Colloidal semiconductor nanocrystals (NCs) have generated significant attention as building blocks for optoelectronic and energy conversion devices. The structure of the surface atoms strongly impacts these properties since the surface atoms are a significant fraction of the nanocrystals. However, the molecular level characterization of surfaces, including termination and coordination of the different atom types is an unresolved challenged. Here, dynamic nuclear polarization (DNP) 113Cd and 77Se solid-state NMR (SSNMR) spectroscopy provides detailed surface structures of archetypal zinc blende CdSe nanocrystals with plate or spheroidal morphology, capped by carboxylic acid ligands. 113Cd cross-polarization magic-angle-turning experiments identified CdSe3O, CdSe2O2, and CdSeO3 Cd coordination environments on the surface the NCs. The oxygen atoms are presumably from coordinated carboxylate ligands, and the differences in the coordination number arises from Cd atoms on {100}
facets and {111} facets. The sensitivity gain from DNP enabled natural isotopic abundance 77Se-113Cd heteronuclear correlation SSNMR experiments reveal the connectivity of the Cd and Se atoms and quantified the number of Cd atoms bonded to Se atoms. Experiments confirmed the major Cd and Se surface species have CdSe2O2 and SeCd4 stoichiometries, respectively. The conclusion that the surface of the spheroidal CdSe NCs is primarily composed of {100} facets is supported by the crystal structure of zinc blende CdSe and the similarity of the SSNMR data for platelet and spheroidal NCs. The methods are generally applicable to surface structures of main group semiconductor nanoparticles. Knowledge of surface structure in these materials will enable precision synthesis and prediction of structure-property relationships.
Chen, Y., Dorn, R. W., Hanrahan, M. P., Wei, L, Blome-Fernández, R, Medina-Gonzalez, A. M, Adamson, M. A. S, Flintgruber, A. H, Vela, J, Rossini, A. J., Revealing the Surface Structure of CdSe Nanocrystals by Dynamic Nuclear Polarization-Enhanced 77Se and 113Cd Solid-State NMR Spectroscopy. J. Am. Chem. Soc. 143, 8747-8760, (2021); DOI: 10.1021/jacs.1c03162.