Electronic Energy Gap Research Articles

Electronic resonance with anticorrelated pigment vibrations drives photosynthetic energy transfer outside the adiabatic framework

The delocalized, anticorrelated component of pigment vibrations can drive nonadiabatic electronic energy transfer in photosynthetic light-harvesting antennas. In femtosecond experiments, this energy transfer mechanism leads to excitation of delocalized, anticorrelated vibrational wavepackets on the ground electronic state that exhibit not only 2D spectroscopic signatures attributed to electronic coherence and oscillatory quantum energy transport but also a cross-peak asymmetry not previously explained by theory. A number of antennas have electronic energy gaps matching a pigment vibrational frequency with a small vibrational coordinate change on electronic excitation. Such photosynthetic energy transfer steps resemble molecular internal conversion through a nested intermolecular funnel.

Kinetic Isotope Effect in the Hydrogenation and Deuteration of Graphene

AbstractTime‐dependent photoemission spectroscopy is employed to study the kinetics of the hydro‐genation/deuteration reaction of graphene. Resulting in an unusual kinetic isotope effect, the graphene deuteration reaction proceeds faster than hydrogenation and leads to substantially higher maximum coverages of deuterium (D/C≈35% vs H/C≈25%). These results can be explained by the fact that in the atomic state H and D have a lower energy barrier to overcome in order to react with graphene, while in the molecular form the bond between two atoms must be broken before the capture on the graphene layer. More importantly, D has a higher desorption barrier than H due to quantum mechanical zero‐point energy effects related to the C–D or C–H stretch vibration. Molecular dynamics simulations based on a quantum mechanical electronic potential can reproduce the experimental trends and reveal the contribution of the constituent chemisorption, reflection, and associative desorption processes of H or D atoms onto graphene. Regarding the electronic structure changes, a tunable electron energy gap can be induced by both deuteration and hydrogenation.

Finite-temperature electronic simulations without the Born-Oppenheimer constraint

The adiabatic approximation, typically assumed when performing standard Born-Oppenheimer (BO) molecular dynamics, can become unreliable at finite temperature, and specifically when the temperature is larger than the electronic energy gap between the ground state and the low-lying excited states. In this regime, relevant for many important chemical processes, the non-adiabatic couplings between the electronic energy states can produce finite temperature effects in several molecular properties, such as the geometry, the vibrational frequencies, the binding energy, and several chemical reactions. In this work, we introduce a novel finite-temperature non-adiabatic molecular dynamics based on a novel covariant formulation of the electronic partition function. In this framework, the nuclei are not constrained to move in a specific electronic potential energy surface. Then, by using a rigorous variational upper bound to the free energy, we are led to an approximate partition function that can be evaluated numerically. The method can be applied to any technique capable to provide an energy value over a given wave function ansatz depending on several variational parameters and atomic positions. In this work, we have applied the proposed method within a quantum Monte Carlo (QMC) scheme. In particular, we consider in this first application only classical ions, but we explicitly include an electronic correlation (Jastrow) term in the wave function, by extending in this way the standard variational QMC method, from ground state to finite temperature properties. We show that our approximation reduces correctly to the standard ground-state Born-Oppenheimer (gsBO) at zero temperature and to the correct high temperature limit. Moreover, at temperatures large enough, this method improves the upper bound of the free energy obtained with a single BO energy surface, since within our approach it is possible to estimate the electron entropy of a correlated ansatz in an efficient way. We test this new method on the simple hydrogen molecule, where at low temperature we recover the correct gsBO low temperature limit. Moreover, we show that the dissociation of the molecule is possible at a temperature much smaller than the one corresponding to the gsBO energy surface, in good agreement with experimental evidence. Several extensions of the proposed technique are also discussed, as for instance the inclusion of quantum effects for ions and the calculation of critical (magnetic, superconducting) temperatures.

