He Shan, Guo Mengfan, Dan Zhenkang, Lan Shun, Ren Weibin, Zhou Le, Wang Yue, Liang Yuhan, Zheng Yunpeng, Pan Jiayu, Shen Yang
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.
Sci Bull (Beijing). 2021 Jun 15;66(11):1080-1090. doi: 10.1016/j.scib.2021.02.004. Epub 2021 Feb 6.
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.
铁电超薄朗缪尔-布洛杰特（LB）薄膜中的大粗糙度和结构无序导致电学、铁电和压电特性出现严重的空间散射，从而限制了纳米器件的纳米尺度研究及其可靠性。然而，迄今为止，尚未报道过针对大面积均匀有机铁电LB薄膜的有效方法。在此，我们提出一种简便的热压策略，以制备相对大面积的聚偏二氟乙烯（PVDF）LB薄膜，其在30μm×30μm区域内的表面均方根（RMS）粗糙度为0.3nm，与金属基板相当，这最大限度地发挥了LB技术控制厚度分布的潜力。更重要的是，与传统退火的LB薄膜相比，热压LB薄膜表现出显著改善的结构均匀性、铁电特性波动更小以及更高的介电和压电响应，这归因于均匀的偶极取向和更高的结晶质量。此外，研究了不同的表面电荷弛豫行为，并根据纳米尺度非均匀开关情况下表面电荷与极化电荷的相互作用解释了其潜在机制。我们相信，我们的工作不仅提出了一种新颖的策略，赋予PVDF LB薄膜前所未有的可靠性和改进的性能，使其成为未来铁电隧道结（FTJ）和纳米机电系统（NEMS）的有竞争力候选者，而且还揭示了表面电位分布与纳米尺度非均匀开关行为之间引人注目的耦合效应，这对于理解由能带结构调制的局部输运特性、数据存储应用中的位信号稳定性以及相关的表面电荷研究（如基于在偏置铁电畴上收集表面电荷的电荷梯度显微镜（CGM））至关重要。
