Lone-pair Band Research Articles

Ab Initio Discovery of Stable Double Perovskite Oxides Na2BIO6 (B = Bi, In) with Promising Optoelectronic Properties.

Recently, oxide perovskites are garnering tremendous attention from the scientific community as possible alternatives to the currently used active materials in photovoltaic (PV) and photoelectrochemical (PEC) devices. Herein, we report the stability and promising optoelectronic properties of a few previously unexplored periodates A2BIO6 (A = alkali metal; B = Bi, Sb, In, Tl, Ga). Our compositional phase diagram analysis reveals two compounds Na2BIO6 (B = Bi, In) that stabilize in monoclinic phase at thermodynamic equilibrium, showing band gaps (Eg) in the visible region. Band engineering via alloying Bi in Na2InIO6 introduces Bi 6s lone pair bands above the valence band maxima (VBM), while alloying Tl in Na2BiIO6 introduces an intermediate band (Tl s character) below the conduction band minima. These alloys, Na2Bi0.25In0.75IO6 and Na2Tl0.25Bi0.75IO6, acquire Eg's of 1.66 and 1.78 eV, respectively. Apart from band gap, the antibonding VBM, favorable optical absorption, highly dispersive band edges, and well-positioned VBM (for efficient oxygen evolution reaction) make these compounds highly promising for PV and PEC applications.

How High Valence Transition Metal Spreads Its Activity over Nonmetal Oxoes: A Proof-of-Concept Study

Here we present a comprehensive survey on identification of the active sites for n-butane activation over binary vanadium phosphorus oxides. Density functional theory calculations show that the activity can be spread over all P═O sites through −OP– chain(s). With an increase in the −OP– chain(s), the activities are gradually decayed. We demonstrate that such a tendency can be quantitatively described by the center of P═O lone-pair band (elp).

The origin of electronic band structure anomaly in topological crystalline insulator group-IV tellurides

AbstractGroup-IV tellurides have exhibited exotic band structures. Specifically, despite the fact that Sn sits between Ge and Pb in the same column of the periodic table, cubic SnTe is a topological crystalline insulator with band inversion, but both isovalent GeTe and PbTe are trivial semiconductors with normal band order. By performing first-principles band structure calculations, we unravel the origin of this abnormal behaviour by using symmetry analysis and the atomic orbital energy levels and atomic sizes of these elements. In group-IV tellurides, the s lone pair band of the group-IV element is allowed by symmetry to couple with the anion valence p band at the L-point, and such s–p coupling leads to the occurrence of bandgap at the L-point. We find that such s–p coupling is so strong in SnTe that it inverts the band order near the bandgap; however, it is not strong enough in both GeTe and PbTe, so they remain normal semiconductors. The reason for this is the incomplete screening of the core of the relatively tight-binding Ge 4s orbital by its 3d orbitals and the large atomic size and strong relativistic effect in Pb, respectively. Interestingly, we also find that the rhombohedral distortion removes the inversion symmetry and the reduced s–p coupling transforms the α-SnTe back to a normal semiconductor. Our study demonstrates that, in addition to spin–orbital coupling, strain and interface dipole fields, inter-orbital coupling is another effective way to engineer the topological insulators.

Accurate calculation of the ionization energies of the chlorine lone pairs in 1,1,1-trifluoro-2-chloroethane (HCFC-133a).

The vertical ionization energies of the chlorine lone pairs in HCFC-133a have been calculated at the SCF (via Koopmans' theorem and including orbital relaxation) and correlated (ROMP2, OVGF, and ROCCSD(T)) levels. Dunning aug-cc-pVXZ (X = D, T, and Q) basis sets were employed, and the ROMP2 and ROCCSD(T) results were extrapolated to the complete basis set (CBS) limit. Our highest-level results (obtained at the ROCCSD(T)/CBS level) were 11.99 and 12.08 eV for the Cl lone pairs of A″ and A' symmetry, respectively. The values obtained at the computationally much less demanding ROMP2/CBS level were just 0.10 and 0.13 eV higher than the highest-level ones. Using the Cl lone-pair band of the photoelectron spectrum of the HCF(2)Cl and CF(3)Cl molecules as a guide, it is considered very unlikely that these two lone pairs can be discriminated in the photoelectron spectrum of the title molecule. The use of the calculated IPs to estimate the energies of the Rydberg states of HCFC-133a is also discussed.

