Synthesis of Hexagonal-Phase NaYF4:Yb,Er and NaYF4:Yb,Tm Nanocrystals with Efficient Up-Conversion Fluorescence

IR‐to‐visible up‐conversion fluorescent nanocrystals of hexagonal‐phase NaYF4:20 %Yb,2 %Er and NaYF4:20 %Yb,2 %Tm have been synthesized by decomposition of multiprecursors of CF3COONa, (CF3COO)3Y, (CF3COO)3Yb, and (CF3COO)3Er/(CF3COO)3Tm in oleylamine at 330 °C. The average particle size is 10.5 ± 0.7 nm (from random measurements of 200 particles from five transmission electron microscopy images) and 11.1 ± 1.3 nm (from dynamic‐light‐scattering measurements). The up‐conversion fluorescence intensity of the hexagonal nanocrystals in this work is much higher than that of other cubic‐phase NaYF4:Yb,Er nanocrystals, including the ones in this work (by a factor of 7.5). Mechanisms for nucleation and growth of the hexagonal‐phase nanoparticles are proposed. These nanocrystals are easily dispersed in organic solvents, producing a transparent colloidal solution. The hydrophobic surfaces of the particles are made hydrophilic using a bipolar surfactant. These nanoparticles and their dispersions in various media have potential applications in optical nanodevices and bioprobes. IR‐to‐visible up‐conversion fluorescent nanocrystals of hexagonal‐phase NaYF4:20%Yb,2%Er and NaYF4:20%Yb,2%Tm have been synthesized. They form a transparent colloidal solution and produce intense green (NaYF4:20%Yb,2%Er) and blue (NaYF4:20%Yb,2%Tm) up‐conversion fluorescence (see figure) under 980 nm near‐IR excitation. They have potential applications in optical nanodevices and bioprobes.