Ferroelectric materials are called “smart materials” as they can be configured to store, release, or interconvert electrical and mechanical energy in a well-controlled manner via their respective characteristics. They have large piezoelectric compliances, pyroelectric coefficients, dielectric susceptibilities, and electro-optic properties. Ferroelectric polyvinylidene fluoride (PVDF) is utilized in a wide range of devices due to their excellent mechanical properties, optical, high thermal and chemical stability, piezoelectricity, pyroelectricity, and ferroelectric responses [1]. In the present investigation, the pristine and neodymium oxide (Nd₂O₃) nanoparticles embedded polyvinylidene fluoride (PVDF) thick films were fabricated via solution casting/intercalation technique. The Nd₂O₃ nanoparticles were synthesized via hydrothermal technique. The Fourier Transform Infrared Spectroscopy (FTIR) has been recorded in the range 400 to 4000 cm⁻¹, and functional groups were identified in the nanocomposite films. Infrared vibrational spectroscopy (FTIR) revealed the presence of ferroelectric β-phase intrinsically in the annealed nanocomposite films. All-important optical constants have been determined for the first time via UV-VIS transmission spectroscopy for the nanocomposite films in the ferroelectric phase.

With the rapid growth of technologies, electroactive polymers (EAPs) are gaining an immense amount of interest as smart materials. There are different EAPs, namely, Nylon-11, polylactide and aniline pentamer copolymer, poly(lactic-co-glycolic acid) (PLGA), and poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE) [2] - [7]. Amongst them, PVDF and PDVF-TrFE exhibit the best electroactive properties, such as piezo-, pyro- and ferroelectricity and optoelectronic. As a result, PVDF and its copolymers are the polymer of choice for the increasing number of possible microelectronics applications, such as electro-optic transducers, waveguides, sensors, actuators, energy harvesting, electro-optic memory, and biomimetic robotic, etc. [2]. PVDF is inherently polar. With respect to the carbon atom in the unit, the hydrogen atoms are positively charged, and the fluorine atoms are negatively charged. The repeat unit of (?CH₂?CF₂) or CH₂CF₂)n, in which the carbon-hydrogen bonds with the electrically polar carbon-fluorine, can take up a number of stable configurations determined by the treatment of the polymer. PVDF can be crystallized into at least four different polymorphous phases, depending on the film preparation conditions. Two forms β (or I) and α (or II) are of particular importance. In form α, PVDF (formed from slow-cooled melts or an acetone solution) is non-piezoelectric, the bonds are in a trans-gauche-trans-gauche configuration, and the molecules are stacked, giving a nonpolar unit cell. Form α can be converted into form β by stretching and electrically poling with a suitable field [3]. In form β, the molecular groups are in an all-trans configuration, and the molecules are assembled to give a polar unit cell. Form β is orthorhombic with point group A mm².

In the recent past, PVDF has attracted interest in the photonics area because of the possibility to be used as hosts for lanthanides photonics ions [4]. In designing the above mentioned emerging optical devices and sensors, the variation of all the standard basic parameters and constants with photon energy are warranted and justified to be investigated. However, few works can be found in the literature about the study of the complete optical properties of Nd₂O₃ nanoparticles embedded PVDF thick films in UV-Vis range [5]. Thus, this publication presents all the essential optical constants, including optical bandgap of nanocomposite PVDF films with various amounts of Nd₂O₃ nanoparticles.

FTIR spectra of the film are illustrated in Figure 1. The FTIR analysis revealed the electroactive β phase of the fabricated films at around 845 cm⁻¹.

Figure 2(a) to Figure 2(b) show the absorption and transmittance spectra of NdO films, respectively. The figure indicates that the transmittance of PVDF films decreases with the addition of Nd₂O₃ nanoparticles. The decrease in transmittance may be due to light scattering of caused by nanoparticles. Figure 3 shows the variation of absorption coefficient with wavelength of Nd-0, Nd-2 and Nd-4 films fabricated.

