Optical and electrical performance of copper chloride doped polyvinyl alcohol for optical limiter and polymeric varistor devices

Physica B: Condensed Matter 572 (2019) 256–265

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Optical and electrical performance of copper chloride doped polyvinyl alcohol for optical limiter and polymeric varistor devices

T

H. Elhosiny Alia,b,∗, Yasmin Khairyb a Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia b Physics Department, Faculty of Science, Zagazig University, 44519, Zagazig, Egypt

A R T I C LE I N FO

A B S T R A C T

Keywords: Copper chloride doped PVA XRD/FTIR DTA EDX/SEM Optical and electrical characteristics

PVA composite films with different wt.% of CuCl2, by the cast method, were prepared. The semi-crystalline phases of PVA have been reduced by more addition of CuCl2 as confirmed with the help of X-ray diffraction, and Fourier transforms infrared spectroscopy. The thermal stability is surveyed by differential thermal analysis. The morphology of CuCl2 particles is studied by scanning electron microscopy, which shows the semi-spherical particles of size varied from (0.31–1.19) μm to (3.1–5) μm for 0.037 wt% CuCl2/PVA and 3.700 wt% CuCl2/PVA (CPVA5) films, respectively. With the increment of the additive, the absorptive property of the films is increased. The indirect and direct energy gap were estimated. The power with 632.8 nm and 533 nm of the laser sources has been reduced to 40% and 47% via CPVA5 films. The AC electrical conductivity follows the Jonscher's power law. The forward lnI (A) – lnV (V) behaviors show that the doped films have high voltage breakdown.

1. Introduction One of the wide distribution materials in natural, industrial, and academic research fields is the polymer. It has been present since the beginning of life and has attracted much attention to the enormous development in the industry as a result of its distinctive characteristics. There are many kinds of polymers like polyethylene, polyester, Teflon as synthetic materials as well as silk, wool, DNA, cellulose, and proteins as natural materials. Their applications and functions depend on the properties of each of them that determined by the polymer micro/ macro-structure [1]. The synthetic process and conditions have much influence on the structure of the polymers and consequence the mechanical, optical, and electrical response [1,2]. For using the polymer materials in various applications and superior their properties, some of the structural defects are needed by natural fillers and manufactured composite. The polyvinyl alcohol (PVA) is a kind of polymers that have a keen interest because of scientists and researchers due to their potential applications when mixed with other composites [3]. PVA is a semicrystalline and a water-soluble material. Also, it has unique chemical and optical properties; therefore, it is used in versatile applications such as optoelectronics, laser filtering/limiting, sensors, ophthalmology, prosthetic materials and others [3]. Also, it has good storage capacity, high dielectric strength, additive-dependent optical properties, as well ∗

as high thermal and chemical stability [4]. The non-covalent forces such as hydrogen and coordination bonds play a significant role in the creation of super-molecules inside PVA which is important for their applications. Up to now, there are many papers described the structure, optical, and electrical properties of polymer materials after being doped with lanthanide ions, fillers, and composites [5,6]. So, the transition metal salts, or oxides can induce pronounced changes in the PVA properties which lead to the use of the resulting compositions in widespread applications, such as UV-protector and laser filter. It was reported that by adjusting the concentration of nano-composite graphene oxide in PVA, a desired optical limiter could be provided [7]. Also, the light in the UV–Vis regions between 200 and 560 nm was completely CUT-OFF by 37 wt% fluorescein sodium salt doped PVA sample [3]. Copper (Cu) is an essential heavy and important nonferrous metal that has many known functions in biological systems and industrial applications. However, at elevated concentrations, it becomes toxic. Nevertheless, the industrial uses of copper led to their widespread in silt, soil, and wastewater with significant environmental problems that require an efficient solution. Therefore, for remediation the polluted sites, many of physical-chemical approaches were used; these, however, are inefficient and costly. On the other hand, the ceramic varistor has been used widely for electronic equipment protection and voltage stabilization. One of the

Corresponding author. Physics Department, faculty of science, Zagazig University, 44519 Zagazig, Egypt. E-mail addresses: [email protected], [email protected], [email protected] (H.E. Ali).

https://doi.org/10.1016/j.physb.2019.08.014 Received 10 April 2019; Received in revised form 24 July 2019; Accepted 8 August 2019 Available online 08 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.

