Antibonding Character Research Articles

Absolute deformation potentials and absolute energy levels of III-N, ZnO, and II-IV-N2 semiconductors for optoelectronic applications

Absolute deformation potentials and absolute energy levels for III-N, ZnO, and II-IV-N2 semiconductors are systematically determined from hybrid-functional calculations. Separate bulk and slab calculations are combined and the vacuum level is taken as the common reference. The trends in the absolute deformation potentials are rationalized by the kinetic energy effect and the bonding (or antibonding) character of the band edge states. The calculated absolute energy levels can be used to obtain the natural band alignment between these semiconductors and are in accordance with the available results. The determined parameters are of practical importance to the optoelectronic devices designs.

Superlattices, Bonding-Antibonding, Fermi Surface Nesting, and Superconductivity

Raman and synchrotron THz absorption spectral measurements on MgB2 provide experimental evidence for electron orbital superlattices. In earlier work, we have detected THz spectra that show superlattice absorption peaks with low wavenumbers, for which spectral density evolves and intensifies after cooling below the superconducting transition temperature for MgB2. In this work, we show how these observations indicate a direct connection to superconducting properties and mechanisms. Bonding–antibonding orbital character is identified in calculated electronic band structures and Fermi surfaces consistent with superlattice structures along the c-axis. DFT calculations show that superlattice folding of reciprocal space generates Brillouin zone boundary reflections, Umklapp processes, and substantially enhances nesting relationships. Tight binding equations are compared with expected charge density waves from nesting relationships and adjusted to explicitly accommodate these linked processes. Systematic analysis of electronic band structures and Fermi surfaces allows for direct identification of Cooper pairing and the superconducting gap, particularly when the k-grid resolution of a calculation is suitably calibrated to structural parameters. Thus, we detail a robust and accurate DFT re-interpretation of BCS superconductivity for MgB2.

Lattice Dynamics and Thermal Transport in Semiconductors with Anti-Bonding Valence Bands.

Achieving high thermoelectric performance requires efficient manipulation of thermal conductivity and a fundamental understanding of the microscopic mechanisms of phonon transport in crystalline solids. One of the major challenges in thermal transport is achieving ultralow lattice thermal conductivity. In this study, we use the anti-bonding character of the highest-occupied valence band as an efficient descriptor for discovering new materials with an ultralow thermal conductivity. We first examined the relationship between anti-bonding valence bands (ABVBs) and low lattice thermal conductivity in model systems PbTe and CsPbBr3. Then, we conducted a high-throughput search in the Materials Project database and identified over 600 experimentally stable binary semiconductors with an anti-bonding character in their valence bands. From our candidate list, we conducted a comprehensive analysis of the chemical bonds and the thermal transport in the XS family, where X = K, Rb, and Cs are alkaline metals. These materials all exhibit ultralow thermal conductivities less than 1 W/(m K) at room temperature despite simple structures. We attributed the ultralow thermal conductivity to the weakened bonds and increased phonon anharmonicity due to their ABVBs. Our results provide chemical intuitions to understand lattice dynamics in crystals and open up a convenient venue toward searching for materials with an intrinsically low lattice thermal conductivity.

Elastic, optoelectronic and photocatalytic properties of semiconducting CsNbO3: first principles insights

The cubic phase of CsNbO3 (CNO) perovskite has been hypothesized to investigate the elastic, electronic, photocatalytic, and optical properties for various technological applications using first-principles method. The pressure dependent structural stability has been confirmed from computed elastic constants. Relatively high value of elastic moduli, large hardness and toughness suggested that CNO would be applicable to design industrial machineries. The ductile to brittle transition is noticed at 20 GPa. The indirect bandgap of CNO proclaims its suitability for photovoltaic and IR photodetector applications. The total and partial density of states are calculated to show in evidence the contribution of individual atomic orbitals in the formation of bands. The pressure changes orbitals hybridization which can be substantiated by the change in the bandgap. Strong covalency of the Nb–O bond and antibonding character of Cs–O have been anticipated by the Mulliken population analysis and by the contour maps of electron charge density. The low carrier effective mass and high mobility carriers predict the good electrical conductivity of the material. The calculated values of conduction and valance band edge potential illustrate the excellent water-splitting and environmental pollutants degradation properties of CNO.

