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The spin Hamiltonian parameters and defect structures are theoretically studied for the substitutional Mn2+ at the core of CdSe nanocrystals and in the bulk materials from the perturbation calculations of spin Hamiltonian parameters for trigonal tetrahedral 3d5 clusters. Both the crystal‐field and charge transfer contributions are taken into account in the calculations from the cluster approach. The impurity‐ligand bond angles are found to be about 1.84° larger and 0.10° smaller in the CdSe:Mn2+ nanocrystals and bulk materials, respectively, than those (≈109.37°) of the host Cd2+ sites. The quantitative criterion of occupation (at the core or surface) for Mn2+ in CdX (X = S, Se, Te) nanocrystals is presented for the first time based on the inequations of hyperfine structure constants (HSCs). This criterion is well supported by the experimental HSCs data of Mn2+ in CdX nanocrystals. The previous assignments of signals SI as Mn2+ at the core of CdS nanocrystals are renewed as Mn2+ at the surface based on the above criterion. The present studies would be helpful to achieve convenient determination of occupation for Mn2+ impurities in CdX semiconductor nanocrystals by means of spectral (e.g., HSCs) analysis. [ABSTRACT FROM AUTHOR]

The spin Hamiltonian parameters ( g factors and hyperfine structure constants) are theoretically studied for the two interstitial Cu 2+ centers I and II in the ZnO nanocrystals from the perturbation formulas of these parameters for a tetragonally elongated octahedral 3d 9 cluster. The ligand spin–orbit coupling and orbital contributions are involved from the cluster approach in view of moderate covalency. The two Cu 2+ centers demonstrate slight tetragonal elongation distortions (denoted by the relative elongation parameters ρ ≈ 3.4% and 3.2% for centers I and II, respectively) due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters and three optical transition bands are in good agreement with the observed values. The differences in EPR and optical spectra for both centers can be ascribed to the weaker covalency (higher N ), stronger tetragonal elongation distortion (larger ρ ) and slightly larger core polarization constant κ as well as almost identical crystal-fields (nearly the same Dq ) for center I as compared to center II. The above delicate discrepancies in spectroscopic behaviors may be attributed to the tiny differences in local structures and electronic states for Cu 2+ at dissimilar parts of the ZnO nanocrystals. [ABSTRACT FROM AUTHOR]

Thegfactors and local structures for Cu2+in the ZnX (X = O, S and Se) nanocrystals at room temperature are theoretically investigated by the perturbation calculations for a tetragonally distorted tetrahedral 3d9cluster in a consistent way, and the isotropicgfactor is predicted for the ZnS:Cu2+nanocrystals at room temperature. The bond angles θ between the four equivalent Cu2+−X2−bonds and the C4axis are found to be about 1.26°, 1.24° and 1.07°, respectively, larger in the ZnO, ZnS and ZnSe nanocrystals than that (θ0≈ 54.74°) for an ideal tetrahedron, inducing tetragonally compressed tetrahedra. The declining tendency (ZnO > ZnS > ZnSe) of the tetragonal angular distortion Δθ (= θ − θ0) can be ascribed to the decreasing strength of the dynamic Jahn–Teller effect via the vibration interactions of the [CuX4]6−groups due to the weakening Cu2+−X2−bonding. The isotropicgfactors are attributable to the appropriate Δθ due to the dynamic Jahn–Teller effect and the internal stress. The slightly increasing (ZnO < ZnS < ZnSe)gfactors can be illustrated by the declining cubic field parameterDq, angular distortion Δθ and covalency factorNof the systems. [ABSTRACT FROM PUBLISHER]

