Structural and optical investigations on ZnO-PVDF-NiO advanced polymer composites for modern electronic devices

Polymer composites are recently introduced as flexible candidates for modern electronic devices. Transition metals oxides incorporated PVDF polymer composite thin films were successfully synthesized and investigated for an optical response. The effect of ZnO and NiO nanoparticles as PVDF fillers is studied in this work. Experimentally pure and doped PVDF uniform and evenly distributed thin films were synthesized by sol-gel based spin coating method. Structural studies were carried out with x-ray diffraction analysis which reveals the sharp traces of ZnO and NiO and endorses the presence of crystalline fillers in PVDF polymer composite thin films. The Field emission scanning electron microscope was used to examine the surface morphology of prepared thin films containing a smooth, uniform distribution and compact polymer traces. Refractive index, extinction, absorption coefficient, reflectivity, optical conductivity and real epsilon were extracted using spectroscopic ellipsometry. Enhanced response of optical parameters was observed in nanoparticles containing PVDF polymer composites. The absorption coefficient seems to increase with the increment of nano-filler contents, which makes these materials suitable for photovoltaic applications. The maxima of the refractive index were recorded as 1.6 for pure PVDF and 2.1 for maximum nano-filler content with an incredible shift to higher energy values. The optical conductivity was observed to increase with the incorporation of nanoparticles in PVDF. The maximum values of real epsilon were recorded as 2.3 for pure PVDF and 3.9 for the highest nano-filler containing composition. The increasing trend of real epsilon in nano-fillers containing compositions is attributed to the enhanced polarization and storage capability of these composites. The outcomes of this work are considered that ZnO-PVDF-NiO advanced polymer composites are promising candidates for enhanced modern electronic devices.


Introduction
Polyvinylidene fluoride (PVDF) synthesis procedures for wide-ranging applications can be selected rendering to the preferred property, porosity, and crystalline phases. The mixing of solution and phase-induced separation is the widely employed method for the production of homogenous and asymmetric porous films [1]. PVDF has excellent hydrophobic and porous polymeric nature and is extensively used in distillation and filtration in form of membranes due to having high thermal and mechanical properties [2]. Polymer composites stimulated charge carrier transport in the matrix which is favorable for electronic applications for polymer-based devices [3]. These are also considered important for interesting piezoelectric responses in these composites for various applications [4]. Previously, remarkable progress has been made in the improvement and development of piezoelectric ceramics such as lead zirconium titanate (PZT), polyvinylidene fluoride (PVDF), zinc oxide (ZnO), Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. and BaTiO 3 in terms of their novel sensing, actuation, and energy harvesting properties [5]. Among these, PVDF and its copolymers show the most efficient piezoelectric, pyroelectric and ferroelectric properties for energy harvesting [6,7].
Recent research on nanomaterials has significantly addressed the environmental and energy issues [8]. Polymer-based dielectric nanomaterials are preferred to the conventional ceramic dielectric materials due to their low loss factor and excellent flexibility [9]. PVDF-based membranes are extensively employed for different applications because of their high thermal and mechanical stability [10]. Researchers have recently observed that the introduction of semiconducting Zinc oxide (ZnO) in the PVDF matrix can significantly enhance the electrical properties of these polymer composites [11], because ZnO has a wide bandgap (3.37 eV), which makes it an efficient UV absorber with an absorption edge of about 368 nm. The stability of the electron-hole pairs is constant even at room temperature due to its large exciton binding energy (60 meV), which is much larger than the room-temperature thermal energy (26 meV) [12]. Furthermore, ZnO is being used in coatings, paints, and Sunscreens due to having high resistance to ultraviolet radiation [13].
ZnO is a well-known semiconductor having promising thermal stability and provides enhanced permittivity and piezoelectric response of PVDF. Therefore, a hybrid PVDF matrix with a ZnO nano-filler may enable the fabrication of a hybrid piezoelectric/pyroelectric sensor [14]. The mechanical properties of ZnO-PVDF composite membranes were accurately studied using the nano-indentation technique [15]. ZnO nanowires incorporated PVDF matrix has been presented as a convenient polymer composite for improved piezoelectric and nano-generator applications [16]. PVDF/ZnO nanofibers (CPZNs) for remote control of gestures in the human-machine interactive system [17]. The photocatalytic activity of ZnO-GO-PVDF polymer composite membranes was studied using methylene blue degradations and de-colorization [18]. PVDF-ZnO nanocomposite synthesized by the electrospinning method was also applied in nano-generator fabrication [19]. Polymer/ZnO nanocomposites are thoroughly investigated according to the impact of ZnO on various polymers including synthesis, processing, characterization methods, and different applications [20].
On the other hand, PVDF-NiO nanocomposite material has immense potential for sensor, actuator, and energy harvesting applications [21]. The Ni-coated PVDF-GO conductive polymer composite membranes were introduced and studied for wastewater solution [22]. Several chemical and physical methods have been employed for the synthesis of doped PVDF samples [11,17,19,22]. As reported in literature, different properties of polymer composites are strongly influenced by the type, density, distribution, and aspect ratio of the fillers [23,24]. Taking this into consideration, the main aim of this work is to synthesize ZnO-PVDF-NiO polymer composite thin films for the investigation of the optical response using chemically derived sol-gel based a spin coated method. According to the literature, no studies were found for such compositions of polymer composites in thin-film form. Different optical parameters like refractive index, extinction, absorption coefficient, reflectivity, optical conductivity, and real epsilon were extracted for detailed study and practicability of these compositions for modern device applications.

