Surface characterization is crucial for understanding how the atomic-level structure affects the chemical and photophysical properties of semiconducting nanoparticles (NPs). Solid-state nuclear magnetic resonance spectroscopy (NMR) is potentially a powerful technique for the characterization of the surface of NPs, but it is hindered by poor sensitivity. Dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) has previously been demonstrated to enhance the sensitivity of surfaceselective solid-state NMR experiments by one to two orders of magnitude. Established sample preparations for DNP SENS experiments on NPs require the dilution of the NPs on mesoporous silica. Using hexagonal boron nitride (h-BN) to disperse the NPs doubles DNP enhancements and absolute sensitivity as compared to standard protocols with mesoporous silica. Alternatively, precipitating the NPs as powders, mixing them with h-BN, then impregnating the powdered mixture with radical solution leads to further four-fold sensitivity enhancements by increasing the concentration of NPs in the final sample. This modified procedure provides a factor 9 improvement in NMR sensitivity as compared to previously established DNP SENS procedures, enabling challenging homonuclear and heteronuclear 2D NMR experiments on CdS, Si and Cd3P2 NPs. These experiments allow NMR signals from the surface, subsurface and core sites to be observed and assigned. For example, we demonstrate that the acquisition of DNP-enhanced 2D 113Cd113Cd correlation NMR experiments on CdS NPs and natural isotropic abundance 2D 13C29Si HETCOR of functionalized Si NPs. These experiments provide a critical understanding of NP surface structures.

Lead halide perovskites possess unique characteristics that are well-suited for optoelectronic and energy capture devices, however, concerns about their long-term stability remain. Limited stability is often linked to the methylammonium cation, and all-inorganic CsPbX 3 (X=Cl, Br, I) perovskite nanocrystals have been reported with improved stability. In this work, the photostability and thermal stability properties of CsPbX 3 (X=Cl, Br, I) nanocrystals were investigated by means of electron microscopy, X-ray diffraction, thermogravimetric analysis coupled with FTIR (TGA-FTIR), ensemble and single particle spectral characterization. CsPbBr 3 was found to be stable under 1-sun illumination for 16 h in ambient conditions, although single crystal luminescence analysis after illumination using a solar simulator indicates that the luminescence states are changing over time. CsPbBr 3 was also stable to heating to 250°C. Large CsPbI 3 crystals (34 � 5 nm) were shown to be the least stable composition under the same conditions as both XRD reflections and Raman bands diminish under irradiation; and with heating the γ (black) phase reverts to the nonluminescent δ phase. Smaller CsPbI 3 nanocrystals (14 � 2 nm) purified by a different washing strategy exhibited improved photostability with no evidence of crystal growth but were still thermally unstable. Both CsPbCl 3 and CsPbBr 3 show crystal growth under irradiation or heat, likely with a preferential orientation based on XRD patterns. TGA-FTIR revealed nanocrystal mass loss was only from liberation and subsequent degradation of surface ligands. Encapsulation or other protective strategies should be employed for long-term stability of these materials under conditions of high irradiance or temperature.[a] B.

J( 77 Se, 113 Cd), heteronuclear 77 Se{ 113 Cd} spin echo (J-resolved) NMR experiments were then used to determine the number of Cd atoms bonded to Se atoms and vice versa.The J-resolved experiments directly confirmed that major Cd and Se surface species have CdSe2O2 and SeCd4 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>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 ¹¹³Cd and ⁷⁷Se 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. ¹¹³Cd cross-polarization magic-angle-turning (CP-MAT) experiments identified CdSe₃O, CdSe₂O₂, and CdSeO₃ 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 ¹¹³Cd-¹¹³Cd and ⁷⁷Se-⁷⁷Se and heteronuclear ¹¹³Cd-⁷⁷Se scalar correlation solid-state NMR experiments that reveal the connectivity of the Cd and Se atoms. Importantly, ⁷⁷Se{¹¹³Cd} scalar heteronuclear multiple quantum coherence (<i>J</i>-HMQC) experiments were used to selectively measure one-bond ⁷⁷Se-¹¹³Cd scalar coupling constants (¹<i>J</i>(⁷⁷Se, ¹¹³Cd)). With knowledge of ¹<i>J</i>(⁷⁷Se, ¹¹³Cd), heteronuclear ⁷⁷Se{¹¹³Cd} 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 CdSe₂O₂ and SeCd₄ 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>

<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 ¹¹³Cd and ⁷⁷Se 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. ¹¹³Cd cross-polarization magic-angle-turning (CP-MAT) experiments identified CdSe₃O, CdSe₂O₂, and CdSeO₃ 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 ¹¹³Cd-¹¹³Cd and ⁷⁷Se-⁷⁷Se and heteronuclear ¹¹³Cd-⁷⁷Se scalar correlation solid-state NMR experiments that reveal the connectivity of the Cd and Se atoms. Importantly, ⁷⁷Se{¹¹³Cd} scalar heteronuclear multiple quantum coherence (<i>J</i>-HMQC) experiments were used to selectively measure one-bond ⁷⁷Se-¹¹³Cd scalar coupling constants (¹<i>J</i>(⁷⁷Se, ¹¹³Cd)). With knowledge of ¹<i>J</i>(⁷⁷Se, ¹¹³Cd), heteronuclear ⁷⁷Se{¹¹³Cd} 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 CdSe₂O₂ and SeCd₄ 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>