Zhi Ren Xiao

All-in-one light-tunable borated phosphors with chemical and luminescence dynamical control resolution.

Single-composition white-emitting phosphors with superior intrinsic properties upon excitation by ultraviolet light-emitting diodes are important constituents of next-generation light sources. Borate-based phosphors, such as NaSrBO3:Ce(3+) and NaCaBO3:Ce(3+), have stronger absorptions in the near-ultraviolet region as well as better chemical/physical stability than oxides. Energy transfer effects from sensitizer to activator caused by rare-earth ions are mainly found in the obtained photoluminescence spectra and lifetime. The interactive mechanisms of multiple dopants are ambiguous in most cases. We adjust the doping concentration in NaSrBO3:RE (RE = Ce(3+), Tb(3+), Mn(2+)) to study the energy transfer effects of Ce(3+) to Tb(3+) and Mn(2+) by comparing the experimental data and theoretical calculation. The vacuum-ultraviolet experimental determination of the electronic energy levels for Ce(3+) and Tb(3+) in the borate host regarding the 4f-5d and 4f-4f configurations are described. Evaluation of the Ce(3+)/Mn(2+) intensity ratios as a function of Mn(2+) concentration is based on the analysis of the luminescence dynamical process and fluorescence lifetime measurements. The results closely agree with those directly obtained from the emission spectra. Density functional calculations are performed using the generalized gradient approximation plus an on-site Coulombic interaction correction scheme to investigate the forbidden mechanism of interatomic energy transfer between the NaSrBO3:Ce(3+) and NaSrBO3:Eu(2+) systems. Results indicate that the NaSrBO3:Ce(3+), Tb(3+), and Mn(2+) phosphors can be used as a novel white-emitting component of UV radiation-excited devices.

Investigation of possible half-metallic antiferromagnets on double perovskites LaABB′O6 (A=Ca,Sr,Ba; B,B′=transition elements)

We thoroughly investigated all the possible (C229=406) candidates of half-metallic (HM) antiferromagnet (AFM) with the structure LaSrBB′O6, where BB′ pairs are any combination of two 3d, 4d, and 5d transition elements. Sr was also replaced by Ca or Ba whenever HM-AFM was found and similar calculation was performed in order to probe more possibilities. We have found that LaAWXO6, where A=Ca,Sr,Ba and X=Tc,Re are potential candidates of HM-AFM. Among those HM-AFM candidates, LaAWReO6 (A=Sr,Ba) are rather robust than others for the structural optimization. Including previous Wang and Guo’s work in 2006 and this work, we believe that we have found all possible HM-AFM candidates for double perovskites structure LaABB′O6 (A=Ca,Sr,Ba and B,B′=transition elements).

Oxygen vacancy induced ferromagnetism in V2O5-x

Ab initio calculations within density functional theory with generalized gradient approximation have been performed to study the effects of oxygen vacancies on the electronic structure and magnetism in undoped V 2 O 5- x (0 < x < 0.5). It is found that the introduction of oxygen vacancies would induce ferromagnetism in V 2 O 5- x with the magnetization being proportional to the O vacancy concentration x . The calculated electronic structure reveals that the valence electrons released by the introduction of oxygen vacancies would occupy mainly the neighboring V d x y -dominant band which then becomes spin-polarized due to intra-atomic exchange interaction, thereby giving rise to the half-metallic ferromagnetism.