Long-Chain 3,4-Ethylenedioxythiophene/Thiophene Oligomers and Semiconducting Thin Films Prepared by Their Electropolymerization

A series of soluble H-terminated conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) combined with a small number of thiophene units and ranging in length from four to eight EDOT/thiophene groups was prepared with the ultimate goal to investigate if facile formation of a reactive trication radical species would enable electrochemical polymerization of such long-chain oligomers. Spectroscopic and electrochemical studies of the oligomers revealed some general dependencies of their electronic properties on the total number and position of EDOT groups. It was the number of consecutive EDOT units rather than total number of these units which was found to have the most profound effect on electronic energy gap and conjugation length. This influence originates from the especially strong planarization induced in the conjugated backbone by the incorporation of EDOT units. In contrast, incorporation of thiophene units was found to result in loss of the conformational stabilization. This phenomenon was analyzed using the natural bond orbital computational approach, which revealed the predominantly hyperconjugative nature of the EDOT-induced conformational stabilization. Whereas shorter oligomers, in agreement with the general consensus, were found to be inert toward electrochemical polymerization due to low reactivity of electrochemically generated cation radical and dication species, the longest oligomer showed an unprecedentedly efficient electropolymerization to yield a stable thin film of an electroactive polymer. The efficient electropolymerization of the long-chain oligomer was found to be in agreement with the formation of a reactive trication radical species. The electronic and spectral properties of the resulting semiconducting polymer film were studied by various electrochemical and spectroelectrochemical methods, as well as conductive probe AFM technique, and revealed a number of unusual features (such as electrical rectifying switching behavior) consistent with the possibility of increased molecular order in this material.

Amino acid 2-aminopropanoic CH3CH(NH2)COOH crystals: materials for photo- and acoustoinduced optoelectronic applications

Photo-induced treatment of l-alanine single crystals grown by slow evaporation method at an ambient temperature was performed using a 25 ps Nd:YAG pulsed laser in the presence of an external acoustic filed. The changes of the absorption were studied for the wavelength 265 nm near the energy band gap edge at acoustical power density varying within 4–6 W/cm2. The observed absorption changes indicate that the external optical electric field strengths and acoustical power densities may be efficient parameters for the characterization of photo-optical and acousto-optical treatment of the samples. From the X-ray diffraction data we have optimized the atomic positions assuming that force on the atoms is around 1 mRy/au. These are used to calculate the electronic structure and the chemical bonding for the amino acid l-alanine single crystals. The calculated electronic band structure and densities of states confirms the experimental results that this compound possesses a relatively large energy band gap. The upper valence band has its maximum at the Z point of the Brillouin zone while the conduction band minimum is located at Γ point in the zone center, resulting in an indirect energy band gap. The electronic energy gap is equal to 4.19 eV within a framework of the used local density approximation and 4.54 eV with the Engel–Vosko generalized gradient approximation as the exchange correlation potential. This is in an agreement with our experimentally measured energy band gap ~4.67 eV. The existence of O-p character in the upper valence band has a significant consequence for the optical band gap. From our calculated electron charge density distribution, we obtain a space electron charge density distribution in the average unit cell of the crystal. The chemical bonding features of l-alanine amino acid were analyzed.

Fingerprints of order and disorder in the electronic and optical properties of crystalline and amorphous TiO2

We have investigated the structural and electronic properties as well as the linear optical response of amorphous TiO${}_{2}$ within density functional theory and a numerically efficient density functional based tight-binding approach as well as many-body perturbation theory. The disordered TiO${}_{2}$ phase is modeled by molecular dynamics. The equivalence to experimentally characterized amorphous phases is demonstrated by atomic structure factors and radial pair-distribution functions. By density functional theory calculations, using both the semilocal Perdew-Burke-Ernzerhof functional and the nonlocal Heyd-Scuseria-Ernzerhof screened hybrid functional, the electronic energy gap is found to be larger than in the crystalline TiO${}_{2}$ phases rutile and brookite but close to the anatase band gap. The quasiparticle energy gap of amorphous TiO${}_{2}$ is determined to be $\ensuremath{\gtrsim}$3.7 eV, while the optical gap is estimated to $\ensuremath{\lesssim}$3.5 eV. The disorder-induced formation of localized electronic states has been analyzed by the information entropy of the charge density distributions. The frequency-dependent optical constants, calculated from the complex dielectric function, have been determined in independent particle approximation. Besides similar absorption characteristics between the most common crystalline phases and amorphous TiO${}_{2}$, we find distinct differences in the optical spectra in the energy region between 5 eV and 8 eV. These differences can be assigned to the loss of symmetry in the local atomic structure of the disordered material. While the composition of the crystalline phases rutile, anatase, and brookite is well described by periodic arrangements of distorted TiO${}_{6}$ octahedra building blocks, the amorphous phase is characterized by partial loss of this octahedral coordination and the disorder-induced formation of under- and over-coordinated Ti ions. This leads to the absence of the characteristic crystal-field splitting of unoccupied Ti${}_{3d}$ states into ${e}_{g}$ and ${t}_{2g}$ like subbands. The optical characteristics of the amorphous phase are interpreted as a superposition of optical transitions that reflect the various local symmetries of the manifold of synthesizable crystalline TiO${}_{2}$ phases. The linear optical properties, calculated within the independent-particle approximation, are found to be in good agreement with the available experimental data.

