Enhanced oscillator strengths and optical parameters of doped ZnS bulk and nanophosphors

We synthesize and investigate the oscillator-strength (OS), dipole-moment (DM), and integrated cross-section values (ICSVs) of singly (Mn) and doubly (Mn and Co or Ni) doped samples of ZnS bulk and nanophosphors. The oscillator-strength values (OSVs) are found to enhance by three orders of magnitude when Co or Ni dopant is incorporated in ZnS:Mn bulk phosphors, which strongly suggests that a quencher dopant triggered an energy-transfer process in the host ZnS material. Nevertheless, although these quencher dopants were previously considered to kill the luminescence from the host material, we used these dopants in ZnS:Mn to create an additional pathway for the relaxation of the carrier, and to initiate the energy-transfer mechanism. On the other hand, a four orders of magnitude enhancement in the OSVs was observed on incorporating the quencher dopants in ZnS:Mn nanophosphors, which is attributed to the fact that our nanosamples are related to a strong-confinement case while the quencher dopant played a significant role in the variation of OSVs. Moreover, the analysis of OSVs showed that excitonic/defect level emission from ZnS host is due to a weak electric dipole transition (WEDT), while a magnetic dipole transition (MDT) dominates in the case of ZnS:Mn nanophosphors. Based on the present investigations, we clearly obtained an origin of excitonic- and impurity-related emission from the doped ZnS bulk and nanophosphor samples, which were almost vague in the previous studies of other researchers.