Structural Evolution Controls Photoluminescence of Post-Synthetically Modified Doped Semiconductor Nanoparticles

This work discusses the photoluminescence properties of doped semiconductor nanoparticles by adding cadmium­(II) nitrates post-synthetically to the terbium cation incorporated zinc sulfide [Zn­(Tb)­S] nanoparticles at room temperature to generate the Zn­(Tb)­S/Cd nanoparticles. The evolution of nanoparticle’s emission is monitored as a function of amount of Cd²⁺, with [Zn­(Tb)­S]/[Cd²⁺] = 1:10^–4 to 1:10, providing an opportunity to access materials of different chemical compositions. Structural features, as evaluated by X-ray diffraction and energy-dispersive X-ray spectroscopy, indicate a partial cation exchange of zinc by cadmium. No apparent replacement of terbium is noticed throughout the post-synthetic modification of the Zn­(Tb)S nanoparticles until the relative reactant ratio reaches 1:10, and this only becomes noticeable with [Zn­(Tb)­S]/[Cd²⁺] = 1:50. Remarkable differences in both broad and sharp emissions of nanoparticles and Tb³⁺, respectively, have been observed in the post-synthetic modification. The reaction initiates with a blue shift of nanoparticle’s broad emission, and a further increase in Cd²⁺ content results in a red shift. Tb³⁺ emission, despite its insensitivity in the spectral band position due to the intra-configurational 4f transitions, shows a decrease in emission efficiency following post-synthetic modification. Formation of alloyed particles, however, significantly improved excitation contribution approaching the visible spectral region. Lifetime measurements of nanoparticles and Tb³⁺ emission support the exchange of cations and the role of competitive non-radiative deactivation pathways, respectively. Collectively, nanoparticles with [Zn­(Tb)­S]/[Cd²⁺] = 1:10^–4 to 1:10^–3, 1:10^–2, 1:10^–2 to 1:10, and 1:50 are argued to form Cd²⁺-induced surface trap-passivated Zn­(Cd)­(Tb)­S, onset of Zn_1–<i>x</i>Cd<i>_x</i>(Tb)S alloy formation, Zn_1–<i>x</i>Cd<i>_x</i>(Tb)S alloys of varying compositions, and Zn_1–<i>x</i>Cd<i>_x</i>S nanoparticles, respectively. Finally, this work provides a foundation to tune the properties of any emissive doped semiconductor nanoparticles in a lesser synthetically demanding fashion and has important implications in developing such materials.