Achieving High Quantum Efficiency in Mn5+ Activated Phosphors for NIR‐II Deep Bioimaging Application

Abstract

AbstractMn5+ emission is a promising candidate for imaging deep tissue structures (e.g., vessels, tumors) in the second near‐infrared (NIR‐II, 1000–1350 nm) region. However, its practical application is impeded by the limited quantum efficiency of the available phosphors due to the unstable valence state of Mn5+. Herein, a novel strategy involving site competition is proposed to stabilize the Mn5+ state by the introduction of valence‐unstable Bi2+/3+. The results demonstrate that Bi3+ ions tend to occupy two different Ca2+ ion sites within the Ca6Ba(PO4)4O lattice. The incorporation of a small amount of Bi3+ effectively suppresses the amount of Mn2+ in Ca2+ sites. This is also confirmed by spectroscopic experiments and density function theory calculations. Notably, an ultra‐high internal quantum efficiency of 82.3% is achieved under excitation at 653 nm, surpassing more than twofold the previously reported value of 37.5% in Ca6Ba(PO4)4O: Mn5+. As a proof of concept, deep tissue imaging with a penetration depth of ≈2.8 cm is achieved using a self‐produced NIR‐II light‐emitting diodes device embedded with Ca6Ba(PO4)4O: 0.003Mn5+/0.003Bi3+ powder. These findings provide valuable insights into improving the luminescent properties associated with Mn5+ ions and pave the way for deep tissue imaging with high spatiotemporal resolution.