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most well-known and major varistor ceramics was based on ZnO, because of its highly nonlinear I–V (current-voltage) characteristic with large nonlinearity coefficient and a highly resistive in the breakdown region [8,9]. Conventional varistors are synthesized by mixing the ZnO powder with oxide additives. However, most of the scientist focused on decreasing the sintering temperatures of varistor ceramic for decreasing grain size and high breakdown voltage applications. Currently, most researchers are looking for materials with multifunctions and synthesized at low temperatures. Thus, the main goal of the present work is to synthesize copper chloride doped PVA composite films for the new trend of the low-cost varistor device. To study the influence of the concentration of CuCl2, on the optical properties for laser limiter and UV-protector, XRD, FTIR, and SEM were used for structural and morphological surface analysis. In addition, the differential thermal (DTA) analyses were performed. UV–Vis-IR spectroscopy, electrical, and dielectric characterization of the as-prepared CuCl2 doped PVA films were, also, examined.

the crystalline order inside the films. The FWHM (full width at half maximum) i.e. β in radian, and the diffraction angle of the main peak θ (in degree) were calculated from Gaussian fitting. These two parameters are used for calculating the crystallite size by Scherrer equation D = 0.9λ/(β cos θ), as well as the internal strain ε = β/(4 tan θ) [10,11]. The second measurement was carried out, by Thermo Nicolet FTIR spectrometer in the gauge wavenumber range 500–4000 (10−2 m)−1, for determining the complex interactions and the functional groups that are appearing due to the influence of CuCl2 in PVA matrix. Differential thermal, DTA, the analysis was performed via Shimadzu DTA-50 analyzer at a rate of rising temperature equal to 15 °C/ min, over the range room temperature - 600 °C, in the atmosphere of nitrogen environment with flow 20 ml/min. Moreover, the effect of CuCl2 concentration on PVA morphology was analyzed through the scanning electron microscopy (SEM) instrument with model JSM-6360. Furthermore, Cu and Cl elementals inside the PVA matrices were tested by EDX analysis.

2. Experimental procedures

2.2.2. UV–Vis–NIR spectroscopy of CuCl2 doped PVA polymeric films Transmission (T), and Absorption (A) optical spectra of CPVA films were measured via dual-beam JASCO V-570 UV (ultraviolet)- Vis (Visible)- NIR (near-infrared) spectrophotometer ranging from 200 to 2000 nm, where samples are mounted in the sample holder inside the spectrophotometer. The incident light in the spectrophotometer was split into two beams. One of these beams reach the polymeric film and the other beam passes through the air as a reference. The two beams are measured at the same time and the resultant transmission is the ratio of the two beam intensities.

2.1. Preparation of the samples CuCl2-doped PVA films were prepared using the conventional casting method according to the following experimental procedures. (I) 0.045 kg of PVA powder, [–CH2CH(OH)–]n chemical structure and 4 N purity supplied from Alfa Aesar-Karlsruhe-Germany, was dissolved in distilled water at 65 °C during 48 h, after that, an equal volume of the pure solution is poured into equal bottles. (II) A cupric chloride anhydrous (CuCl2.2H2O) of 3 N purity, supplied from Fisher Scientific UK, with different concentrations (X = 0, 0.037, 0.185, 0.37, 1.85, and 3.7 wt%) were used as inorganic doping fillers. (III) Each weight was mixed with a homogeneous solution of pure PVA and they well stirred for 10 min to a complete the dissolution. (IV) The mixtures were uniformly spread over very clean and dried Petri dishes with an equal volume. All sample solutions were placed in an oven for approximately four days at 40 °C, and then the dried films were peeled off from the glass plate. Thick films of CuCl2-doped PVA synthesized by casting method were obtained after drying. The mean value of the thickness of each film is equal to 0.1 mm. For further characterizations, all samples are cut to small parts with lateral dimensions of 2 × 2 cm2. The prepared films are named here as shown in Table 1.

