Precise Tuning of Flexoelectricity in SrTiO by Ion Irradiation.

Flexoelectric coefficient is a tetradic and its introduction enables centrosymmetric materials to exhibit piezoelectricity. However, the flexoelectric paradigm currently lacks a strategy to effectively tune the strain gradient for optimal electro-mechanical coupling. This study proposes a quantized collision model accessible through ionic irradiation technology to explore the flexoelectricity and precisely modulate the strain gradient. The lattice strain is introduced in SrTiO (STO) single crystals and tuned broadly by irradiation with ions of He, C, and P at dose of 1 × 10 and 2 × 10 ion cm, respectively. Under C ion irradiation at a dose of 2 × 10 ion cm, thin-film X-ray diffraction reveals a strain gradient up to ≈0.65% nm. The resulted polarization is found to orient out-of-plane, as observed through X-ray reciprocal space mapping and high-angle annular dark field scanning transmission electron microscopy. Piezoresponse force microscopy characterization reveals that the electric-induced out-of-plane polarization reversal emerges at room temperature, corresponding to a stain gradient ≈0.05% nm in STO's flexoelectric response. This study demonstrates that ion irradiation is an effective strategy for precisely tuning the flexoelectric properties.