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

Abstract

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 (⁴G_7/2 → ⁴I_11/2) emitted before 654 nm (⁴F_9/2 → ⁴I_15/2) with Yb³⁺ dramatically increasing (opposite to the normal low Yb³⁺ doping cases), representing a new four-photon pathway (⁴I_15/2 → ⁴I_11/2 → ⁴F_7/2 → ²H_11/2 →⁴S_3/2 → ²G_7/2 → ⁴G_11/2 → ²H_9/2 → ²D_5/2 → ⁴G_7/2). 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.