Exploring the Unusual Nanotribology of Ferroelectric Domains and Domain Walls

Ferroelectric materials can host a wide range of unusual structural features, often linked to novel functional properties, potentially useful for nanoelectronics applications. At domain walls or in regions with high strain gradients, in particular, the complex interaction between polarisation, electrostatics, and strain can lead to localised chiral polarisation textures, electrical conductivity, local mechanical responses, and charge or chemical segregation. We use a broad spectrum of primarily scanning probe microscopy techniques, coupled with machine learning analysis to investigate and disentangle the many complex and correlated emergent phenomena in these materials.

These have allowed us to observe the switchable, polarisation-dependent friction and wear behaviour of ferroelectrics at the scale of domains — down domains have lower friction coefficients and show slower wear rates than up domains. This asymmetry is enabled by flexoelectrically coupled polarisation in the up and down domains under a sufficiently high contact force. This polarisation-dependent tribological asymmetry is a general feature across different ferroelectric families with varying chemical compositions and crystalline symmetry.
Such switchable tribological properties of ferroelectrics offer an alternative route to visualise and control ferroelectric domains and can be combined with multi-pass patterning as domain-based dynamic smart masks, with which we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains. Such patterning can, furthermore, be scaled up to technologically relevant (mm–cm) size.

In addition, investigating ferroelastic twins in ferroelectric materials, which present strongly localised and very high strain gradients, we have found distinct mechanical properties and enhanced electrical conduction at the heart of the twin crossing, and a characteristics localised piezoelectric response and conductive superstructure in the twins themselves, reflecting their complex polarisation textures revealed in transmission electron microscopy.