Role of Interchain Interaction in Determining the Band Gap of Trigonal Selenium: A Density Functional Theory Study with a Linear Combination of Bloch Orbitals

The role of interchain interactions in determining the band gap of trigonal selenium (t-Se) is studied by expanding the orbital of t-Se as a linear combination of Bloch orbitals (LCBO) of isolated Se chains (IC-Se) within the plane-wave density functional theory method. Using the LCBO, it is revealed that anisotropic interchain bonds are formed by charge transfer from the lone-pair band to the σ* band and that the band gap is located at the point in reciprocal space where the repulsion interaction is large and the charge transfer interaction is at a minimum. The interchain interactions contributing to the band orbitals responsible for the band gap are satisfactorily interpreted based on the interorbital interactions among IC-Se with a 3-fold helical structure in the Brillouin zone of the hexagonal symmetry.

Spectroscopic properties of nitrogen doped hydrogenated amorphous carbon films grown by radio frequency plasma-enhanced chemical vapor deposition

Nitrogen doped hydrogenated amorphous carbon thin films have been deposited by rf plasma-enhanced chemical vapor deposition using CH4 as the source of carbon and with different nitrogen flow rates (N2/CH4 gas ratios between 0 and 3), at 300 K. The dependence modifications of the optical and the structural properties on nitrogen incorporation were investigated using different spectroscopic techniques, such as, Raman spectroscopy, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, ultraviolet-visible (UV-VIS) spectroscopy, electron spin resonance (ESR), photoluminescence (PL) and spectroscopic ellipsometry (SE). Raman spectroscopy and IR absorption reveal an increase in sp2-bonded carbon or a change in sp2 domain size with increasing nitrogen flow rate. It is found that the configuration of nitrogen atoms incorporated into an amorphous carbon network gradually changes from nitrogen atoms surrounded by three (σ bonded) to two (π bonded) neighboring carbons with increasing nitrogen flow rate. Tauc optical gap is reduced from 2.6 to 2.0 eV, and the ESR spin density and the peak-to-peak linewidth increase sharply with increasing nitrogen flow rate. Excellent agreement has been found between the measured SE data and modeled spectra, in which an empirical dielectric function of amorphous materials and a linear void distribution along the thickness have been assumed. The influence of nitrogen on the electronic density of states is explained based on the optical properties measured by UV-VIS and PL including nitrogen lone pair band.

Long-range sigma-pi interactions in tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes). Photo-electron spectroscopy, ab initio SCF MO calculations, and natural bond orbital analyses

Long-range sigma-pi interactions in tetrahydro4H-thiopyran end-capped oligo(cyclohexylidenes) were identified by He(I) photoelectron spectroscopy (PES) and ab initio RHF/6-31G* calculations. The vertical ionization energies Ivj of the highest occupied molecular orbitals (MO's) were assigned using Koopmans' theorem (Iv,j = -epsilonj) and by correlation with the ionizations of related reference compounds. The experimental (PES) and theoretical (RHF/6-31G*) results are in good agreement. For tercyclohexylidene derivatives which contain two nonconjugated pi-bonds splittings deltaIv,j of the pi-bands in the range from approximately 0.5 to 0.7 eV (delta-epsilonj approximately 0.6 to 0.9 eV). For the bi- and tercyclohexylidene compounds containing two sulfur atoms at their alpha- and omega-end positions the pi-type sulfur lone pair bands [Lppi(S)] split significantly by deltaIvj approximately 0.3 to 0.4 eV (delta-epsilonj approximately 0.3 to 0.4 eV), i.e. sigma-pi interactions over distances of ca. 8 and 12 A, respectively, occur. The magnitude of the interactions and the observed splittings are independent of the anti and syn conformations of the oligo(cyclohexylidene) hydrocarbon skeletons. RHF/6-31G* Natural Bond Orbital analyses reveal that the Hax-C-C-Hax precanonical MO's (PCMO's) centered on the cyclohexyl-type rings are paramount for the relay of the through-bond sigma-pi interactions; no through-space sigma-pi interactions were identified.

Structural and Electronic Properties of Expanded Selenium in the Liquid-Vapor Supercritical Region

The purpose of this work is to give a theoretical explanation of the nonmetal-to-metal (NM-to-M) transition observed in liquid Se when the system is expanded by increasing temperature in the liquid-vapor supercritical region. The NM-to-M transition accompanying the expansion of the system is unexpected since it is in contradiction to the established understanding of NM-to-M transitions. We introduce structural models for expanded Se and calculate the electronic states and the wave functions of these models by the simulated annealing method. From the results of our calculations, we show that, when some of Se–Se bonds are weakened in the process of expanding the system or equivalently in the process of decreasing the density, the splitting between the bonding and anti-bonding level is reduced and consequently the anti-bonding band is relatively lowered, which leads to the disappearance of the band gap between the anti-bonding and lone-pair band, or in other words to the occurrence of the NM-to-M transition.

