The Synthesis Of Mn²⁺Doped ZnS Nanocrystals Via Hydrothermal Method

Semiconductor nanocrystals (NCs) have attracted a great of attention due to their unique optical properties and potential applications in various fields, such as lighting emitting diodes, lasers, solar cells and biomedical labeling. Transition metal ions doped Ⅱ-Ⅵ semiconductor nanocrystals have been extensively studied in view of their intense, stable, and tunable emission. In addition, these doped nanocrystals have several advantages over undoped nanocrystals, for example, thermal and environmental stability. As a typical semiconductor Ⅱ-Ⅵ compound, zinc sulfide (ZnS) is suitable for use as a host material for a large variety of dopants because of its wide band gap. Among this category, doping of Mn ions in zinc sulfide has been widely investigated. The doped Mn ions act as recombination centers for the excited electron-hole pairs and result in characteristic luminescence. Currently, various synthetic approaches have been developed for Mn-doped ZnS nanocrystals. The organometallic hot injection method has been adopted to prepare high-quality ZnS:Mn NCs with excellent optical properties, but this approach has disadvantages such as high-cost, unfriendly and rigorous experimental conditions, furthermore, these as-prepared ZnS:Mn NCs are insoluble in water and, therefore, are not useful in biological systems. In comparison with organometallic route, different methods have been used to prepare Mn-doped ZnS nanocrystals, including aqueous synthesis reverse micelle route and chemical precipitation method. However, these as-synthesized nanocrystals often suffer several drawbacks such as poor crystallinity and agglomeration, which hinder their potential applications.In this work, we report a hydrothermal method to synthesize water-soluble ZnS:Mn NCs using ethanol as solvent. No any capping agent was used in the reaction. The simple hydrothermal approach was adopted here because of relatively low temperature, capability of large-scale production of materials and environmental friendliness. On the other hand, by controlling temperature, pressure, reaction time, precursor chemicals and solvent, one can get various morphology, dimensions and structure of the final products. The as-prepared ZnS:Mn NCs are highly crystalline and nearly monodisperse, possessing zinc blende structure with an average particle size of about3.5nm. The obtained samples exhibit bright orange emission under excitation of ultraviolet light, the highest PL quantum yield of the obtained ZnS:Mn nanocrystals can be up to10.6%. The influence of Mn ions concentration on the PL intensity was also studied in detail.