Core Electronic Structure Research Articles

Enhancing detection of thermal runaway gases: Exploiting doping and vacancy effects in GeS quantum dots

This study investigates chemical modifications on Germanium Sulfide (GeS) quantum dots (QDs) for selective detection of thermal runaway gases (TRGs). We examine how doping with oxygen, phosphorus, silicon, and introducing sulfur vacancies at edge and surface regions affect the structural, electronic, and optical properties of GeS-QDs and their derivatives. Our findings reveal significant changes in bond parameters, formation energy, electronic band gap, and light absorption behavior. Oxygen doping enhances stability, while other dopants and sulfur vacancies increase reactivity towards TRGs (H2, CO, CH4, C2H4). All materials show favorable adsorption for these gases, with C2H4 displaying the strongest binding affinity. Adsorption minimally affects core electronic structure (HOMO) but shifts surface electronic states (LUMO). Sulfur vacancies reduce the energy gap, enhancing conductivity and facilitating TRGs detection. These results suggest that GeS-QDs can be effectively tuned for selective TRG detection by manipulating their chemical composition and surface characteristics.

First-principles investigation of a-line Shockley partial dislocations in wurtzite GaN: core reconstruction and electronic structure

Αtomistic simulations based on density functional theory were performed to investigate Shockley partial dislocations, parallel to the <11–20> direction, in wurtzite gallium nitride. The cores of both 30° and 90° Shockley partial dislocations, their possible reconstructions and their electronic structures were analyzed and compared systematically. Shockley partial dislocations were found to have a variety of core structures while exhibiting different bonding states like: deformed Ga–N bonds, Ga–Ga and N–N homo-nuclear bonds, as well as Ga– and N–dangling bonds. We have demonstrated that the core reconstruction of these dislocations is not always energetically favourable in wurtzite GaN, as it is the case in elemental semiconductors. It results from interplay between the energy gain from eliminating dangling and forming homo-nuclear bonds and the energy excess from the induced strain of hetero-nuclear Ga–N bonds within the core. Depending on their core configurations, Shockley partial dislocations are expected to have different electronic behaviours in wurtzite GaN, as they can act either as sources of parasitic luminescence or non-radiative recombination centres, or give pathways to leakage currents. We have demonstrated that deep states associated with dislocations are not due to the dangling bonds, as generally believed, but rather to Ga–Ga bonds in their cores, and that N–N bonds do not lead to any gap states. These results are of high relevance as guidelines in defect engineering strategies aiming at producing dislocations with a particular electronic behaviour in wurtzite GaN.

Dehydrogenation of Ammonia Borane Impacts Valence and Core Electrons: A Photoemission Spectroscopic Study.

Ammonia borane (H3BNH3) is a promising material for hydrogen storage and release. Dehydrogenation of ammonia borane produces small boron–nitrogen hydrides such as aminoborane (H2BNH2) and iminoborane (HBNH). The present study investigates ammonia borane and its two dehydrogenated products for the first time using calculated photoemission spectra of the valence and core electrons. It is found that a significant decrease in the dipole moment was observed associated with the dehydration from 5.397 D in H3BNH3, to 1.942 D in H2BNH2, and to 0.083 D in HBNH. Such reduction in the dipole moment impacts properties such as hydrogen bonding, dihydrogen bonding, and their spectra. Dehydrogenation of H3BNH3 impacts both the valence and core electronic structure of the boron–nitrogen hydrides. The calculated valence vertical ionization energy (VIE) spectra of the boron–nitrogen hydrides show that valence orbitals dominated by 2p-electrons of B and N atoms exhibit large changes, whereas orbitals dominated by s-electrons, such as (3a14a15a1/3σ4σ5σ) remain less affected. The first ionization energy slightly increases from 10.57 eV for H3BNH3 to 11.29 eV for both unsaturated H2BNH2 and HBNH. In core space, the oxidative dehydrogenation of H3BNH3 affects the core electron binding energy (CEBE) of borane and nitrogen oppositely. The B1s binding energies increase from 194.01 eV in H3BNH3 to 196.93 eV in HBNH, up by 2.92 eV, whereas the N1s binding energies decrease from 408.20 eV in H3BNH3 to 404.88 eV in HBNH, dropped by 3.32 eV.