Controlling the Topology of Fermi Surfaces in Metal Nanofilms

The properties of metal crystals are governed by the electrons of the highest occupied states at the Fermi level and determined by Fermi surfaces, the Fermi energy contours in momentum space. Topological regulation of the Fermi surface has been an important issue in synthesizing functional materials, which we found to be realized at room temperature in nanometer-thick films. Reducing the thickness of a metal thin film down to its electron wavelength scale induces the quantum size effect and the electronic system changes from three to two-dimensional, transforming the Fermi surface topology. Such an ultrathin film further changes its topology through one-dimensional (1D) structural deformation of the film when it is grown on a 1D substrate. In particular, when the interface has 1D metallic bands, the system is additionally stabilized by forming an electron energy gap by hybridization between 1D states of the film and substrate.

Effects of Zinc Binding on the Structure and Stability of Glycylglycine Dipeptide: A Computational Study

Energies and structures of six different isomers resulted of complex between glycylglycine dipeptide and zinc cation were calculated at the BLYP and B3LYP levels of theory with 6-311G** and 6-311++G** basis sets. The energies of the glycylglycine-Zn2+different complexes at the BLYP/6-311++G** obtained more stable than another methods. In this method, the most stable complex was zinc cation bound to the two oxygen sites of the glycylglycine zwitterion and acted as a bidentate ligand. The vibrational frequencies, thermodynamic and electronic properties, HOMO, LUMO, and energy gap for the most stable isomer in different solvents were calculated at the BLYP/6-311++G** level of theory.

Study of molecule spectra of carotenoid

Carotenoid is a short-chain polyene biomolecule of 9 CC conjugate double-bonds. Due to its special structure carotenoid is used not only in photoprotection and anti-cancer, but also in molecule wires, light switch, and light filter. In this paper molecule spectra of carotenoid are studied. The carotenoid has the broadband fluorescence, the broadband stimulated Raman scattering is obtained when the fluorescence of carotenoid is used to enhance stimulated Raman scattering. Electron energy gap of carotenoid becomes narrow with temperature decreasing, and absorption spectra are red-shifted. These characteristics can make the carotenoid a high-quality semiconductor component. Raman active is intensive and the 3rd order optical nonlinear coefficient is large. The Raman scattering cross section of CC bond is about 10 orders of magnitude larger than that of common molecule. The Raman scattering intensity of overtone is intensive: the ratio of overtone to basic frequency is around 0.5 at low temperature. These spectrum characteristics of carotenoid are significant for studying polyene molecule structure, property and non-biology domain.

Theory of local heating in single molecular bridge junctions

Inelastic scattering of conduction electrons due to vibronic coupling gives arise local heating of the single molecular junction. We have discussed the problem based on a self-consistent theory treating inelastic heat generation and heat diffusion/transport to thermalized electrodes on equal footing, including both electron-phonon coupling and electron correlation effects. We found that the local heating is suppressed by reducing phonon damping either via a large electronic energy gap or electron correlation. Theoretical results were compared with available experimental results in literature, especially the case of alkanedithiol single-molecular scanning tunneling microscope (STM) break junctions.

Self-Interaction Corrected Electronic Structure and Energy Gap of CuAlO2 beyond Local Density Approximation

We implemented a self-interaction correction (SIC) into first-principles calculation code to go beyond local density approximation and applied it to CuAlO 2 . Our simulation shows that the valence band width calculated within the SIC is narrower than that calculated without the SIC because the SIC makes the d -band potential deeper. The energy gap calculated within the SIC expands and is close to experimental data.

