Investigating the effect of Yb3+ and Er3+ concentration on red/green luminescent ratio in β-NaYF4: Er, Yb nanocrystals using spectroscopic techniques

β-NaYF4 co-doped with Er3+ and Yb3+ ions were successfully synthesized at the reaction temperature as low as 180 °C by the two steps procedure using the rare-earth stearates as precursors. The XRD and TEM data show that the NaYF4 nanocrystals were relatively uniform with the approximate size of 50 nm and the dopants led to shrinking of NaYF4 crystals. Moreover, the EDX mapping analysis indicate that the Yb3+, Er3+ ions were distributed homogeneously in host matrix even the Yb3+ doping concentration as high as 25%. The PL spectroscopy was used to investigate the optical properties of these systems. The red/green emission ratio of Er3+ was controlled by changing the doping concentration in matrix. An increase of Yb3+ contents from 2 to 25 mol % induced a remarkably tunable emission from green to red. Addition, the highest red/green ratio was also achieved for the sample of 1% Er3+ and 20% Yb3+. Combining the results of the up-conversion emission intensity as a function of the pump power and luminescence data, the energy transfer mechanism between the Yb3+ sensitizers and Er3+ activators was demonstrated. The intense efficiency of red emission under 980 nm excitation provides the potential applications in bio-labeling and bio-imaging.