Surface Sites and Ligation of Amine-capped CdSe Nanocrystals

The conversion of colloidal nanocrystals (NCs) into various applications is usually facilitated by designing and controlling their surface properties. A key strategy for tailoring surface properties is to modulate surface ligands. Amines, as L-type ligands, have been universally introduced into various surface modifications, promoting NCs synthesis, processing and performances. However, a full understanding of the nature of the amine-capped NCs surface has yet to be established, due to several challenges, such as the presence of various surface, subsurface and bulk sites along with the presence of surface dynamic. In this work, we addressed the issue on CdSe NCs, looking at both NC surface sites and the ligation of amine based on the combination of 113Cd, 77Se and 15N NMR spectroscopy, respectively. In particular, dynamic nuclear polarization (DNP) enhanced 113Cd and 77Se NMR allowed acquisition of high quality 1D spectra, thus enabling the identification of bulk and surface sites on different facets. Amines terminated sites on both Se-rich facets and nonpolar facets were resolved by 113Cd 2D NMR spectroscopy, augmented with density function theory (DFT) calculations. In addition to directly bonding to surface sites, amines interaction through hydrogen-bonding with absorbed water and carboxylate surface ligands were further revealed by 15N NMR. The insight into the complexity of amine-capped CdSe NCs surface and the characterization methodology developed in this work is an important step towards rational design, assessing surface modification as well as improving the processing of colloidal NCs towards their exploitation in various applications.