Effects of added thiol ligand structure on aggregation of non-aqueous ZnO dispersions and morphology of spin-coated films

The ability to control the morphology of spin-coated semiconducting nanocrystal films using solution based methods is potentially important for optoelectronic device applications. In this study we sought to establish relationships between added bifunctional thiol ligand structure, triggered nanocrystal aggregation and spin-coated nanocrystal film morphology using ZnO nanocrystals. The latter nanocrystals were studied because they do not rely on adsorbed long-chain ligands for dispersion stability, which simplifies the analysis, and have potential application in solar cells. The dithiol ligands used in this study were 1,2-ethanedithiol, 1,2-benzenedithiol and 1,4-benzenedithiol. Dispersion stability was assessed visually and using turbidity measurements. A colloid stability phase diagram for ZnO/1,2-ethanedithiol dispersions in chlorobenzene was constructed and the ability of 1,2-ethanedithiol and 1,2-benzenedithiol to trigger dispersion aggregation was compared. The morphology of spin-coated nanocrystal films was investigated using optical microscopy, SEM and TEM. The data show that added 1,2-benzenedithiol was far more effective at triggering aggregation than 1,2-ethanedithiol and this effect was attributed to stronger inter-nanocrystal linkages. Moreover, spin-coated ZnO/1,2-benzenedithiol films strongly scattered light in the visible region which was attributed to the formation of polydisperse sub-micrometre aggregates. A mechanism for ligand triggered aggregation of dispersed ZnO nanocrystals was proposed. We propose design rules for bifunctional thiol ligand selection for controlling triggered aggregation and achieving enhanced light scattering of ZnO nanocrystal films. The latter may enable simpler processing of efficient ZnO photoelectrodes for dye sensitised solar cells.