Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Indium phosphide quantum dots (InP QDs) have attracted great interest, due to their size-tunable emission in the visible and near-infrared spectral range and low intrinsic toxicity compared to Cd-based QDs. These properties make InP QDs promising candidates as narrow red band emitters in white light LEDs. Unfortunately, bare InP QDs usually exhibit poor optical properties. Main problems are low emission efficiencies (PLQY<1%), broad emission profiles (FWHM 50-100 nm) due to a poor control of the size distribution, and the presence of trap emission related to surface defects.
Using a recently developed synthesis method, we are now able to synthesize monodisperse InP QDs using a less toxic aminophosphine precursor [1]. The resulting InP QDs show well-resolved absorption features throughout the visible spectrum, indicating a narrow size distribution. To further improve the optical properties of the QDs, passivation of the nanocrystal surface is essential. The most suitable material for shell growth is zinc sulfide (ZnS), because of its low intrinsic toxicity and the type-I band alignment. However, several previous studies have shown that the growth of a ZnS shell around InP QDs induces a blue shift in both the absorption and emission spectra of the resulting InP/ZnS QDs. This blue shift is unfavorable for the use of InP/ZnS QDs as narrow red band emitters. In the literature several possible explanations for this blue shift are proposed, such as cation exchange, surface etching, and anion exchange [2].
To get more insight in the shell growth processes involved, we studied the optical properties of InP/ZnS QDs synthesized with various sulfur and zinc precursors at a constant reaction temperature. We show that the emission of the resulting InP/ZnS QDs can be tuned from the green to the red spectral range starting with InP QDs with a bandgap emission of 650 nm by variation of the shell precursors. The blue shift increases as the reactivity of the S-precursor increases and the PLQY of the resulting particles depends on the Zn-precursor used. All resulting InP/ZnS QDs show a narrow band emission, but the PLQY varies from 5-70% while the emission maximum shifts from 480 to 615 nm depending on the combination of the precursors used. Further insight in the characteristics of the luminescence for the different InP/ZnS QDs is obtained by time-resolved and temperature-dependent photoluminescence spectroscopy.
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