Up-conversion luminescence of the NaYF4:Yb3+,Er3+ nanomaterials prepared with the solvothermal synthesis

The presence of the oleic acid was observed in the FT-IR spectra. The TEM images showed small quasi-spherical nanoparticles as well as long nanorods. The XPD measurements revealed that both cubic and hexagonal forms of NaYF4 were present in the materials. The crystallite sizes ranged from ca. 20 to over 150 nm for the cubic and hexagonal phases, respectively. The characteristic up-conversion luminescence of Er3+ in red (640-685 nm; 4F9/2 → 4I15/2) and green (515-560 nm; 2H11/2, 4S3/2 → 4I15/2 transitions) wavelengths was observed. The most intense luminescence and the longest luminescence emission lifetime were obtained with the material annealed for 12 h at 177 °C with 1.8 MPa pressure due to the predominance of the well-crystallized hexagonal form of NaRF4 (R: Y, Yb, Er).