Revealing the Surface Structure of CdSe Nanocrystals by Dynamic Nuclear Polarization-Enhanced 77Se and 113Cd Solid-State NMR Spectroscopy [post]

Yunhua Chen, Rick Dorn, Michael Hanrahan, Lin Wei, Rafael Blome-Fernandez, Alan Medina-Gonzalez, Marquix Adamson, Anne Flintgruber, Javier Vela, Aaron Rossini
2021 unpublished
<p>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 and which are capped by carboxylic acid ligands. 1D <sup>113</sup>Cd and <sup>77</sup>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.
more » ... Cd cross-polarization magic-angle-turning (CP-MAT) experiments identified CdSe<sub>3</sub>O, CdSe<sub>2</sub>O<sub>2</sub>, and CdSeO<sub>3</sub> 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 <sup>113</sup>Cd-<sup>113</sup>Cd and <sup>77</sup>Se-<sup>77</sup>Se and heteronuclear <sup>113</sup>Cd-<sup>77</sup>Se scalar correlation solid-state NMR experiments that reveal the connectivity of the Cd and Se atoms. Importantly, <sup>77</sup>Se{<sup>113</sup>Cd} scalar heteronuclear multiple quantum coherence (<i>J</i>-HMQC) experiments were used to selectively measure one-bond <sup>77</sup>Se-<sup>113</sup>Cd scalar coupling constants (<sup>1</sup><i>J</i>(<sup>77</sup>Se, <sup>113</sup>Cd)). With knowledge of <sup>1</sup><i>J</i>(<sup>77</sup>Se, <sup>113</sup>Cd), heteronuclear <sup>77</sup>Se{<sup>113</sup>Cd} spin echo (<i>J</i>-resolved) NMR experiments were then used to determine the number of Cd atoms bonded to Se atoms and vice versa. The <i>J</i>-resolved experiments directly confirmed that major Cd and Se surface species have CdSe<sub>2</sub>O<sub>2</sub> and SeCd<sub>4</sub> 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 semiconductors.</p>
doi:10.26434/chemrxiv.14224148 fatcat:kkrvqlclc5fwxcssdftzqooeom