Improvement of luminescence properties of NaYF :Yb /Er upconversion materials by a cross-relaxation mechanism based on co-doped Ho ion concentrations.

NaYF :Yb /Er /Ho nanophosphors were successfully synthesized using a solvothermal method and with various concentrations of Ho ions. The crystal structure, grain size, morphology, and luminescence properties were analyzed by X-ray diffraction, field-emission scanning electron microscopy, and photoluminescence measurements. All samples were crystallized as a cubic structure; it was confirmed that all samples exhibited strong green emission and weak red emission generated at a particular level of the activated ions. The strongest upconversion fluorescence intensity was observed in the Ho and Er ions co-doped NaYF materials with a Ho ion concentration of 0.005 mol. Only the green fluorescence intensity at the 542 nm centre increased strongly due to the S → I energy transfer. This increase in upconversion fluorescence intensity at a selected wavelength was described as a cross-relaxation mechanism due to the addition of Ho ions.