Experimental method 2.1. Materials and synthesis
Experimentally, pure, Zn, Ni oxides doped PVDF polymer composite thin films were prepared on silicon (Si) substrates using spin coating technique, which is suitable and always allows the soft and uniform synthesis of nano-sized materials. Polyvinylidene Fluoride (PVDF) beads, Zinc nitrate hexahydrate [Zn( , and Citric acid were supplied by Sigma-Aldrich, USA. All the precursors were of analytical grade and used without further purification. The required amounts of beads of PVDF were dissolved in water under continuous stirring at 40°C awaiting obtaining its thick gel. The required percentages of synthesized ZnO and NiO compositions of metal oxide were homogeneously mixed in PVDF gels with ultrasonic agitation. The Contents were dissolved in the requisite quantity of distilled water. To attain a homogenous solution, the assortment was stirred for 45 min by placing it on the hot plate at 50°C. This homogeneous solution was transferred to the silicon substrate (10 × 10 mm) using spin-coating at 3000 rpm for 45 s. After that this spin-coated substrates were baked at 100°C for 5 min and then placed in a muffle furnace for annealing at 200°C for one hour in ambient conditions.The final products of ZnO-NiO/PVDF nanocomposites were obtained in the form of dry plain thin films.

Characterizations
ZnO-NiO/PVDF was investigated for structural studies using Panalytical X'Pert Multipurpose diffractometer (MPD) by employing Cu K α x-rays using wavelength 0.154 nm. The x-ray diffractometer was operated at 40kV voltage and 40 mA current. Morphology of samples was examined through FEI Nano-SEM 450 field emission scanning electron microscopy (FESEM) using a built-in high resolution through lens detector (TLD).
Micrographs were obtained using 5 kV electron beam energy at magnification of ×25K. Elemental composition of synthesized nanoparticles and dispersive nanocomposites polymer matrix were performed using Oxford Inca X'Act energy dispersive x-ray spectrometer (EDS) with 15kV electron beam energy. Alpha-SE spectroscopic ellipsometer was used to extract the highly accurate optical parameters of thin films.

