Precise Tuning of Flexoelectricity in SrTiO₃ 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 SrTiO₃ (STO) single crystals and tuned broadly by irradiation with ions of He⁺, C⁺, and P⁺ at dose of 1 × 10¹⁴ and 2 × 10¹⁵ ion cm^-2, respectively. Under C⁺ ion irradiation at a dose of 2 × 10¹⁵ ion cm^-2, thin-film X-ray diffraction reveals a strain gradient up to ≈0.65% nm^-1. 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^-1 in STO's flexoelectric response. This study demonstrates that ion irradiation is an effective strategy for precisely tuning the flexoelectric properties.