The complex refractive index is the representation of the optical constants of

material and in represented by n^* = n + ik. The real part “n” is the index of refraction, defines the phase velocity of light in material: v = c/n. where v is the speed of light in materials and c is the speed of light in vacuum. The imaginary part “k” is the extinction coefficient, determines how fast the amplitude of the wave decreases. The variation of n and k both decrease with wavelength as shown in Figure 4(a) and Figure 4(b), respectively. This may be due to a change in density and/or the polarization in the film.

The optical conductivity is one of the powerful tools for studying the electronic states in materials. The plot between the optical conductivity (O.C) and optical density (OD) against photon energy is depicted in Figure 5(a) and Figure 5(b), respectively. The spectrum indicates that optical conductivity increases with the increase of photon energy. The behavior of optical density (O.D) is similar to the variation of O.C with photon energy in pure PVDF film.

The dielectric function ( ε T = ε ′ + i ε ″ ) is a complex quantity and a fundamental intrinsic property of the material which consists of both the real and imaginary parts ( ε ′ and ε ″ ). The real part indicates how the speed of light in the material can be slowed down while the imaginary part deals with the absorption of energy by a dielectric from the electric field due to dipole motion.

From the optical dielectric curve shown in Figure 6, it is evident that ε' increases with an increase of photon energy. The calculated dielectric loss tangent (tanδ) of the PVDF film represents the depletion of the electrical energy [2] [6]. The study of the absorption edge is essential in connection with the theory of electronic structure, which leads to the prediction of whether the band structure is affected near the band extreme. So the main aim is to determine the magnitude and the nature of the optical energy band gap of pure PVDF thin film. The optical energy gap of the films is determined by the absorption spectra. According to the Tauc relation, the absorption coefficient α for the material is given by [2] [6] :

A h ν = A ( h ν − E g ) n (7)

where Eg is the optical energy gap; A is a constant and is different for different transitions; hν, is the energy of photon; and n is an index which assumes the

values 1/2, 3/2, 2 and 3 depending on the nature of electronic transitions responsible for absorption. Thus, the bandgap is obtained by plotting (αhν)^1/n versus hν in the high absorption range followed by extrapolating the linear region of the plots to (αhν)^1/n = 0. The analyses of our data showed that the plots of (αhν)^1/n against hν with n = 1/2. This indicates the allowed direct transition is responsible for the inter-band transition in the doped crystal. Figures 7(a)-(c) show the

plot of (αhν)², against photon energy hν for the Nd-0, Nd-2 and Nd-4 films. As shown in Figure 7, the values of the optical bandgaps (Eg) for (αhν)², of pure PVDF film (Nd-0), Nd-2 and Nd-4, were found to be 4.0, 4.2, and 4.3 eV respectively. Table 1 summarizes the optical constants obtained at the wavelengths 500

nm, 600 nm and 700 nm respectively.

In this study, optical quality PVDF nanocomposites film with has been fabricated and the optical constants, such as the direct and indirect optical bandgaps (Eg), absorption coefficient (α), refractive index (n), extinction coefficient (k), and real and imaginary dielectric constants (ε' and ε'') and the electric loss tangent (tanδ) were calculated to analyze the optical properties of PVDF thick films with embedded Nd₂O₃ nanoparticles. The results of the UV-VIS spectroscopy measurements show the absorption and transmission property of the PVDF films, and the Raman analysis revealed the electroactive β phase of the fabricated films at around 845 cm⁻¹.

The authors gratefully acknowledge support for this work through the title III program at Alabama A & M University. Authors thank Dr. M. D. Aggarwal for his support and encouragement.

The authors declare no conflicts of interest regarding the publication of this paper.

Batra, A., Showe, A., Sampson, J., Kassu, A., Curley, M. and Arun, K. (2019) Fabrication and Optical Characterization of Polyvinylidene Fluoride/ Neodymium Oxide Nanocomposite Films. Open Access Library Journal, 6: e5979. https://doi.org/10.4236/oalib.1105979