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

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journal contribution
posted on 20.11.2019 by Saoni Rudra, Madhumita Bhar, Prasun Mukherjee
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 Zn1–xCdx(Tb)S alloy formation, Zn1–xCdx(Tb)S alloys of varying compositions, and Zn1–xCdxS 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.