Example of an advanced characterization study of a complex system (thin film solar cell) that may serve as an instructive handbook to help building up the full picture of multilayer-based devices for a broad spectrum of readers and researchers.
The atomic structure of nanoparticles can be easily determined by transmission electron microscopy. However, obtaining atomic-resolution chemical information about the individual atomic columns is a rather challenging endeavor. Here, crystalline monodispersed spinel FeO/MnO core-shell nanoparticles have been thoroughly characterized in a high-resolution scanning transmission electron microscope. Electron energy-loss spectroscopy (EELS) measurements performed with atomic resolution allow the direct mapping of the Mn/Mn ions in the shell and the Fe/Fe in the core structure. This enables a precise understanding of the core-shell interface and of the cation distribution in the crystalline lattice of the nanoparticles. Considering how the different oxidation states of transition metals are reflected in EELS, two methods of performing a local evaluation of the cation inversion in spinel lattices are introduced. Both methods allow the determination of the inversion parameter in the iron oxide core and manganese oxide shell, as well as detecting spatial variations in this parameter, with atomic resolution. X-ray absorption measurements on the whole sample confirm the presence of cation inversion. These results present a significant advance toward a better correlation of the structural and functional properties of nanostructured spinel oxides.
Cation exchange reactions have been exploited in the last years as an efficient tool for the controlled chemical modification of pre-made nanocrystals. In this work, the gradual transformation of Ag2S nanocrystals into Au2S analogues is performed by sequential cation exchange reactions that allow for a fine control of the chemical composition, delivering also two intermediate ternary sulfides based exclusively on noble metals. The role of two different surfactants in the reaction medium has been studied: while dodecylamine is favoring the heterogeneous nucleation of metallic Au on the surface of the semiconductor domains in detriment of the cation exchange reaction, the use of tetraoctylammonium bromide turns out to be crucial for the enhancement of the exchange in order to reach full cation substitution, if desired. The presence of Branions in the reaction medium represents an additional tool to modulate the morphology of the final nanocrystals, being either solid or hollow depending on their concentration. The synthetic protocol has been successfully conducted in both spherical and rod-like nanocrystals with identical results, leading to a wide variety of binary, ternary and/or hybrid nanostructures that have been carefully characterized.
Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysis, offering easier recyclability and a number of potentially advantageous functionalities, such as tunable band gaps, plasmonic properties, or a magnetic moment. Using high-throughput printing technologies, colloidal NCs can also be supported onto substrates to produce cost-effective electronic, optoelectronic, electrocatalytic, and sensing devices. For both catalytic and technological application, NC surface chemistry and supracrystal organization are key parameters determining final performance. Here, we study the influence of the surface ligands and the NC organization on the catalytic properties of InS, both as a colloid and as a supported layer. As a colloid, NCs stabilized by inorganic ligands show the highest photocatalytic activities, which we associate with their large and more accessible surfaces. On the other hand, when NCs are supported on a substrate, their organization becomes an essential parameter determining performance. For instance, NC-based films produced through a gelation process provided five-fold higher photocurrent densities than those obtained from dense films produced by the direct printing of NCs.
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