Computational Comparative Analysis of Small Atomically Precise Copper Clusters.

Atomically precise copper clusters (APC) have attracted attention for their promise in sensing, water remediation, and electrochemical technologies. However, smaller-sized APCs and the evolution of their properties as a function of size and composition are not clearly understood. Here, we have performed an investigation into the electronic structure, geometry, and optical properties of small atomically precise copper clusters using density functional theory (DFT) and time-dependent DFT. Through comparative analysis, we show that the electronic structures of the experimentally characterized clusters, Cu4(PN(C6H5)2CH)4 and Cu4(SN2C7H11)4, are similar with the closed-shell superatom character 1S21P2. By changing the ligand on Cu4(PN(C6H5)2CH)4 and Cu4(SN2C7H11)4, there were no major changes observed in the tetrahedral Cu4 core geometry, electronic structure, or optical spectra. However, a change in the anchor atom causes an increase in the electronic gap and induces a hypochromic shift in the onset peak in the optical spectrum of the small clusters. Increasing the copper core size showed small changes Cu-Cu bond lengths, lower electronic gap values, and a bathochromic shift in the optical spectra. Computational results not only provide detailed physical insight into APCs but also aid in identifying compound compositions of small atomically precise nanoclusters from data collected in the experiment.

Experimental and Theoretical Soft X-ray Study of Nicotine and Related Compounds.

The valence and core electronic structure of nicotine, nicotinic acid, and nicotinamide have been studied by photoelectron and soft X-ray absorption spectroscopy, supported by theoretical calculations, which take into account conformational isomerism. The core-level photoionization spectra of all molecules have been assigned, and theory indicates that the effects of conformational differences are small, generally less than the natural line widths of the core ionic states. However, in the case of nicotinamide, the theoretical valence ionization potentials of cis and trans conformers differ significantly in the outer valence space, and the experimental spectrum is in agreement with the calculated outer valence cis conformer spectrum. In addition, the C, N, and O K edge near-edge absorption fine structure spectra are reported and interpreted by comparison with reference compounds. We find evidence at the N and O K edges of interaction between the delocalized orbitals of the pyridine ring and the substituents for nicotinic acid and nicotinamide. The strength of the interaction varies because the first is planar, while the second is twisted, reducing the extent of orbital mixing.

Origin of itinerant carriers in antiferromagnetic LaFe1−xMoxO3 studied by x-ray spectroscopies

We report on the electronic structure of doped $\mathrm{LaFe}{\mathrm{O}}_{3}$ at the crossover from an insulating-to-metallic phase transition. Comprehensive x-ray spectroscopic methodologies are used to understand core and valence electronic structure as well as crystal structure distortions associated with the electronic transition. Despite the antiferromagnetic (AFM) ordering at room temperature, we show direct evidence of itinerant carriers at the Fermi level revealed by resonant photoemission spectroscopy (RPES) at the Mo ${L}_{3}$ edge. RPES data taken at the Fe ${L}_{3}$ edge show spectral weight near the valence band edge and significant hybridization with O $2p$ states required for AFM ordering. Resonant inelastic x-ray scattering spectra taken across Fe ${L}_{2,3}$ edges show electron correlation effects ($U$) driven by Coulomb interactions of $d$ electrons as well as broad charge-transfer excitations for $x\ensuremath{\ge}0.2$ where the compound crosses over to a metallic state. Site substitution of Fe by Mo ions in the Fe-${\mathrm{O}}_{6}$ octahedra enhances the separation of the two Fe-O bonds and Fe-O-Fe bonding angles relative to the orthorhombic $\mathrm{LaFe}{\mathrm{O}}_{3}$, but no considerable distortions are present to the overall structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. This insulator-to-metal phase transition with AFM stability is primarily understood through intermediate interaction strengths between correlation ($U$) and bandwidth ($W$) at the Fe site, where an estimation of this ratio is given. These results highlight the important role of extrinsic carriers in stabilizing a unique phase transition that can guide future efforts in antiferromagnetic-metal spintronics.