Structure and bonding in liquid tellurium.

The atomic structure and bonding mechanism in liquid tellurium have been investigated by a tight-binding Monte Carlo simulation. On melting, the chain structure of the crystal is preserved in spite of some significant changes in the local atomic environment. A third covalent bond appears with a bond length (widely distributed around 3.15 \AA{}) intermediate between those characteristic of the crystal. A short-long alternation of the bonds takes place within the chains, in agreement with the most recent extended x-ray-absorption fine structure measurements. In addition, the bond angle within the chains is reduced. Our calculations clearly prove that these effects are due to the electronic interaction between the lone pair orbitals. The subsequent broadening of the lone pair band is responsible for the semiconductor to metal transition that takes place upon melting. \textcopyright{} 1996 The American Physical Society.

The optical properties of liquid chalcogens

The temperature and pressure variations of the optical reflectivity of liquid Te, Se, Se 80Te 20 and Se 80S 20 have been measured in the energy range of 0.5–5.4 eV. The optical conductivity, σ(ω), has been derived by using an oscillator fitting analysis. The σ(ω) curve of liquid Te, which has a stacked planar zigzag geometry, has a broad peak around 2.2 eV and a Drude term on the low-energy side. The peak can be mainly ascribed to the transition from the bonding σ band to the antibonding σ ∗ band and from the filled σ ∗ band to the unfilled σ ∗ band. On the other hand, the σ(ω) curve of liquid Se at 500°C has a broad peak around 3.5 eV, maintaining a semiconducting character. The peak is assigned to the transition from the lone-pair band to the σ ∗ band. With increasing temperature, the peak shifts to the lower energy side. The optical gap closes between 1100 and 1300°C and a Drude term appears. This implies that the structure of liquid Se changes from the spiral chain geometry to the geometry of the metallic liquid Te. The temperature of the semiconductor-metal transition is lowered by replacing Se with Te.

Electronic structure of silicon nitride

Silicon Nitride is found to have a valence band maximum of nitrogen lone pair p electrons because of the planar nitrogen site. This contrasts with the usual lone pair semiconductors, such as SiO2, caused by a p4 valence configuration. Consequently although the valence band density of electron states shows a lone pair band and a deeper bonding band as usual, impurities have a greater effect in the nitride than in conventional lone pair semiconductors. Hole transport is also discussed.

Study of microstructure of amorphous Ge-Se films using spectroscopic ellipsometry

The dielectric function spectra ε ̃ = ε 1 − iε 2 of evaporated amorphous Ge-Se films in the energy range 2–5.2 eV have been studied using spectroscopic ellipsometry (SE). The ε̃ spectra of Ge xSe 1 − x (0.2 ⩽ x ⩽ 0.33) films exhibit a two-peak behaviour showing the presence of a well-separated lone-pair band on top of the valence band. The effect of composition, angle of deposition and photoexposure have been studied. Detailed analyses of SE data have been carried out to yield quantitative information on the microstructure of the films.

Surface band dispersion of Ge(111)c(2×8) and Ge(111):As 1×1

The surface band dispersions of Ge(111)c(2×8) and Ge(111):As 1×1 were determined experimentally using angle resolved photoemission. The clean c(2×8) reconstructed Ge(111) surface exhibited a surface dispersion with a 2×2 periodicity, suggesting that the surface has a 2×2 local geometry. Addition of As to the surface converted the surface symmetry to 1×1 and produced a strongly dispersing surface state with a bandwidth of 1.7 eV. Energy minimization calculations based on the first principles pseudopotential method and the local density approximation were used to determine the structure of Ge(111):As 1×1. The dispersion of the lone pair band was calculated and compared to the experimental results. The agreement found between experiment and theory allows us to conclude that arsenic interaction with the Ge(111) surface results in the replacement of the outer Ge layer with a single As layer.

A Through Space/Bond Interaction Analysis on the Shape of Band Structure of Polynitrilomethylidyne

Abstract The concept of the through space/bond interaction presented by Hoffmann et al. was extended to study electronic states of polymers. The band shape of lone pair orbitals was investigated by truncating converged Fock matrices and overlap integrals except for the terms corresponding to the through bond and particular through space interactions. This method was used by applying the ab initio tight-binding crystal orbital method to three isomers of one-dimensional polynitrilomethylidyne, for which the dependences of the lone pair orbital energy on the wave number vector differ significantly from one another. The band shape of the lone pair orbital is related intimately to the interaction with H atoms. The effect of each through space interaction on the band shape of the lone pair orbital may be reasonably interpreted in terms of the symmetry of crystal orbitals. This method seems expedient for analyzing qualitatively lone pair bands of various polymers at the ab initio level.

