Surface Modification and Gradient Polarization Synergistically Enhance the Piezoelectric Properties of P(VDF-TrFE)...

High frequency, broadband ultrasonic transducers are pivotal for precise superficial vascular imaging and pathological diagnosis due to their superior spatial resolution. Achieving high frequency detection fundamentally necessitates reducing the piezoelectric layer thickness to the micrometer scale. For ultrathin ferroelectric polymers, this physical scaling significantly amplifies the "surface dead layer" effect, where inherent low crystallinity and disordered dipole orientation severely suppress polarization switching, resulting in a drastic deterioration of device sensitivity. To resolve the trade-off between thickness and performance, we propose a synergistic enhancement strategy combining plasma surface modification with Curie temperature gradient polarization (CT-GP). Plasma treatment is employed to introduce polar functional groups to reactivate the inert surface layer, while the CT-GP process leverages the cooperative effects of thermal activation and electric fields near the Curie temperature to induce robust molecular chain rearrangement and domain stabilization. The β phase crystallinity of the 5 μm-thick film is elevated to 33.01%, with a piezoelectric coefficient (d₃₃) of 20.6 pC N^-1. Focused transducers fabricated from these optimized films exhibit outstanding acoustic performance, characterized by a center frequency of 39.5 MHz and a -6 dB bandwidth of 54 MHz. The high signal-to-noise ratio (19 dB) in vivo imaging of murine microvasculature was successfully demonstrated, achieving lateral resolutions of 39.4 μm. This work elucidates the physical mechanism by which surface layers limit the performance of ultrathin films and provides a new theoretical and technical pathway for the development of high-resolution vascular imaging and flexible acoustic devices.