Masahiko Isobe

Rare earth-free high color rendering white light-emitting diodes using CsVO3 with highest quantum efficiency for vanadate phosphors

We have developed a reliable water-assisted solid-state reaction for synthesizing CsVO3, which is an excellent rare earth (RE)-free phosphor. The polycrystalline CsVO3 had a large particle size of 4.3 μm and a very smooth crystal surface. The maximum internal quantum efficiency (IQE) value of CsVO3 at 345 nm excitation was 95.8%, which is the highest value reported for vanadate phosphors. The IQE averaged for 10 samples was 92.3%, which was much higher than that of CsVO3 prepared by the conventional solid-state reaction (84.4%). Moreover, the photoluminescence excitation band edge was red-shifted by 10 nm, resulting in the large increase of IQE in the near-ultraviolet range; the IQE at 400 nm remained at 85%. We prepared RE-free white light-emitting diodes (WLEDs) from the CsVO3 powders and the red phosphor Mg2TiO4:Mn4+ for emission color tuning. The emission of the WLED was white at CIE (x, y) = (0.330, 0.382), the averaged color rendering index (CRI) was 91.1, and all CRIs exceeded 83. Thus, we fabricated high color rendering RE-free WLEDs by using polycrystalline CsVO3 with an increased IQE.

Crystallographic features related to a van der Waals coupling in the layered chalcogenide FePS3

We investigated the crystallographic structure of FePS3 with a layered structure using transmission electron microscopy and powder X-ray diffraction. We found that FePS3 forms a rotational twin structure with the common axis along the c*-axis. The high-resolution transmission electron microscopy images revealed that the twin boundaries were positioned at the van der Waals gaps between the layers. The narrow bands of dark contrast were observed in the bright-field transmission electron microscopy images below the antiferromagnetic transition temperature, TN ≈ 120 K. Low-temperature X-ray diffraction showed a lattice distortion; the a- and b-axes shortened and lengthened, respectively, as the temperature decreased below TN. We propose that the narrow bands of dark contrast observed in the bright-field transmission electron microscopy images are caused by the directional lattice distortion with respect to each micro-twin variant in the antiferromagnetic phase.