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the band gap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states, providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of the QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.

Understanding Electron Transport in Disk-Shaped Triphenylene-Tris(naphthaleneimidazole)s through Structural Modification and Theoretical Investigation.

Disk-shaped molecules with large aromatic π-surfaces are a class of organic semiconductors in which the charge-carrier transport properties could be greatly facilitated by preferred intermolecular stacking of the π-surfaces. The optical and electronic properties are not only determined by the core aromatic structure of these disk-shaped molecules but are also strongly dependent on the side chains, which directly impact the molecular self-assembly behavior in condensed phases. Triphenylene-tris(naphthaleneimidazole) (TP-TNI) is a recently reported n-type semiconductor featuring a large π-core and branched side chains, with an electron-transporting mobility reaching 10-4 cm2 V-1 s-1. To further improve material performance, a detailed study is needed to understand the dependence of carrier transport properties on both the core electronic structure and side chain. Here, we present the detailed synthesis and characterization of a TP-TNI derivative bearing linear side chains, which has demonstrated a field-effect electron-transport mobility of up to 1.3 × 10-3 cm2 V-1 s-1. The more than 1 order improvement in electron-transport properties over the branched side chain homologue can be correlated to ordered twisted packing in the thin film, as revealed by in situ variable temperature grazing incidence wide-angle X-ray scattering studies. In-depth theoretical understanding of the frontier orbitals, reorganization energies, and charge-transfer integrals of TP-TNI molecules has provided further insight into the relationship between the molecular stacking geometry and charge-transport properties.

Multidimensional resonant nonlinear spectroscopy with coherent broadband x-ray pulses

New x-ray free electron laser (XFEL) and high harmonic generation (HHG) light sources are capable of generating short and intense pulses that make x-ray nonlinear spectroscopy possible. Multidimensional spectroscopic techniques, which have long been used in the nuclear magnetic resonance, infrared, and optical regimes to probe the electronic structure and nuclear dynamics of molecules by sequences of short pulses with variable delays, can thus be extended to the attosecond x-ray regime. This opens up the possibility of probing core-electronic structure and couplings, the real-time tracking of impulsively created valence-electronic wavepackets and electronic coherences, and monitoring ultrafast processes such as nonadiabatic electron-nuclear dynamics near conical-intersection crossings. We survey various possible types of multidimensional x-ray spectroscopy techniques and demonstrate the novel information they can provide about molecules.

Simulating electron momentum spectra of iso-C2H2Cl2: A study of the core electronic structure

Electron momentum spectroscopy (EMS) has been used for the first time to study core electronic structure of iso-C2H2Cl2. In the present work, the pronounced difference between ionization energies of two C1s core orbitals (2A1 and 3A1) is seen as a chemical shift of 3 eV, which is due to different chemical environments of the related carbon atoms. Both the calculated spherically averaged core electron momentum distributions (MDs) and three-dimensional electron momentum density maps show that these core molecular orbitals (MOs) 2A1 and 3A1 exhibit strong atomic orbital characteristics in real and momentum space. However, the core states 2B2 and 4A1, which are almost degenerate and related to two equivalent atoms, exhibit notable differences between the momentum and position depictions. In contrast to the position space, the momentum density maps of these two core MOs highlight the interference effects which are due to the nuclear positions. The 2B2 orbital of iso-C2H2Cl2 is the antisymmetric counterpart of the 4A1 core orbital in real space. However, it relates to the 4A1 orbital by an exchange of maxima and minima in momentum space. Due to interference effects between electrons scattered from different atomic centers, modulations with a periodicity of 1.12 a.u. can be seen in the computed momentum densities, which tend to decay with increasing electron momenta. Accordingly, the EMS can not only effectively image the electronic structure of compounds by studying valence orbitals, but also provides direct information on the nature of the nuclear geometry by investigating the core states.