Negligible Bowing Effect of Bandgap and Lattice Constant in a Variety of Compositions using Large Tilt Distortion in a Cesium–Lead Mixed‐Halide System

AbstractHalide perovskites have broad bandgap tunability, making them suitable for diverse applications in optoelectronics and photovoltaics. The optical bandgap of halide perovskites varies almost linearly with the halide ionic size, and therefore, it can be controlled through compositional engineering. However, the mechanism underlying this low‐bandgap variation is not yet fully understood. Thus, this study comprehensively investigates the bandgap bowing of cesium–lead mixed‐halide perovskites using compositional engineering and demonstrates that bandgap bowing is extremely small in a variety of compositions including Cl, Br, and I. Subsequently, through density functional theory calculations, it is suggested that the antibonding character of the valence band maximum, tilt distortion of halide ions, and entropy effect leading to equal participation of various halide ions around Pb are collectively responsible for the small bandgap bowing.

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4.

The new phase Mg7Pt4Ge4 (≡Mg8□1Pt4Ge4; □ = vacancy) was prepared by reacting a mixture of the corresponding elements at high temperatures. According to single crystal X-ray diffraction data, it adopts a defect variant of the lighter analogue Mg2PtSi (≡Mg8Pt4Si4), reported in the Li2CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg7Pt4Ge4. However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg2PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg2PtGe" reveal potential electronic instabilities at EF in the band structure and significant occupancy of states with an antibonding character resulting from unfavorable Pt-Ge interactions. These antibonding interactions can be eliminated through introduction of Mg defects, which reduce the valence electron count, leaving the antibonding states empty. Mg itself does not participate in these interactions. Instead, the Mg contribution to the overall bonding comes from electron back-donation from the (Pt, Ge) anionic network to Mg cations. These findings may help to understand how the interplay of structural and electronic factors leads to the "hydrogen pump effect" observed in the closely related Mg3Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.

Self-Accommodation Induced Electronic Metal-Support Interaction on Ruthenium Site for Alkaline Hydrogen Evolution Reaction.

Tuning the metal-support interaction of supported metal catalysts has been found to be the most effective approach to modulating electronic structure and improving catalytic performance. But practical understanding of the charge transfer mechanism at the electronic level of catalysis process has remained elusive. Here, it is reported that ruthenium (Ru) nanoparticles can self-accommodate into Fe3 O4 and carbon support (Ru-Fe3 O4 /C) through the electronic metal-support interaction, resulting in robust catalytic activity toward the alkaline hydrogen evolution reaction (HER). Spectroscopic evidence and theoretical calculations demonstrate that electronic perturbation occurred in the Ru-Fe3 O4 /C, and that charge redistribution directly influenced adsorption behavior during the catalytic process. The RuO bond formed by orbital mixing changes the charge state of the surface Ru site, enabling more electrons to flow to H intermediates (H* ) for favorable adsorption. The weak binding strength of the RuO bond also reinforces the anti-bonding character of H* with a more favorable recombination of H* species into H2 molecules. Because of this satisfactory catalytic mechanism, the Ru-Fe3 O4 /C supported nanoparticle catalyst demonstrated better HER activity and robust stability than the benchmark commercial Pt/C benchmark in alkaline media.

Why does thionating a carbonyl molecule make it a better electron acceptor?

The past decade has witnessed a surge of biomedical and materials applications of thiocarbonyl molecules (R2CS), such as in photodynamic therapy, organic field-effect transistors, and rechargeable batteries. The success of these applications originates from thiocarbonyl's small optical gap in the visible region and the enhanced electron affinity compared to the carbonyl analogues (R2CO). Although these observations seem to be contrary to the implication based on a simple electronegativity consideration (2.58 for sulfur and 3.44 for oxygen), a natural bond orbital (NBO) analysis gives a straightforward explanation for the LUMO-lowering effect of CO → CS substitution. In comparison to the valence (2p)C/(2p)O interactions in CO, the higher 3p orbital of sulfur and its weaker overlap with the 2p level of carbon result in a weaker antibonding interaction in NBO, a prominent contributor to the LUMO. Such an analysis also provides a semi-quantitative understanding of the electronic effect of substituents on or in π-conjugation with a (thio)carbonyl functionality. The intuitive concepts uncovered here offer a simple rule to predict the electronic properties of π-conjugated molecules that incorporate heavy heteroelements and would facilitate materials development.

Valence-Inverted States of Nickel(II) Complexes Perform Facile C-H Bond Activation.