Alignment, Vibronic Level Splitting, and Coherent Coupling Effects on the Pump−Probe Polarization Anisotropy

The pump-probe polarization anisotropy is computed for molecules with a nondegenerate ground state, two degenerate or nearly degenerate excited states with perpendicular transition dipoles, and no resonant excited-state absorption. Including finite pulse effects, the initial polarization anisotropy at zero pump-probe delay is predicted to be r(0) = 3/10 with coherent excitation. During pulse overlap, it is shown that the four-wave mixing classification of signal pathways as ground or excited state is not useful for pump-probe signals. Therefore, a reclassification useful for pump-probe experiments is proposed, and the coherent anisotropy is discussed in terms of a more general transition dipole and molecular axis alignment instead of experiment-dependent ground- versus excited-state pathways. Although coherent excitation enhances alignment of the transition dipole, the molecular axes are less aligned than for a single dipole transition, lowering the initial anisotropy. As the splitting between excited states increases beyond the laser bandwidth and absorption line width, the initial anisotropy increases from 3/10 to 4/10. Asymmetric vibrational coordinates that lift the degeneracy control the electronic energy gap and off-diagonal coupling between electronic states. These vibrations dephase coherence and equilibrate the populations of the (nearly) degenerate states, causing the anisotropy to decay (possibly with oscillations) to 1/10. Small amounts of asymmetric inhomogeneity (2 cm(-1)) cause rapid (130 fs) suppression of both vibrational and electronic anisotropy beats on the excited state, but not vibrational beats on the ground electronic state. Recent measurements of conical intersection dynamics in a silicon napthalocyanine revealed anisotropic quantum beats that had to be assigned to asymmetric vibrations on the ground electronic state only [Farrow, D. A.; J. Chem. Phys. 2008, 128, 144510]. Small environmental asymmetries likely explain the observed absence of excited-state asymmetric vibrations in those experiments.

Heterogeneous Fenton oxidation of dissolved organics in salt-laden wastewater from leather industry without sludge production

This is the first report on the heterogeneous oxidation of dissolved organics in wastewater with little consumption of Fenton’s reagent and without sludge production. The salt-laden wastewater discharged from leather industry has poor biodegradability due to the presence of high total dissolved solids. This wastewater is evaporated in solar evaporation pans. The evaporated residue faces disposal problem due to high concentration of organic and inorganic salts. The present study demonstrates the preparation, characterization and application of mesoporous activated carbon matrix along with Fenton’s reagent for the removal of dissolved organics in wastewater. Free electron-rich mesoporous activated carbon matrices (MAC700, MAC800 and MAC900) were prepared and characterized for surface area, C, H, N, ash content, free electron density and energy gap value. Results show that heterogeneous catalytic oxidation of dissolved organics in the wastewater decreased biochemical oxygen demand, chemical oxygen demand, total organic carbon and dissolved protein, respectively, by 74, 69, 61 and 80% at optimum pH (3.5), H2O2 (7.5 mmol/l), FeSO4.7H2O (0.3 mmol/l) and MAC800 (10 g/l).

Electronic energy gaps and optical properties of LaMnO3

We investigate electronic structure and optical properties of LaMnO3 through density-functional-theory calculations with a recent improved exchange potential. Calculated gaps are consistent with recent experimental values, and calculated optical conductivities and dielectric constants as functions of photon energy are in excellent agreement with low-temperature experimental results. These lead to a satisfactory theoretical understanding of the electronic gaps and optical properties of LaMnO3 without tuning atomic parameters and can help elucidate electronic structures and magnetic properties of other manganite materials.

Adsorption Properties of O2 onCr5 Nanostructures: A DFT Study

Based on the density functional theory (DFT) adsorption properties of oxygen molecule on Cr5 nanostructures at various positions (top, bridge and central sites) were investigated. Geometry optimizations and vibrational frequencies of them were carried out at B3LYP level of theory with LANL2DZ and 6‐31G* basis sets using Gaussian 98 suites of program. Their thermodynamic properties were estimated. We have also investigated the total electronic properties, HOMO, LUMO and energy gap for Cr5 nanostructures with O2. The result showed that the adsorption of O2 on Cr5 nanocluster in horizontal bridge site is the most stable state of adsorption and its binding energy is strong, ‐145.88 kcal/mol.

