Selective Growth of Upconverting YbPO₄:Ln (Ln = Er or Tm) Crystals in a Micro Reaction Cell

Abstract

Highly crystalline upconversion fluorescence crystals of ytterbium phosphate (YbPO 4 ) doped with Er 3+ or Tm 3+ were selectively and directly grown on a hydrophilic/hydrophobic self-assembled monolayer (SAM) template patterned by vacuum ultraviolet (VUV, λ = 172 nm) light irradiation. First, a gelatin gel with an inner solution [(NH 4 ) 2 HPO 4 ] was selectively adsorbed on a hydrophilic surface, and the adsorbed gel was successfully used as the micro reaction cell between the inner solution and the outer solution [(CH 3 COO) 3 Yb·4H 2 O]. The low crystalline YbPO 4:Ln (Ln = Er or Tm) nanospheres, which are densely aggregated, were selectively and directly grown on the hydrophilic surface at 5 °C for 5 days. After annealing at 1100 °C for 10 h, their shape changed dramatically from an aggregation of nanospheres to an aggregation of well-formed nanocubes (200−600 nm). In addition, atoms and molecules of Yb, PO 4, and Ln were highly and uniformly distributed in the YbPO 4:Ln crystal. The YbPO 4:Er and YbPO 4:Tm crystals clearly show green and blue upconversion fluorescence, respectively, under 980 nm (near-infrared) laser irradiation.