Valence-inverted reactivity (VIR) is discovered here through high-level computations of excited states of Ni(II) complexes that are generated by triplet energy transfer. For example, the so-generated 3[(Ar)(bpy)NiII(Br)] species possesses a valence-inverted occupancy, dxy1dxz1dx2-y22, wherein the uppermost dx2-y2 orbital is metal-ligand antibonding. This state promotes C-H bond activation of THF and its cross-coupling to the aryl ligand. Thus, due to the metal-ligand antibonding character of dx2-y2, the dxy1dx2-y22 subshell opens a Ni-coordination site by shifting the bidentate bipyridine ligand to monodentate plus a dangling pyridine. The tricoordinate Ni(II) intermediate inserts into a C-H bond of THF, transfers a proton to the dangling pyridine moiety, and eventually generates an arylated THF by reductive-coupling. The calculated high kinetic isotope effect is in accord with experiment, both revealing C-H activation. The VIR pattern is novel, its cross-coupling reaction is highly useful, and it is generally expected to occur in other d8 complexes.

Effect of bonding and antibonding character of electronic states on their tunneling spectra

We argue that the bonding or antibonding character of electronic states of a substrate can influence significantly the observed scanning tunneling microscopy (STM) spectra. In particular, the antibonding states formed by nonorthogonal orbitals of the substrate present much stronger extension in the vacuum compared to corresponding bonding combinations and are therefore much more visible in STM experiments. A clear example is provided by black phosphorus (BP), which is one of the most interesting two-dimensional layered materials nowadays. The pronounced asymmetry in its conductance spectra, with significant increase at positive voltage, can be attributed to the antibonding nature of the conduction band compared to the bonding valence band, despite the same ${p}_{z}$-orbital character. Furthermore, this asymmetry can be at the origin of different broadenings of frontier molecular orbitals observed experimentally for molecules on BP.

Photoinduced bond oscillations in ironpentacarbonyl give delayed synchronous bursts of carbonmonoxide release

Early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed mechanistic details in the sub-picosecond regime. Hence, in this study the photoexcitation of ironpentacarbonyl Fe(CO)5 is simulated with semi-classical excited state molecular dynamics. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces synchronous Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)5 acts as a CO geyser, as a result of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered (MC) excited states. The predominant release of axial CO ligands and delayed release of equatorial CO ligands are explained in a unified mechanism based on the σ*(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states.

Theoretical study of generalized oscillator strengths for the low-lying electronic excitations of CH3Cl and CF3Cl

We report a theoretical study of electronic excitation in CH3Cl and CF3Cl by electron impact. Momentum-transfer-dependent generalized oscillator strengths (GOSs) are calculated for transitions to low-lying excited singlet-states at the equation-of-motion coupled-cluster singles and doubles level. The influence of molecular vibration is taken into account in the calculation. The theoretical results show reasonable overall agreement with experimental data reported in the literature. The shapes of the GOS profiles reveal that the 1 1E state of CH3Cl has a valence-Rydberg mixed nature, while that of CF3Cl is of a predominant C–Cl antibonding character. A comparison with the experimental GOSs of CH3Cl provides unambiguous evidence that the 3pe state is lower in energy than the 3pa1 state. Optical oscillator strengths are also calculated and comparison is made with available experimental and other theoretical results.

Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering.

The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

Local ordering and interatomic bonding in magnetostrictive Fe[formula omitted]Ga[formula omitted]X (X=Ni,Cu,Co,La) alloy

The electronic structure of doped Fe–Ga alloy is investigated theoretically using methods of the density functional theory. The work aims to uncover the difference in the material’s magnetostrictive properties when doping with transition metals or La as the simplest representative of rare-earth elements. The effect understudy was investigated by considering the change in two features of the electronic structure caused by doping: the density of d-states on Fe atoms and the nature of the Fe–Fe bonding near the doping atom. The transition metal atoms are surrounded by Fe atoms with a low-density d-states, eliminating the doping effect. The interatomic bonds of transition metals with the nearest Fe atoms have an antibonding character, while bonds between Fe atoms of the first and second coordination spheres are more binding than in an undoped alloy. This effect leads to magnetostriction decrease. In the La-doped alloy, the opposite picture is found. An increase in magnetostriction should occur due to the enhancement of the antibonding character of bonds between Fe atoms in the first and second coordination spheres of the dopant.

