Temperature- and Mn²⁺ Concentration-Dependent Emission Properties of Mn²⁺-Doped ZnSe Nanocrystals

Mn²⁺-doped ZnSe nanocrystals (Mn:ZnSe d-dots) with high optical quality-high dopant emission quantum yield with monoexponential dopant-emission decay dynamics-enable systematic and quantitative studies of temperature- and Mn²⁺ concentration-dependent optical properties of the dopant emission, especially its relationship with magnetic coupling. While temperature-dependent steady-state and transient dopant emission of d-dots with dilute Mn²⁺ concentrations originated from isolated Mn²⁺ ions, and can be quantitatively treated as a result of exciton-phonon coupling of isolated paramagnetic emission centers. Dopant emission of d-dots with high Mn²⁺ concentrations (up to 50% of Zn²⁺ ions being replaced by Mn²⁺ ions in the core nanocrystals) are found solely related to magnetically coupled Mn²⁺ emission. Magnetic coupling effects on steady-state dopant emission of d-dots with high Mn²⁺ concentrations are much stronger than those observed for doped bulk semiconductors, which is found to follow a strong and universe shell-thickness dependence for the epitaxial ZnSe and/or ZnS shells of the d-dots. By exciting the magnetically coupled Mn²⁺ ions directly, dopant-emission of d-dots with high Mn²⁺ concentrations exhibit monoexponential decay dynamics. In addition to this emission channel, a minor channel with slightly longer decay lifetime appears when the host nanocrystals with high Mn²⁺ concentrations are excited, which is barely visible at room temperature and increases its fraction by decreasing temperature.