Enhancement of photoluminescence of ZnS:Mn nanocrystals by hybridizing with polymerized acrylic acid

Mn²⁺-doped ZnS (ZnS:Mn) nanocrystals are hybridized with polymerized acrylic acid. By using either acrylic acid (AA) monomer or poly(acrylic acid) (PAA) polymer as a starting material, differences in optical and chemical properties of hybrid nanocrystals are compared. The enhancement of photoluminescence at 580 nm due to the d-d transition of Mn²⁺ is larger for ZnS:Mn nanocrystals modified by AA and aged at 80°C for t_agehours for polymerization (the sample ZSAA5-t_age, 0 ≤ t_age ≤ 24), as compared with that modified by PAA (the sample ZSPAA). The C=O groups and ZnS are excited simultaneously by a light of 350 nm to stimulate energy transfer to Mn²⁺ ions jointly. In the samples ZSAA5-t_age, oxygen atoms of carboxyl groups form -S-O-C(=O)- bonds and the valence number of sulfur increases from -2 to +6, as confirmed by infrared absorption and X-ray photoelectron spectra. As a result, electrons of sulfur atoms are pushed toward carboxyl groups to enhance the emissions from both C=O groups and Mn²⁺ ions. In contrast, the sample ZSPAA exhibits predominant interaction of C=O groups with metallic ions to form -C-O-Me- bonds (Me=Zn²⁺ and Mn²⁺). This decrease in the amount of C=O groups reduces energy transfer to Mn²⁺ ions.