2.2.3. OL (optical limiting) measurement of CuCl2 doped PVA polymeric films Two sources of lasers measured the limiting characteristic of the visible light via CPVA samples, (I) 15.396 mW average power of the green laser (Baran laser company) with length 533 nm, as well as (II) 0.5 mW low power of He–Ne laser with 632.8 nm. The tested film was placed in a location = the focal length of the convex lens = 0.1 m, where the sample and the lens were fixed amongst the output and entering beams, respectively. Both beams are noted by using a digital sensitive laser power meter. 2.2.4. Electrical characteristics of CuCl2 doped PVA polymeric films AC electrical conductivity and dielectric capacitance of the as-prepared CPVA films were carried out using semiconductor characterization framework 4200-SCS KEITHLEY via the applied voltage in sine waveform, over the wide region of frequencies (3 kHz −10 MHz) at room temperature. Also, the same equipment was used for testing the forward non-linear current-voltage (I–V) curves. All the electrical measurements were recorded by fixing the films inside the sample holder which have two electrodes of brass. The bottom electrode was used as a bottom contact for the polymeric films, while the upper electrode is connected to spring for good contact with the upper part of the polymeric sample. The upper electrode has a 0.01 m diameter. No

2.2. Characterization and devices 2.2.1. Structure, thermal, and surface morphology studies of CuCl2-doped PVA polymeric films The crystalline structure, as well as the complex interactions of all prepared CPVA films, were studied by using two techniques functioned at the temperature of an area. The primary measurement was operated by using CuKα radiation from Shimadzu model XRD-6000 X-ray diffractometer at 40 kV and 0.03 A. The scan with speed of 60/min, in the angle 2θ range start from 100 to 600, was used to detect all the peaks of

Table 1 Indirect energy gaps and the slope of ln I-ln V curves for CPVA polymeric composite samples. Samples

(eV)

(eV)

(eV)

(eV)

Eg1 from

Eg2 from

P1

P2

P3

References

CPVA0 (Pure PVA) CPVA1(0.037 wt% CuCl2/PVA) CPVA2(0.185 wt% CuCl2/PVA) CPVA3(0.370 wt% CuCl2/PVA) CPVA4(1.850 wt% CuCl2/PVA) CPVA5(3.700 wt% CuCl2/PVA) 10 wt % Cu (NO3)2/PVA 15 wt % Cu (NO3)2/PVA 20 wt % Cu (NO3)2/PVA

5.05 4.00 3.31 3.04 2.39 2.34 2.97 2.90 2.85

– 3.23 2.16 2.05 0.65 0.65 – – –

– – 0.99 0.74 – – – – –

5.80 5.58 3.69 3.77 3.27 3.01 – – –

5.28 3.91 3.80 3.67 2.91 2.83 – – –

– – – – 0.56 0.52 – – –

0.20 0.71 0.83 0.87 0.85 1.21 – – –

– 3.29 4.23 5.09 5.24 5.36 – – –

– 1.11 1.10 1.23 1.25 1.31 – – –

Present Present Present Present Present Present [3] [3] [3]

257

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Fig. 1. XRD patterns of CuCl2-doped PVA composite films. Fig. 2. Gaussian fitting of XRD peaks for pure PVA, CPVA1, CPVA3, and CPVA5 composite films.

external pressure was made on the two electrodes. Teflon was used to be an isolated material to separate the two contacts. The experimental was arranged as the following consequence: First bottom electrode, the tested film, Upper electrode, and finally Teflon bar connected between the upper and down electrode.

2.3.2. FT-IR spectral analysis of CuCl2 doped PVA polymeric films One of the valuable tools to provide useful information about the impact and the complex interaction, of different concentration of CuCl2 on the structure of the PVA samples, is the FTIR spectroscopy. Fig. 3(a& b) represent the FTIR transmission profile spectra of the pure PVA and CPVA composite films under the investigated wavenumber and in 400–1750 (10−2 m)−1 ranges. The samples exhibit various stretching and bending characteristic bands. A broad absorption band extended from approximately 3508–3106 (10−2 m)−1 to 3630–2561 (10−2 m)−1 in the pure PVA and CPVA5 spectra, respectively. It is signified to the vibration of –OH stretching in the PVA matrix [16–18]. Also, this vibrational mode is corresponding to the band detected at 625 (10−2 m)−1. The remarkable weak peak that observed at 2940 (10−2 m)−1 and 2182 (10−2 m)−1 are indicated to the group of asymmetric stretching of –CH2 and the combination vibration of (CH + CC) in all samples. With more magnification of the spectra as shown in Fig. 3b, there are two absorption bands referred to twin modes of C]C at 1711 and 1656 (10−2 m)−1 [19]. The C–H bending mode of –CH2 at 1564 (10−2 m)−1 is observed at a low concentration of CuCl2 particles, as it is approximately disappeared in the spectra of CPVA4 and CPVA5 films. Moreover, the absorption band positioned at 1437 (10−2 m)−1, and 853 (10−2 m)−1 in the pure PVA spectra are attributed to –CH2 bending and stretching, while 1330 (10−2 m)−1 and 1094 (10−2 m)−1 are signified to the combination vibration of (CH + OH) groups and C–O stretching vibrations, respectively. The only possible reason for intensity changes and broadening of the functional groups in the backbone of the PVA matrix is the strong incorporations with Cu2+ions. This result, also, confirms the degradation in the crystallinity degree of the PVA with increasing the salt level.

