Manganese doped ZnS nanoparticles

Abstract

Nanostructured materials can be defined as those materials whose structural elements - clusters, crystallites or molecules - have dimensions in the 1-100 nm range. The explosion in both academic and industrial interest over the past 20 years arises from the remarkable variations in fundamental electrical, optical and magnetic properties that occur as one progresses from an infinitely extended solid to a particle of material consisting of a countable number of atoms. In this work the attention was laid on the optical properties of Mn2+ doped ZnS nanoparticles with respect to a possible application as bio sensor in medical and biological analytics. A synthesis for this type of particles has been developed and the nanocrystals obtained have been investigated in terms of their composition, particle size and optical properties. The particles synthesized were stabilized with the amino acid L-cysteine and showed the typical ZnS:Mn emission spectrum with its orange emission due to the 4T1 –> 6A1 transition within the d-orbitals of the Mn2+ at 585 nm and its blue emission due to recombination over the ZnS band gap from shallow electron traps at 380 nm. The onset of the absorption was blue shifted from 340 nm (bulk) to 310 nm, indicating a quantum size effect. The particle sizes and particle size distributions were determined with methods such as transmission electron microscopy, photon correlation spectroscopy, analytical ultra centrifugation and field flow fractionation. The particle size has also been calculated from the shift in the band gap with the help of the effective mass approximation. The limits of the applied techniques are discussed and the particle size distribution determined to go from 3-20 nm with a first maximum at about 5 nm and a second maximum at about 17 nm. The particles' luminescence quantum yields have been determined between 1.5-2 %. They could be enhanced by a factor of three by coating the particles with a SiO2 shell. The quantum yields showed a strong dependence on the dispersion concentration and Mn2+ content. These effects have been investigated with quantum yield measurements at different temperatures and Mn2+ contents, and lifetime measurements. The lifetime measurements showed decay times in the ns region for the blue and in the μs-ms region for the orange emission, indicating a florescence mechanism for the ZnS and a phosphorescence mechanism for the Mn2+ radiation.