Density Functional Theory Electronic Structure Research Articles

Synergetic Experimental and Theoretical Investigation of Molecular Structure – Optical Properties Relationships of Anthrazoline-Based Polymeric Chains

The photoluminescence and UV-Vis absorption spectra of copolyamides containing 1,9-anthrazoline with para- and meta-[(substituted carbonyl)amino]phenyl-1-ene moieties in a polymer backbone were studied by a combination of experimental and theoretical approaches. The investigation was accomplished through time-dependent density functional theory electronic structure calculations of small-molecule models mimicking a polymer chain. Theoretical absorption and luminescence spectra of ten atomistic models were compared with corresponding experimental data, and the optical properties of two new luminescent molecules with bromine auxochrome were predicted. An analysis of the optical properties demonstrate an identical effect of the type and position of a substituent on the spectra for para- and meta-[(substituted carbonyl)amino]phenyl-1-ene moieties. It was found that the absorption and luminescence spectra of theoretical para-models demonstrate red shifts relative to the corresponding meta- ones. The same phenomenon was observed in experimental spectra of low-molecular-mass compounds and corresponding copolyamides in solution and bulk. Unique optical properties allow anthrazoline-based polymer compounds to be used in numerous advanced optoelectronic applications with desired optical and electronic characteristics.

DFT electronic structure investigation of chromium ion-implanted cupric oxide thin films dedicated for photovoltaic absorber layers

This study explores the enhancement of cupric oxide (CuO) thin films for photovoltaic applications through chromium doping and subsequent annealing. Thin films of CuO were deposited on silicon and glass substrates using reactive magnetron sputtering. Chromium was introduced via ion implantation, and samples were annealed to restore the crystal structure. The optical and structural properties of the films were characterized using X-ray diffraction, spectrophotometry, and spectroscopic ellipsometry. Results indicated that implantation reduced the absorbance and conductivity of the films, while annealing effectively restored these properties. Sample implanted with 10 keV energy and 1 × 1014 cm−2 dose of Cr ions, after annealing had sheet resistance of 1.1 × 106 Ω/sq compared to 1.7 × 106 Ω/sq for non implanted and annealed CuO. Study of crystalline structure confirmed the importance of annealing as it reduced the stress present in the material after deposition and implantation. Density Functional Theory (DFT) calculations were performed to investigate the electronic structure and optical properties of CuO with varying levels of chromium doping. Calculations revealed an energy gap of 1.8 eV for undoped CuO, with significant changes in optical absorption for doped samples. Energy band gap determined using absorbance measurement and Tauc plot method had value of 1.10 eV for as deposited CuO. Samples after implantation and annealing had energy band gap value increased to about 1.20 eV. The study demonstrates that chromium doping and subsequent annealing can enhance the optical and electronic properties of CuO thin films, making them more efficient for photovoltaic applications.

CALCULATION of HOMA AND BIRD AROMATICITY INDICES, NLO and NBO PROPERTIES OF BENIDIPINE

Benidipine hydrochloride, being the derivate of 1,4-dihydropyridine is a calcium channel blocker antihypertensive drug. The aromaticity, natural bond orbital (NBO) and Nonlinear Optical (NLO) parameters properties of Benidipine (BEN) and BENHCl compounds were theoretically investigated using density functional theory (DFT) electronic structure method. Among the values of common aromaticity indices, the Harmonic Oscillator Aromaticity Model (HOMA) and BIRD's (Aromaticity Index) for electronic delocalization show that the aromaticity of the nitrophenyl group is higher in BEN and BENHCl compounds, and also the aromaticity in the gas phase is higher than that in water, octanol and DMF phase. In compound BEN, it was observed that as the dielectric constant of the medium increased, the aromaticity difference increased slightly. In the compound BEN, the correlation coefficient between HOMA values and aromatic fluctuation index (FLU), para delocalization index (PDI) and para-linear response (PLR) for the nitrophenyl group is higher than that the phenyl group. Donor-acceptor transitions, stabilization energies, intramolecular charge transfer were determined by natural population analyses. NLO parameters such as dipole moment, polarisibility and first order hyperpolarizability values of the BEN and BENHCl compound were also studied.

