**2.1. Spin coating**

Spin coating is a desirable method to achieve the large area uniform nanofilms. However, the surface of traditional spin coating PVDF film is too rough for the application in microelectronics. The acquisition homogeneous and smooth PVDF nanofilms with small roughness is a major breakthrough for spin coating and electron device [10, 23, 24]. Smooth PVDF films have been obtained by humidity-controlled spin coating, dimethyl formamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), or dimethylacetamide (DMAc) used as polar solvents to dissolve PVDF [10, 12, 23]. The micrographs were obtained using the JEOL JSM-IT100 of thin films SEM were under different relative humidity by spin coating were evaluated, as shown in **Figure 1** [25].

various applications [1, 2]. In particular, the β-phase PVDF (expressed in TTTT conformation) is mostly desired for ferroelectric memories in the data storage fields. The dipole moments in the molecule stem from the strongly electronegative fluorine atoms predominantly, inducing

A number of methods have been proposed to prepare the PVDF films with the thickness up to several microns such as electric poling, hygroscopic salts, mechanical stretching, epitaxy with KBr, and solvent evaporation [5–7]. In contrast, the spin coating and Langmuir–Blodgett deposition techniques are two main methods to obtain the ferroelectric β-phase PVDF films with the thickness down to 300 nm or more less [8, 9]. Furthermore, spin coating can be used to fabricate the large area uniform films from the industrial point of view [10]. Langmuir– Blodgett (LB) deposition technique can prepare the ultra-thin PVDF films with the thickness of several nanometers [11]. In this chapter, we firstly report the main results about the fabrications of β-phase PVDF films by spin coating method and Langmuir–Blodgett deposition techniques. Then, the typical applications of β-phase PVDF films in several organic devices

Microelectronic devices using β-phase PVDF film generally require nanoscale thickness and pin hole-free to avoid the electrical shorts [12]. Many works have been done to discuss the preparation and application of PVDF thin films. The purpose of these studies is preparation

The high coercive field is one drawback for PVDF and its copolymers used in the devices, which determines the minimum electric field that needed to reverse the polarization state. With the coercive fields of 50 MV/m and higher [16], the thickness of β-phase PVDF films must be less than 100 nm to allow for low operation voltage. The early devices based on β phase PVDF [17], required an operation voltage up to 200 V, while the more recent reports still need 30 V or more to operate [18–20]. The ultrathin films of P(VDF-TrFE) obtained by Langmuir–Blodgett deposition on silicon wafers has produced one nanometer ferroelectric films [21] and operation voltage less than 10 V for nonvolatile memory devices [22]. The

Spin coating is a desirable method to achieve the large area uniform nanofilms. However, the surface of traditional spin coating PVDF film is too rough for the application in microelectronics. The acquisition homogeneous and smooth PVDF nanofilms with small roughness is a major breakthrough for spin coating and electron device [10, 23, 24]. Smooth PVDF films have been obtained by humidity-controlled spin coating, dimethyl formamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), or dimethylacetamide (DMAc)

of smooth thin films or achieving a ferroelectric phase (β, δ, or γ phase) [13–15].

β-phase PVDF nanofilms are mainly prepared by spin coating and LB deposition.

the ferroelectricity of β-phase PVDF [3, 4].

134 Ferroelectrics and Their Applications

are introduced.

**2. Preparation**

**2.1. Spin coating**

The percentage of β-phase PVDF (β%) in films fabricated by spin coating method is mainly studied by Fourier transform infrared (FTIR) (BRUKER) spectroscopy and X-ray diffraction (XRD) (RIGAKU) techniques. It is found that the processing conditions such as solution concentration, spin rotation speed (rpm) and annealing temperature obviously affect the percentage of β-phase PVDF [26]. The value of β% was calculated and the different percentages for the corresponding films at different annealing temperatures were plotted in **Figure 2**.

Meanwhile, Cardoso et al. successfully prepare thin PVDF films with high β-phase content by thermally annealing at 70°C [27]. A 160 nm thick PVDF film mainly consists of the ferroelectric β phase was prepared by rapid thermal annealing and humidity controlled spin coating method [10]. Ramasundaram et al. reported the fabrication of PVDF films dominantly with β phase using a heat-controlled setup of spin coating [9].

**Figure 1.** Scanning electron microscopy (SEM) micrographs of PVDF films prepared by spin coating with the relative humidity at (a) 20%, (b) 40%, (c) 60%, and (d) 80% [25]. This research was conducted at Center of High Technology Materials New Mexico in 2017.

**2.2. Langmuir-Blodgett method**

layer-by-layer [29].

University of Japan in 2014.

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

Preparation and Device Applications of Ferroelectric β-PVDF Films

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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

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

spin-coated film are randomly oriented, as schematically illustrated in **Figure 3b** [8].

**Figure 2.** β-phase dependence of 20% PVDF films on annealing temperatures [26]. This research was conducted at Center of High Technology Materials New Mexico in 2017.

**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, Technology and Research of Singapore in 2012.
