Nonstatistical Dopant Distribution of Ln3+-Doped NaGdF4 Nanoparticles

Abstract

Oleate-stabilized NaGdF4 nanoparticles codoped with 20% Y3+ and 5% Tb3+ (NaGdF4:Y,Tb), with 20% Nd3+ (NaGdF4:Nd), and with 20% Tb3+ (NaGdF4:Tb) were prepared in organic medium. The distribution of dopant ions was studied using synchrotron radiation X-ray photoelectron spectroscopy along with X-ray powder diffractometry, transmission electron microscopy, energy dispersive X-ray spectroscopy, and luminescence spectroscopy. Results show that these nanoparticles do not have the intended statistical dopant distribution despite the fact that different synthesis procedures and dopant ions with different ionic radii were used. NaGdF4:Y,Tb nanoparticles have a subtle gradient structure with Gd3+ more concentrated toward the center and Y3+ more concentrated toward the surface of the nanoparticles. NaGdF4:Nd nanoparticles have a steep gradient structure with Gd3+ more concentrated toward the center and Nd3+ more concentrated toward the surface of the nanoparticles. Even NaGdF4:Tb nanoparticles have a steep gradient structure with Tb3+ more concentrated toward the center and Gd3+ more concentrated toward the surface of the nanoparticles in spite of the very similar ionic radius of Gd3+ and Tb3+. This work shows that the general assumption that Ln3+ dopant ions in lanthanide-based nanoparticles are statistically distributed in the whole nanoparticle may not be true.