Peculiarities of Phase Formation in Mn-Based Na SuperIonic Conductor (NaSICon) Systems: The Case of Na1+2x Mn x Ti2-x (PO4)3 (0.0 ≤ x ≤ 1.5).

NAtrium SuperIonic CONductor (NASICON) structured phosphate framework compounds are attracting a great deal of interest as suitable electrode materials for “rocking chair” type batteries. Manganese-based electrode materials are among the most favored due to their superior stability, resource non-criticality, and high electrode potentials. Although a large share of research was devoted to Mn-based oxides for Li- and Na-ion batteries, the understanding of thermodynamics and phase formation in Mn-rich polyanions is still generally lacking. In this study, we investigate a bifunctional Na-ion battery electrode system based on NASICON-structured Na1+2xMnxTi2–x(PO4)3 (0.0 ≤ x ≤ 1.5). In order to analyze the thermodynamic and phase formation properties, we construct a composition–temperature phase diagram using a computational sampling by density functional theory, cluster expansion, and semi-grand canonical Monte Carlo methods. The results indicate finite thermodynamic limits of possible Mn concentrations in this system, which are primarily determined by the phase separation into stoichiometric Na3MnTi(PO4)3 (x = 1.0) and NaTi2(PO4)3 for x < 1.0 or NaMnPO4 for x > 1.0. The theoretical predictions are corroborated by experiments obtained using X-ray diffraction and Raman spectroscopy on solid-state and sol–gel prepared samples. The results confirm that this system does not show a solid solution type behavior but phase-separates into thermodynamically more stable sodium ordered monoclinic α-Na3MnTi(PO4)3 (space group C2) and other phases. In addition to sodium ordering, the anti-bonding character of the Mn–O bond as compared to Ti–O is suggested as another important factor governing the stability of Mn-based NASICONs. We believe that these results will not only clarify some important questions regarding the thermodynamic properties of NASICON frameworks but will also be helpful for a more general understanding of polyanionic systems.

Function of Doping Ru Element in the Hydrogen Evolution Reaction in Rare-Earth Transition-Metal Intermetallics.

Transition metal-based intermetallics are promising electrocatalysts for replacing the commercial Pt metal in the hydrogen evolution reaction (HER). In this work, RENi2 and RERu0.25Ni1.75 (RE = Pr, Tb, and Er) were synthesized and their electrocatalytic HER activities were explored. Among undoped compounds, PrNi2 exhibits the best performance and requires an overpotential of 55 mV, while partially replacing Ni with Ru element (PrRu0.25Ni1.75) can greatly reduce the overpotential to 20 mV at a current density of 10 mA/cm2. Such enhancement was recognized that belongs to their extrinsic property, and their intrinsic HER activities were similar after normalizing the electrocatalytic surface area. Further investigation on ScM2 and ScRu0.25M1.75 (M = Co and Ni) suggests that doping Ru element in ScCo2 will significantly enhance antibonding character around the Fermi level (EF) and weaken hydrogen adsorption energy. On the other hand, the antibonding population for ScNi2 and ScRu0.25Ni1.75 is similar at EF, which accounts for their close intrinsic HER activities.

Temperature-dependent structural fluctuation and its effect on the electronic structure and charge transport in hybrid perovskite CH3 NH3 PbI3.

The recently discovered hybrid organic-inorganic perovskites have been suggested for high-performance optoelectronic applications. Owing to the mechanical flexibility of these compounds, they demonstrate structural fluctuation at finite temperatures that have been widely discussed with respect to their optical properties. However, the effect of temperature-induced structural fluctuation is not clear until now, with respect to the equally important charge transport properties. In the present study, through ab initio molecular dynamics simulations of cubic-phase CH3 NH3 PbI3 at different temperatures, the temperature-dependent electronic structure and charge carrier transport properties are examined. Compared with the significant structural fluctuation of organic cations, the structural change of the inorganic framework is minor. In addition, because the band edge states at R point are mainly influenced by the anti-bonding character of the Pb-I bond, CH3 NH3 PbI3 demonstrates relatively small deformation potentials as well as low temperature dependence of band gaps (ΔEg ≈ 50 meV from 330 K to 400 K) and electron-phonon coupling strengths, despite the large structural fluctuation of organic cations. Furthermore, the effective mass of the valence band increases with the increase of temperature. The predicted mobilities of CH3 NH3 PbI3 can reach above 75 cm2 V-1 s-1 near room temperature, exhibiting an appropriate optoelectronic potential, while the temperature dependence is steeper than T-1.5 of the traditional semiconductors because of the enhanced effective masses.

