Influence of the cationic composition on the optical properties and photostability of AgInS2 and AgInS2/ZnS quantum dots

Abstract

Semiconductor I-III-VI nanocrystals are highly luminescent low toxic materials with direct band gap and tunable emission in visible and near infrared region which can be synthesized in aqueous media via simple procedure making them competitive object in compare with well developed and investigated II-VI quantum dots. However the nature of radiative recombination pathways definition and effect of synthesis conditions on spectral characteristics of the nanocrystals investigation remains an important task. In the present work influence of the cation precursors ratio as well as the heat treatment duration on the composition, optical properties and photostability of AgInS2 and AgInS2/ZnS nanocrystals synthesized in aqueous media have been investigated. Due to the low reactivity of indium salts in aqueous solution coused by formation of a stable complexes with stabilizer or hydroxycomplexes the nearest stoichiometric Ag1,1InSx quantum dots are formed at the initial [In]:[Ag] ratio 7:1. Under high excess of Ag formation of Ag2S/AgInS2 core/shell quantum dots confirming by presence of large 12-15 nm nanocrystals and red shift of the photoluminescence maximum with increasing [In]:[Ag] ratio from 1 to 3 (shell thickness should increase proportionally) is possible. With a further increase of the [In]:[Ag] ratio the absorption edge and the photoluminescence maximum are green shifted indicating increase of the quantum dots band energy. That can be explained by lowering of the valence band ceiling energy and rising of the conduction band bottom energy due to decrease of density of states of Ag 4d orbitals and increase of density of states of In 5s and 5p orbitals involved in the AgInS2 band gap formation. Increase of Indium content leads to significant increase of the photoluminescence intensity of AgInS2 nanocrystals eliminating nonradiative defects such as Agi. It have been shown that the ZnS shell epitaxial grow occurs due to the cation exchange between Zn2+ and In3+ and takes place only at low concentration of Zinc precursor ([Zn]:[Ag] ≤ 4). With further enhancement of the ratio the solid solution of AgInS2-ZnS is forming. To achieve the highest possible photoluminescence intensity and energy the AgInS2-ZnS nanocrystals should be heated at 950C at least for 120 minutes. It have been shown that the nearly stoichiometric Ag1,1InSx nanocrystals posess the highest photostability under UV light irradiation.