Copper doping induced band structure and morphology transformation in CaTiO3 for textile dye photodegradation applications

Semiconductor metal oxides with a wide bandgap like CaTiO3 can be exploited into an efficient visible light photocatalyst via cation doping. The type of dopant and the site of doping is known to greatly influence the photocatalytic activity of a material. Based on the intricacies of the density functional theory electronic structure study, we delve into the optimization of one-pot solvothermal synthesis to obtain Cu doped CaTiO3 nanocuboids. Doping of copper not only resulted in change in the electronic structure of the material but also led to change in the morphology. The uneven nanostep architecture resulted in increase in the surface area of the catalyst, which led to more active sites for the adsorption of the dyes and subsequent degradation. The reduced band gap and decreased recombination of charge carriers made the copper doped calcium titanate an efficient photocatalyst for degradation of both cationic (99.7% degradation of MV dye in 120 minutes) and anionic (99.8% degradation of RB in 45 minutes) dyes.

Honeycomb-structured S and N-codoped highly graphitized carbon as a catalyst support for Rh nanoparticles: A new benchmark electrocatalyst for hydrogen evolution reaction

State-of-the-art electrocatalysts are based on catalytically active metal deposited on conductive porous carbon. Herein, we report a new strategy of engineering of a honeycomb-structured S and N dual-doped graphene-like carbon (SNG) as a supporting material for Rh catalysts by a simple low-temperature (850 °C) pyrolysis of S-doped carbon nitride (S-CN) in the presence of Mg. Interestingly, here Mg plays a marvelous dual role not only as a reducing agent for graphitizing the S-CN but also as a precursor for new Mg3N2 and MgS products, which play as pore-generating templates for honeycomb-like structures. This features highly robust graphitized carbon with excellent electrical conductivity, proper S and N content, and porosity, making it highly desirable as catalyst support. Supporting Rh (5.2 wt.%) on SNG outperforms state-of-the-art commercial Pt/C electrodes for hydrogen evolution reaction (HER) under alkaline settings (1.0 M KOH) with an extremely lower overpotential of 13 mV at 10 mA cm−2, a lower Tafel value, and a higher turnover frequency. In addition, Rh/SNG as the cathode for industrial water electrolysis in 6.0 M KOH electrolyte at 70 °C shows excellent performance with only 1.76 Vcell to achieve 500 mA cm−2 and maintains long-term durability with negligible decay. The distinctive properties of SNG, including S and N dual doping, high graphiticity, superior electrical conductivity, and honeycomb-like hierarchical meso‑ and macropore structure, are credited with this remarkable HER performance. The S and N dopants in the SNG framework optimize the electronic structure of the Rh by synergistic interaction between them as illustrated by first-principal density functional theory calculations and electronic structure analysis.

Microcanonical treatment of HCl dissociative chemisorption on Au(111): Reactive dampening through inefficient translational energy coupling and an active surface.

Microcanonical unimolecular rate theory is applied to Shirhatti and Wodtke's recent supersonic molecular beam experiments examining the activated dissociative chemisorption of HCl on Au(111). A precursor mediated microcanonical trapping (PMMT) model (where the surface vibrates and HCl rotations, vibration, and translation directed along the surface normal are treated as active degrees of freedom) gave dissociative sticking coefficient predictions that are several orders of magnitude higher than experimental values but in good accord with prior quantum and molecular dynamics simulations. Density functional theory (DFT) electronic structure calculations using the Perdew-Burke-Ernzerhof (PBE) functional served to fix the vibrational frequencies of the reactive transition state and the threshold energy for dissociation, E0 = 72.9 kJ/mol. To explore the possibilities of varying threshold energy, coupling to phonons, and dynamics, a three-parameter [E0, s, ɛn] dynamically biased (d-) PMMT model was fit to the experiments. A dynamical bias was introduced using an efficiency, ɛn, of normal translational energy to contribute to the active exchangeable energy capable of promoting reactivity. To achieve the low sticking probabilities observed in experiment, severe normal translational energy dampening (ɛn → 0.26) was imposed, leading to a large vibrational efficacy of ηv = εv/εn = 3.85. The optimal threshold energy for dissociation was E0 = 30.88 kJ/mol, some 40 kJ/mol below the PBE-DFT prediction, and the optimal number of Au surface oscillators was s = 1. The d-PMMT modeling indicates that HCl/Au(111) reactivity can be consistent with electronically adiabatic passage across a relatively low and late transition state that dynamically disfavors normal translational energy.

