**2.2. Langmuir-Blodgett method**

**Figure 3.** (a) The ψ dependence of the XRD peak β (110) (200) of the PVDF LB film and a spin-coated PVDF film. (b) Cross-sectional view to compare the molecular chain orientation of the LB PVDF film and spin-coated PVDF film. The black bars are used to denote the PVDF molecular chains [8]. This research was conducted at Agency for Science,

**Figure 2.** β-phase dependence of 20% PVDF films on annealing temperatures [26]. This research was conducted at

Technology and Research of Singapore in 2012.

Center of High Technology Materials New Mexico in 2017.

136 Ferroelectrics and Their Applications

As an effective technique, LB deposition was usually used to fabricate nanoscale films, in which the interaction of water with PVDF molecules is involved [8, 28]. This could deposit PVDF films at room temperature on any substrate material because the films were grown layer-by-layer [29].

The orientation of dipoles of the PVDF LB film was analyzed by the XRD peak intensity (β (200) (110)) compared with a spin-coated PVDF film using an XRD system (D8-ADVANCE, Bruker AXS GmbH, Karlsruhe, Germany) as shown in **Figure 3a**. It was found that LB deposition process can directly format β crystalline phase of PVDF, with the molecular chains parallel and the dipoles aligned perpendicular to the substrate. The results of ψ-scan XRD indicated that the molecular chains in the LB films were parallel with the substrates, but chains in the spin-coated film are randomly oriented, as schematically illustrated in **Figure 3b** [8].

Zhu et al. reported that the 5–35 layers PVDF nanofilms could be prepared by the LB method at 20°C [30]. The dependence of root-mean square (RMS) surface roughness on the thickness of PVDF nanofilms is shown in **Figure 4**. The high surface roughness of 35 and 81 nm thick films is expected to be related to a sudden decrease of polarization. A Pr of 6.6 μC cm−2 was acquired for 81 nm thick PVDF homopolymer film with no post-treatment. They also firstly achieved the ferroelectricity in a 12 nm thick PVDF film, indicating a potential low-voltage application of PVDF nanofilms [11]. The thickness of each PVDF layer could be thin as 2 nm. Therefore, an ultra-thin PVDF film can be expected through layer-by-layer growth using LB method. Recently, the β-phase PVDF films was reported with the thickness around five nanometers [31].

**Figure 4.** The dependence of root-mean square (RMS) surface roughness on different thickness of PVDF LB nanofilms. The inset is an atomic force micropy image of PVDF film at 81 nm thickness [11]. This research was conducted at Tohoku University of Japan in 2014.
