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Nanoscale horizons 5(7), 1065 - 1072 (2020). doi:10.1039/D0NH00117A, The self-assembly of nanoparticles into highly ordered crystals is largely influenced by variations in the size and shape of the constituent particles, with crystallization generally not observed if their polydispersity is too large. Here, we report on size selectivity in the self-assembly of rounded cubic maghemite nanoparticles into three-dimensional mesocrystals. Different X-ray scattering techniques are used to study and compare a nanoparticle dispersion that is used later for self-assembly, an ensemble of mesocrystals grown on a substrate, as well as an individual mesocrystal. The individual μm-sized mesocrystal is isolated using a focused-ion-beam-based technique and investigated by the diffraction of a micro-focused X-ray beam. Structural analysis reveals that individual mesocrystals have a drastically smaller size dispersity of nanoparticles than that in the initial dispersion, implying very strong size selectivity during self-assembly. The small size dispersity of the nanoparticles within individual mesocrystals is accompanied by a very narrow lattice parameter distribution. In contrast, the lattice parameter distribution within all mesocrystals of an ensemble is about four times wider than that of individual mesocrystals, indicating significant size fractionalization between mesocrystals during self-assembly. The small size dispersity within each mesocrystal has important implications for their physical properties., Published by Royal Society of Chemistry, Cambridge

International audience; Understanding the assembly of nanoparticles into superlattices with well-defined morphology and structure is technologically important but challenging as it requires novel combinations of in-situ methods with suitable spatial and temporal resolution. In this study, we have followed evaporation-induced assembly during drop casting of superparamagnetic, oleate-capped gamma-Fe2O3 nanospheres dispersed in toluene in real time with Grazing Incidence Small Angle X-ray Scattering (GISAXS) in combination with droplet height measurements and direct observation of the dispersion. The scattering data was evaluated with a novel method that yielded time-dependent information of the relative ratio of ordered (coherent) and disordered particles (incoherent scattering intensities), superlattice tilt angles, lattice constants, and lattice constant distributions. We find that the onset of superlattice growth in the drying drop is associated with the movement of a drying front across the surface of the droplet. We couple the rapid formation of large, highly ordered superlattices to the capillary-induced fluid flow. Further evaporation of interstitital solvent results in a slow contraction of the superlattice. The distribution of lattice parameters and tilt angles was significantly larger for superlattices prepared by fast evaporation compared to slow evaporation of the solvent

In this work a variety of different systems of transition metal oxides ABO$_{3}$ (perovskite materials, where B stands for a transition metal and A for a rare earth element) were produced as thin films and heterostructures and analyzed for the structural, magnetic and ferroelectric properties. For the epitaxial film preparation mostly pules laser deposition (PLD) was applied. For one series high pressure oxide sputter deposition was used as well. The bulk multiferroics TbMnO$_{3}$ and DyMnO$_{3}$, which develop their electric polarization due to a cycloidal magnetic order, have been prepared as single layers with thicknesses between 2 and 200 nm on YAlO$_{3}$ substrates using PLD and sputter deposition. The structural characterization of the surfaces and crystal structure where performed using x-ray reflectometry and diffraction, respectively. These yielded low surface roughness and good epitaxial growth. The magnetic behavior was macroscopically measured with SQUID magnetometry and microscopically with polarized neutron diffraction and resonant magnetic x-ray scattering. While all investigated samples showed antiferromagnetic order, comparable with the collinear magnetic phase of their bulk materials, only the sputter deposited samples exhibited the multiferroic low temperature cycloidal order. The investigation of the optical second harmonic generation in a TbMnO$_{3}$ sample could proof the presence of a ferroelectric order in the low temperature phase. The respective transition temperatures of the thin films have been very similar to those of the bulk materials. In contrast an increase in the rare earth ordering temperature has been observed, which reduces the Mn order slightly, an effect not known from bulk TbMnO$_{3}$ crystals. The coupling of the antiferromagnetic order in TbMnO$_{3}$ to ferromagnetic layers of LaCoO$_{3}$ was investigated in super-lattices containing 20 bilayers produced with PLD on the same substrates. The SQUID magnetometry yielded a strong influence of the antiferromagnetic transitions on the overall magnetization. Element specific magnetization measurement from XMCD showed that the ferromagnetism has only Co contributions. The analysis of a polarized neutron diffraction experiment resulted in a complex magnetic model, which requires a strong coupling of the magnetic moments of Mn and Co. The investigation of a system build of EuTiO$_{3}$-BaTiO$_{3}$ super-lattices, produced with PLD on GdScO$_{3}$ substrates, for a magnetic order of the Eu system with polarized neutron reflectometry could eliminate ferromagnetism down to 0.01 $\mu_{B}$. Nevertheless, the system could be used to test simulation models, build for reflectivity and diffraction on the other systems. All simulations showed very good agreement with the experiments. In the context of this thesis and in addition to the already mentioned simulations the evaluation software Plot.py was developed, which was used for most of the data analysis.

In this work a variety of different systems of transition metal oxides ABO3 (perovskite materials, where B stands for a transition metal and A for a rare earth element) were produced as thin films and heterostructures and analyzed for the structural, magnetic and ferroelectric properties. For the epitaxial film preparation mostly pules laser deposition (PLD) was applied. For one series high pressure oxide sputter deposition was used as well. The bulk multiferroics TbMnO3 and DyMnO3, which develop their electric polarization due to a cycloidal magnetic order, have been prepared as single layers with thicknesses between 2 and 200 nm on YAlO3 substrates using PLD and sputter deposition. The structural characterization of the surfaces and crystal structure where performed using x-ray reflectometry and diffraction, respectively. These yielded low surface roughness and good epitaxial growth. The magnetic behavior was macroscopically measured with SQUID magnetometry and microscopically with polarized neutron diffraction and resonant magnetic x-ray scattering. While all investigated samples showed antiferromagnetic order, comparable with the collinear magnetic phase of their bulk materials, only the sputter deposited samples exhibited the multiferroic low temperature cycloidal order. The investigation of the optical second harmonic generation in a TbMnO3 sample could proof the presence of a ferroelectric order in the low temperature phase. The respective transition temperatures of the thin films have been very similar to those of the bulk materials. In contrast an increase in the rare earth ordering temperature has been observed, which reduces the Mn order slightly, an effect not known from bulk TbMnO3 crystals. The coupling of the antiferromagnetic order in TbMnO3 to ferromagnetic layers of LaCoO3 was investigated in super-lattices containing 20 bilayers produced with PLD on the same substrates. The SQUID magnetometry yielded a strong influence of the antiferromagnetic transitions on the overall magnetization. Element specific magnetization measurement from XMCD showed that the ferromagnetism has only Co contributions. The analysis of a polarized neutron diffraction experiment resulted in a complex magnetic model, which requires a strong coupling of the magnetic moments of Mn and Co. The investigation of a system build of EuTiO3-BaTiO3 super-lattices, produced with PLD on GdScO3 substrates, for a magnetic order of the Eu system with polarized neutron reflectometry could eliminate ferromagnetism down to 0.01 muB.Nevertheless, the system could be used to test simulation models, build for reflectivity and diffraction on the other systems. All simulations showed very good agreement with the experiments. In the context of this thesis and in addition to the already mentioned simulations the evaluation software Plot.py was developed, which was used for most of the data analysis.