Ln ³⁺ -ion-mediated enhancement in UV/X-ray-induced optical emission from Mn ²⁺ -doped ZnSe nanocrystals

Strategically selecting energy-transfer agents for photoemitting Mn²⁺ sites in semiconductor quantum dots (QDs) and precisely controlling their local chemical environment is important to reduce nonradiative pathways and enhance emission efficiency. This study unravels the impacts of different lanthanide dopants (La³⁺, Y³⁺, Nd³⁺, Yb³⁺, Tm³⁺, and Lu³⁺) on the UV- and X-ray-induced luminescence properties of ZnSe:Mn²⁺ QDs. In particular, co-doping these QDs with Yb³⁺, La³⁺, Tm³⁺, and Nd³⁺ at a 2 mol% concentration significantly enhanced the emission intensity of Mn²⁺ ions. This resulting enhancement in emission intensity was attributed to the change in the local chemical environment around the photoemitting Mn²⁺ sites caused by the incorporation of Ln³⁺ ions, which was probed by comprehensive X-ray absorption fine structure (XAFS) analysis. Although certain Ln³⁺ ions, such as Tm³⁺ and Nd³⁺, possess promising energy level structures for efficient energy transfer to Mn²⁺ ions, yet defects/traps induced by the mismatch in oxidation states between Zn and Ln, the localization of the electron-hole pair near the photoemitting dopant site in the ZnSe lattice, and the quenching of band edges' radiative decay during electron-hole recombination were found key factors that improved the excitation efficiency of photoemitting Mn²⁺ ions. Therefore, considerable enhancement in Mn²⁺ emission was revealed using synchrotron radiation X-ray-excited optical luminescence analysis of the Ln³⁺-doped ZnSe:Mn²⁺ QDs, suggesting that they would be promising candidates as scintillating layers in X-ray detection for imaging and tomography applications.