The impact of Yb3+ concentration on multiband upconversion in a single NaYF4:Yb/Er microcrystal determined via nanosecond time-resolved spectroscopy

In lanthanide-sensitized upconversion (UC) nanomaterials, the typical sensitizer Yb3+ can significantly modulate the codoped activator (such as Er3+ ions) to generate multiband transitions. However, the kinetics of these multiband emissions affected by the Yb3+ concentration remains unclear. In this work, we employ nanosecond time-resolved spectroscopy to investigate a single optically trapped NaYF4:Yb/Er microcrystal (MC) doped with different Yb3+ concentrations. Interestingly, high doping Yb3+ ions shorten the UC response time by a factor of 10 (from ∼1 μs to ∼100 ns) compared to low doping Yb3+. Meanwhile, 430 nm light (4G7/2 → 4I11/2) emitted before 654 nm (4F9/2 → 4I15/2) with Yb3+ dramatically increasing (opposite to the normal low Yb3+ doping cases), representing a new four-photon pathway (4I15/2 → 4I11/2 → 4F7/2 → 2H11/2 →4S3/2 → 2G7/2 → 4G11/2 → 2H9/2 → 2D5/2 → 4G7/2). Furthermore, we observed a broad range of time-dependent emission color, changing from being initially green over near-white to red. Particularly, the red emission is dominated in the decay process, yielding insight into the fundamental multiband UC kinetics. The result shows that varying the Yb3+ doping concentration provides a way to control the energy flow between the energy levels of Er3+ ions, which can modulate the response time and color evolution of UC on Yb/Er doped materials. This can direct the prospective development of fast-response optoelectronic devices, photocatalysis, and UC displays in the future.