The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

https://iopscience.iop.org/article/10.1088/0953-8984/9/4/002/pdf

First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+ U method

Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

Nano‐photocatalytic Materials: Possibilities and Challenges

We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++‐Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susc...

Diluted magnetic semiconductors

Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organicdevices have not yet reached those of their inorganic counterparts (η ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400094498

Organic solar cells: An overview

We report new insights into the electrochemical reduction of CO2 on a metallic copper surface, enabled by the development of an experimental methodology with unprecedented sensitivity for the identification and quantification of CO2 electroreduction products. This involves a custom electrochemical cell designed to maximize product concentrations coupled to gas chromatography and nuclear magnetic resonance for the identification and quantification of gas and liquid products, respectively. We studied copper across a range of potentials and observed a total of 16 different CO2 reduction products, five of which are reported here for the first time, thus providing the most complete view of the reaction chemistry reported to date. Taking into account the chemical identities of the wide range of C1–C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multicarbon products involving enol-like surface intermediates as a possible pathway, accounting for the observed selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcohols, and carboxylic acids.

New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces

The electronic structure of copper oxides has been investigated by photoelectron (x-ray photoemission, ultraviolet photoemission), Auger electron, and bremsstrahlung isochromat spectroscopies. The experimental results are compared with one-electron band-structure calculations as well as with a cluster configuration interaction model. It is demonstrated that the results for ${\mathrm{Cu}}_{2}$O agree well with band theory, whereas those for CuO clearly show strong deviations which we argue are due to electron-correlation effects in the open-shell $d$ bands. From the comparison to cluster calculations we extract values for the $\mathrm{Cu} d\ensuremath{-}d$ and $\mathrm{O} p\ensuremath{-}p$ Coulomb interactions, the O to Cu charge transfer energy, and the degree of $\mathrm{Cu} d\ensuremath{-}\mathrm{O} 2p$ hybridization. From this we demonstrate that CuO is a charge-transfer gap insulator.

Electronic structure of Cu2O and CuO

The oxygen 1s x-ray-absorption edges of a series of 3d-transition-metal oxides have been measured. The structures at the edge arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of mainly metal character. The spectra can be divided into two regions: The first is a double-peaked sharp structure near threshold, which can be related to the metal 3d states; the second is a broader structure 5--10 eV above the edge and is related to the metal 4s and 4p bands. We attribute the oxygen p character up to 15 eV above threshold to mainly oxygen 2p character. The data are analyzed in terms of ligand-field and exchange splittings. It is shown that the splitting between the two sharp peaks near threshold is related closely to the ligand-field splitting, but the relative intensities of the peaks are not fully explained at the present time.

Oxygen 1s x-ray-absorption edges of transition-metal oxides

Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1-biphenyl1-4,4′-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Although both electrode surfaces have a similar work function (∼5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules.

/pdf/conjugated-organic-molecules-on-metal-versus-polymer-2acnl4o5us.pdf

Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism

The electronic structure of ${\mathrm{Ag}}_{2}$O has been investigated by photoelectron, Auger-electron, and bremsstrahlung isochromat spectroscopy. The experimental results are compared with model one-particle band-structure and two-particle cluster calculations. We extract values for the on-site Ag 4d and O 2p energies, the Ag 4d--O 2p transfer integrals, as well as the on-site Coulomb Ag 4d-4d and O 2p-2p interactions. A comparison with ${\mathrm{Cu}}_{2}$O and CuO is made, and the differences are discussed in view of high-${T}_{c}$ superconductivity and the instability of AgO.

Electronic structure of Ag2O.

Using oxygen $K$-edge x-ray-absorption spectroscopy we show that the holes compensating the ${\mathrm{Li}}^{1+}$ impurity charge in ${\mathrm{Li}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ are located primarily in the O $2p$ states rather than in the conventionally assumed Ni $3d$ states These holes are bound to the ${\mathrm{Li}}^{1+}$ impurities forming states in the NiO band gap We also argue that these O $2p$ holes have a very large antiferromagnetic exchange interaction with neighboring ${\mathrm{Ni}}^{2+}$ (${d}^{8}$) spins, which makes them appear like low-spin ${\mathrm{Ni}}^{3+}$ states in macroscopic magnetic measurements This is similar to what could be happening in the Cu-based high-${T}_{c}$ superconductors

Jacques Ghijsen

Papers

Electronic structure of Cu2O and CuO

Oxygen 1s x-ray-absorption edges of transition-metal oxides

Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism

Electronic structure of Ag2O.

Character of holes in lixni1-xo and their magnetic-behavior