Photoluminescence properties of manganese-doped zinc selenide quantum dots

Mn²⁺-doped II-VI semiconductor quantum dots reveal remarkably intense photoluminescence with a short lifetime associated with the ⁴T₁(⁴G) → ⁶A₁( ⁶S) transition, which is spin-forbidden and is allowed because of crystal field effects. We explored the photophysical properties of high-quality, narrow-size-distribution Mn²⁺-doped ZnSe (ZnSe:Mn²⁺) quantum dots. ZnSe:Mn²⁺ quantum dots with varying amounts of dopant were studied at temperatures down to 10 K. Substitutional incorporation of Mn²⁺ in ZnSe quantum dots was confirmed by electron paramagnetic resonance measurements as well. Photoluminescence emission (PL) and photoluminescence excitation (PLE) spectroscopies at low temperature were employed to examine the sp-d interactions. PL measurements of ZnSe:Mn ²⁺ quantum dots show Mn²⁺-related orange luminescence. PLE measurements were carried out at a fixed emission wavelength related to Mn ²⁺ orange luminescence. Five excited states corresponding to Mn ²⁺ d-d transitions were observed. The crystal field strength (10Dq) increases with increasing Mn²⁺ concentration, increasing size, and decreasing temperature. In contrast to earlier conjectures about transition-metal-doped quantum dots, Mn²⁺-related photoluminescence feature could be observed in ZnSe:Mn²⁺ quantum dots even when the excitation energy was lower than the forbidden gap but was equal to the energy of the d-d transitions. The behavior of ZnSe:Mn²⁺ quantum dots was also compared with that of their bulk counterpart.