Modulation of magnetic behavior vialigand-field effects in the trigonal clusters (PhL)Fe3L*3(L*= thf, py, PMe2Ph)

Utilizing a hexadentate ligand platform, a series of trinuclear iron clusters (PhL)Fe3L*3 (PhLH6 = MeC(CH2NPh-o-NPh)3; L* = tetrahydrofuran (1), pyridine (2), PMePh2 (3)) has been prepared. The phenyl substituents on the ligand sterically prohibit strong iron–iron bonding from occurring but maintain a sufficiently close proximity between iron centers to permit direct interactions. Coordination of the weak-field tetrahydrofuran ligand to the iron centers results in a well-isolated, high-spin S = 6 or S = 5 ground state, as ascertained through variable-temperature dc magnetic susceptibility and low-temperature magnetization measurements. Replacing the tetrahydrofuran ligands with stronger σ-donating pyridine or tertiary phosphine ligands reduces the ground state to S = 2 and gives rise to temperature-dependent magnetic susceptibility. In these cases, the magnetic susceptibility cannot be explained as arising simply from superexchange interactions between metal centers through the bridging amide ligands. Rather, the experimental data are best modelled by considering a thermally-induced variation in molecular spin state between S = 2 and S = 4. Fits to these data provide thermodynamic parameters of ΔH = 406 cm−1 and Tc = 187 K for 2 and ΔH = 604 cm−1 and Tc = 375 K for 3. The difference in these parameters is consistent with ligand field strength differences between pyridine and phosphine ligands. To rationalize the spin state variation across the series of clusters, we first propose a qualitative model of the Fe3 core electronic structure that considers direct Fe–Fe interactions, arising from direct orbital overlap. We then present a scenario, consistent with the observed magnetic behaviour, in which the σ orbitals of the electronic structure are perturbed by substitution of the ancillary ligands.

SiGe nanocrystals core and surface electronic structure from ab initio large unit cell calculations

Oxidised surface and core parts of SiGe nanocrystals are investigated using first-principle techniques that incorporate self-consistent Hartree–Fock method coupled with large unit cell (LUC) method to obtain the electronic structure of Bravais and primitive LUC multiples-based nanocrystals. Results show that lattice constant and ionicity of the core part show decreasing order as nanocrystals grow up in size. The smallest investigated nanocrystal is 2.5% larger in lattice constant and 15% larger in ionicity than the converged value of largest investigated nanocrystal. The lattice constant of both the surface and core has nearly the same value indicating good adherence of oxidised surface. Surface oxygen atoms absorb some of the electronic cloud of germanium and silicon surface atoms. Contrary to what one can expect, Si is the cation in this compound. The core energy gap increases and fluctuates as nanocrystals increase in size. The oxidised surface part on the other side has a dramatically lower gap. Oxygenated Ge- and Si-terminated nanocrystals have entirely different behaviour and properties. This includes a wider gap at the Si-terminated surface and highly ionised atoms. A bilayer with positively charged Ge and Si atoms are located at the Ge-terminated surface behind the oxygen layer. The present work suggests the addition of ionicity and lattice constant to the quantities that are affected by quantum confinement phenomenon. Some quantities, such as ionisation potential and affinity, continue to fluctuate indefinitely as nanocrystals grow up in size because of the generation of new shapes and surfaces.

Expanded redox accessibility via ligand substitution in an octahedral Fe6Br6 cluster

Oxidation of the nominally all-ferrous hexanuclear cluster ((H)L)(2)Fe(6) with six equivalents of ferrocenium in the presence of bromide ions results in a six-electron oxidation of the Fe(6) core to afford the nominally all-ferric cluster ((H)L)(2)Fe(6)Br(6). The hexabromide cluster is also structurally characterized in a 4+ core oxidation state. A structural comparison of these two clusters provides an insight into the Fe(6) core electronic structure.