Temperature Dependence of Regioselectivity in Nucleophilic Photosubstitution of 4-Nitroanisole. The Activation Energy Criterion for Regioselectivity

Photosubstitution of the nitro group vs the methoxy group of triplet 4-nitroanisole by hydroxide ion in water leads to product yields of about 80% 4-methoxyphenol and 20% 4-nitrophenol. The ratio depends slightly on temperature from 3 to 73 °C. The slight temperature variation in the yield ratio is reproduced almost perfectly with a simple Arrhenius model for a mechanism involving bonding of hydroxide ion with the triplet state of 4-nitroanisole. The competing transition states have activation energies of 2.2 and 2.6 kcal/mol, respectively. Correct prediction of regioselectivity can be done for this case by quantum chemical calculation of the competing triplet transition-state energies, or those of the corresponding triplet σ-complexes. Other models for aromatic photosubstitution regioselectivity in mechanisms of the S(N)2Ar* type, such as those based on calculated electron densities, HOMO/LUMO coefficients, or energy gap sizes, are discussed and shown to be inferior to the relative activation energies model. The photoreaction in alcohol solvents, claimed by others to generate the same products as in water and to have an exceedingly large variation of the product ratio with temperature, may reflect chemical changes other than those reported.

Controlling Energy Gap of Bilayer Graphene by Strain

Using the first principles calculations, we show that mechanically tunable electronic energy gap is realizable in bilayer graphene if different homogeneous strains are applied to the two layers. It is shown that the size of the energy gap can be simply controlled by adjusting the strength and direction of these strains. We also show that the effect originates from the occurrence of strain-induced pseudoscalar potentials in graphene. When homogeneous strains with different strengths are applied to each layer of bilayer graphene, transverse electric fields across the two layers can be generated without any external electronic sources, thereby opening an energy gap. The results demonstrate a simple mechanical method of realizing pseudoelectromagnetism in graphene and suggest a maneuverable approach to fabrication of electromechanical devices based on bilayer graphene.

Low-Energy Electronic States of Carbon Nanocones in an Electric Field

The low-energy electronic states and energy gaps of carbon nanocones in an electric field are studied using a single-π-band tight-binding model. The analysis considers five perfect carbon nanocones with disclination angles of 60°, 120°, 180°, 240° and 300°, respectively. The numerical results reveal that the low-energy electronic states and energy gaps of a carbon nanocones are highly sensitive to its geometric shape (i.e. the disclination angle and height), and to the direction and magnitude of an electric field. The electric field causes a strong modulation of the state energies and energy gaps of the nanocones, changes their Fermi levels, and induces zero-gap transitions. The energy-gap modulation effect becomes particularly pronounced at higher strength of the applied electric field, and is strongly related to the geometric structure of the nanocone.

Electron-withdrawing groups and strong-field ligands containing iridium(III) complexes and their efficient blue light-emitting

Abstract Two new heteroleptic iridium(III) complexes [Ir(4,6-dfppy)2(PPh3)L] (4,6-dfppy = 2-(4,6-difluorophenyl)pyridyl, PPh3 = triphenylphosphine, L = NCS−, 1; NCO−, 2) have been synthesized and fully characterized. By introduction of electron-withdrawing groups such as fluorine atoms on the 4- and 6-positions of 2-phenylpyridyl (ppy) and using strong-field ligands for instance PPh3 and pseudohalogen as ancillary ligands, the HOMO–LUMO electronic energy gaps of 1 and 2 have been increased sufficiently. The photoluminescence (PL) spectra of 1 and 2 in solution show emission maxima at 456 and 458 nm, respectively, corresponding to efficient blue light-emitting. X-ray analyses show that intra- and intermolecular π–π interactions exist in the solid state of 1 and 2. The PL spectra of 1 and 2 in solid state exhibit about 30 nm spectral red shifts compared with those in solution.

Low-energy electronic states of carbon nanocones in an electric field

Abstract The low-energy electronic states and energy gaps of carbon nanocones in an electric field are studied using a single-π-band tight-binding model. The analysis considers five perfect carbon nanocones with disclination angles of 60°, 120°, 180°, 240° and 300°, respectively. The numerical results reveal that the low-energy electronic states and energy gaps of a carbon nanocones are highly sensitive to its geometric shape (i.e. the disclination angle and height), and to the direction and magnitude of an electric field. The electric field causes a strong modulation of the state energies and energy gaps of the nanocones, changes their Fermi levels, and induces zero-gap transitions. The energy-gap modulation effect becomes particularly pronounced at higher strength of the applied electric field, and is strongly related to the geometric structure of the nanocone.