ d (R=Y, Eu) and (La 0.7Ca 0.3) 1− xMn 1+ xO 3

The valence band and core level states of R 1− x Ca x Ba 2Cu 3O 6+ d (R = Y, Eu) and (La 0.7Ca 0.3) 1− x Mn 1+ x O 3 systems were studied by means of angle integrated photoemission. The measurements have been performed on deoxygenated R 1− x Ca x Ba 2Cu 3O 6.1 samples which were insulators or superconductors depending on the calcium content as well as on oxygenated high T c RBa 2Cu 3O 6.9 superconductors. The goal was to study the effect of doping on the valence band and core level states and observe the change in the electronic structure. The studies of the valence band show an increase in the number of states located at 2.5 eV below the Fermi level. The parameters of the electronic structure are estimated (by cluster model calculations) to be: Δ∼1 to 2 eV, T∼2.5–3.5 eV and U dd∼6.5 eV. The valence band spectra of (La 0.7Ca 0.3) 1− x Mn 1+ x O 3 ( x=0, 0.1) are very similar showing two peaks at about 3.5 and 5.7 eV for x=0 and at 6.0 and 7.7 eV for x=0.1 below the upper edge of the valence band. These indicate partly hybridized Mn 3d–O 2p states. From the Mn 2p spectrum and its comparison with the relevant Auger spectrum values of the charge transfer energy Δ, the hybridization energy between Mn 3d and O 2p states T and the on site Coulomb correlation energies U dd in Mn 3d and U pp in O 2p states were estimated to be about 5, 4, 4 and 6.5 eV, respectively. These results indicate the mixed character of the ground state.

XPS and XAS investigation of condensed and adsorbed n-octane on a Cu(110) surface

The electronic structure of n-octane adsorbed on Cu(110) is studied by using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) in combination with cluster model calculations in the framework of density functional theory (DFT). The molecule is found to be well oriented on the surface, which is seen from the high degree of XAS dichroism. Saturated hydrocarbons are commonly considered to physisorb on metals such as Cu(110), but still the C 1s XAS spectra reveal large changes in the electronic structure of the adsorbed octane relative to the free molecule. We find that the XAS resonances corresponding to the molecular Rydberg-valence states are strongly quenched upon adsorption and that there is a significant hybridization of the molecular valence orbitals with the metal bands. In addition to a precise interpretation of the XAS spectra, we present details on the molecular orbital structure of the adsorbed octane molecule. We also discuss shifts in the relative binding energies of the chemically inequivalent carbon atoms in octane upon adsorption, which lead to a narrower XPS spectrum for the adsorbate than the condensed phase spectrum due to the existence of a new relaxation channel.