Heterogeneity Induced Dual Luminescent Properties of AgInS 2 and AgInS 2 -ZnS Alloyed Nanocrystals
Résumé
We have elaborated a new preparation method of ternary AgInS2 and alloyed quaternary AgInS2-ZnS nanocrystals which consisted of two consecutive injections of sulfur (S dissolved in oleylamine, OLA) and then silver (AgNO3 or Ag2CO3 dissolved in dichlorobenzene, DCB) precursors to a mixture of indium(III) chloride, zinc stearate and 1-dodecanethiol (DDT) dissolved in 1-octadecene (ODE). In these conditions nucleation of cubic In2S3 seeds took place followed by the growth of orthorhombic AgInS2 or alloyed AgInS2-ZnS cubic phases to yield a heterodimer type of nanocrystals. In both types of nanocrystals clearly separated photoluminescence peaks could be observed, confirming their heterogenic nature. The first one at 430 nm originated from the luminescence of the In2S3 phase. The second one was ascribed to the presence of ternary Ag-In-S or quaternary Ag-In-Zn-S phases and its position, within the spectroscopic range from 515 nm to 710 nm, strongly depended on the nanocrystals composition. The registered two-dimensional excitation-emission topographical maps clearly indicated that the observed emissions in two different spectral regions were related to the excitations in the same spectral range (300-400 nm), however their photoluminescence mechanisms were distinctly different. The photoluminescence lifetime of 3 ns measured for the emission at shorter wavelengths was typical of the simple mechanism of excitons radiative recombination in the In2S3 phase. Significantly longer lifetime of the longer wavelengths emission (26 s) seemed to clearly indicate that in this case the photoluminescence mechanism was more complex, involving exciton trap states whose positions depended on the composition of the ternary (or quaternary) phase.