White-light emission from zinc chalcogenide alloy quantum dots with gradient compositions

Zn(Te_1−xS_x) quantum dots (QDs) surrounded by a ZnS shell were synthesized using a standard hot-injection method. The obtained Zn(Te_1−xS_x) QDs consisted of a Te-rich center and an outer layer with a gradient composition in which the S concentration increased with increasing distance from the center. The Te-rich portion of the Zn(Te_1−xS_x) QDs had a composition range of 0.2 ≤ x ≤ 0.5 and a diameter <2 nm, while the compositionally graded S-rich region had a thickness of ~0.5 nm. The optical gaps of the Zn(Te_1−xS_x) QDs were smaller than that of unalloyed ZnTe QDs owing to the large band gap bowing that appears in ZnTe–ZnS alloys. The Zn(Te_1−xS_x)/ZnS core/shell QDs exhibited photoluminescence (PL) covering the entire visible range, with a narrow band emission at the blue region arising from exciton recombination and a broad band emission centered in the region from green to red. The alloying level, x, moved the values of the chromaticity coordinates (CIE 1931), where both x = 0.10 and x = 0.18 corresponded to warm white-light and daylight with the CIE color coordinates of (0.405, 0.408) and (0.318, 0.352), and a correlated color temperature of ~3400 and 6000 K, respectively. Based on the point defects that commonly appear in zinc chalcogenides and PL emission decay trace, the electronic transition responsible for the broad band emission was attributed to the recombination of the electron at the 1S_e quantum level and the hole at the localized zinc vacancy acceptor level.