Adsorption and Binding of Ligands to CdSe Nanocrystals

CdSe nanocrystals (NCs) capped by organic ligands are studied at the atomistic level using classical molecular simulations. We show for the first time that the NC-ligand bond strength can be explained using a simple model based on electrostatic interactions. The computed binding energies in vacuum for amine, thiol, thiolate, and phosphine oxide ligands are 86.8, 34.7, 1283, and 313.6 kJ/mol, respectively. These values are in good agreement with available quantum chemical calculations and experiments. It is crucial that one corrects for the dielectric constant of the solvent used in the experiment. We also show that the amine capping layer is formed in two stages: first, amine molecules binds to a single surface cation each, and then additional amines bind to less favorable sites forming hydrogen bonds with already adsorbed ligands. The crossover between these mechanisms can occur at ambient conditions. We speculate that this crossover may be responsible for transitions in optical properties reported earlier. The calculated adsorption isotherms show that amine ligands desorb from the nanocrystal surface under ultra-high vacuum at ambient temperatures.