In situ reversible tuning of photoluminescence of Mn²⁺-doped ZnS quantum dots by redox chemistry

Abstract

Herein we report the development of a new method for in situ reversible tuning of photoluminescence properties of quantum dots (Qdots) by partial oxidation of population of the emitting species and subsequent chemical reduction of the oxidized form. The concept has been demonstrated using Mn²⁺-doped ZnS Qdots stabilized by acetyl acetonate. Treatment of an aqueous solution of the Qdots (with Mn(OAc)₂ being the source of Mn used for the synthesis of the Qdots) by potassium peroxodisulfate (KPS) led to reduction of intensity of emission due to Mn²⁺ (⁴T₁-⁶A₁). Subsequent treatment of the solution containing KPS-treated Qdots with NaBH₄ led to regaining of intensity, thus providing reversibility to the tuning, which was possible for more than one cycle. Electron spin resonance (ESR) spectroscopic investigations revealed reduction of the population of Mn²⁺ upon treatment with KPS, whereas it went back up upon further treatment with NaBH₄. Interestingly, a mixed population of oxidation states of Mn was indicated to be present in the Qdots prepared using KMnO₄ as the source of Mn. The fluorescence intensity of the Qdots so prepared increased upon treatment with NaBH₄ following synthesis, which was not possible when the source of Mn was Mn(OAc)₂. Transmission electron microscopic and X-ray diffraction studies indicated that oxidation and reduction did not change the sizes of Qdots significantly.