Large-area atomic-smooth polyvinylidene fluoride Langmuir-Blodgett film exhibiting significantly improved ferroelectric and piezoelectric responses

Abstract

Large roughness and structure disorder in ferroelectric ultrathin Langmuir-Blodgett (LB) film results in severe space scatter in electrical, ferroelectric and piezoelectric characteristics, thus limiting the nanoscale research and reliability of nano-devices. However, no effective method aiming at large-area uniform organic ferroelectric LB film has ever been reported to date. Herein, we present a facile hot-pressing strategy to prepare relatively large-area poly(vinylidene fluoride) (PVDF) LB film with ultra-smooth surface root mean square (RMS) roughness is 0.3 nm in a 30 μm × 30 μm area comparable to that of metal substrate, which maximized the potential of LB technique to control thickness distribution. More importantly, compared with traditionally annealed LB film, the hot-pressed LB film manifests significantly improved structure uniformity, less fluctuation in ferroelectric characteristics and higher dielectric and piezoelectric responses, owing to the uniform dipole orientation and higher crystalline quality. Besides, different surface charge relaxation behaviors are investigated and the underlying mechanisms are explained in the light of the interplay of surface charge and polarization charge in the case of nanoscale non-uniform switching. We believe that our work not only presents a novel strategy to endow PVDF LB film with unprecedented reliability and improved performance as a competitive candidate for future ferroelectric tunnel junctions (FTJs) and nano electro mechanical systems (NEMS), but also reveals an attracting coupling effect between the surface potential distribution and nanoscale non-uniform switching behavior, which is crucial for the understanding of local transport characterization modulated by band structure, bit signal stability for data-storage application and the related surface charge research, such as charge gradient microscopy (CGM) based on the collection of surface charge on the biased ferroelectric domains.