Spectrally Narrow Blue-Light Emission from Nonstoichiometric AgGaS₂ Quantum Dots for Application to Light-Emitting Diodes

Luminescence color tuning of less toxic I-III-VI-based quantum dots (QDs) has been intensively investigated for application in wide-color-gamut displays. However, the emission peaks of these multinary QDs are relatively broad in the blue-light region compared to those in the green and red regions. Here, we report the synthesis of AgGaS₂ (AGS) QDs that show a narrow blue emission peak through nonstoichiometry control and surface defect engineering. While as-prepared AGS QDs with angular shapes primarily exhibited a weak green photoluminescence (PL) peak at 520 nm assigned to defect-site emission, treatment with chloride ions resulted in the appearance of a sharp band-edge PL peak at 442 nm, with the number of surface defect sites decreasing as a result of rounding off the angles of the QD shape. Further coating of the QDs with a gallium sulfide (GaS_x) shell selectively enhanced the band-edge PL peak at 446 nm with a narrow full width at half-maximum of 22 nm, where the defect-site emission was almost eliminated due to the removal of surface defect sites. The PL quantum yield (QY) significantly increased from 5.5% for chloride-treated AGS QDs to 12% for AGS core-GaS_x shell QDs (AGS@GaS_x). QD light-emitting diodes fabricated with AGS@GaS_x QDs exhibited a sharp emission peak at 450 nm, slightly red-shifted from that of the PL spectrum of the QD films, accompanied by the reappearance of a weak broad defect-site emission peak at around 560 nm.