Engineering Surface Chemistry to Enhance Ferroelectric Phase Formation in Ultrathin PVDF-TrFE Films

The development of flexible, lightweight electronic devices has driven growing interest in ferroelectric polymers, with a focus on poly-(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) copolymers. These materials offer solution processability, mechanical flexibility, and high remanent polarization, making them well-suited for applications in sensors, nonvolatile memories, and energy harvesters. However, as film thickness is reduced below 50 nm, crystallization becomes increasingly sensitive to interfacial interactions, leading to variations in phase composition and surface morphology. This work investigates how controlled modifications to substrate surface chemistry influence the crystallization behavior of ultrathin PVDF-TrFE films. To this end, polymer brushes of varying polarity were grafted onto silicon oxide substrates to create a systematic gradient in surface energy, spanning from hydrophilic to hydrophobic regimes. PVDF-TrFE copolymers with VDF:TrFE ratios of 80:20, 75:25, and 70:30 were spin-coated onto these surfaces to produce uniform films with thicknesses below 50 nm. Ellipsometry and contact-angle measurements were used to confirm brush coverage and film thickness. Crystalline phase composition was quantified using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and grazing-incidence wide-angle X-ray scattering (GIWAXS), while atomic force microscopy (AFM) was employed to characterize nanoscale surface topography. Results demonstrate that both brush chemistry and copolymer composition significantly affect β-phase content and crystalline texture. Hydrophobic surfaces consistently promoted superior film coverage, larger crystalline domains, and higher electroactive β-phase content compared to hydrophilic counterparts. These findings provide a detailed framework for controlling phase behavior and morphology in nanoconfined PVDF-TrFE films. By controlling the interface rather than the material itself, this study offers a simple and effective strategy for improving ultrathin ferroelectric films, providing useful design guidelines for flexible electronics and opening new directions for research in nanoscale polymer engineering.