Piezotronic Bicrystals and Hexagonal Nano-Platelets: A TEM Study on Structure and Chemistry of Functionalized Zinc Oxide

Project Leaders

  • Maximilian Trapp and Hans-Joachim Kleebe

Description

In zinc oxide (ZnO) varistor ceramics, the typical non linear electrical characteristics are attributed to back to back double Schottky barriers along grain boundaries. These potential barriers are formed due to the trapping of charge carriers by dopants and associated defect states at the interface. Zinc oxide’s non centrosymmetric wurtzite structure provides piezoelectric properties, which can be utilized to mechanically tune the height of these barriers. In doing so, mechanical stress causes compensating or accumulating piezoelectric charges at the varistor grain-boundaries, depending on the orientation of the crystal structure and whether tensile or compressive strain is applied.

In close cooperation with the division Nonmetallic-Inorganic Materials of Prof. Jürgen Rödel, ZnO bicrystals with specific orientations and doping strategies are chemically and structurally investigated by atomic resolved transmission electron microscopy (TEM) methods with an emphasis on the crystallographic grain-boundary characteristics, the dopant segregation and the effect of different synthesis procedures. The TEM results are compared and correlated with load dependent electrical measurements. Since bicrystals are ideal model systems for the investigation of specific grain-boundary configurations and hence specific potential barriers, these studies contribute to both the development of novel piezotronic devices and the fundamental research on zinc oxide varistor ceramics.

In addition, the applied TEM characterization methods are also performed on related ZnO materials, such as nanowires, nano sized inversion twins and hexagonal nanoplatelets.

Fig. 1: a) Two-beam TEM BF image of a strained, meandering grain boundary in a ZnO bicrystal, b) HR-STEM image of a Bi-doped grain boundary in a diffusion bonded ZnO bicrystal and c) TEM BF image of hexagonal ZnO platelets.
Fig. 1: a) Two-beam TEM BF image of a strained, meandering grain boundary in a ZnO bicrystal, b) HR-STEM image of a Bi-doped grain boundary in a diffusion bonded ZnO bicrystal and c) TEM BF image of hexagonal ZnO platelets.

Keil, P., Trapp, M., Novak, N., Frömling, T., Kleebe, H. J., Rödel, J. (2018). Piezotronic tuning of potential barriers in ZnO bicrystals. Advanced Materials, 30(10), 1705573.

Trapp, M., Keil, P., Frömling, T., Rödel, J., Kleebe, H. J. (2020). Segregation and properties at curved vs straight (000-1) inversion boundaries in piezotronic ZnO bicrystals. Journal of the American Ceramic Society, 103(4), 2817-2827.

Hoffmann, R. C., Trapp, M., Erdem, E., Kratzer, M., Teichert, C., Kleebe, H. J., Schneider, J. J. (2020). Synthesis and Assembly of Zinc Oxide Microcrystals by a Low‐Temperature Dissolution–Reprecipitation Process: Lessons Learned About Twin Formation in Heterogeneous Reactions. Chemistry, 26(42), 9319.

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