First Principles Study of Electron Structure of La x Ba1- x MnO3 Crystals

In the framework of the density functional theory electronic band structure and density of states of LaxBa1-xMnO3 solid solutions containing different concentrations of La and Ba atoms, were studied using different exchange correlation functionals. Our calculation show that the band gap, which characterizes the properties of semiconductors, varies depending on the concentration of La and Ba atoms in the compounds. Using such a dependence, it is possible to make changes in physical properties depending on the concentration of cations.

Kinetic studies of surfactant-aspirin micellization by means of ultrasonic relaxation spectroscopy

We report on the dynamics of the Sodium dodecyl sulfate (SDS) – aspirin micellization process taking place in aqueous environment by combining vibrational and ultrasonic spectroscopies. The experimental vibrational spectra have been compared with the corresponding theoretically predicted spectra obtained by density functional theory electronic structure calculations. The formation of a swollen SDS@aspirin micelle above a specific surfactant concentration significantly affects several physical properties of the system including surface tension and isentropic compressibility allowing the estimation of the critical micelle concentration. In the acoustic spectra, a single ultrasonic relaxation process has been detected in SDS-aspirin-water solutions at concentrations slightly above critical micelle concentration. The variation of the sound absorption coefficient and velocity as a function of frequency at different concentrations reveals the presence of a relaxation process attributed to the perturbation of the monomer-micelle dynamic equilibrium. We progress on the comprehensive understanding of the micellization kinetics, and the results are discussed in the context of several theoretical models of the field.

Adsorption and sensor performance of transition metal-decorated zirconium-doped silicon carbide nanotubes for NO2 gas application: a computational insight.

Owing to the fact that the detection limit of already existing sensor-devices is below 100% efficiency, the use of 3D nanomaterials as detectors and sensors for various pollutants has attracted interest from researchers in this field. Therefore, the sensing potentials of bare and the impact of Cu-group transition metal (Cu, Ag, Au)-functionalized silicon carbide nanotube (SiCNT) nanostructured surfaces were examined towards the efficient detection of NO2 gas in the atmosphere. All computational calculations were carried out using the density functional theory (DFT) electronic structure method at the B3LYP-D3(BJ)/def2svp level of theory. The mechanistic results showed that the Cu-functionalized silicon carbide nanotube surface possesses the greatest adsorption energies of -3.780 and -2.925 eV, corresponding to the adsorption at the o-site and n-site, respectively. Furthermore, the lowest energy gap of 2.095 eV for the Cu-functionalized surface indicates that adsorption at the o-site is the most stable. The stability of both adsorption sites on the Cu-functionalized surface was attributed to the small ellipticity (ε) values obtained. Sensor mechanisms confirmed that among the surfaces, the Cu-functionalized surface exhibited the best sensing properties, including sensitivity, conductivity, and enhanced adsorption capacity. Hence, the Cu-functionalized SiCNT can be considered a promising choice as a gas sensor material.

Polar Polymorphism: A New Intermediate Structure toward the Thin-Film Phase in Asymmetric Benzothieno[3,2-b][1]-benzothiophene Derivatives.

Understanding structure and polymorphism is relevant for any organic device optimization, and it is of particular relevance in 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) since high carrier mobility in Ph-BTBT-10 thin films has been linked to the structural transformation from the metastable thin-film phase to the thermodynamically stable bilayer structure via thermal annealing. We combine here a systematic nanoscale morphological analysis with local Kelvin probe force microcopy (KPFM) that demonstrates the formation of a polar polymorph in thin films as an intermediate structure for thicknesses lower than 20 nm. The polar structure develops with thickness a variable amount of structural defects in the form of individual flipped molecules (point defects) or sizable polar domains, and evolves toward the reported nonpolar thin-film phase. The direct experimental evidence is supported by electronic structure density functional theory calculations. The structure of the film has dramatic effects on the electronic properties, leading to a decrease in the film work function (by up to 1 eV) and a considerable broadening of the occupied molecular orbitals, attributed to electrostatic disorder. From an advanced characterization point of view, KPFM stands out as a valuable tool for evaluating electrostatic disorder and the conceivable emergence of polar polymorphs in organic thin films. The emergence of polar assemblies introduces a critical consideration for other asymmetric BTBT derivatives, which may be pivotal to understanding the structure-property relationships in organic field-effect transistors (OFETs). A precise determination of any polar assemblies close to the dielectric interface is critical for the judicious design and upgrading of high-performance OFETs.

