Synthesis and Characterization of ZnS:Eu$^{\mathrm{3+}}$ - CMC nanophosphors emitting white light over broad excitation range

In this paper we report for the first time the synthesis and characterization of nanophosphors of ZnS:Eu$^{\mathrm{3+}}$ - embedded in sodium carboxymethyl cellulose matrix (CMC) that emits high quality white light over broad range of excitation. The nano-phosphors of cubic (zinc blende) structure were synthesized using precipitation technique with doping concentrations of Eu$^{\mathrm{3+}}$ ions 1 mol{\%} and 5 mol{\%}. The crystal sizes were 2.56 nm and 2.91 nm respectively. Annealing at 300 $^{\mathrm{o}}$C in a sulfur-rich atmosphere altered the crystal size to 4.35 nm and 3.65 nm respectively and the band gap from 4.2 eV to 3.76 eV and 3.81 eV respectively. The as-synthesized samples gave pure orange-red emission when excited at wavelengths of 394 nm and 465 nm. After thermal annealing of the samples, a broad emission band in the blue-green region assigned to defect related states emerged or were enhanced. Also enhanced were the emission lines of Eu$^{\mathrm{3+}}$ ions in the orange-red region. A combination of these two transitions gave white light of different shades (recorded on the CIE 1931 chromaticity diagram) from cool white through Day-light to warm white light, depending on Eu$^{\mathrm{3+}}$ concentration and the excitation wavelengths (UV-330 to blue 465 nm), thus showing great potential applications of these nano-phosphors.