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Structural Studies of Liquid Surface Self-Assembled Iron Oxide Nanoparticle Monolayers


The phenomenon of self--assembly has become increasingly relevant due to the role it can play in nanofabrication and the emergence of macro--scale structure. This phenomenon has been seen in a broad range of material systems. For example, iron oxide nanoparticles undergo self-assembly into well-ordered monolayer films of macroscopic size at an air-water interface. It is this system that is the topic of the work herein. The self-- assembly process is the result of the van der Waals forces between the constituent particles. For roughly spherical particles the monolayer is a 2D hexagonal close packed lattice. Using a variety of X-Ray scattering techniques including Grazing Incidence X-Ray Diffraction (GID), X-Ray Reflectivity (XR), and Grazing Incidence X-Ray Off- Specular Scattering (GIXOS) the structure of some of the emergent characteristics of these films is studied. Namely, the microscale multilayering that occurs when these films are laterally compressed, exhibits varying morphologies which depend on the size of the constituent particles. Additionally, when these films are formed from bi-- dispersed mixtures containing 10 and 20 nm particles, the particles phase separate into well--ordered patches during the self--assembly process. The domain sizes of these phase separated regions are at most a factor of 2-3 times smaller than that of a film comprising only mono-- dispersed particles, and their degree of disorder is comparable. Finally, using these iron oxide nanoparticle monolayers as a test system, a novel analysis is demonstrated that allows a direct comparison between Scanning Electron Microscopy (SEM) of ex-situ films and GID measurements of in-situ films: the results demonstrate that structural information comparable to that contained in the GID measurements can be obtained by Fourier transform analysis of the SEM images taken of the film after it has been transferred to a silicon substrate. Furthermore this comparison suggests that the Langmuir- Schaefer method of transferring the liquid surface film to a solid substrate preserves the average hexagonal structure and inter-particle spacing of the film

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