Insight into the pyrolysis mechanism of α-O-4 linked lignin: The role of bimolecular interactions and substituents

Pyrolysis can convert lignin into various high-value phenolic derivatives, whereas the pyrolysis mechanism of lignin, particularly the α-O-4 linked lignin, has not been clearly revealed. Herein, unsubstituted α-O-4 lignin dimer benzyl phenyl ether (BPE) and Cα-substituted α-CH2OH-BPE, α-(CH2)2OH-BPE and α-CHOHCH2OH-BPE were employed to study the radical and concerted mechanisms of α-O-4 type lignin pyrolysis by density functional theory (DFT) calculation and electronic structure analysis. A pyrolysis system was constructed to carefully consider unimolecular and bimolecular reactions, and the influence of substituent groups was investigated. The results indicate that unsubstituted α-O-4 structure can only undergo unimolecular Cα–O homolysis without hydrogen transfer, while various substituents provide more hydrogen sites, leading to the occurrence of radical chain reactions and concerted reactions. Cβ–dehydrogenated radicals derived from substituted α-O-4 dimers can be easily generated by intermolecular Cβ–hydrogen abstraction, which tends to break the Cα–O bond. Similarly, hydroxyl-assisted hydrogen transfer (AHT) can promote Cα–O cleavage by inducing Cβ/Cγ–hydrogen exchange of substituted hydroxy groups. Comprehensively, both radical hydrogen abstraction and concerted hydroxyl-AHT interactions have positive effects on the Cα–O decomposition of α-O-4 linkage. This study provides a more reasonable explanation for the initial decomposition of α-O-4 linked lignin and the preliminary generation of phenolic hydroxy and phenoxy radical derivatives.

Strongly Bound Polynuclear Anions Comprising Scandium Fluoride Building Blocks.

The stability of polynuclear anions composed of ScF3 building blocks was studied by using ab initio and density functional theory electronic structure methods and flexible basis sets. Thorough exploration of ground state potential energy surfaces of (Sc2F7)-, (Sc3F10)-, and (Sc4F13)- anions which may be viewed as comprising ScF3 fragments and the additional fluorine atom led to determining the isomeric structures thereof. It was found that the most stable isomers which are predicted to dominate at room temperature correspond to the compact structures enabling the formation of a large number of Sc-F-Sc bridging linkages rather than to the chain-like structures. The vertical electron detachment energies of the (ScnF3n+1)- anions were found to be very large (spanning the 10.85-12.29 eV range) and increasing with the increasing number of scandium atoms (n) and thus the ScF3 building blocks involved in the structure. Thermodynamic stability of (ScnF3n+1)- anions (i.e., their susceptibility to fragmentation) was also verified and discussed.

Stannane in extreme ultraviolet lithography and vacuum technology: Synthesis and characterization

In extreme ultraviolet (EUV) lithography, tin droplets evaporate and subsequently coat various surfaces including the collector mirrors. To clean off the tin, a hydrogen plasma is often used, but as a result, an unstable by-product, stannane (SnH4) is formed. The physicochemical characteristics of this gas, its formation in a plasma process, and its interaction with various materials have not been explored and understood completely. Here, the electron ionization mass spectrum of SnH4 is presented. All ten natural abundance isotopes were observed experimentally for each fragment, i.e., Sn+, SnH+, SnH2+, and SnH3+. Density functional electronic structure theory was used to calculate the optimized ground state geometries of these gas phase species and their relative stabilities and helped explain the absence of SnH4+ in the observed signals. The density of the liquid, its cracking pattern, and the surface morphology of its deposits were examined. The surface of the deposited tin film resulting from the decomposition and subsequent oxidation was characterized by x-ray photoelectron spectroscopy. The main species found at the surface were metallic tin and tin (II) oxide (SnO). The detailed characterization of stannane should help correctly identify it in EUV lithographic processes and develop approaches in the future to mitigate its decomposition and redeposition on the collector mirrors or vacuum chamber walls.

