Microstructure Study And Linear/Nonlinear Optical Properties of Bi-Embedded PVP/ PVA Films For Optoelectronic And Optical CUT-OFF Applications


 Hybrid polymer films of polyvinyl pyrrolidone (PVP)/ polyvinyl alcohol (PVA) embedded with gradient levels of Bi-powder were prepared using a conventional solution casting process. X-ray diffraction (XRD), Fourier transform infrared spectroscopic (FT-IR), and Scanning electron microscope (SEM) techniques have been used to examine the micro/molecular structural and morphological of the synthesized flexible films. The XRD peak intensity and FTIR spectra of the PVA gradually decline as Bi-metal is introduced. In addition, filler changes the microstructure surface of the pure film. The modification in the microstructure leads to an enhancement in the optical characteristic of the blend films. The indirect allowed transition energy was calculated via Tauc's and ASF models. The decrease in the hybrid film’s bandgap (\({E}_{gi}^{opt}\)) returns to the localized states in the forbidden region, as illustrated by the Urbach energy (Eu) values. The optical absorption parameters, including absorption coefficient (α) and edge (Ee), have also been evaluated. The relation between the transition energy and the refractive index (n) was studied. The rise of Bi-metal concentrations leads to an improvement in the nonlinear (χ(3)) susceptibility and (n2) refractive parameters. The optical limiting characteristics (OLC) revealed that the higher concentration dopant films reduce the light transmission intensity. The results suggest that hybrid films are promising materials in a wide range of applications, especially optical devices, and optoelectronics.


Abstract:
Hybrid polymer films of polyvinyl pyrrolidone (PVP)/ polyvinyl alcohol (PVA) embedded with gradient levels of Bi-powder were prepared using a conventional solution casting process. X-ray diffraction (XRD), Fourier transform infrared spectroscopic (FT-IR), and Scanning electron microscope (SEM) techniques have been used to examine the micro/molecular structural and morphological of the synthesized flexible films. The XRD peak intensity and FTIR spectra of the PVA gradually decline as Bi-metal is introduced. In addition, filler changes the microstructure surface of the pure film. The modification in the microstructure leads to an enhancement in the optical characteristic of the blend films. The indirect allowed transition energy was calculated via Tauc's and ASF models. The decrease in the hybrid film's bandgap intensity. The results suggest that hybrid films are promising materials in a wide range of applications, especially optical devices, and optoelectronics.

Introduction
Polymer materials have fascinated scientists due to their safe, economical price, plentiful, eco-sustainable properties, and their extensive application in technical and scientific study. They can be used in optoelectronics, solar cells, UV-filters, coatings, photovoltaics, lightemitting diodes (LEDs), laser production, as well as several other potential applications [1,2].
The synthesis and examination of various polymer blends recently had a tremendous degree of concern. A polymer blend is a substance that combines two or sometimes more polymers to improve the product's behavior with appropriate properties. It may have a distinct feature, enhancing the advantages of both additive polymers and improving their desirable characteristics. Also, they are dependent on the miscibility degree of the host polymers [3][4][5][6].
Their properties can be modified by adding different substances and particles [3][4][5]. Therefore, composites can be produced because the polymeric materials create a miscible solvent of strong hydrogen bonding between the molecules of the constituents [7].
Poly(vinyl alcohol) (PVA) [(C2H4O)n] is an exciting polymer due to its physical, chemical, mechanical, and thermal characteristics. Moreover, PVA has particular features such as semi-crystalline, adhesive properties, and water-soluble. It is suitable for a wide range of scientific, biomedical, and technological applications [8][9][10][11]. However, another polymer material with an amorphous structure, biological compatibility, soft processing capability, good environmental stability, and outstanding solubility is poly (vinyl pyrrolidone) (PVP) [(C6H9ON)n], which makes it suitable for a variety of applications such as optics and photonics [2,6].
PVP and PVA are considered the famous and desired polymers as perfect and operative binders in the production of optical responsive materials employed in the designing of sensor systems, optoelectronics, and organic electronic systems [12].

Experimental techniques
Pieces of regular thickness polymer films were carefully placed on a specimen holder of Shimadzu diffractometer (XRD-6000)-copper target (λCu-kα = 1.54108 Å). It was working at a and wavelengths 632.8 and 533 nm, respectively, were operated to show the influence of Bi content on the film's absorption.

