Engineering Er³⁺-sensitized nanocrystals to enhance NIR II-responsive upconversion luminescence

An Er³⁺-sensitized system with a high response to 1550 nm radiation in the second near-infrared window (NIR II) has been considered for a new class of potential candidates for applications in bio-imaging and advanced anti-counterfeiting, yet the achievement of highly efficient upconversion emission still remains a challenge. Here, we constructed a novel Er³⁺-sensitized core-shell-shell upconversion nanostructure with a Yb³⁺-enriched core as the emitting layer. This designed nanostructure allows the Yb³⁺ emitting layer to more efficiently trap and lock excitation energy by combining the interfacial energy transfer (IET) from the shell (Er³⁺) to the core (Yb³⁺), high activator Yb³⁺ content, and minimized energy back-transfer. As a result, the NIR II emission at 1000 nm is remarkably enhanced with a high quantum yield (QY) of 11.5%. Based on this trap and lock-in effect of the excitation energy in the Yb³⁺-enriched core, highly efficient 1550 nm-responsive visible and NIR upconversion emissions are also achieved by co-doping with other activator ions (e.g., Ho³⁺ and Tm³⁺). Our research provides a new functional design for improving NIR II-responsive upconversion luminescence, which is significant for developing practical applications.