Structural, morphology and optical properties of NaYF4 thin films doped with trivalent lanthanide ions

NaYF4 and NaYF4:Ln(3+) (Ln(3+) = Nd3+, Eu3+, Yb3+, Er3+) thin films were prepared by electrodeposition technique. After deposition, all samples were thermally annealed and then fully characterized in order to know their structural, morphology and optical properties. Results have shown the formation of cubic phase NaYF4, i.e. no additional phases were detected even when doping with Ln(3+) ions due to the direct substitutional mechanism between Y3+ Ln(3+) ions. Although, when Ln(3+) ions enter into the crystal lattice small differences are detected in the lattice parameter due to differences in ionic radius size of the Ln(3+) ions. The differences in ionic radius size also affect the particle size and the agglomerated structures size of the Ln(3+) doped NaYF4 thin films, which is related to small variations in pH of electrolytic solutions during the electrodeposition process. Being NaYF4 a disordered matrix, when doped with Ln(3+) ions; variations on the local crystal field arise for the second nearest neighbors of the Ln(3+) ions, imposing thus a slightly different crystal field environment for each subset of Ln(3+) ions. Such differences imply an inhomogeneous broadening of their bands corresponding to 4f-4f transitions in the photoluminescence spectra unlike those from ordered crystals that exhibit narrow bands.