X-ray diffraction (XRD) investigation
The XRD pattern of pristine and Bi-blend composites can be seen in Fig 1. The pattern of the PVA demonstrates two diffraction peaks at approximately 2θ = 20° and 41.7°. This is attributed to the semi-crystalline feature of the pristine PVA [9]. Due to the -OH groups within the main matrix of PVA, it involves good intermolecular and intramolecular hydrogen bonding [19]. However, the blend film's pattern shows that the intensity of the main crystalline peak of the PVA was considerably shifted to 19.86 o , and the broadest of the bandwidth. This means the amorphous portion in the mixed sample is more significant than that in the pristine PVA sample.
It is known that PVP has an amorphous structure, which is an appropriate polymer for various applications [20]. Therefore, the mixture between two polymers causes a reduction in PVA crystallinity and an amorphous increment in the blend matrix. This demonstrates the better miscibility and connectivity between the -OH groups of PVA and C=O of PVP groups [16].
Moreover, this decline becomes more significant with Bi-metal content due to the disruption of particles in the crystalline portion of the blend matrix, making the amorphous performance major in the composite/hybrid films. Therefore, there is a direct correlation between the crystallinity degree and the peak intensity, as recognized via El-Naggar and coauthors [21]. They noticed that the intensity of PVA/PVP diffraction decreased by raising the amorphous nature by adding filler.
The XRD pattern of Bi-nanoparticle shows reflection peaks corresponding to the rhombohedral structure with space group R3m (#166). This diffraction was compatible with the file JCPDS: 44-1246 for pure Bi-metal [22,23]. The addition of a low ratio of Bi-particles to the blend presented no specific peak relating to the crystal structure of the powder. However, the The degree of crystallinity of the pure and composite films was estimated by fitting their diffraction patterns of the main peak via Fityk 0.8.9 software ( Fig. 2(a-f)). The crystalline fraction (XCryst.) was calculated using the next relation [26].
Acryst. and A(cryst.+amoph.) are represent the area under the crystalline and all curves. The values are reported in Table 1. A significant decrease in crystallinity was observed in Bi-blend composite films. However, for 4Bi-blend composite film, the crystallinity increased again due to the presence of a high ratio of Bi-crystalline particles. This behavior was reported in various studies of doping polymeric martial with different fillers [26][27][28].

FTIR investigations
Fourier transforms infrared (FTIR) spectroscopy is an effective widespread technique applied to acquire a broad range of infrared spectra that describes and identifies the interactions of polymer matrix with dopant materials [29].  [8,10]. Moreover, the peaks located at 1647, 1375, and 1290 cm -1 match with C=O stretching, -CH2 bending, and CH2 twisting or wagging vibrational modes of the PVP chains, respectively [30,31]. Also, the band centered at 1495 cm -1 corresponds to the characteristic vibration of C=N (pyridine ring) of PVP [32]. The intensity of such peaks slightly decreased by increased Bi-metal powder concentration in the blend relative to the pure films. This is similar to the result of the XRD study. Thus, it can be deduced that the Bi-particles interact with the backbone chains of the hydroxyl groups in PVA and carbonyl (C=O) groups in PVP [33].

Morphological analysis
The surface morphology of the synthesized pure and hybrid films was investigated using the scanning electron microscope (SEM). Pure and different Bi-contents loaded blend composite images are shown in Fig 4(a-d). The blend film has a homogeneous and smooth surface without cracks (Fig 4a). These results are consistent with PVA/PVP blend film [2,12,31]. The bright area on the surface of the composites is related to the Bi-particles that are randomly distributed over the polymer blend surface. This induces a significant change in the surface morphology of the PVA-PVP matrix. There is a little agglomeration when the dopant contents rise to 3.7 wt.% [2Biblend] (Fig 4c). This agglomeration increased in 4Bi-blend film. These clusters dispersed throughout the film's surface, suggesting proper polymer-particle interactions; the organicinorganic components in the polymer composites are compatible [34]. Thus, the hybrid film's surface roughness rises as the concentration of Bi-metal increases.

Optical properties
The analysis of optical characteristics of the materials is a valuable method for investigating the band structure as well as the density of electronic states [35]. dispersed leads to a decrease in the transmissions significantly [18,36]. The feature can be considered a novel implementation for UV block and laser attenuation. In the pristine PVA, a tiny peak at about 278 nm was noticed, indicating the presence of an electronic movement from n to π* [37]. However, this peak is wholly disappeared in the pristine blend and Bi-blend samples.
Regarding the optical absorption spectra in Fig 5b, a high absorption level was improved by the 4Bi-blend film. This confirms the creation of levels between VB and CB, leading to easy electron transfer throughout the structure [38]. The absorption edge of pristine and Bi-blend composites moves to lower energy (higher wavelength) than pristine PVA film. Therefore, the bandgap changes with Bi-metal in the blend suggest the complex interaction.
One of the significant parameters for investigating the variation of the polymer material's band structure is the absorption coefficient [39].
The variations of the α-spectra with the photon energy, hυ, are shown in Fig 6. The α values for all films are evaluated within the range of 10 4 m -1 . This suggests the energy required is sufficient to excite the electrons from LUMO (lowest unoccupied molecular orbital) to HOMO (highest occupied molecular orbital) [41]. By extrapolating the sharp part of the absorption graph to intersect the hυ axis at the point of α = 0, the absorption edge's energy (Ee) could be calculated.

