Photoacoustic and Photoluminescence Spectra of ZnS:Mn Nanocrystals

The optical absorption spectra of ZnS:Mn nanocrystals can be obtained by applying photoacoustic (PA) spectroscopy, which is a powerful technique for detecting small amounts of highly scattered materials. The peak position in PA difference spectra increases linearly within experimental accuracy with the decrease of Mn²⁺ ion concentration. These results correspond to the following possibilities, \ding172 the increase of the higher excited energy states relative to the ground state of ⁶A₁, \ding173 the increase of state density for the higher excited states and \ding174 the increase of transition probability from the ⁶A₁ state to the higher excited states. These possibilities suggest the hybridization of the s-p states of the ZnS nanocrystal host and the d state of the Mn²⁺ ion impurity. The peak position of the PL spectrum (the transition from ⁴T₁ to ⁶A₁ states) is ∼ 2.08 eV with a half width of 0.24 eV, similar to that of the bulk ZnS:Mn, and it is independent of the Mn²⁺ ion concentration. This result might be attributed to the fact that the Mn²⁺ emission is due to phonon assisted transition. Hence, there might be a slight shift of the peak position with increase of Mn²⁺ ion concentration due to hybridization of the s-p states of the ZnS nanocrystal host and the d state of the Mn²⁺ ion impurity. The peak intensity of the PL spectrum shows a maximum at a certain value of Mn²⁺ ion concentration and it decreases with the increase of concentration of Mn²⁺ ion impurities (concentration quenching).