A high-performance white-light-emitting-diodes based on nano-single crystal divanadates quantum dots

We report a high-performance phosphors-free white light-emitting-diodes (w-LEDs) using Ba(2)V(2)O(7) or Sr(2)V(2)O(7) quantum dots that directly heteroepitaxially grown on common quartz substrates by polymer assisted deposition (PAD). The quantum efficiency of quantum dots is as high as 95%. More importantly, electronic local functions, band structure and partial density of states have been firstly calculated to study the luminescent and heteroepitaxial growth mechanisms by the Ab-initio Simulation. Additionally, the glaring white light excited at a wavelength of 325 nm was experimentally observed, which unambiguously demonstrated that such quantum dots can be efficient w-LEDs for solid state lighting

Quantum chemical study of Co 3+ spin states in LaCoO 3

Ab initio quantum-chemical cluster calculations are performed for the perovskite LaCoO 3. The main concern is to calculate the energy level ordering of different spin states of Co 3+ , which is an issue of great controversy for many years. The calculations performed for the trigonal lattice structure at T=5 K and 300 K, with the structural data taken from experiment, display that the low-spin (LS, S=0) ground state is separated from the first excited high-spin (HS, S=2) state by a gap >100 meV, while the intermediate-spin (IS, S=1) state is located at much higher energy ≈0.5 eV. We suggest that the local lattice relaxation around the Co 3+ ion excited to the HS state and the spin-orbit coupling reduce the spin gap to a value ~10 meV. Coupling of the IS state to the Jahn-Teller local lattice distortion is found to be rather strong and reduces its energy position to a value of 200 $\div$ 300 meV. Details of the quantum-chemical cluster calculation procedure and the obtained results are extensively discussed and compared with those reported earlier by other authors. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010

Spontaneous cationic ordering in chemicalsolution- grown La2CoMnO6 double perovskite thin films

Double perovskite oxides are of interest because of their electric, magnetic, and elastic properties; however, these properties are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Therefore, many efforts have been made to improve the level of cationic ordering to obtain optimal properties while suppressing antisite defect formation. Here, epitaxial double perovskite La2CoMnO6 thin films were grown on top of (001)-STO oriented substrates by a polymer-assisted deposition chemical solution approach. Confirmation of the achievement of full Co/Mn cationic ordering was found by scanning transmission electron microscopy (STEM) measurements; EELS maps indicated the ordered occupancy of B–B′ sites by Co/Mn cations. As a result, optimal magnetic properties (Msat ≈ 6 µB/f.u. and Tc ≈ 230 K) are obtained. We show that the slow growth rates that occur close to thermodynamic equilibrium conditions in chemical solution methods represent an advantageous alternative to physical deposition methods for the preparation of oxide thin films in which complex cationic ordering is involved.We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496), COACHSUPENERGY project (MAT2014-51778-C2-1-R) and MAT2015-71664-R, cofinanced by the European Regional Development Fund. Support from the European Union′s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie grant agreement no. 645658 (DAFNEOX Project) is also acknowledged. H.W. acknowledges financial support from the China Scholarship Council (CSC). J.G. also acknowledges the Ramon y Cajal program (RYC-2012-11709). The STEM–EELS analysis was sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The authors would like to thank Anna Crespi and Francesc Xavier Campos for assistance with the 3D reciprocal space tomography and reciprocal space map measurements.Peer reviewe