Improved Route for the Synthesis of Colloidal NaYF₄ Nanocrystals and Electron Spin Resonance of Gd³⁺ Local Probe

This paper presents the synthesis and characterization of colloidal NaYF₄ and NaYF₄:20% Gd lanthanide nanocrystals. The nanoparticles were prepared by chemical route using co-thermolysis of Na(CF₃COO), Y(CF₃COO)₃ and Gd(CF₃COO)₃ precursor in oleylamine surfactant/phenylether at T _s = 250 °C. By tuning the precursor/surfactant molar ratio during the process, it was possible to control the crystalline phase, chemical order and size of the nanocrystals. The nanocrystals were characterized by Transmission Electron Microscopy, Small Angle X-ray Scattering, powder X-ray Diffraction, dc-magnetization and Electron Spin Resonance (ESR) techniques. The ESR experiments show the so called U-spectrum for the Gd³⁺ ions in bulk counterpart materials, where characteristic powder spectra of cubic and lower crystal field symmetries were observed.