Effect of europium on the blue-green emission of ZnS thin films by polyol and dip-coating technique

This work reports the effect of annealing treatment (550, 650, and 750 degrees C) and europium concentration (1-7 at.%) on the structural, morphological, and optical properties of ZnS:Eu3+ thin films synthesized by the polyol method and dip-coating technique. The films were indexed to the zinc-blende phase of ZnS with crystallite size ranging from 2-7 nm and exhibited preferential orientation at the (111) plane according to the XRD analyses. The SEM and AFM images indicate homogeneous, flat, crack-free, compacted films, with pyramidal grain shape and particle size distribution about similar to 25 nm. High transmittance in the visible region (above 80 %) and band gap (3.75-3.9 eV) computed by the Tauc model were obtained by UV-VIS spectroscopy. The PL spectra show the emission bands related to radiative recombination involving defect states, particularly, zinc and sulfur vacancies. The PL spectra also indicate that the higher the europium concentration, the higher the emission light intensity in the blue-green regions. At the same time, the typical D-5(0) -> F-7(2) transition from Eu3+ is barely noticeable due to a poor energy transfer from the ZnS host to the Eu ions. On the other hand, by increasing the annealing temperature from 550 to 750 degrees C, a 2.5-fold decrease in the emission intensity is recorded.