Characterization of Mn²⁺ coordination states in ZnS nanocrystal by EPR spectroscopy and related photoluminescence properties

Coordination states of Mn²⁺ inside and near the surface of ZnS:Mn nanocrystal (NC) (ca. 1.8 nm in particle radius) coated with poly(acrylic acid) (PAA) were examined by the detailed analysis of electron paramagnetic resonance (EPR). The symmetry of both Mn²⁺ sites inside and near the surface of NC is lower than that of submicron particles (0.125 μm in particle radius), because of larger lattice distortion and larger zero field splitting constant. Temperature dependence of line width (ΔH_pp) of EPR signals from Mn²⁺ inside and near surface sites of ZnS:Mn disappears when the particle radius of ZnS:Mn decreases from 0.125 μm to 1.8nm. These indicate increasing extent of d-d transition and stronger interaction between Mn²⁺ and ZnS as well as between Mn²⁺ and PAA in NC, leading to more effective energy transfer from ZnS and PAA to Mn²⁺. These phenomena explain the high luminescence intensity of ZnS:Mn nanocrystal coated with PAA.