The Surface Chemistry of Colloidal HgSe Nanocrystals, toward Stoichiometric Quantum Dots by Design

The prevalent structure of binary semiconductor nanocrystals is a crystallite enriched in metal cations and terminated by anionic surface species that are classified as X-type ligands. Here, we demonstrate that HgSe NCs synthesized from selenourea and mercury chloride in oleylamine have a stoichiometry close to that of bulk HgSe and feature a surface terminated by oleylammonium chloride: a combination of composition and ligand capping that preserves charge neutrality. We demonstrate that oleylammonium chloride can be formed as a side product of the formation of HgSe in the particular reaction mixture used. We complement the experimental work with a detailed investigation by density functional theory of a [HgSe]55 model system. This analysis confirms that the combination of a stoichiometric HgSe nanocrystal and alkylammonium chloride ligands forms a stable structure. Moreover, DFT predicts that stoichiometric HgSe nanocrystals prefer binding of methylammonium chloride over binding of mercury chloride. We thus conclude that HgSe NCs are retrieved with this unconventional surface termination because it is (1) possible by synthesis and (2) preferred by thermodynamics. Finally, we argue that the identification of surface ligands as acids or bases provides a convenient alternative to the covalent bond classification for describing NC-ligand binding motifs involving ion-pairs.