2.3. Results and discussion 2.3.1. X-ray diffraction (XRD) analysis of CuCl2 doped PVA polymeric films The XRD patterns of pure PVA, as well as CPVA composite films with various concentrations percentages, are shown in Fig. 1. It is clear from this diffractogram that (I) a relatively broad peak close to 19.600 diffraction angles has been seen in all polymeric samples, corresponding to (101) crystal reflection planes [12], indicates that they have a structure in nature between crystalline and amorphous states i.e. semi-crystalline [13]. (II) with the increment of doping, the intensity and the sharpness of the peak decreases, i.e. the main diffraction from the crystalline plane become broadens since the internal distance between C–C is enlarged [14]. (III) No other peaks are detected from the CuCl2 in the CPVA polymeric films which describe the complete dissolution of salt in the PVA amorphous regions. These results are strongly established the complex intermolecular interaction between the Cu2+-ions and hydrogen bond of PVA chains which observed in the decrease of the CPVA composite intensity. A consequence of a significant reduction in the crystalline phase with increasing of the amorphous degree was, as well, detected [11]. The relationship between the peak intensity and the crystalline fractions in the doped PVA films were, also, noticed and interpreted by Omed et al. [15]. Fig. 2 presents the Gaussian fitting of the main diffraction peak for pure PVA and CPVA composite films with the estimated cluster size D and internal strain ε values. It is revealed that the CuCl2 has a relatively small effect on the peak positions of the PVA. On the other hand, the D values are reduced from 4.63 nm to 4.27 nm corresponding to pure PVA and CPVA5 samples, respectively, whereas the ε increased gradually with increasing doping level. These results can be ascribed to the effect of CuCl2 particles, as the layers of PVA become far apart and consequently the degree of crystallinity was decreased due to the strong incorporation [5].

2.3.3. Differential thermal analyses (DTA) of CuCl2 doped PVA polymeric films The thermal stability of undoped and CuCl2 doped PVA composites was studied from the curves of the DTA technique. Fig. 4(a–f) present the DTA thermograms of PVA, CPVA1, CPVA2 CPVA3, CPVA4, and CPVA5, respectively. Three endothermic and four exothermic events were detected in the PVA thermograms as depicted in Fig. 4a. The weak peak of endothermic at 128 °C is associated with the elimination of the 258

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[19]. The 402 °C, 444 °C, and 479 °C of the exothermic peaks indicate the beginning and gradual degradation, whereas the large peak positioned at the 534 °C presents the full degradation temperature of the PVA. The glass transition temperature increased to 156 °C for CPVA5 sample which demonstrates the strong branching effect in the CPVA complexes [20]. The large endothermic peak of the melting point of the PVA decreased from 228 °C to 191 °C for CPVA1 and CPVA5 films, respectively, while the third peak of the PVA endothermic was shifted to higher temperature with increasing the wt.% of CuCl2 which indicate the increment of decomposition state that correlated to the side chain (C–O scission reaction). It is, also, clear that the peak of the third endothermic became exothermic at 264 °C. However, other exothermic peaks appeared, and the maximum exothermic peak shifted from 534 °C to 543 °C with the increment of doping level which indicates the complete degradation occurred at high temperature. 2.3.4. Morphological surface study and EDX analysis of CuCl2 doped PVA polymeric films Although, there are no peaks coming from CuCl2 in PVA samples were detected by XRD. However, the presence of Cu and Cl elemental particles in PVA can be confirmed by EDX analyses as displayed in Fig. 5 for CPVA3 sample. The EDX results confirm that there is no one of the particles was evaporated during the synthesis or casting. The change in the morphological images for CuCl2 raw and CPVA composite films are shown in Fig. 6(a–f). The average particle size of the powder of CuCl2 raw material is about 288 nm as seen in Fig. 6a. Moreover, the displayed images specify that the morphological surfaces of PVA changed with CuCl2 percentages (Fig. 6b-f). The gray area referred to the base pure PVA, whereas the whitish regions are depicted to the CuCl2 filler. Non-homogeneous particles shape and size were observed in all the synthesized films. In addition, an agglomeration form clusters with rising the wt.% CuCl2 enter the PVA matrix, as visibly seen in the images of CPVA films. The size of the particles, in the CPVA1, CPVA2, CPVA3, CPVA4, and CPVA5 samples, is ranging over 0.31–1.19 μm, 0.9–2.5 μm, 1.8–4 μm, 1.4–5.5 μm, and 3.1–5 μm, respectively, which may be owing to ligand-ion complex formation and the agglomeration of the particles with rising the dopant level. This is, also, recently found in the polymeric films of KMnO4 doped PVA [21].

