Nanoscale Ferroelastic Writing of Ferroelectric Domains

There is a growing effort towards nanomechanics approaches using a scanning probe microscopy (SPM) tip in order to define patterns of ferroic properties. Next to micromachining, pressure-induced phase transitions or the flexoelectric effect, there is the emerging field of ferroelastic nanoscale manipulation. Ferroelasticity can provide access to mechanical manipulation of a local ferroic order parameter (such as magnetization or ferroelectric polarization), if the order parameter is coupled to a distinct lattice distortion. After giving an introduction to recent advances in the field, the talk addresses our work on mechanical tip-induced definition of ferroelectric domains in a prototype ferroelectric copolymer, P(VDF-TrFE). While there is an increasing track record of nanomechanical manipulation by SPM tip in fully crystalline ferroic films, semi-crystalline polymers offer a unique advantage because of their elasticity supporting damage-free strains of several percent. <br/><br/>The ferroelectric copolymer P(VDF-TrFE) (here with 22% TrFE) is one of the most often applied ferroelectric organic materials. The pseudo-hexagonal lattice of P(VDF-TrFE) supports six stable ferroelectric polarization orientations, and, strikingly, the lattice shrinks substantially along the polarization direction. As-prepared films show multiple few-nanometer-wide ferroelectric domains. The electric dipolar disorder is highly critical, leading to strong suppression of the functional electric responses such as dielectric permittivity, direct or inverse piezoelectric effects. In addition, electric poling and switching of P(VDF-TrFE) films require relatively large electric fields, preventing precise electric nanoscale domain writing because of the tip stray fields. <br/><br/>Our spin-coated P(VDF-TrFE) films consist of very densely packed fine crystalline lamellae of the ferroelectric beta-phase. There is a well-defined (<u>1</u>10) out-of-plane texture, but no in-plane texture. After vertical electric poling, complex deliberate in-plane domain patterns with tip-related resolution of 50 nm have been defined [1] by scanning with moderate mechanical tip forces (100-300 nN). The mechanical treatment leaves the semicrystalline lamellar structure essentially unchanged, and the surface roughness remains below 3 nm (rms). Importantly for various functional devices, the in-plane piezoelectric response in written domains is massively enhanced, indicating the achievement of efficient ordering of electric dipoles. I will discuss opportunities to exploit mechanically defined nanoscale polarization patterns in functional devices. This work was partially funded by Deutsche Forschungsgemeinschaft (DFG), CRC 762 and TRR 102.<br/><br/>[1] R. Roth et al., <i>Adv. Electron. Mater</i>. 2022, <b>8</b>, 2101416