Synthesis and polarity driven alignment of ZnO crystallites
Solid State and Surface Chemistry, Material Synthesis
Final Report Abstract
The synthesis of ceramic materials by ordered assembly of nanoscale objects offers a unique possibility to control the resulting granular arrangement on a microscopic level. In this way, in materials with polar crystal structure like zinc oxide, stacks can be obtained in which the crystallites exhibit concordant or opposing orientation of the polar axis. Possible applications include switching devices, in which charge transport between the crystalline grains is influenced by mechanical stress (compression or bending). The present project explored which techniques come into consideration and are suited for such directed assembly. An important pre-condition is furthermore the availability of highly uniform zinc oxide particles as building blocks in the assembly process, since larger deviations lead inevitably to stacking faults. In particular Electrophoretic (EPD) and Langmuir-Blodgett (LB) Deposition were applicable for the before-mentioned purpose and further investigated in detail. The alignment is hereby induced by electrical fields or surface forces, respectively. For the EPD process stable dispersions of monodisperse nanoparticles were synthesized by means of microwave-assisted decomposition of suitable molecular single-source precursors. Although oriented attachment seems to occur in areas close to the surface, the overall film consists of randomly oriented ZnO crystallites. An in-depth study of the deposition process was carried out by means of positron annihilation lifetime spectroscopy. No distinction could be made between the packing densities in areas closer to the interface in comparison to those at the surface. This suggests that EPD process allows a very uniform deposition of the ZnO. As an alternative approach we also investigated the LB technique, which is a versatile method, facilitating the formation of ordered arrays with nanoscale building blocks. For this purpose, however, single crystalline, micrometer-sized hexagonal platelets are required. The synthesis of such 2D building blocks of zinc oxide is surprisingly difficult and was only understood incompletely. In the course of our works an easy to use hydrothermal route was developed which controls the crystallization by cobalt doping and using ethanol/water mixtures. The formation of monolayers containing domains with ordered tessellation was carried out on glass functionalized with polyelectrolyte multilayers. No major differences are found between deposits on substrates with anionic or cationic surface modification. The adherence to the substrates was sufficient to determine the absolute orientation of the deposited polar single crystals by Piezoresponse Force Microscopy (PFM) as well as Kelvin Probe Force Microscopy (KPFM).
Publications
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Zinc diketonates as single source precursors for ZnO nanoparticles: microwave-assisted synthesis, electrophoretic deposition and field-effect transistor device properties. J Mater Chem C 2016;4:7345-7352
Hoffmann RC, Sanctis S, Erdem E, Weber S, Schneider JJ
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Molecular precursors for ZnO nanoparticles: Field-assisted synthesis, electrophoretic deposition, and field-effect transistor device performance. Inorg. Chem. 2017;56:7550-7557
Hoffmann RC, Sanctis S, Schneider JJ
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Metal oxide double layer capacitors by electrophoretic deposition of metal oxides. Fabrication, electrical characterization and defect analysis using positron annihilation spectroscopy. J Mater Chem C 2018;6:9501-9509
Hoffmann RC, Koswloski N, Sanctis S, Liedke MO, Wagner A, Butterling M, Schneider JJ
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Synthesis and assembly of zinc oxide microcrystals by a low temperature dissolution– reprecipitation process: Lessons learned about twin formation in heterogeneous reactions. Chem Eur J 2020; 26: 9319-9329
Hoffmann RC, Trapp M, Erdem E, Kratzer M, Teichert C, Kleebe HJ, Schneider JJ
