Ligand Dynamics Time Scales Identify the Surface–Ligand Interactions in Thiocyanate-Capped Cadmium Sulfide Nanocrystals

The nanocrystal surface, which acts as an interface between the semiconductor lattice and the capping ligands, plays a significant role in the attractive photophysical properties of semiconductor nanocrystals for use in a wide range of applications. Replacing the long-chain organic ligands with short inorganic variants improves the conductivity and carrier mobility of nanocrystal-based devices. However, our current understanding of the interactions between the inorganic ligands and the nanocrystals is obscure due to the lack of experiments to directly probe the inorganic ligands. Herein, using two-dimensional infrared spectroscopy, we show that the variations in the inorganic ligand dynamics within the heterogeneous nanocrystal ensemble can identify the diversities in the inorganic ligand–nanocrystal interactions. The ligand dynamics time scale in SCN– capped CdS nanocrystals identifies three distinct ligand populations and provides molecular insight into the nanocrystal surface. Our results demonstrate that the SCN– ligands engage in a dynamic equilibrium and stabilize the nanocrystals by neutralizing the surface charges through both direct binding and electrostatic interaction.