Effects of Intra-Base Pair Proton Transfer on Dissociation and Singlet Oxygenation of9-Methyl-8-Oxoguanine-1-Methyl-Cytosine Base-Pair Radical Cations.

8-Oxoguanosine is the most common oxidatively generated base damage and pairs with cytidine within duplex DNA. The 8-oxoguanosine-cytidine lesion, if not recognized and removed, not only leads to G-to-T transversion mutations but renders the base pair being more vulnerable to the ionizing radiation and singlet oxygen (1O2) damage. Herein, reaction dynamics of a prototype Watson-Crick base pair [9MOG·1MC]·+, consisting of 9-methyl-8-oxoguanine radical cation (9MOG·+) and 1-methylcystosine (1MC), was examined using mass spectrometry coupled with electrospray ionization. We first detected base-pair dissociation in collisions with the Xe gas, which provided insight into intra-base pair proton transfer of 9MOG·+·1MC[[EQUATION]][9MOG - HN1]··[1MC + HN3']+ and subsequent non-statistical base-pair separation. We then measured the reaction of [9MOG·1MC]·+ with 1O2, revealing the two most probable pathways, C5-O2 addition and HN7-abstraction at 9MOG. Reactions were entangled with the two forms of 9MOG radicals and base-pair structures as well as multi-configurations between open-shell radicals and 1O2 (that has a mixed singlet/triplet character). These were disentangled by utilizing approximately spin-projected density functional theory, coupled-cluster theory and multi-referential electronic structure modeling. The work delineated base-pair structural context effects and determined relative reactivity toward 1O2 as [9MOG - H]· > 9MOG·+ > [9MOG - HN1]··[1MC + HN3']+ ≥ 9MOG·+·1MC.

Spin Polarization and Flat Bands in Eu-Doped Nanoporous and Twisted Bilayer Graphenes.

Advanced two-dimensional spin-polarized heterostructures based on twisted (TBG) and nanoporous (NPBG) bilayer graphenes doped with Eu ions were theoretically proposed and studied using Periodic Boundary Conditions Density Functional theory electronic structure calculations. The significant polarization of the electronic states at the Fermi level was discovered for both Eu/NPBG(AA) and Eu/TBG lattices. Eu ions' chemi- and physisorption to both graphenes may lead to structural deformations, drop of symmetry of low-dimensional lattices, interlayer fusion, and mutual slides of TBG graphene fragments. The frontier bands in the valence region at the vicinity of the Fermi level of both spin-polarized 2D Eu/NPBG(AA) and Eu/TBG lattices clearly demonstrate flat dispersion laws caused by localized electronic states formed by TBG Moiré patterns, which could lead to strong electron correlations and the formation of exotic quantum phases.

Electronic and Vibrational Properties of Episulfide-Graphene Materials

Our group has investigated the stability of sulfur adatoms on graphene obtained by a direct reaction of highly oriented pyrolytic graphite (HOPG) with elemental sulfur. With the help of density functional theory (DFT) calculations, we have developed different models of the structure and evaluated the electronic and vibrational properties of synthesized graphene-sulfur (GS) materials. Our structural models are based on sulfur atoms covalently attached to graphene. The DFT electronic structure calculations support the formation of a strong and stable sulfur bonding to graphene and give a chemical composition in good agreement with the experimental proportion of sulfur to carbon. In addition, ab-initio molecular dynamics simulations allow us to confirm the stability on S-G bonds. The vibrational modes calculated with DFT support the vibrational signatures of the GS materials observed using Raman spectroscopy.

Effects of Enol-imine/Keto-amine tautomerism and conformational changes on the electronic spectra of a novel 1,2,4-triazole ortho-hydroxyaryl Schiff base in different solvents

In this study, a novel o-hydroxyaryl Schiff base bearing the 1,2,4-triazole heterocyclic core, (E)-2-(((4H-1,2,4-triazol-4-yl)imino)methyl)-6-methoxyphenol, was synthesized and structurally characterized by NMR (1H- and 13C-) and IR spectroscopies as well as by Density Functional Theory (DFT) electronic structure calculations. The compound was then investigated by UV–Vis absorbance and fluorescence emission spectroscopies in different solvents. The wavelength shifts observed in the band corresponding to the lowest energy transition in the absorbance spectra upon changing the solvent, as well as in the emission bands observed in the fluorescence spectra (upon excitation at 310 nm) were explained considering the effects of solvent-induced conformational changes and photoinduced enol-imine/keto-amine tautomerism. The interpretation of the experimental results was supported by structural data calculated for the different isomeric forms of the compound and of their electronic spectra in the gas phase and in solvents of different polarity. Solvent effects were also evaluated at the light of the Kamlet-Taft and Catalán solvatochromic models.

