Improvement of luminescence properties of NaYF 4 :Yb 3+ /Er 3+ upconversion materials by a cross‐relaxation mechanism based on co‐doped Ho 3+ ion concentrations

NaYF 4 :Yb 3+ /Er 3+ /Ho 3+ nanophosphors were successfully synthesized using a solvothermal method and with various concentrations of Ho 3+ 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 3+ and Er 3+ ions co‐doped NaYF 4 materials with a Ho 3+ ion concentration of 0.005 mol. Only the green fluorescence intensity at the 542 nm centre increased strongly due to the 4 S 3/2 → 4 I 15/2 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 3+ ions.