Fig. 3. (a &b): FTIR spectra of CPVA polymeric films, (a) over a wide range of wavenumber (500 cm−1- 4000 cm−1), and (b) in the range 1750 cm−11300 cm−1.

2.3.5. Optical spectroscopy of CuCl2 doped PVA polymeric films The spectra of transmission and absorption for pure and doped PVA samples at room temperature are displayed in Fig. 7(a&b). The pure

Fig. 4. DTA (differential thermal analysis) for pure PVA and CuCl2-doped PVA composite films.

adsorbed water and the glass phase transition temperature, while the 228 °C and 304 °C peaks are related to the melting temperature and the C–O decomposition reaction of the semi-crystalline PVA, respectively

Fig. 5. EDX spectra for CPVA3 sample. 259

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Fig. 6. (a–f): SEM morphology images of (a) the raw powder of CuCl2, (b) CPVA1, (c) CPVA2, (d) CPVA3, (e) CPVA4, and (f) CPVA5 composite films.

PVA film showed maximum transmittance (about 92%) at the highest wavelength, while at far UV region, the light gradually was absorbed. This describes that there is no significant effect with small doping of CuCl2 particles in PVA, as the closer behavior was observed by F.M. Ali et al. [3]. However, with raising the doping level in the PVA matrix, the transmittance reached zero, as well as the light, was fully absorbed/ impassable in the region of UV-band from 190 nm to 359 nm. Another absorbance peak in the visible region at 706 nm for CPVA2 film was raised and shifted to a high wavelength of 859 nm for CPVA5 sample. Therefore, PVA samples with a high doping concentration of CuCl2 particles may be applied as laser CUT-OFF, as well as various applications of optoelectronic [22,23]. The variation of the transmittance values in the CPVA films may be due to the creation of layers via the intermolecular interface between CuCl2 and the PVA chains via H- bond, which causes a significant decline in the transmittance of the light with

increasing the doping level. The shoulder peaks that exist below the wavelength of 359 nm are assigned to the presence of the electronic transition π- π* in the carbonyl groups [4,24]. Moreover, the peaks at wavelengths longer than 700 nm are assigned to the bi-polaronic state caused by doping of CuCl2 [25]. The remarkable rise of the peak in the range of 706–859 nm can be due to the molecular aggregation of CuCl2 particles, as seen from SEM images. On the other hand, the shift of the absorption edges to higher wavelength signifies the semi-crystalline structure of the CPVA samples. The same absorbance performance was recognized in PVA films with enormously doped dye concentrations [26]. So, due to the promising results of the optical absorbance property in the UV–Vis region, the PVA films with high CuCl2-concentration level can be an excellent for the blocking (i.e. CUT-OFF) visible laser filters, as well as UV-protector. One of the vital factors for determining the characteristic properties 260

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Fig. 7. (a &b): (a) Optical transmission, and (b) absorbance spectra of CPVA polymeric composite films.

where h, A, and υ are of traditional meanings. Eid and Ed for as-prepared films are listed in Table 1. These results are estimated through intercepting straight line from the linear part of the curve with the axis of the incident photon energy (hυ) at α (hυ) = 0, as seen in Fig. (8). It is markedly that Eid and Ed decrease with raising the doping percentage of CuCl2 in the PVA matrix. Moreover, these values are lower than the pure PVA sample. A significant influence of the small CuCl2 doping level, is comparable with high Cu(NO3)2.3H2O concentrations in PVA, led to a decrease in the energy gap (Table 1) [3]. Also, there is more than one magnitude of Eid are recorded with a significant reduction of it in CPVA5 film. This can be explained as the strong incorporation between CuCl2 particles and the chains of PVA which consequently create trapped levels in the region between LOMO (valence band) – HOMO (conduction band). These attributed to the generation of more dipoles in disorder manner especially for large wt.% in accord to XRD data.

