Positron annihilation study of lattice defects and nanoporous structures in Mn4+ doped K2SiF6 nanophosphors exhibiting high quantum yield

The present paper reports an application positron annihilation spectroscopy (PAS) to systematically study the formation of lattice defects, nanoporous structures and sites of Mn4+ ions doped into the K2SiF6 (KSFM) nanophosphors prepared using four different solvents. Three main techniques of PAS, including positron annihilation lifetime (PAL), Doppler broadening (DB) and electron momentum distribution (EMD) measurements, were simultaneously carried out under the same vacuum condition. The structural characteristics and quantum yield (QY) of KSFM nanophosphors were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), coupled plasma mass spectroscopy (ICP-MS), photonic multichannel analyzer-spectrophotometer (PMAS) and fluorescence spectrophotometer (PLS). The obtained results indicated that decreasing the dielectric constant of the solvents led to the reduction of the particle sizes in KSFM. The occupation of Mn4+ to the sites of Si4+ in the SiF62– octahedron structures of KSFM as well as the high concentration of lattice defects and nanopores (average size of 0.390–0.621 nm) in the doped materials that used isopropanol and diethyl ether solvents were indicated by PAS. Moreover, increasing the concentration of doped Mn4+ caused a decrease of the occupation of Mn4+ to the adjacent Si-vacancy and K-divacancy and at the same time, the number of Si-vacancies in KSFM increased. Combining PAS and QY measurements, we found that the QY of KSFM nanophosphors depended on not only the porous structures (nanopores) but also the concentration of doped Mn4+ in the sites of Si as well as the concentration of Si-vacancies.