Results and discussion
3.1. Structural analysis X-ray diffraction (XRD) spectra of PVDF, PVDF-ZnO, and PVDF/ZnO-NiO thin films are shown in figure1. To investigate the crystallinity of pure and filler-containing PVDF thin films, XRD spectra were recorded in 2θ range of 20 to 50 degrees [21]. XRD spectrum of pure PVDF thin film contains the minute peak at 2θ value of 20.68 degrees of its (110) plane. The crystalline nature of PVDF is also reported in some earlier works. However, the spectrum reveals the traces of crystalline and amorphous combination because the peak is not sharp. XRD peak at 29.92 degrees in other spectra is associated with the (100) plane of wurtzite ZnO. The crystalline nature of ZnO fillers in homogenously mixed PVDF can enhance the piezoelectric response [25]. While the peak at 33.46 degrees belongs to the (400) plane of NiO [21]. The sharp traces of ZnO and NiO endorses the presence of crystalline fillers in PVDF polymer composite thin films. The peak of NiO(400) becomes sharper with the increasing content of NiO particles in composites as shown in the spectrum of figure 1(d). This spectrum contains some other minute peaks which are associated with the other plane traces of NiO. The homogeneity of the fillers in PVDF nanocomposite is directly influenced by the physical properties of the material [26]. Figures 2(a)-(d) presents the field emission scanning electron microscopy (FESEM) micrographs of pure and nano-fillers containing PVDF polymer composite thin films. The micrographs were acquired with an electron beam energy of 5kV using a through lens detector under secondary electron (SE) mode at a magnification of 25000x. FESEM micrograph of pure PVDF ( figure 2(a)) shows the uniform deposition without agglomeration. The surface morphology contains smooth and compact polymer traces. Figure 2(b)-(c) shows the FESEM micrographs of fillers containing PVDF thin films. The particles are uniformly oriented and distributed in PVDF as can be seen in micrographs. The size of ZnO nanoparticles is smaller ( figure 2(b)) and increased in compositions containing NiO nanoparticles. Further, some agglomeration was observed in the composition containing the highest content of NiO particles as presented in figure 2(d). Elemental compositions of polymer nanocomposite thin films were studied using energy dispersive x-ray spectroscopy (EDX) analysis. EDX spectra were obtained using electron beam energy of 15 kV and spot size of 5 um to obtain the maximum x-ray counts at the detector. EDX spectra of all compositions (figure 3) confirm the presence of expected elemental traces in relevant polymer composite thin films without the existence of any other impurity elements. This reveals the high purity of synthesized nano-fillers containing PVDF polymer composite thin films which are considered necessary for the detailed study of exact physical properties.

