Strong enhancement of the upconversion emission in ZrO2: Yb3+, Er3+, Gd3+ nanocubes synthesized with Na2S

In this work, the structural, morphological and luminescent characterization of ZrO2:Yb3+,Er3+, Gd3+ nanocrystals prepared with Na2S by precipitation method is reported. The XRD spectra showed a mixture of tetragonal (88%) and monoclinic phases (12%) in samples fabricated with and without Na2S. The tetragonal phase (100%) was stabilized by the introduction of the Gd3+. According to TEM images, samples showed a cubic morphology with an average size of 150 nm, when both, Na2S and Gd3+ were added. Red (678 nm), green (545 nm) and ultraviolet (275, 285 nm) emissions were obtained as a result of the upconversion process due to the energy transfer between Yb3+-Er3+-Gd3+ ions, under 970 nm of excitation. The integrated red band showed an increment of 40% and 125%, for samples prepared with Na2S and combined with Gd3+, respectively, compared to the ZrO2:Yb3+-Er3+ sample. Such increments are explained in terms of the surface pasivation by using Na2S during the synthesis process and the role of Gd3+ as a reservoir of energy from Er3+.