Investigations on the defect structures for Mn2+ in CdSe nanocrystals and bulk materials and the criterion of occupation for Mn2+ in CdX (X = S, Se, Te) nanocrystals

Abstract

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.

Keywords: CdSe; Mn2+; defect structures; electron paramagnetic resonance (EPR); hyperfine structure constants (HSCs); nanocrystals.