Link between interlayer hybridization and ultrafast charge transfer in WS2-graphene heterostructures

Ultrafast charge separation after photoexcitation is a common phenomenon in various van-der-Waals (vdW) heterostructures with great relevance for future applications in light harvesting and detection. Theoretical understanding of this phenomenon converges towards a coherent mechanism through charge transfer states accompanied by energy dissipation into strongly coupled phonons. The detailed microscopic pathways are material specific as they sensitively depend on the band structures of the individual layers, the relative band alignment in the heterostructure, the twist angle between the layers, and interlayer interactions resulting in hybridization. We used time- and angle-resolved photoemission spectroscopy combined with tight binding and density functional theory electronic structure calculations to investigate ultrafast charge separation and recombination in WS2-graphene vdW heterostructures. We identify several avoided crossings in the band structure and discuss their relevance for ultrafast charge transfer. We relate our own observations to existing theoretical models and propose a unified picture for ultrafast charge transfer in vdW heterostructures where band alignment and twist angle emerge as the most important control parameters.

Identifying the Local Atomic Environment of the "Cu3+" State in Alkaline Electrochemical Systems.

CuO-based catalysts are active for the oxygen evolution reaction (OER), although the active form of copper for the OER is still unknown. We combine operando Raman experiments and density functional theory (DFT) electronic structure calculations to determine the form of Cu(O)xOHy present under OER conditions. Raman spectra show a distinct feature related to the active "Cu3+" species, which is only present under highly oxidizing conditions. DFT is used to produce theoretical Raman standards and match the unique Raman feature of copper under OER potentials. This method identifies a range of Cu3+-containing compounds which match the distinct Raman feature. We then integrate experimental electrochemistry to progressively eliminate possible structures and determine the stoichiometry of the active form as CuOOH, which likely takes the form of a surface-adsorbed hydroxide on a CuO surface. Bader charge analysis, site-projected wavefunctions, and density of states analysis show that electron density is removed from O 2p orbitals upon hydroxide adsorption, suggesting that the electronic structure exhibits d9L Cu2+ behavior rather than the local electronic structure of a formal Cu3+.

Synthesis and Characterization of Zinc(II), Cadmium(II), and Palladium(II) Complexes with the Thiophene-Derived Schiff Base Ligand.

Zn(II), Pd(II), and Cd(II) complexes, [L TH MCl 2 ] (M = Zn, Pd; X = Br, Cl) and [L TH Cd(μ-X)X] n (X = Cl, Br; n = n, 2), supported by the (E)-N 1,N 1-dimethyl-N 2-(thiophen-2-ylmethylene)ethane-1,2-diamine (L TH ) ligand are synthesized and structurally characterized. Density functional theory (DFT) electronic structure calculations and variable-temperature NMR support the presence of two conformers and a dynamic interconversion process of the minor conformer to the major one in solution. It is found that the existence of two relevant complex conformers and their respective ratios in solution depend on the central metal ions and counter ions, either Cl- or Br-. Among the two relevant conformers, a single conformer is crystallized and X-ray diffraction analysis revealed a distorted tetrahedral geometry for Zn(II) complexes, and a distorted square planar and square pyramidal geometry for Pd(II) and Cd(II) complexes, respectively. It is shown that [L TH MCl 2 ]/LiO i Pr (M = Zn, Pd) and [L TH Cd(μ-Cl)Cl] n /LiO i Pr can effectively catalyze the ring-opening polymerization (ROP) reaction of rac-lactide (rac-LA) with 94% conversion within 30 s with [L TH ZnCl 2 ]/LiO i Pr at 0 °C. Overall, hetero-enriched poly(lactic acid)s (PLAs) were provided by these catalytic systems with [L TH ZnCl 2 ]/LiO i Pr producing PLA with higher heterotactic bias (P r up to 0.74 at 0 °C).