Photoacoustic and Photoluminescence Spectra of ZnS:Mn Nanocrystals

Abstract

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).