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

Abstract

<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. <sup>113</sup>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>

Mere Anarchy is Loosed: Structural Disorder in Cu2Zn1–xCdxSnS4

“…If a disorder of Cu, Zn, and Cd atoms occurs, then the chemical environment around the Cd sites will undergo relatively large changes (e.g., the second coordination sphere may contain a mixture of Cu and Cd atoms as well as Sn atoms). 113 Cd NMR spectroscopy ( I = 1/2; 12.2% natural abundance, Ξ = 22.2%) was used to probe these environments as it is sensitive to local structural variations within various solids. − …”

Mapping out the Aqueous Surface Chemistry of Metal Oxide Nanocrystals: Carboxylate, Phosphonate, and Catecholate Ligands

“…The latter has proven to be a very powerful tool to study nanocrystal surface chemistry, 41−45 providing complementary information to solidstate NMR spectroscopy. 44,46 Unfortunately, iron oxide NCs interfere with magnetic fields and cannot be studied in NMR. HfO 2 NCs are thus an ideal starting point also because their surface chemistry has already been extensively studied in nonpolar solvents using NMR spectroscopy.…”

The Role of Protons in and Around Biradicals for Cross-Effect Dynamic Nuclear Polarization

“…spectra with high resolution and sensitivity. [3][4][5][6][7][8][9][10] DNP has revolutionized the field of solid-state NMR and enabled numerous applications both for biological and material samples, [11][12][13][14][15][16][17][18][19][20][21][22][23] in particular at natural isotopic abundance. [24][25][26][27] As of today, MAS-DNP is best carried out using biradicals as polarizing agents, 28,29 that generate high nuclear spin hyperpolarization via a mechanism called cross-effect (CE).…”