Er³⁺ Sensitized Photon Upconversion Nanocrystals

Lanthanide doped upconversion nanocrystals, showing bright future in diverse fields, are typically excited by ≈700–1000 nm light when Nd³⁺ and Yb³⁺ are used as sensitizers. Thus far, extending the excitation range of upconversion nanocrystals is still a formidable challenge. Herein, a new type of upconversion nanocrystals is reported, using Er³⁺ ions as sensitizers, which can be excited by 1532 nm light located in the second near-infrared biological window. Through Er³⁺ sensitization, upconversion emission from a series of activators, including Nd³⁺, Ho³⁺, Eu³⁺, and Tm³⁺, is obtained and can be modulated by Yb³⁺ codoping. In addition, Er³⁺ sensitized photon upconversion of Ho³⁺ and Tm³⁺ can be further enhanced by shell coating. It is found that Er³⁺ sensitized upconversion processes are mainly dependent on the energy transfer between Er³⁺ ions and activators. Considering the demonstration of anticounterfeiting by using this newly designed nanocrystal, it is anticipated that these results can bring more opportunities to upconversion nanomaterials in other aspects, ranging from lasing to super resolution imaging.