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Register a new userMany-Body electronic structure calculations of Eu doped ZnO
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The formation energies and electronic structure of europium doped zinc oxide has been determined using DFT and many-body $GW$ methods. In the absence of intrisic defects we find that the europium-$f$ states are located in the ZnO band gap with europium possessing a formal charge of 2+. On the other hand, the presence of intrinsic defects in ZnO allows intraband $f-f$ transitions otherwise forbidden in atomic europium. This result coorroborates with recently observed photoluminescence in the visible red region [1].
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Based on density functional theory calculations, we systematically investigate the behaviors of a H atom in Ag-doped ZnO, involving the preference sites, diffusion behaviors, the electronic structures and vibrational properties. We find that a H atom can migrate to the doped Ag to form a Ag-H complex by overcoming energy barriers of 0.3 - 1.0 eV. The lowest-energy site for H location is the bond center of a Ag-O in the basal plane. Moreover, H can migrate between this site and its equivalent sites with energy cost of less than 0.5 eV. In contrast, dissociation of such a Ag-H complex needs energy of about 1.1 - 1.3 eV. This implies that the Ag-H complexes can commonly exist in the Ag-doped ZnO, which have a negative effect on the desirable p-type carrier concentrations of Ag-doped ZnO. In addition, based on the frozen phonon calculation, the vibrational properties of ZnO with a Ag-H complex are predicted. Some new vibrational modes associated with the Ag-H complex present in the vibrational spectrum of the system.
Doping is one of the most common strategies for improving the photocatalytic and solar energy conversion properties of TiO$_2$, hence an accurate theoretical description of the electronic and optical properties of doped TiO$_2$ is of both scientific and practical interest. In this work we use many-body perturbation theory techniques to investigate two typical n-type dopants, Niobium and Hydrogen, in TiO$_2$ rutile. Using the GW approximation to determine band edges and defect energy levels, and the Bethe Salpeter equation for the calculation of the absorption spectra, we find that the defect energy levels form non-dispersive bands %associated with localized states lying $simeq 2.2 eV$ above the top of the corresponding valence bands ($simeq 0.9 eV$ below the conduction bands of the {it pristine} material). The defect states are also responsible for the appearance of low energy absorption peaks that enhance the solar spectrum absorption of rutile. The spatial distributions of the excitonic wavefunctions associated with these low energy excitations are very different for the two dopants, suggesting a larger mobility of photoexcited electrons in Nb-TiO$_2$.
We have studied the electronic structure of Zn$_{0.9}$Fe$_{0.1}$O nano-particles, which have been reported to show ferromagnetism at room temperature, by x-ray photoemission spectroscopy (XPS), resonant photoemission spectroscopy (RPES), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). From the experimental and cluster-model calculation results, we find that Fe atoms are predominantly in the Fe$^{3+}$ ionic state with mixture of a small amount of Fe$^{2+}$ and that Fe$^{3+}$ ions are dominant in the surface region of the nano-particles. It is shown that the room temperature ferromagnetism in the Zn$_{0.9}$Fe$_{0.1}$O nano-particles is primarily originated from the antiferromagnetic coupling between unequal amounts of Fe$^{3+}$ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of $sim$ 2-3 nm.
Cyclometalled Ir(III) compounds are the preferred choice as organic emitters in Organic Light Emitting Diodes. In practice, the presence of the transition metals surrounded by carefully designed ligands allows the fine tuning of the emission frequency as well as a good efficiency of the device. To support the development of new compounds the experimental measurements are generally compared with ab-initio calculation of the absorption and emission spectra. The standard approach for these calculations is TDDFT with hybrid exchange and correlation functional like the B3LYP. Due to the size of these compounds the application of more complex quantum chemistry approaches can be challenging. In this work we used Many Body Perturbation Theory approaches (in particular the GW approximation with the Bethe-Salpeter equation) implemented in gaussian basis sets, to calculate the quasiparticle properties and the adsorption spectra of six cyclometalled Ir(III) complexes going behind TDDFT. In the presented results we compared standard TDDFT simulation with BSE calculations performed on top on perturbative G 0 W 0 and accounting for eigenvalue self consistency. Moreover, in order to investigate in detail the effect of the DFT starting point, we concentrate on Ir(ppy) 3 performing GW-BSE simulations starting from different DFT exchange and correlation potentials.
We present results for the electronic structure of alpha uranium using a recently developed quasiparticle self-consistent GW method (QSGW). This is the first time that the f-orbital electron-electron interactions in an actinide has been treated by a first-principles method beyond the level of the generalized gradient approximation (GGA) to the local density approximation (LDA). We show that the QSGW approximation predicts an f-level shift upwards of about 0.5 eV with respect to the other metallic s-d states and that there is a significant f-band narrowing when compared to LDA band-structure results. Nonetheless, because of the overall low f-electron occupation number in uranium, ground-state properties and the occupied band structure around the Fermi energy is not significantly affected. The correlations predominate in the unoccupied part of the f states. This provides the first formal justification for the success of LDA and GGA calculations in describing the ground-state properties of this material.
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