Revealing the Surface Structure of CdSe Nanocrystals by Dynamic Nuclear Polarization-Enhanced Se and Cd Solid-State NMR Spectroscopy.

Dynamic nuclear polarization (DNP) solid-state NMR (SSNMR) spectroscopy was used to obtain detailed surface structures of zinc blende CdSe nanocrystals (NCs) with plate or spheroidal morphologies which are capped by carboxylic acid ligands. 1D Cd and Se cross-polarization magic angle spinning (CPMAS) NMR spectra revealed distinct signals from Cd and Se atoms on the surface of the NCs, and those residing in bulk-like environments, below the surface. Cd cross-polarization magic-angle-turning (CP-MAT) experiments identified CdSeO, CdSeO, and CdSeO Cd coordination environments on the surface of the NCs, where the oxygen atoms are presumably from coordinated carboxylate ligands. The sensitivity gain from DNP enabled natural isotopic abundance 2D homonuclear Cd-Cd and Se-Se and heteronuclear Cd-Se scalar correlation solid-state NMR experiments which revealed the connectivity of the Cd and Se atoms. Importantly, Se{Cd} scalar heteronuclear multiple quantum coherence (-HMQC) experiments were used to selectively measure one-bond Se-Cd scalar coupling constants ((Se, Cd)). With knowledge of (Se, Cd), heteronuclear Se{Cd} spin echo (-resolved) NMR experiments were used to determine the number of Cd atoms bonded to Se atoms and vice versa. The -resolved experiments directly confirmed that major Cd and Se surface species have CdSeO and SeCd stoichiometries, respectively. Considering the crystal structure of zinc blende CdSe and the similarity of the solid-state NMR data for the platelets and spheroids, we conclude that the surface of the spheroidal CdSe NCs is primarily composed of {100} facets. The methods outlined here will generally be applicable to obtain detailed surface structures of various main group semiconductor nanoparticles.