Single-band red upconversion luminescence of β-NaLuF4:Gd, Yb, Er@SiO2 submicron particles via annealing without agglomeration

Although the effect of annealing on upconversion luminescence (UCL) of lanthanide-doped NaREF4 has been studied extensively, it is either for inefficient -phase samples or for agglomerated β-phase ones. In this work, β-NaLuF4:Gd, Yb, Er submicron particles with efficient UCL were synthesized first by a facile solvothermal route, and then were coated with agglomeration-blocking SiO2 shell by a modified Stöber procedure, last UCL of the core@shell particles after different annealing treatments was studied, and we found large increase of red-to-green intensity ratio (R/G) up to 42 after annealing at 400 °C for 8 h, accompanying with significant shortening of decay time of the red UCL. Relationship between particles' UCL and structure changes after annealing treatment was investigated in detail using in-situ spectroscopic measurement, in combination with in-situ temperature-change x-ray diffraction measurement. The initial increase of intensity for particles after annealing below 350 °C was ascribed to the crystal refinement of the particle cores; as the annealing temperature increase to between 350 and 400 °C, nearly single-band red UCL is achieved due to the particle cores gradual transition from single crystal to many much smaller poly-crystals accompanying with shortening of Er3+-Er3+ distance, indicated by the appearance of nano-pores; as the annealing temperature increases further, UCL weakens rapidly because of phase transition from hexagonal to cubic phases in the center area of the particle core and transition from crystalline phase to amorphous phase around the periphery of the core. In addition, the SiO2 shell coated on particles' surface is able to prevent particles from agglomeration and delay the phase transition from hexagonal to cubic phases.