Stability of fast elaborated small CdS quantum dots
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Stability of fast elaborated small CdS quantum dots M. Fregnaux1,2, S. Dalmasso1, J.-J. Gaumet2, J.-P. Laurenti1 1 Laboratoire de Physique des Milieux Denses 2 Laboratoire de Spectrométrie de Masse et de Chimie Laser Institut Jean Barriol, Université Paul Verlaine – Metz, 1 Boulevard Arago, F-57078 Metz Cedex 03 ABSTRACT Small size CdS QDs were synthesized by (i) the single source precursor methodology and by (ii) the microwave synthetic route. The consequences of CdS QD direct exposure to air for a period of 7 days were investigated by following the evolution of the photoluminescence (PL) and absortion spectra. For QDs obtained by (i), the excitonic emission band (3.0 - 3.1 eV) decreases in intensity, relatively to the low energy one (2.2 - 2.5 eV) tentatively associated to midgap surface states. This suggests arising of new recombination path(s) associated to degradations during aging, possibly an oxidative formation of a CdO surface layer. On the other hand, no significant change is observed in the absorption spectra. For QDs obtained by (ii), no degradation is revealed by the PL spectra which remain unchanged. On the other hand, the absorption spectra are dominated by an unexplained broad band around 3.6 eV which tends to hide the fundamental excitonic transition one and increases in intensity with aging. INTRODUCTION The synthesis of high quality II-VI semiconductor nanocrystals (NCs) is fundamental in developing new devices in several applications such as biomarkers, solar cells or blue-UV lasers. These emerging technologies are funded on the size-dependent properties of the NCs. Consequently, it is a crucial aspect to get insight about different ways for synthesis of NCs and to closely control the chemical, structural and optoelectronic properties. The solid-state method is an easy and efficient way to produce such kind of clusters. Previously, we reported on growth of CdSe and CdS quantum dots (QDs) by the single source precursor methodology [1-3]. High quality small QDs (2.2 – 4.5 nm in diameter) were obtained. The technique was proved to be simple and fast: growth durations did not exceed 2 hours. Joint physical and chemical characterization methods provided consistent data on quality of these small size QDs. Recently, we extended this study to CdS QDs produced by microwave (MW) chemistry [4-6] in the same size range. The growth durations did not exceed 25 min. So this technique was proved to be a worthwhile alternative one to the single source precursor methodology. However, characterization measurements, mainly optical spectrometry, were systematically performed just after removing the stabilizing agent surrounding the QDs after the growth process. In this work, we get insight about the evolution of optical spectra (photoluminescence - PL – and absorption) of CdS QDs kept directly exposed to air during several days. We compare the trends observed depending on the growth techniques mentioned above. The effects addressed in this study involve mainly surface phenomena. Due to a large surface-to-volume ratio
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