Abstract:We report about the fabrication and characterization of iron oxide nanoparticle thin film superlattices. The formation into different film morphologies is controlled by tuning the particle plus solvent-to-substrate interaction. It turns out that the wetting vs. dewetting properties of the solvent before the self-assembly process during solvent evaporation plays a major role to determine the resulting film morphology. In addition to layerwise growth also three-dimensional mesocrystalline growth is evidenced. The understanding of the mechanisms ruling nanoparticle self-assembly represents an important step toward the fabrication of novel materials with tailored optical, magnetic or electrical transport properties.The advent of controlled thin film growth about seven decades ago revolutionized many areas of science and technology such as optical coatings [1,2], magnetic layers and multilayers [3,4] or semiconductor thin films [5,6]. In the early stage of research on thin films it soon became clear that it was imperative to understand the mechanisms which control and define the growth of thin films to gain control over the physical properties of these novel artificial materials. Hence huge efforts of the scientific community were dedicated to characterize, optimize and understand film growth. Thin films are evidently composed of atoms, which are considered as zero-dimensional building blocks. Extending this concept to the case of films composed of nanoparticles, the assumption is made that nanoparticles (also termed 'nanocrystals') can also serve as zero-dimensional building blocks. By self-assembly, these

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