Hydrothermal synthesis and upconversion luminescence properties of β-NaGdF4:Yb3+/Tm3+ and β-NaGdF4:Yb3+/Ho3+ submicron crystals with regular morphologies

Abstract

Single phase β-NaGdF(4):Yb(3+)/Tm(3+) and β-NaGdF(4):Yb(3+)/Ho(3+) submicron crystals with various morphologies including hexagonal prisms, spindles, and spheres were synthesized via the one-step hydrothermal method by controlling the pH values and sort of chelators (EDTA and citric acid). The prepared products showed intense up-converted luminescence (UCL) pumped by infrared laser at 980 nm. The hexagonal prisms that meaning high degree crystallinity demonstrated strong UCL in comparison with other morphologies such as spindles and spheres. In β-NaGdF(4):Yb(3+)/Tm(3+), UCL not only appeared transitions from (1)G(4), (1)D(2), and (1)I(6) states to the lower lying states of Tm(3+), but also (6)P(J)→(8)S(7/2) transition (310 nm) of Gd(3+). These UCL were responsible for three, five, and six photons processes determined by pump power dependence of UCL intensities. The observation of UCL of Gd(3+) implied occurrence of energy transfer from Tm(3+):(1)I(6) to Gd(3+):(6)P(J).