Impact of LaF3 and silica shell formation on the crystal, optical and photo-luminescence properties of LaF3:Ce/Tb nanoparticles
LaF3:Ce/Tb and LaF3:Ce/Tb@LaF3 core and core/shell nano-structures were synthesized by the polyol process at low temperature. X-ray diffraction patterns, transmission electron microscopy images, energy dispersive X-ray, FTIR, UV/Vis, and photoluminescence spectra and band gap energy were used to investigate the impact of shell formation on the crystal structure, opto-electronic, photoluminescence and lifetime properties of the core-nanoparticles (core-NPs). The TEM image shows that the as-prepared luminescent core and core/shell/SiO2-NPs consist of polycrystals aggregated with a narrow size distribution, which can be easily dispersed in aqueous and non-aqueous solvents to form a transparent colloidal solution. The TEM image illustrated that the core and core/shell/SiO2 NPs are irregular hexagonals with a mean diameter of 20–35 nm. It is observed that the band gap energy gradually decreased after shell formation which may be due to the decreased crystallinity of the luminescent nanoproducts. The excitation spectra show a characteristic charge transfer transition of Ce3+ 4f–5d(275) and all excitations of Tb3+ 7F6 → 5Dj(7Dj = 1–6) ions, respectively. The excitation, emission and decay time clearly revealed that the luminescence efficiency was greatly enhanced after inert shell formation, whereas after silica surface modification the luminescence efficiency decreased because non-radiative decay is higher in core and core/shell NPs.