Electronic structure of germanium nanocrystals core and (001)-(1×1) oxidised surface

The electronic structure of (001)-(1×1) oxidised surface and inner core of germanium nanocrystals (1.5–2.3 nm in diameter) is studied separately using the self-consistent Hartree–Fock method coupled with the large unit cell (LUC) method. The separate evaluation of surface and core electronic structure leads to the exact discrimination between nanocrystal surface and core properties. The results show that the lattice constant and the energy gap of the core part decrease as nanocrystals increase in size in agreement with previous studies. The small differences between the core and the surface lattice constant reflects good adherence of the oxide layer. The nanocrystal oxidised surface is characterised by a smaller gap and a wider valence band than the core. This result is owing to the splitting of the highly degenerate core states as they approach the surface in addition to the newly added levels of oxygen surface atoms. The present two gap results resolved the ambiguity in different gap and photoluminescence results of Ge nanocrystals. The surface layers have oscillatory negative and positive charges that diminish as they reach the inner fourth germanium layer. LUC method predicts nearly direct transitions in small germanium nanocrystals that explain their suitability for light-emitting applications.

Surface and Core Electronic Structure of Oxidized Silicon Nanocrystals

Ab initio restricted Hartree‐Fock method within the framework of large unit cell formalism is used to simulate silicon nanocrystals between 216 and 1000 atoms (1.6–2.65 nm in diameter) that include Bravais and primitive cell multiples. The investigated properties include core and oxidized surface properties. Results revealed that electronic properties converge to some limit as the size of the nanocrystal increases. Increasing the size of the core of a nanocrystal resulted in an increase of the energy gap, valence band width, and cohesive energy. The lattice constant of the core and oxidized surface parts shows a decreasing trend as the nanocrystal increases in a size that converges to 5.28 Ǻ in a good agreement with the experiment. Surface and core convergence to the same lattice constant reflects good adherence of oxide layer at the surface. The core density of states shows highly degenerate states that split at the oxygenated (001)‐(1 × 1) surface due to symmetry breaking. The nanocrystal surface shows smaller gap and higher valence and conduction bands when compared to the core part, due to oxygen surface atoms and reduced structural symmetry. The smaller surface energy gap shows that energy gap of the nanocrystal is controlled by the surface part. Unlike the core part, the surface part shows a descending energy gap that proves its obedience to quantum confinement effects. Nanocrystal geometry proved to have some influence on all electronic properties including the energy gap.

Effects of Core Distances, Solvent, Ligand, and Level of Theory on the TDDFT Optical Absorption Spectrum of the Thiolate-Protected Au25 Nanoparticle

Density functional theory calculations are employed to calculate geometries (R = H, CH(3), CH(2)CH(3), CH(2)CH(2)Ph) and excitation energies (R = H, CH(3), CH(2)CH(3)) for the Au(25)(SR)(18)(-) nanoparticle. The splitting between the first two peaks in the optical absorption spectrum is known to arise as a result of ligand-field splitting of superatom D orbitals, and the value of this splitting is found to be a very sensitive probe of gold-gold distances in the Au(25)(SH)(18)(-) nanoparticle core. LDA functionals such as Xalpha with a triple-zeta basis set are found to predict core geometries in good agreement with experiment, which suggests that this level of theory may be useful in future structural predictions. Asymptotically correct potentials SAOP and LB94 with triple-zeta basis sets yield excitation energies within 0.15-0.20 eV of experimental values; LB94 with a frozen-core basis set is found to be an inexpensive alternative to the preferred SAOP potential. The size of the ligand plays a minor role on the optical absorption spectrum and solvent effects on geometries and excitation energies are negligible, which demonstrates that the core geometric and electronic structure is primarily responsible for the discrete optical absorption exhibited by this nanoparticle.

