![]() ![]() The first example are nanoheterostructures combining ferrites with coinage metals (Figure 1b) exhibiting combined plasmonic and magnetic properties. (36) Two examples for such nanoheterostructures with properties not reachable in bulk are presented in the following to demonstrate the cation exchange procedure. (32-35) Synthesizing them as nanoheterostructures allows obtaining physical properties which are unreachable in bulk materials. In particular, ferrite-based single component nanocrystals and multicomponent core/shell nanocrystal hererostructures were chosen as starting materials for the cation exchange treatments (Figure 1), because of their high potential in biomedical sciences (21-31) and their highly reproducible synthesis by facile solution-phase thermolysis routes. ![]() While the high potential for tailoring of material properties by the cation exchange treatment has been demonstrated for ionic semiconductors (primarily chalcogenides), (8-20) here it is applied to ionic magnetic oxide materials. Treating PbX (X = S, Se, Te) with Cd 2+ ions results, e.g., in the formation of protective CdX shells, causing a significant increase of the photoluminescence quantum yield of these materials. Cation exchange reactions have been applied also to improve the properties of semiconductor nanocrystals. (17) As will be discussed below, also the coordinating solvent applied during cation exchange might contribute to the anionic framework conservation. (12, 14, 16) The latter is caused by an anionic framework conservation, which might result from the larger ionic size of the anions in the lattice, causing their lower diffusion velocity as compared to that of the cations. In contrast to the galvanic replacement where often substantial morphology changes are observed (e.g., hollow structures are obtained from solid nanoparticles) (3, 4) during cation exchange the nanocrystal shape is nearly preserved. (8-14) By cation exchange procedures, e.g., nanorod superlattices of regularly spaced Ag 2S quantum dots in CdS colloidal quantum rods, (9) nonepitaxial hybrid nanostructures with gold nanoparticle cores and CdS shells, (15) or branched nanocrystals with either CdSe or Cu 2- xSe central cores and Cu 2S pods, (16) have been demonstrated. (1-7) The cation exchange is a similar process, however, usually applied to compound semiconductors. The galvanic replacement is predominantly applied to form noble metal nanocrystals, such as Au nanocages and nanoboxes, by reacting solutions of appropriate salts with nanocrystals as “nanotemplates”. Postsynthetic substitution reactions, including galvanic replacement and cation exchange, applied to colloidal nanocrystals, (1-14) represent a simple and versatile tool to achieve nanoarchitectures not readily accomplishable by other methods. By applying the cation exchange to FeO/CoFe 2O 4 core/shell nanocrystals the Neél temperature of the core material is increased and exchange-bias effects are enhanced so that vertical shifts of the hysteresis loops are obtained which are superior to those in any other system. For core/shell nanoheterostructures a selective doping of either the shell or predominantly of the core with Co 2+ is demonstrated. In homogeneous magnetite nanocrystals and in gold/magnetite core shell nanocrystals the cation exchange increases the coercivity field, the remanence magnetization, as well as the superparamagnetic blocking temperature. This allows tracing of the compositional modifications by systematic and detailed magnetic characterization. While the cation exchange procedure, performed in solution phase approach, was restricted so far to chalcogenide based semiconductor nanocrystals, here ferrite-based nanocrystals were subjected to a Fe 2+ to Co 2+ cation exchange procedure. For three types of colloidal magnetic nanocrystals, we demonstrate that postsynthetic cation exchange enables tuning of the nanocrystal’s magnetic properties and achieving characteristics not obtainable by conventional synthetic routes. ![]()
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