(342c) Alignment and Orientation of ZnO Nanorod Assemblies

Nanoparticulate thin films are of high interest in various industries. In this contribution we focus on electronic applications, e.g. as semiconducting or conducting layer in field effect transistors (FETs), solar cells or displays^{1, 2}. The properties of solution processed thin films are varying from highly porous to very densely packed nanoparticles according to their field of application. Despite the importance of these films, the mechanisms of film formation and assembly of nanoparticles is far from being understood. Guenther et al.^{3, 4}, Mahajeri et al. ⁵ and Reindl et al.⁶ investigated dispersions and formation of thin films of spherical nanoparticles. Starting from these works we go one step forward to try to understand the basic concepts of tailoring nanoparticulate thin film properties which consist of anisotropic nanoparticles like nanorods. Therefore the control of the particle interactions, the drying behaviour of the dispersions and the wetting of the substrate is of high interest. Moreover, the structural evolution of the particulate network as a function of solid concentration and stability parameters is studied. ZnO nanorods produced by wet chemical synthesis with a mean diameter of about 90 nm in length and about 20 nm in width are dispersed in ethanolic and aqueous solutions. With steric stabilization mechanisms of short carbonic acids we produce stable dispersions against agglomeration and sedimentation which are investigated by dynamic light scattering and UV-vis spectroscopy. On patterned silica substrates with hydrophilic and hydrophobic areas droplets of these dispersions are deposited. The drying behaviour is investigated in ambient conditions and in controlled gas atmospheres. By monochromatic light interference we identify the thickness of the droplets in situ while drying occurs. The dried films are investigated by electron microscopy and profilometry. We show that the stability of the dispersion is strongly influencing self-assembly processes of the nanorods. With stabilized dispersions we produce oriented assemblies of nanorod structures in parallel and perpendicular to the substrate with lateral extension of several µm.  

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