Nano-twist: Twisted Epitaxial Perovskite Heterostructures

• Applicants: Dr. Jens Martin; Dr. Jutta Schwarzkopf
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 461735383

The intentional incorporation of lattice strain in perovskites has emerged as an exciting opportunity to tune their functional properties like ferroelectricity. Typically, this is performed by the growth of ferroelectric thin films on lattice mismatched substrates. The imposed strain impacts on crystal symmetry, domain formation and functional properties of perovskite materials. An alternative way is given by the exploitation of strain gradients inducing a strong flexoelectric effect. This is particularly interesting on the nanometer scale where large strain gradients could be present. Ferroelectrics are of fundamental interest and drivers for a plethora of innovations, yet the control over specific phases and the stabilization of particular domain patterns is crucial for their exploitation. Strained perovskite heterostructures offer advanced manipulation of domain (wall) formation and phase transitions. Further, tuning of the crystal structure of oxide materials provides the possibility to improve existing properties, or even create novel functionality. With the ongoing drive for miniaturization in device applications, this requires not only the deposition of ultrathin layers with high crystalline perfection, but also the manipulation of their properties on the nanometer scale. Defects can be utilized for strain engineering on the local scale since every crystal defect is associated with lattice deformation, hence causing a local strain field. Since shear stress plays an important role in the formation of ferroelectric domains and in phase transitions, we aim to create a network of defects producing shear strain, namely via screw dislocations. This cannot be achieved with traditional epitaxial growth methods, yet dislocations occur at grain boundaries with appropriate semi-coherent interfaces in which lattice-matched regions are separated by 1D-defects. Indeed, ideal twist boundaries result in 2D-networks of in-plane screw dislocations. For the utilization of screw dislocation networks, we require methods which produce twist boundaries on demand. With bulk materials, this can be achieved via wafer bonding. However, with this technique it is not possible to create uniform ultrathin films which are needed to fully exploit the functional properties generated by strain gradients and dislocations.In this project, we will employ layer transfer of epitaxial thin films to create small angle twist boundaries in ferro- and dielectric perovskites. The main research objectives are to investigate the formation mechanism of screw dislocation networks in artificial twist boundaries of epitaxial perovskite thin films, to identify the detailed atomic structure of screw dislocation cores, and to determine the structure-function relationship of ferroelectric domains patterns in twisted perovskites.

• DFG Programme: Research Grants