Origin of anomalous band-gap bowing in two-dimensional tin-lead mixed perovskite alloys

The origin of the pronounced and composition-dependent band-gap bowing in Sn/Pb mixed perovskite alloys has been under debate for a long time. Previous studies reported conflicting results on whether the chemical or structural effect is the dominant mechanism. In this paper, the band-gap bowing effect and its possible origins in recently synthesized two-dimensional (2D) ${\mathrm{Cs}}_{2}{\mathrm{Pb}}_{x}{\mathrm{Sn}}_{1\text{\ensuremath{-}}x}{\mathrm{I}}_{2}{\mathrm{Cl}}_{2}$ alloys are investigated from first-principles calculations. In agreement with experiments, a large and composition-dependent bowing coefficient is observed. By analyzing the contribution from volume deformation, charge exchange, structural relaxation, and short-range order, it is found that the dominant mechanism causing the anomalous gap bowing is the structural relaxation-induced wave-function localization, forming isovalent-defect-like states, despite the negligible octahedral distortion and small lattice mismatch between the two end compounds. This is understood by the s-p repulsion-induced strong antibonding character of the valence-band maximum which leads to a large deformation potential, thus even a small atomic displacement can result in a large shift of the energy level. These results thus highlight the critical role of strong deformation potential and structural relaxation effect in unusual band evolution of 2D Sn/Pb perovskite alloys, and can be helpful to the modulation of their band gap for optoelectronic applications.

First-principles analysis of desired inherent photovoltaic functionalities of tetragonal CuAlX2 (X=O, S, Se and Te)

Ternary AⅠBⅢC semiconductors are significant for today's electronic equipment and solar cell applications due to their intriguing properties. In this article, first-principles calculations are employed to comprehensively study the fundamental photovoltaic properties, such as the lattice thermodynamics and phonon stability, band gap, antibonding character of band-edge states, electron and hole effective masses, exciton binding energy and optical absorption coefficient of CuAlX2 (X ​= ​S, Se and Te) materials. Outcomes demonstrate that CuAlS2, CuAlSe2 and CuAlTe2 compounds have nice mechanical stability, thermodynamic and phonon stability. Compared with CuAlS2 and CuAlSe2 crystals, CuAlTe2 possesses the more appropriate band gap value, smaller binding energy, higher light absorption and utilization efficiency. Due to their high exciton binding energy and Bohr radius, CuAlS2 and CuAlSe2 are limited to solar cell absorber applications. Certainly, other properties of CuAlS2 do not meet conditions like wide band gap value, heavy carrier effective masses and low absorption coefficient. According to direct photovoltaic functionality screening, we find CuAlTe2 crystal should be a good photovoltaic absorber.

Photostability of luminescent europium(III) hexafluoroacetylacetonates: Combined theoretical and experimental study

Photostability is a critical problem during development of luminescent lanthanide matherials. In this work, four europium hexafluoroacetylacetonates has been investigated. The photostability of the compounds increased in the following sequence: K[Eu(Hfaa)4] – Rb[Eu(Hfaa)4] – Cs[Eu(Hfaa)4] – Eu(Hfaa)3∙2TPPO (Hfaa – hexafluoroacetylacetonate-anion, TPPO – triphenylphosphine oxide). Molecules in ground and excited states have been studied using the DFT/TDDFT/PBE0 method. Natural bond orbital (NBO) analysis has been carried out to understand the nature of different interactions responsible for the electron delocalization and the intramolecular charge transfer between the orbitals of ligands and Eu(III) ions. Quantum chemistry modeling has explained the reason of unique behavior of one of the coordinated Hfaa ligands, which noticeably distances from a central atom during photoexcitation. This ligand, on which HOMO is localized, has appeared to be the most photoreactive fragment of the complex and directly affect the photostability of the studied series of compounds. The maximum photostability in a number of compounds has been observed for a complex that has a TPPO ligand in its composition, which provides decreasing of HOMO's antibonding character during photoexcitation, unlike other complexes, in which this ligand is absent and the antibonding character HOMO rises. The results obtained can be used in the development of new promising optical materials with increased photostability.