Optical properties
An optical response was observed to explore the optical properties of polymer composite thin films using spectroscopic ellipsometry. It is considered helpful in the determination of radiation interaction effect with the material. The various optical parameters like refraction, extinction, absorption, optical conductivity, reflectivity, and real epsilon can be formulated using well-known Kramers-Kronig relations [27].
The absorption coefficient was calculated by fitting the energy-dependent values of Psi and delta extracted through spectroscopic ellipsometry. Figure 4 presents the trend of absorption coefficient curves with the variation of photon energy. The values of absorption coefficient were found to increase with photon energy in all compositions and got the maxima at the highest values of energy. An absorption edge appeared in the visible regime which was found to shift at higher energies with the incorporation of ZnO and NiO nano-fillers in PVDF.  This shift reveals the emergence of excitonic transitions at specific values of energies [28]. Further, absorption coefficient curves were undergone an increasing trend with NiO concentration which makes these compositions favorable for photovoltaic applications.
Refraction (n) of a material is another crucial optical parameter for the determination of optoelectronic response. The refraction depends upon the velocity of radiation inside and outside of the medium. This can be determined through the following relation [29];   The extinction (k) of a material is directly proportional to the absorption coefficient and can be presented as [29]; The extinction curves present the broader image of the absorption of material which is shown in figure 6. A broader single exciton peak was observed in pure PVDF thin film at an approximate energy value of 2 eV. The number of exciton peaks increased and appeared at various energy values in ZnO-PVDF and ZnO-PVDF-NiO thin films. Further, the intensity of the extinction curves and exciton peaks were found to increase with the introduction of nan-particles in the PVDF matrix. The exciton peaks are associated with the transitions and are strongly dependent upon the size of the nanoparticles in the matrix [30]. Overall increasing trend of k value due to nano-fillers reveals the enhanced absorption of polymer composite thin films. The grain boundaries in thin films also play a vital role in the shift of absorption edge and optical conductivity which can be observed in this case. The mean free path in polymer composites varies due to the size of nano-fillers and hence directly influenced the optical parameters like extinction and absorption of thin films.
The real epsilon (ε r ) of a material assists to calculate the optical response upon light propagation in it and can be determined using Kramers-Kronig relations. This is mathematically presented as [29]; Figure 7 presents the experimentally obtained curves of real epsilon of pure and nano-fillers containing PVDF polymer composite thin films. These curves describe the polarization within the material upon propagation of radiations and show a steep decline in curves after attaining maxima at the lowest energy regimes. This mechanism arises due to the dipole orientation in a material with a variation of photon energy. The value of real epsilon was observed as 2.18 at the lowest value of energy (1.4 eV) for pure PVDF thin film and increased to 3.56 for ZnO-PVDF-NiO composition. These curves then present a steady decrease in values due to polarization. The curves show a significant enhancement of the real part of dielectric in nano-fillers containing composition revealing the improved polarization due to semiconducting nanoparticles in the matrix.
The incorporation of nano-fillers in PVDF has directly influenced the optical conductivity. The optical conductivity can be determined using the following relation [31]; nc 4 s a p = The optical conductivity curves were recorded with the variation of photon energy for all thin films as presented in figure 8. Optical conductivity was showing an increasing trend for all compositions with the increase of photon energy. Further, the values of optical conductivity increased due to the incorporation of nano-fillers in PVDF. The maximum values were obtained in a composition having the highest content of NiO along with ZnO in the PVDF matrix. Some exciton peaks have also appeared in nano-fillers containing PVDF thin films due to transitions at some specific values of energies. The maxima of exciton peaks indicate the availability of maximum carriers suitable for optical conductivity at these energies. The reflectivity of a material describes the behavior of the surface in response to incident radiations. It also resembles the response of material for the ratio of incident to reflected intensity of radiations. Reflectance is strongly associated with surface mechanism and hence dependent upon the incident angle of radiations, the polarization of material, and the morphology of thin films. It can be mathematically expressed using the following equation [29]; w = -+ + + Figure 9 presents the reflectivity curves as a function of photon energy for pure and nano-fillers containing PVDF polymer composite thin films. The curves show an increasing trend first up to photon energy of approximately 2.5 eV and then experienced a sharp decrease at higher energy regimes. Meanwhile, reflectivity values are also enhanced with the incorporation of ZnO and NiO nanoparticles in the PVDF matrix. The maxima of the reflectivity curves first shifted to higher energy values with ZnO incorporation and then shifted at lower energy values when introduced NiO in the PVDF matrix. A significant enhancement of reflectivity in nano-fillers containing composition is strongly influenced by the surface morphology as observed in FESEM micrographs and due to the increase of free carriers in the matrix which consequently enhanced the optical conductivity and absorption of polymer composite thin films.

Conclusion
Thin films of pure PVDF and ZnO-PVDF-NiO polymer composites were successfully synthesized with smooth and uniform morphology. The crystallographic information provides the information of crystalline traces of various corresponding planes of nano-fillers in all compositions. FESEM and EDX analysis confirm the uniform distribution of nanoparticles with expected ratios of elemental contents. Nono-fillers in PVDF matric strongly influenced the optical response of thin films. The various optical parameters were found to enhance in polymer composite thin films which make these compositions favorable for different device applications. The extinction and absorption coefficient exhibits the highest values at maximum photon energy. The absorption coefficient curves experienced an increasing trend with the incorporation of NiO content which considers suitable for improved photovoltaic applications. The maximum value of the refractive index for pure PVDF thin film was recorded as 1.59 at 1.3 eV while it increased to 2.05 at 3.3 eV in ZnO-PVDF-NiO composition. The value of real epsilon was observed as 2.18 at the lowest value of energy (1.4 eV) for pure PVDF thin film and increased to 3.56 for ZnO-PVDF-NiO composition revealing the improved polarization due to the presence of semiconducting nano-fillers in the matrix. The high value of the dielectric constant with higher dopant content evidence that it can be used as a superior dielectric medium. Optical studies shows significant increment in absorption coefficient, optical conductivity, and real epsilon with increase in dopant content which makes these materials an excellent choice for applications in modern electronic devices.