Photoluminescence of ZnS Clusters in Zeolite-Y

Two obvious emissions are observed from the ZnS clusters encapsulated in zeolite-Y. The emissionaround 355 nm is sharp and weak, locating at the onset of the absorption edge. The bandaround 535 nm is broad. Strong and Stokes-shifted. Both the two emissions shift to blue andtheir intensities firstly increase then decrease as the loading of ZnS in zeolite-Y or clusters sizedecreases. Through investigation, the former is attributed to the excitonic fluorescence, andthe latter to the trapped luminescence from surface States. The cluster size-dependence of theluminescence may be explained qualitatively by considering both the carrier recombination andthe nonradiative recombination rates. Four peaks appearing in the excitation spectra are assignedto the transitions of 1S-1S, 1S-1P, 1S-1D and sudsce state, respectively. The excitation spectraof the clusters do not coincide with their absorption spectra. The states splitted by quantum-sizeconfinement are detected in the excitation spectra, but could not be differentiated in the opticalabsorption spectra due to inhomogeneous broadening. The size-dependence of the excitationspectra is similar to that of the absorption spectra. Both the excitation spectra of excitonicand of trapped emissions are similar, but change in relative intensity and shift in position areobserved.