Among the different synthesis approaches to colloidal nanocrystals, a recently developed toolkit is represented by cation exchange reactions, where the use of template nanocrystals gives access to materials that would be hardly attainable via direct synthesis. Besides, postsynthetic treatments, such as thermally activated solid-state reactions, represent a further flourishing route to promote finely controlled cation exchange. Here, we report that, upon in situ heating in a transmission electron microscope, Cu2Se or Cu nanocrystals deposited on an amorphous solid substrate undergo partial loss of Cu atoms, which are then engaged in local cation exchange reactions with Cu “acceptor” phases represented by rod- and wire-shaped CdSe nanocrystals. This thermal treatment slowly transforms the initial CdSe nanocrystals into Cu2–xSe nanocrystals, through the complete sublimation of Cd and the partial sublimation of Se atoms. Both Cu “donor” and “acceptor” particles were not always in direct contact with each other; hence, the gradual transfer of Cu species from Cu2Se or metallic Cu to CdSe nanocrystals was mediated by the substrate and depended on the distance between the donor and acceptor nanostructures. Differently from what happens in the comparably faster cation exchange reactions performed in liquid solution, this study shows that slow cation exchange reactions can be performed at the solid state and helps to shed light on the intermediate steps involved in such reactions.

Controlled synthesis of anisotropic iron oxide nanoparticles is a challenge in the field of nanomaterial research that requires an extreme attention to detail. In particular, following up a previous work showcasing the synthesis of magnetite nanorods (NRs) using a two-step approach that made use of polyethylenenemine (PEI) as a capping ligand to synthesize intermediate β-FeOOH NRs, we studied the effect and influence of the capping ligand on the formation of β-FeOOH NRs. By comparing the results reported in the literature with those we obtained from syntheses performed (1) in the absence of PEI or (2) by using PEIs with different molecular weight, we showed how the choice of different PEIs determines the aspect ratio and the structural stability of the β-FeOOH NRs and how this affects the final products. For this purpose, a combination of XRD, HRTEM, and direct current superconducting quantum interference device (DC SQUID) magnetometry was used to identify the phases formed in the final products and study their morphostructural features and related magnetic behavior.

The synthesis of large pentatwinned five-branched gold nanostars (GNS) has been modified so to obtain overall dimensions shrunk to 60% and a lower branches aspect ratio, leading to a dramatic blue shift of their two near-infrared (NIR) localized surface plasmon resonances (LSPR) absorptions but still maintaining one LSPR in the biotransparent NIR range. The interactions of polyethylene glycol (PEG) coated large and shrunk GNS with SH-SY5Y cells revealed that the large ones (DCI - diameter of the circumference in which GNS can be inscribed=76nm) are internalized more efficiently than the shrunk ones (DCI=46nm), correlating with a decreased cells surviving fraction.

Here we show that it is possible to produce different magnetic core−multiple shell heterostructures from monodisperse Fe3O4 spherical magnetic seeds by finely controlling the amount of a manganese precursor and using, in a smart and simple way, a cation-exchange synthetic approach. In particular, by increasing the amount of precursor, we were able to produce nanostructures ranging from Fe3O4/ manganese ferrite core−single-shell nanospheres to larger, flowerlike Fe3O4/manganese ferrite/Mn3O4 core−double-shell nanoparticles. We first demonstrate how formation of the initial thin manganese ferrite shell determines a dramatic reduction of the superficial disorder in the starting Fe3O4, bringing nanomagnets with lower hardness. Then, the growth of the second and most external manganese oxide shell causes magnetic hardening of the heterostructures, while its magnetic exchange coupling with the rest of the heterostructure can be either antiferromagentic or ferromagnetic, depending on the strength of the applied external magnetic field. This response is similar to that of an iron oxide−manganese oxide core−shell system but differs from what is observed in multiple-shell heterostructures. Finally, we report that the most external shell becomes magnetically irrelevant above the ferrimagnetic−paramagnetic transition of the manganese oxide, and the resulting magnetic behavior of the flowerlike structures is studied in depth

Energy Filtered Transmission Electron Microscopy (EFTEM) is an analytical tool that has been successfully and widely employed in the last two decades for obtaining fast elemental maps in TEM mode. Several studies and efforts have been addressed to investigate limitations and advantages of such technique, as well as to improve the spatial resolution of compositional maps. Usually, EFTEM maps undergo post‐acquisition treatments by changing brightness and contrast levels, either via dedicated software or via human elaboration, in order to maximize their signal‐to‐noise ratio and render them as visible as possible. However, elemental maps forming a single set of EFTEM images are usually subjected to independent map‐by‐map image treatment. This post‐acquisition step becomes crucial when analyzing materials that change composition over time as a consequence of an external stimulus, because the map‐by‐map approach doesn't take into account how the chemical features of the imaged materials actually progress, in particular when the investigated elements exhibit very low signals. In this article, we present a facile procedure applicable to whole sets of EFTEM maps acquired on a sample that is evolving over time. The main aim is to find a common method to treat the images features, in order to make them as comparable as possible without affecting the information there contained. Microsc. Res. Tech. 78:1090–1097, 2015. © 2015 Wiley Periodicals, Inc.