Relationship between the coprecipitation mechanism, doping structure and physical properties of Zn(1-x)Co(x)S nanocrystallites.

Zn(1-x)Co(x)S nanocrystallites synthesized by the coprecipitation method have been studied. X-ray powder diffraction analysis shows that the solid solubility limit of sphalerite Zn(1-x)Co(x)S can reach x = 16.4%. It is found that when the Co(2+) percentage in the starting solution (Co:(Co+Zn)(solution)) is less than 15% only 20% of Co(2+) ions can be coprecipitated. Otherwise, around 50% of Co(2+) ions will be effectively coprecipitated. The surface exchange experiments and the optical absorption characteristics reveal that Co(2+) ions are in the form of internal dopants and substitute for tetrahedral Zn sites. Further optical and magnetic measurements indicate significant differences for samples with different coprecipitation ratios. Distorted lattice structure and low temperature super-paramagnetism are observed for high coprecipitation ratio samples, while a crystallized structure with high photoluminescence efficiency and abnormal effective magnetic moment (3.3 mu(B)/Co) are found in low coprecipitation ratio samples. A mechanism relating to different nucleation and crystal growth kinetics in two coprecipitation regions has been proposed to accommodate the differences in structural and physical properties of Zn(1-x)Co(x)S nanocrystallites.