The effect of Er(3+) concentration on the kinetics of multiband upconversion in NaYF(4):Yb/Er microcrystals

In Yb-Er co-doped upconversion (UC) nanomaterials, upconversion luminescence (UCL) can be modulated to generate multiband UCL emissions by changing the concentration of activator Er(3+). Nonetheless, the effect of the Er(3+) concentrations on the kinetics of these emissions is still unknown. We here study the single β-NaYF(4):Yb(3+)/Er(3+) microcrystal (MC) doped with different Er(3+) concentrations by nanosecond time-resolved spectroscopy. Interestingly, different Er(3+) doping concentrations exhibit different UCL emission bands and UCL response rates. At low Er(3+) doping concentrations (1 mol%), multiband emission in β-NaYF(4):Yb(3+)/Er(3+) (20/1 mol%) MCs could not be observed and the response rate of UCL was slow (5–10 μs) in β-NaYF(4):Yb(3+)/Er(3+). Increasing the Er(3+) doping concentration to 10 mol% can shorten the distance between Yb(3+) ions and Er(3+) ions, which promotes the energy transfer between them. β-NaYF(4):Yb(3+)/Er(3+) (20/10 mol%) can achieve obvious multiband UCL and a quick response rate (0.3 µs). However, a further increase in the Er doping concentration (80 mol%) makes MCs limited by the CR process and cannot achieve the four-photon UC process ((4)F(5/2) → (2)K(13/2) and (2)H(9/2) → (2)D(5/2)). Therefore, the result shows that changing the Er(3+) doping concentration could control the energy flow between the different energy levels in Er(3+), which could affect the response time and UCL emission of the Yb/Er doped rare earth materials. Our work can facilitate the development of fast-response optoelectronics, optical-sensing, and display industries.