Urbach's tail energy and optical energy gap calculations
The change in the absorption coefficient was contributed to the band tail (Urbach's energy). The tail energy (Eu) value indicates the defects and the disorder in the polymer matrix. It is located inside the prohibited bandgap close to the valence and conduction band's edges and describes the localized states' width [39]. Urbach proposed that this band tail is defined through the following empirical relationship [17]: where βo is constant. Urbach The electron transitions of the materials typically depended on the incident photon's energy, as suggested by Tauc's law. The relationship between the α and the energy of the incident photon (hυ) was calculated in the region of strong absorption using the formula [43]: K is the energy-independent band tail parameter and depends on the probability of electronic transition between the valance and conduction bands.
The forbidden optical gap (eV) was estimated by extrapolating the linear segment of (Abs 1/2 /λ)-1/λ plot to (Abs 1/2 /λ)= 0, as shown in Fig 9. Then, the optical band gap was calculated by using the relationship: = 1240 ⁄ . Finally, the value of each film was recorded in Table 1. The bandgap calculated by the ASF approach is almost close to those from Tauc's model. These observed results represent a typical attitude since the bandgap reduction is due to localized states at the prohibited band's boundaries. For many polymer films, this indirect attitude between Eopt and the Eu was being indicated [47][48][49].

Extinction coefficient and refractive index investigations
The The variation of k with the incident photon wavelength, λ, is shown in Fig 11. It is observed that for those composites is improved [51].
Moreover, the index of refraction, n, is an essential parameter for using the materials in the manufacturing of optical instruments, optoelectronic devices, optical switches, filters, lightemitting diodes, modulation, and waveguides [52]. It would be relevant to the ion electronic polarization and the local field within the optical substances [17]. Furthermore, there is a relation between their values and the energy gap, Eopt. According to their significance in studying material's band structures, these two essential parameters were studied intensively. The energy gap is commonly evaluated by the electromagnetic wave threshold absorption, while the transparency of the material is estimated utilizing the refractive index [53]. The n value was calculated depending on the optical bandgap as [54]:  The negative slope indicates that the refractive index behaves reversely with the energy gap value.
Their values changed with the refractive indices and the optical energy gap, respectively, as reported in Table 2. The result indicates the linear optical parameters changed with the modification of the electronic structure of the blend by the Bi-metal content.

Nonlinear optical parameters
The characteristics of nonlinear optical polymeric materials primarily depend on dopant concentration and host polymer properties. High-value nonlinear optical materials are commonly required to design various optoelectronic instruments [55]. Nonlinear material behavior is induced by large radiation intensities, like lasers [56]. This is due to the induced polarization (P) and the applied electric field (E). Therefore, the nonlinear refractive index (n2), first-order susceptibility ( (1) ), and higher-order susceptibility ( (3) ) of the as-prepared polymer films must be examined. First, (3) can be being determined using the following relation: where C = 1.7x10 -10 esu, and (1) is determined by known the linear refractive index (nav) using the formula: Therefore, it is possible to estimate the n2 value from the below relation: The first-order (1) and nonlinear parameters ( (3) and n2) are summarized in Table 2. The larger values are observed for 3Bi-blend, as shown in Fig 13. At higher Bi-metal concentrations, more particles absorb extra electromagnetic waves, leading to high polarization of the polymer films and an improvement in the nonlinear parameters. This means that as the transition energy gap decreased, the nonlinear properties of the polymeric composite enhanced. Therefore, the asprepared hybrid films could be used in nonlinear optoelectronic devices.

Optical limiting characterization
Optical limiters are devices made to filter the incident electromagnetic waves [11]. The protection of optical sensors and components from laser deterioration is one of the most commonly used application fields of this effect [57]. Therefore, the output power and normalized (output/input) power of two distinct laser sources (green and He-Ne laser sources) with wavelengths of 533 nm and 632.8 nm, respectively, were operated to investigate the optical limiting characteristics (OLC) for the films under examination. Table 3  Hence, the filler concentration plays a significant factor in the OLC. The variation in the output power values between the two sources is due to the composite film's reaction sensitivity to incident light. A sample with greater Bi-metal nanopowder concentrations has more molecules per unit volume in the blend matrix, which participates in the optical interactions during nonlinear absorption mechanisms [11]. Consequently, the OLC of polymer films is correlated with the sample's ability to absorb and the scatter light. As shown in Fig 14b, the 3Bi-blend polymer sample produced the lowest value of normalized power. Therefore, the sample can be used as an Optical limiting laser since the light power is strongly attenuated.