What Role Can Surface Capping Ligand Play To Control Dopant Emission in Semiconductor Nanoparticles?

The role of surface capping ligands in controlling dopant photoluminescence in semiconductor nanoparticles is examined by monitoring emission in terbium cation incorporated zinc sulfide [Zn(Tb)S] nanoparticles, as a function of [H+] that is varied postsynthetically. Increases in Tb3+ emission of similar to 6 and similar to 1.3 times are observed on changing the pH from 4 to 7 and from 7 to 11, respectively. An increased contribution of host sensitization over direct excitation is observed under basic conditions. Subtle structural modification of the capping ligand is argued to be solely responsible for the dopant emission in the acidic-neutral range. The neutral-basic range in addition to this effect has a minor contribution from alteration in band alignment as well. A major outcome from this work relates to identifying the role of the terminally placed functional group in the capping ligand to control emissions from both the host (zinc sulfide nanoparticles) and guest (Tb3+), with a pronounced effect on dopant Tb3+ emission in the 1-thioglycerol capped Zn(Tb)S nanoparticles. These results identify surface engineering as an important modulator, in addition to the primary criteria of (a) band gap engineering and (b) breaking (or optimizing) dopant local site symmetry in maximizing (or guiding) dopant emission in doped semiconductor nanoparticles.