Electronic states of Mn²⁺ in ZnS:Mn nanoparticle examined by EPR measurements and molecular orbital calculation

The discrete variational Xα method was introduced to elucidate the possible factors of luminescence enhancement of ZnS:Mn modified by poly acrylic acid. The effective charge calculation of Mn indicated that the ionicity of Mn and its first neighbors increase when Mn atom is located at the surface site rather than in the bulk, in accordance with the signals observed from EPR measurements. The bond order between Mn and S increased by the stronger exchange interaction when Mn was oxidized by coordination of carboxyl group rather than by adsorption of OH or a SH group. The bond order between Mn and O is higher for the same situation, which may attribute to the energy transfer from poly acrylic acid to ZnS:Mn. As a consequence, the symmetry of the d orbital decreased, judging from the contour maps of the molecular orbital. This relaxed the forbidden d-d transition of Mn²⁺ and resulted in the enhanced photo luminescence.