Photoelectron and Penning ionization electron spectroscopic investigation of some silazanes

Penning ionization electron spectroscopy and CNDO/S calculations have been successfully applied to the analyses of the UV photoelectron spectra of some silazanes. The relative intensities of the lone pair bands are enhanced in the Penning spectra compared with those in the UV photoelectron spectra. The intensity differences between the two spectra give useful informations for the investigation of the σ-electron systems.

Angle-resolved photoelectron spectroscopy of intramolecular hydrogen bond systems: 2-chloroethanol and 2-bromoethanol

He(I) photoelectron angular distributions of typical intramolecular hydrogen bond systems, 2-chloroethanol and 2-bromoethanol, were measured. The intramolecular hydrogen bonds were found not to influence the photoelectron angular distributions for the halogen atom lone pair bands of these compounds. This result is useful for spectral band assignments.

The electronic structure and the solid state properties of the one-dimensional tetraza porphin nickel (II ) system. A self-consistent-field Hartree-Fock crystal-orbital approach based on a semiempirical Hamiltonian

The electronic structure and the solid state properties of tetraza porphin nickel (II), 1, have been studied by means of self-consistent field (SCF) Hartree-Fock (HF) crystal-orbital (CO) calculations that are derived in the framework of a semiempirical INDO Hamiltonian. The iterative SCF procedure is based on an algorithm where Hartree damping of the bond-order matrices and a direct steepest-descent formalism have been combined in order to avoid SCF difficulties in the CO method as a result of the closely spaced occupied and virtual Fermi seas. The diatomic intercell energies are mainly determined by electrostatic contributions. The NiNi coupling is highly repulsive, large stabilising two-centre pairs are found for the interaction between the 3d centre of the reference cell and the heteroatoms (N) in the neighbouring fragments. A large number of ligand, pi , lone-pair and sigma bands are predicted to be on top of dispersion curves with substantial Ni3d character. Oxidation processes in 1 are ligand-centred leading to organic pi radical cations. The INDO CO results for 1 are compared with experimental data derived for nickel phthalocyanine, NiPc, and halogenated NiPc derivatives.

Photoelectron and penning electron spectroscopic investigation of phenylhalosilanes

Abstract Penning ionization electron spectroscopy and CNDO/S calculations have been applied succesfully to the analyses of the UV photoelectron spectra of some phenyl-fluoro- and -chloro-silanes. The relative intensities of the π-type and the lone pair bands are enhanced in the Penning spectra compared with those of the UV photoelectron spectra.

The band structure of tetrathiosquaratonickel(II)

The band structure of tetrathiosquaratonickel(II), TTSqNi, is studied in the framework of the crystal orbital formalism based on the tight-binding approximation. Computational background is an improved semiempirical INDO model for transition metal compounds. The metal-ligand-metal arrangement in the direction of the stacking axis prevents the formation of broader energy bands. The calculated values for the bandwidth of the filled and empty valence bands are found between ca. 0.1 eV and 1.0 eV. Valence and conduction bands of the polymer are ligand π π ∗ functions with negligible Ni 3d admixtures. Various ligand π and sulfur lone-pair bands are predicted on top of the four occupied Ni 3d bands of the square-planar d 8 center. Strong electron-lattice interactions are expected for the conduction processes in the rather narrow (0.13 eV) conduction band.

A CNDO/INDO crystal orbital model for transition metal polymers of the 3d series?the band structure of nickel(II)glyoximate

The band structure of nickel(II)glyoximate has been investigated by means of an INDO crystal orbital formalism based on the tight-binding approximation. Calculations have been performed for unit cell dimensions of 3.223 A which corresponds to the geometry of the partially oxidized chain and of 3.547 A for the unoxidized polymer. The convergence of the lattice sums has been studied. It is shown that resonance and classical electrostatic integrals lead to a fast convergence while the exchange contributions are only slowly reduced with increasing intercell separation. The rotational profile of the title compound shows a pronounced minimum for the staggered 90 ° conformation; this is in line with experimental X-ray data. The conformation of the low-dimensional system is determined by the maximization of stabilizing intercell energies. The intracell stabilization is largest for an orientation where the intercell coupling is most inefficient (α = 50 °−60 °). Ligand π and lone-pair bands are predicted on top of the various Ni bands in the INDO band structure calculations. This sequence is conserved even if the creation of localized 3d states with trapped valences upon partial oxidation is considered. The ground state of the partially oxidized polymer corresponds to a system where charge density from the ligand framework has been removed leading to a typical organic metal or semimetal. The computational results are compared with experimental observations.