Synthesis and luminescent properties of ultrasmall colloidal CdS nanoparticles stabilized by Cd(II) complexes with ammonia and mercaptoacetate

The synthesis of stable water-soluble ultrasmall 1.8 nm CdS nanoparticles co-stabilized with Cd(II) complexes with NH3 and mercaptoacetate ions is reported. The CdS nanoparticles emit broad-band photoluminescence with a peak at 2.3 eV and a quantum yield of up to 15 %. The photoluminescence decay is strongly non-exponential and characterized by an average radiative life-time increasing from 46 to 105 ns as the emission quantum energy decreases from 2.9 to 1.8 eV. The photoluminescence intensity and life-time decrease to about 50 % as the colloid temperature is elevated from 22 to 50 °C. This behavior is completely reversible and assumed to originate from the thermally activated dissociation of Cd(II)–NH3 complexes on the nanoparticle surface and the formation of Cd(II)–H2O complexes acting as the radiationless recombination sites. The strong temperature dependence of the luminescent properties coupled with the small size and reasonably high quantum yields of emission makes CdS nanoparticles co-stabilized with NH3 and mercaptoacetate attractive for bio-imaging, bio-sensing, and other applications.