Due to the large lattice mismatch of 7.7% between zinc blende InP and ZnS it is challenging to obtain a thick shell. The synthesis of InP/ZnS nanocrystals in a single-step one-pot method by adding a sulfur source (dodecanethiol, DDT) during the core synthesis leads to an alloy structure capped with a thin ZnS shell ( Huang et al., 2010). Nonetheless, for achieving high photoluminescence quantum yields (PLQY) and enhanced photostability the precise engineering of core/shell heterostructures is a prerequisite, and ideally, the InP or InZnP core should be covered by a thick shell of a large band gap, chemically stable semiconductor such as ZnS ( Reiss et al., 2009). Concerning the core synthesis, it has been established that the addition of zinc, in form of carboxylate (e.g., stearate, undecylenate, oleate) improves both the size distribution and hence the emission linewidth and the emission efficiency ( Li and Reiss, 2008 Xu et al., 2008 Ung et al., 2010b Stein et al., 2016). The chemical synthesis of InP QDs and their core/shell structures has been strongly developed in the past decade with the goal to bring the optical properties to a level comparable with CdSe based QDs ( Cossairt, 2016 Tamang et al., 2016). In the quest for toxic-heavy-metal-free quantum dots (QDs), indium phosphide has been shown to be a valid alternative to cadmium-based materials ( Reiss et al., 2016 Allocca et al., 2019). However, a trend toward reduced cell proliferation is observed for higher concentrations of gradient shell and CSS QDs with a thin ZnS shell, while CSS QDs with a thicker ZnS shell do not exhibit any impact. Cytotoxicity studies on human primary keratinocytes revealed that exposure for 24 h to 6.25–100 nM of QDs did not affect cell viability. The gradient shell and CSS systems could be transferred to the aqueous phase using surface ligand exchange with penicillamine. These modifications do not occur in the case of the alumina-capped sample, which exhibits excellent chemical stability. By means of XPS studies we identify the degradation of the ZnS outer layer and concomitant oxidation of the emissive InZnP core as the main origins of degradation in the gradient structure. The alumina coated core/shell system exhibits the highest stability in terms of PLQY retention as well as the lowest shift of the PL maximum and lowest increase of the PL linewidth, followed by the CSS QDs and finally the gradient shell system. To assess their photostability they are incorporated into a transparent poly (methyl methacrylate) (PMMA) matrix and exposed to continuous irradiation with simulated sunlight in a climate chamber. All three systems have photoluminescence quantum yields (PLQY) > 50% and similar PL decay times (64–67 ns). With the goal to improve their photostability, InP-based QDs are passivated with three types of inorganic shells, namely (i) a gradient ZnSe xS 1−x shell, (ii) an additional ZnS shell on top of the gradient shell with two different thicknesses (core/shell/shell, CSS), (iii) an alumina coating on top of ZnS. Grenoble Alpes, CEA, CNRS, IRIG, IBS, Grenoble, France Grenoble Alpes, CEA-LITEN L2N, Grenoble, France Grenoble Alpes, CEA, CNRS, IRIG, SyMMES, CIBEST, Grenoble, France Grenoble Alpes, CEA, CNRS, IRIG, SyMMES, STEP, Grenoble, France Karl David Wegner 1 * †, Fanny Dussert 2, Delphine Truffier-Boutry 3, Anass Benayad 3, David Beal 2, Lucia Mattera 1, Wai Li Ling 4, Marie Carrière 2 * and Peter Reiss 1 *
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