Investigation of Photophysical Properties of Ternary Zn-Ga-S Quantum Dots: Band Gap versus Sub-Band-Gap Excitations and Emissions.

Abstract

Highly luminescent ternary Zn–Ga–S quantum dots (QDs) were synthesized via a noninjection method by varying Zn/Ga ratios. X-ray diffraction and Raman investigations demonstrate composition-dependent changes with multiple phases including ZnGa2S4, ZnS, and Ga2S3 in all samples. Two distinct excitation pathways were identified from absorption and photoluminescence excitation spectra; among them, one is due to the band-gap transition appearing at around 375 and 395 nm, whereas another one observed nearby 505 nm originates from sub-band-gap defect states. Photoluminescence (PL) spectra of these QDs depict multiple emission noticeable at around 410, 435, 461, and 477 nm arising from crystallographic point defects formed within the band gap. The origin of these defects including zinc interstitials (IZn), zinc vacancies (VZn), sulfur interstitials (IS), sulfur vacancies (VS), and gallium vacancies (VGa) has been discussed in detail by proposing an energy-level diagram. Further, the time-dependent PL decay curve strongly suggests that the tail emission (appear around 477 nm) in these ternary QDs arises due to donor–acceptor pair recombination. This study enables us to understand the PL mechanism in new series of Zn–Ga–S ternary QDs and can be useful for the future utilization of these QDs in photovoltaic and display devices.