Size Dependent Optical Properties and Structure of ZnS Nanocrystals Prepared from a Library of Thioureas

ZnS nanocrystals (λ_max(1S_e–1S_3/2h) = 260–320 nm, d = 1.7–10.0 nm) are synthesized from Zn­(O₂CR)₂ (O₂CR = tetradecanoate, oleate and 2-hexyldecanoate), N,N′-disubstituted and N,N′,N′-trisubstituted thioureas, and P,P,N-trisubstituted phosphanecarbothioamides. The influence of precursor substitution, ligand sterics, and reaction temperature on the final nanocrystal size was evaluated. Using saturated hydrocarbon solvents and saturated aliphatic carboxylate ligands, polymeric byproducts could be avoided and pure ZnS nanocrystals isolated. Elevated temperatures, slower precursor conversion reactivity, and branched zinc 2-hexyldecanoate yield the largest ZnS nanocrystals. Carefully purified zinc carboxylate, rapidly converting precursors, and cooling the synthesis mixture following complete precursor conversion provide quasispherical nanocrystals with the narrowest shape dispersity. Nanocrystal sizes were measured using pair distribution function (PDF) analysis of X-ray scattering and scanning transmission electron microscopy (STEM) and plotted versus the energy of their first excitonic optical absorption. The resulting empirical relationship provides a useful method to characterize the nanocrystal size from 1.7 to 4.0 nm using optical absorption spectroscopy.