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The search for the new nanoscale functional materials has led to the emergence of the quantum dots (QDs) nanoengineering as a leading field of contemporary science. In recent years, special scientific attention has been devoted to the novel group of ternary I-III-VI QDs from which the chalcopyrite-type AgInS2 (AIS) have become one of the most interesting and important material for further applications, due to its reduced toxicity, red-shifted absorption and emission and high tolerance to off-stoichiometry. Despite their good luminescence properties, there is a need for extensive research to comprehend the physical nature of the electronic transitions in AgInS2 quantum dots. Because of the defect related nature of those structures the donor-acceptor pair (DAP) model is mostly indicated as the origin of emission in AIS QDs. However, limited applicability of the DAP model to highly confined structures indicate different physical mechanisms of luminescence which are still under discussion. The specific problem connected with the AIS QDs is their defect mediated kinetics of excited states relaxation resulting in highly non-exponential luminescence decays, effective trapping of the fundamental excitations and effective non-radiative relaxations [1].
Recent experiments [2,3] show that the incorporation of the pool of non-bonding electrons near the quantum dot can influence the mechanisms of excited state relaxation and significantly reduce dark states resulting in the enhancement of the QDs luminescence. Previously it was realized as the metallic gold shell around the quantum dot [2] or by placing the QDs on the metallic nanoparticles thin film [3]. Due to the known high tolerance of the AgInS2 chalcopyrite structure to off stoichiometry the excess of non-bonding electrons can be accomplished by the synthesis of non-stoichiometric metal-rich AIS QDs. Density functional theory calculations show, that the excessive Ag atoms in Ag-In-S nanoclusters result in the formation of the additional electronic energy levels near the HOMO (highest occupied molecular orbital) level with molecular orbitals of clearly non-bonding characters.
In this paper we show the improvement of the luminescent properties of AIS QDs due to the deviation from stoichiometry. The results of the synthesis and optical measurements for the series of Ag(1+x)InS2 (x = -0.2, -0.1, 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) QDs samples are presented. The red-shift and widening of the luminescence spectrum have been observed with the increasing of the metal amount. Femtosecond time-resolved fluorescence spectroscopy measurements reveal minor deviation from the exponential decay for the Ag-rich AIS QDs in comparison to the stoichiometric ones. It was concluded, that such behavior was inducted by the excess of non-bonding electrons in metal-rich structures.
[1] Cichy B et al. Two blinking mechanisms in highly confined AgInS2 and AgInS2/ZnS quantum dots evaluated by single particle spectroscopy. Nanoscale 2016;8:4151–9.
[2] Ji B et al. Non-blinking quantum dot with a plasmonic nanoshell resonator. Nat Nanotechnol 2015;10:170–5.
[3] Ma X et al. Fluorescence enhancement, blinking suppression, and gray states of individual semiconductor nanocrystals close to gold nanoparticles. Nano Lett 2010;10:4166–74.
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