Multicolor electroluminescent devices using doped ZnS nanocrystals

Alternate-current electroluminescent (ac EL) devices based on doped ZnS nanocrystals emitting blue, green, and orange-red colors are reported. ZnS nanocrystals doped with $Cu+–Al3+$ and $Cu+–Al3+–Mn2+$ combinations were synthesized by wet chemical method at room temperature. The $ZnS:Cu+,$ $Al3+$ nanocrystals show blue (462 nm) and green (530 nm) EL emissions depending upon the presence and absence of sulphur vacancies, respectively. The orange EL emission (590 nm) is realized from $ZnS:Cu+,$ $Al3+,$ $Mn2+$ nanoparticles by way of nonradiative energy transfer from $AlZn–CuZn$ pairs to $MnZn.$ The EL devices show low turn-on voltage of ∼10 V ac @100 Hz. The mechanism of ac EL in ZnS nanocrystals has been explained wherein the excitation is attributed to the electric-field-assisted injection of electron-hole pairs from the surface regions into the interiors and their subsequent recombination therein causes emission.