title: Piezotronic Bicrystals and Hexagonal Nano-Platelets: A TEM Study on Structure and Chemistry of Functionalized Zinc Oxide
Creators: name: Trapp, Maximilian
Publisher: name: UNSPECIFIED
Year: 2023

description: TEM and SEM investigations of piezotronic ZnO bicrystals were conducted with the aim to study the relationship between interface structure and electrical properties. In this context, the term “piezotronic” refers to the manipulation and tuning of electrostatic potential barriers at doped varistor-type grain boundaries via piezoelectric charges generated upon mechanical load. To this end, varistor-type inversion-boundary bicrystals were synthesized in tail-to-tail (000-1)|(000-1) or head to head (0001)|(0001) orientation with respect to the c axis, providing an optimized piezotronic response for load applied in &lt;0001&gt; directions. Different synthesis methods are compared and specific tilt configurations as well as undoped reference bicrystals were examined. Furthermore, ZnO nanocrystals, i.e., hexagonal platelets and rod shaped twins, were investigated, as both the bicrystals and the nanocrystals hold a large potential for functionalization and the development of novel devices. Finally, the dopant-related varistor-effect itself, which underlies the piezotronic applications, is – despite decades of research – still not completely elucidated and the examined bicrystals are well suited model systems for respective studies on specific dopant/grain-boundary situations. Since the varistor-effect is attributed to potential barriers at doped grain-boundaries, respective interfaces were investigated by atomic-resolution HAADF STEM; the main method in this thesis, capable of delivering structural as well as chemical information. These measurements were complemented with conventional TEM, ABF STEM, electron diffraction and EDS methods in order to fully characterize the bicrystals as well as control and verify their successful synthesis. The TEM results were interpreted with respect to the findings from corresponding electrical measurements. In doing so, the focus laid upon the successful doping with bismuth, which is essential to obtain varistor behavior and hence, for the subsequent piezotronic manipulation. As a main result, it was found that doping with Bi, being insoluble in ZnO, is by no means trivial and demands appropriate synthesis procedures and/or bicrystal configurations, which provide suitable segregation sites. Otherwise, Bi retracts from the interfaces and forms electrically inactive secondary phases. The presence of such segregation sites was found to be related to the respective structural coherence of the grain boundary. While highly coherent interfaces did not feature any significant Bi doping, semi- or incoherent interfaces exhibited a clear Bi decoration, which was also reflected by the electrical measurements revealing the absence or occurrence of varistor behavior, respectively. The required incoherency could be introduced either via a special synthesis procedure (epitaxial solid-state transformation) leading to a strongly curved defect-rich interface or by applying specific tilt configurations to diffusion-bonded bicrystals with a flat interface. The latter comprises two different cases: Semi-coherent configurations, where a coincidence-site lattice (CSL) is formed and CSL points act as semi-periodic segregation sites, and highly incoherent situations, where the interface structure is strongly and irregularly disordered. In addition to the structural characterization, thermodynamic aspects were considered, indicating that all three types of Bi-segregation (curved, flat semicoherent, flat incoherent) can be explained qualitatively by applying Gibbs adsorption isotherm, which describes the lowering of surface or interface energies depending on the decoration by impurity atoms. Due to the special case of an insoluble dopant, which is available in a quasi-infinite reservoir with respect to the limited amount of segregation sites, the segregation of Bi depends only on a) the energy difference between the undecorated and decorated interface, and b) the actual quantity of segregation sites. Both a) and b) are higher for incoherent interfaces and approach zero for the case of maximum coherency. The TEM results as well as the thermodynamic considerations were found to be not only in perfect agreement with previous ZnO bicrystal studies but also with comparable situations in other, completely different material systems. In consequence, they are considered to be applicable to all cases of interfacial segregation of insoluble dopants. The main goal behind the synthesis of the hexagonal ZnO platelets is their self-assembled tessellation upon Langmuir-Blodgett deposition. However, a synthesis route needed to be established first. TEM investigations were performed in order to characterize the obtained mineralization products and provide feedback for the optimization of the synthesis. In doing so, the same methods as for the bicrystals could be employed, in particular, ABF STEM and NBED for determining the absolute direction of the c-axis. This was an important issue for both the hexagonal platelets, where inversion twinning needed to be ruled out, as well as for the rod shaped twins, whose twinning character could be identified this way. In addition, the TEM investigations revealed a surface coverage of the hexagonal facets by residuals from the precipitation process. In consequence, an additional calcination step was added to the synthesis procedure, which was shown to successfully remove this coverage and simultaneously increase the quality of the surfaces from rough to almost atomically flat. Regarding the rod-shaped twins, actually a side-product of the synthesis experiments, the TEM findings revealed a situation similar to the piezotronic inversion-boundary bicrystals. In all cases, the crystallites were found to be accurate tail-to-tail inversion twins with anti-parallel c-axes and a disordered interface, probably capable of incorporating dopants such as Bi. In consequence, they are seen as holding potential for further development in their own right, especially against the background that the original “piezotronics” were established based on ZnO nanocrystals (nanowires).