Synthesis and polarity driven alignment of ZnO crystallites

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The project focuses on the fabrication of zinc oxide crystals with an inherent oriented polarity as well as the particular electrical and mechanical properties of this material based on its crystallinity and orientation. The aim is to achieve an anisotropic microstructure which enables the alteration of the polarisation under compressive stress and thus the modulation of charge carrier transport across junctions. For this purpose nano- and micrometre sized crystallites of zinc oxide will be synthesized in a a controlled fashion and deposited in an oriented manner. Several techniques will be applied to obtain objects ranging from thin to thick films on substrates whereby additional processing or sintering steps can be used. If the synthetic conditions are suitably adapted, a polycrystalline structure with meso- and microsized ZnO grains will become accessible. First, ZnO single crystals of suitable size must be synthesized, whereby uniform particles with a narrow size distribution are required. The controlled alignment of the ZnO crystallites is a synthetic challenge and will be studied using three different approaches: (a) induced by electrical fields (b) by capillary forces (convective assembly) or (c) by organic templates (self-assembled monolayers or layer-by-layer stacks of polyelectrolytes). The electrophoretic deposition (a) of piezoelectric materials is established for the deposition of piezoelectric materials yielding free standing ceramic bodies with dimensions in the regime of micro to millimetres. This method will consequently be studied for the deposition of layers with alternating oriented polarity herein for the first time. The alignment and stacking of ZnO platelets by means of convective assembly (b) provides a route for a controlled formation of mono and multilayers. Organic templates (c) could be combined with the deposition of dedicatedly synthesized particles as well as with in-situ formed ceramic ZnO layers from chemical baths. Finding the optimum synthetic deposition techniques which allow a controlled synthesis of ZnO crystals and the assembly of those to yield their desired deposition with oriented polarity are the two central tasks and challenging goals of the project which is part of the joined research effort in the field of piezoresisitive effects and mechanical modulation of electrical conductivity. Therefore the interaction with the partner projects; Klein, Albe, Rödel/Kleebe and Gjonaj/Xu is essential. Our work contributes essentially towards scientific questions as: How do chemical composition, particle orientation and its electronic structure at interfaces affect the dependence of electrical conductivity and strain? Our materials choice will ideally supplement the studies of macroscaled ZnO bicrystals treated by the Rödel/Kleebe project. Likewise this proposal will benefit but will also provide materials expertise for use in the other groups of PAK 928/1.