Role of the defect core in energetics of vacancies

Electronic structure calculations at macroscopic scales are employed to investigate the crucial role of a defect core in the energetics of vacancies in aluminium. We find that vacancy core energy is significantly influenced by the state of deformation at the vacancy core, especially volumetric strains. Insights from the core electronic structure and computed displacement fields show that this dependence on volumetric strains is closely related to the changing nature of the core structure under volumetric deformations. These results are in sharp contrast to mechanics descriptions based on elastic interactions that often consider defect core energies as an inconsequential constant. Calculations suggest that the variation in core energies with changing macroscopic deformations is quantitatively more significant than the corresponding variation in relaxation energies associated with elastic fields. Upon studying the influence of various macroscopic deformations, which include volumetric, uniaxial, biaxial and shear deformations, on the formation energies of vacancies, we show that volumetric deformations play a dominant role in governing the energetics of these defects. Further, by plotting formation energies of vacancies and di-vacancies against the volumetric strain corresponding to any macroscopic deformation, we find that all variations in the formation energies collapse on to a universal curve. This suggests a universal role of volumetric strains in the energetics of vacancies. Implications of these results in the context of dynamic failure in metals through shock-induced spalling are analysed.

Vortex core structure in strongly correlated superfluidity

In order to study the vortex core electronic structure in high- T c superconductors, we examine 1-D Hubbard model with trap potential by using the density matrix renormalization group method. Instead of directly treating the vortex, the approach mimics the carrier density depression inside the vortex core via the trap potential and exactly calculate strong correlation effects on the depressed region. Consequently, we find that the Mott state emerges in the central region and the metallic edge surrounds the Mott region. Furthermore, when adding spin imbalance, the calculations reveal that a local antiferromagnetic order covers the Mott state region, and moreover, the antiferromagnetic order modulates with a long periodicity. We expect that these results closely relate to the vortex core electronic structure while their calculation results can be directly compared with atomic Fermi gases loaded on an optical lattice.

New insights on the electronic and molecular structure of cyanide-ligated iron(III) porphyrinates.

The preparation and characterization of several new cyano-ligated six-coordinate low-spin iron(III) porphyrinates are reported. The synthesis and structure of the new bis(cyanide) derivative K(222)][Fe(TMP)(CN)2] (TMP = tetramesitylporphyrinate) is described. Three mixed-ligand species of the general form [Fe(Porph)(CN)(L)], where L = 1-methylimidazole or pyridine, have also been prepared and structurally characterized. All complexes have been studied with EPR spectroscopy in frozen solution and in microcrystalline form. In some cases, especially those of the bis(cyanide) derivative above and the previously reported [Fe(TPP)(CN)2](-), there are significant differences in the EPR spectra as a result of the state change. These spectral differences can be correlated with changes in the electron configuration that are the likely result of a differing environment of the coordinated cyanide ligands; the core conformation and electronic structure of the porphyrin ligand are unlikely to play a role. All four new complexes and [Fe(TPP)(CN)2](-) have been studied by Mössbauer spectroscopy with variable-temperature and applied magnetic-field measurements. The sign of the quadrupole splitting value has been established as negative. These measurements have allowed us to give estimates of the energy difference between the two close-lying dpi (dxz and dyz) orbitals. These splitting values range from approximately 267 cm-1 for [Fe(TPP)(CN)2](-) to approximately 614 cm(-1) for [Fe(TPP)(CN)(Py)].

Surface characterization of colossal magnetoresistive manganites La 1− xSr xMnO 3 using photoelectron spectroscopy

We have studied the temperature and time dependence of the surface chemical composition and atomic structure of in situ fractured colossal magnetoresistive perovskites La 1− x Sr x MnO ( x = 0.3, 0.4) using core-level photoelectron spectroscopy and diffraction, simultaneous with observing marked changes in both core and valence electronic structure on going above the Curie temperature [N. Mannella et al., Phys. Rev. Lett. 92 (2004) 166401]. Stoichiometric analyses via core-level intensity ratios show that the near-surface composition is very nearly the same as that of the nominal (bulk) stoichiometry and further show that, during duration of our experiments, the degree of surface stoichiometry alteration or contamination has been minimal. The effects of photoelectron diffraction on such analyses are also explored. We comment on the degree to which near-surface composition or atomic-structure alterations might influence spectroscopic investigations of these manganites, or other strongly correlated materials.