Local structures around Mn luminescent centers in Mn-doped nanocrystals of ZnS

X-ray-absorption fine-structure measurements were carried out to probe the local environment surrounding Mn ions in Mn-doped nanocrystals of ZnS with different size distributions ranging from 30 35 to 50 55. The interatomic distances between Mn and neighboring atoms, the coordination number, local disorder, and effective valency determined for the nanocrystals are compared with those in bulk Mn-doped ZnS. The Mn ions are found to substitute for the Zn sites in the host ZnS but with significant size-dependent local structural changes. The questions of Mn-cluster formation and the presence of Mn impurities on the surface of the nanocrystals are addressed. Near-edge x-ray-absorption fine structures indicate that the effective valency of Mn ions in the nanocrystals is close to +2 with a weak size dependence. These local structures are believed to be closely related to the novel optical properties observed in this new class of semiconductors.