By employing the perturbation calculations, the spin Hamiltonian parameters (SHPs) and local structures are uniformly explored for Mn 2+ in the ZnO nanocrystal and bulk. The EPR spectra are satisfactorily explained for both the systems, and the defect structure information is obtained, i.e., the substitutional impurity Mn 2+ may undergo the axial displacements of 0.036 and 0.042 Å away from the oxygen triangles along the C 3 axis in the ZnO nanocrystal and bulk, respectively. The similar SHPs for Mn 2+ in the nanocrystal and bulk can be illustrated as the approximately identical covalency (equal covalency factor N ) as well as the smaller axial displacement, the stronger crystal–field (CF) interactions (larger cubic CF parameter Dq) and Mn 2+ 3d–3s orbital admixture (larger core polarization constant κ ) in the former. [ABSTRACT FROM AUTHOR]

The spin Hamiltonian parameters ( g factors and hyperfine structure constants) and local structures are theoretically studied for the tetragonal Cu 2+ centers in the ZnX (X=O and S) nanocrystals from the perturbation formulas of these parameters for a 3d 9 ion in tetragonally distorted tetrahedra. The ligand orbital and spin–orbit coupling contributions are considered in view of strong covalency. Due to the Jahn–Teller effect, the local Cu 2+ ‒X 2− bond angles between the four equivalent impurity-ligand bonds and the four-fold axis are found to be about 2.47° and 1.68° larger than that (≈54.74°) for an ideal tetrahedron. This induces tetragonally compressed [CuX 4 ] 6− clusters on tetrahedral substitutional Zn 2+ sites, different from the assignments (i.e., Cu 2+ on tetragonally elongated octahedral and tetrahedral substitutional sites in the ZnO and ZnS nanocrystals, respectively) in the previous works. The calculated g factors for both systems and the parallel component of the hyperfine structure constants for the ZnS:Cu 2+ nanocrystals based on the above local angular distortions are in good agreement with the observed values. The validity of the present assignments for the local structures of the Cu 2+ centers is analyzed. [ABSTRACT FROM AUTHOR]

The spin Hamiltonian parameters (zero-field splittingD,gfactors and hyperfine structure constants) are theoretically studied for Mn2+in the ZnS nanocrystals and bulks from the perturbation formulae of these quantities for trigonal and cubic tetrahedral 3d5clusters, respectively. The trigonal Mn2+centre in the ZnS nanocrystals is attributed to the impurity–ligand bond angle related to the C3axis about 0.39° larger than that (≈109.47°) of an ideal tetrahedron. Almost the samegfactors and hyperfine structure constants for the nanocrystals and bulks can be ascribed to similar crystal-field environments (i.e. comparable cubic field parametersDq), nearly the same covalency (i.e. the equal covalency factorsN) and the Mn2+3d–3s orbital admixture (i.e. the identical core polarisation constants κ) in both systems. The ligand orbital and spin–orbit coupling contributions are found to be important and should be included in the electron paramagnetic resonance analysis in view of significant covalency. [ABSTRACT FROM AUTHOR]

Construction of self‐healing materials with improved mechanical performance is a great challenge. A strong and tough self‐healing composite is fabricated via in situ growth of zeolitic imidazole framework‐8 (ZIF‐8) nanocrystals in imidazole‐containing polymer networks. By adjusting the stoichiometric ratio of the zinc salt to 2‐methylimidazole, composites with various mechanical performances are obtained. The existence of ZIF‐8 nanocrystals via in situ growth in the polymer networks is confirmed by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The zinc–imidazole interactions between the ZIF‐8 nanocrystals and the polymer are confirmed by attenuated total reflection Fourier transform infrared (ATR‐FTIR) spectroscopy. The composites can repair themselves under mild conditions owing to dynamic zinc–imidazole interactions. The self‐healing efficiency of composites can reach up to 91% under the condition of 60 °C for 48 h. In contrast to the pure zinc cation crosslinking system, the composite containing ZIF‐8 nanocrystals prepared via in situ growth exhibited enhanced tensile strength and toughness by 43% and 100%, respectively. This study proves that incorporating the metal‐organic frameworks (MOFs) materials into a self‐healing system via an in situ growth strategy is highly promising for designing self‐healing materials with improved mechanical performance. [ABSTRACT FROM AUTHOR]