Studies on the spin Hamiltonian parameters and the defect structures for the two interstitial tetragonal Cu2+ centers in the ZnO nanocrystals

Theoretical investigation on the optical and EPR spectra for Cu2+‐doped ZnO‐CdS nanocomposites

“…Moreover, for Cu 2+ doped in ZnCdO crystal and CdSe nanocrystals, the impurity Cu 2+ occupying the interstitial position site with the local symmetry of [CuO 6 ] 10− clusters in both crystals is tetragonal elongated octahedral, by analysis the EPR parameters, the relative elongation ratio was found to be about ρ ≈ 3.5% and 3%, respectively, which is close to ρ (≈ 4.1%) obtained in this work and can be regarded as reasonable. It is noted that present calculations for interstitial Cu 2+ in ZnO‐CdS nanocomposites are actually different from those for Cu 2+ in the interstitial sites in ZnO nanocrystals, despite the same EPR formulas for the same tetragonally elongated octahedral 3d 9 centers. First, the dissimilar environments can be expected for the interstitial Cu 2+ in present ZnO‐CdS nanocomposites [9] to that in the previous ZnO nanocrystals due to the distinct preparation conditions .…”

“…Our calculations suggest that A ‖ and A ⊥ is negative and positive, respectively (see Table ). There are many reports on the negative A ‖ and positive A ⊥ of the hyperfine structure constants for the tetragonally elongated [CuO 6 ] 10− octahedral clusters . Consequently, the signs of hyperfine structure constants were determined in this work and can be regarded as reasonable.…”

“…For example, the slightly larger D q (≈1215 cm −1 ) of present ZnO‐CdS:Cu 2+ nanocomposites than that (≈1205 cm −1 ) of the previous ZnO:Cu 2+ nanocrystals can reflect the slightly stronger local crystal fields possibly arising from the more compact structure in the former. Second, the relative axial tetragonal elongation ratio of present ZnO‐CdS:Cu 2+ nanocomposites is also higher than that (≈ 3%) of previous ZnO ones, which can be ascribed to the more intense Jahn–Teller effect via the stronger vibration interactions of the Cu 2+ –O 2− bonds in the former systems. Finally, the smaller core polarization constant (κ ≈ 0.2) for present Cu 2+ center in ZnO‐CdS nonocomposites than that (≈ 0.28) in the previous ZnO nonocrystals may indicate the smaller copper 3d‐3 s (4 s) orbital admixtures due to the next or third nearest neighbor sulphur atoms with much larger polarizability.…”

“…Second, the relative axial tetragonal elongation ratio of present ZnO‐CdS:Cu 2+ nanocomposites is also higher than that (≈ 3%) of previous ZnO ones, which can be ascribed to the more intense Jahn–Teller effect via the stronger vibration interactions of the Cu 2+ –O 2− bonds in the former systems. Finally, the smaller core polarization constant (κ ≈ 0.2) for present Cu 2+ center in ZnO‐CdS nonocomposites than that (≈ 0.28) in the previous ZnO nonocrystals may indicate the smaller copper 3d‐3 s (4 s) orbital admixtures due to the next or third nearest neighbor sulphur atoms with much larger polarizability. Thus, the differences between the present and previous results can be understood in view of the dissimilar impurity behaviors owing to the new composition CdS and corresponding preparation conditions…”

“… A 2 k ( R 0 ) ( k = 2, 4) are the intrinsic parameters, and R 0 is the reference bond length. For 3d n ions under octahedra clusters, A 4 ( R 0 ) ≈ 3 D q /4, and the ratio A 2 ( R 0 )/ A 4 ( R 0 ) ≈ 9 ‐ 12 is acquired by studying the optical and EPR data in many crystals; here, we take A 2 ( R 0 ) = 9 A 4 ( R 0 ). The cubic field parameter and D q ≈ 1,000–1,300 cm − 1 is found for [CuO 6 ] 10 − octahedra in many crystals; we take D q ≈ 1,215 cm − 1 here; this value is very close to that D q ≈ 1,205 cm − 1 adopted for Cu 2+ centers in the ZnO nanocrystals and can be regarded as reasonable. When Cu 2+ ion on the interstitial site in ZnO‐CdS nanocomposites, the Cu 2+ –O 2− distance was not reported; we reasonably estimated that the distance R 0 (≈2.04 Ǻ), which is close to the sum of ionic radii of Cu 2+ and O 2− ions .…”