of the solid materials is the energy gap (Eg), as it is playing an important character in optoelectronic design and new solar cell. Thus, its evaluation value is necessary to estimate precisely which can be done by using the analysis of the absorption spectra of the studied optical samples. In order to determine Eg, the absorption coefficient α (hυ ) is first calculated with helping of Abs (the optical absorbance) and thickness (X) of the polymeric film under investigation according to Lambert's relation [27]:

α = 2.303

Abs X (m)

(1)

After that, the expression of Tauc's model for the allowed indirect transition (m = 2) and direct (m = 1/2) was used to identify the value of indirect energy Eid , and direct energy Ed [28]:

(αhυ)1/ m = A (hυ − EgOpt )

(2) 261

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(a)

(b)

Fig. 9. The dielectric loss with the incident photon energy (hν ) for PVA with different concentration of CuCl2 salt.

energy, for all samples, is displayed in Figure (9). The earlier research shows that interception with the photon energy axis from linear components in all sample curves can be considered as a right transition in the band gap. Therefore, the intercept between the linear components (see Fig. 9) and the photon energy axis can be considered a real band. Hence, the kinds of electronic transition can be recognized by matching the plots taken from Tauc's equation (see Fig. 8) with Fig. 9 for optical dielectric loss. It can be decided that the sort of electronic transition in pure PVA and CPVA1 specimens is mainly indirect, while other samples have a direct transition. Therefore, the optical dielectric can be a valuable tool to study the structure of the band for the materials. The reduction in band gap (see Table 1) is linked to the rise in the state density when adding CuCl2 salt to the host polymer [32,33]. Fig. 8. The relation between (αhν )1/2 (a), and (αhν )2 (b) with the incident photon energy (hν ).

2.3.6. Optical limiting properties of CuCl2-doped PVA polymeric films The optical limiting (OL) characteristic, of the pure PVA and CPVA films, was investigated through detecting the output and normalized powers (Output power per the input power) as a function of CuCl2 concentrations using different laser sources of wavelengths (i.e. 533 nm and 632.8 nm). Fig. 10(a &b) show the influence of the doping level on the OL behavior of the CPVA samples. At a low percentage of CuCl2, the films have a small effect to attenuate the laser power (Fig. 10a). On the other hand, a meaningful decrease in the laser intensity occurred by the other samples. CPVA5 film has the capability to reduce the powers of 632.8 nm and 533 nm to approximately 40% and 47%, respectively (Fig. 10b). This gives information about the key role of the wt.% of CuCl2 in PVA to attenuate the visible light i.e the increment of the wt.% causes a decrement of the output power and consequently an improvement of the OL of the polymeric films. This can be due to the agglomeration of molecules (as seen SEM), which is related to the further wt.% of filler in the PVA, and their contribution in the interaction between the sample and the laser light [34]. The difference in the OL behaviors of CPVA composite samples arose due to their different response to the type of the incident laser. The sensitivity of the samples to 632.8 nm laser beam is highly incomparable to that of 533 nm which related to the transmittance performance of the CPVA film. From our previous work [21], it was noted that the reduction in the laser power (OL) depends principally on the transmittance values of the films at a similar wavelength of the beams. The closer of the transmittance to zero, more blocking and attenuation of the power was detected, and thus, the polymeric samples can be utilized as a filter. For that reason, these results forced that the PVA films with a high concentration level of CuCl2 can be used as a UV protector and OL/CUTOFF for visible laser beams.

Thus, with the further addition of CuCl2 in PVA, the band gap reduced and a dramatical change was caused in the dielectric characteristic of PVA to a semi-conductive material as previous reports for PVA films with fluorescein sodium salt [4]. However, the complex of electronic transition and charge transfer are not clearly recognized in conductive/semiconducting polymers [29]. The transitions take place by providing the needed momentum and energy from phonon and photon, respectively. The previous study has shown that Tauc's model and optical dielectric loss can be used, respectively, to predict the electronic transition and band gap [30]. This is because the dielectric function strongly depends on the structure of the material band. Hence, a more accurate description of this relationship requires an adequate view of quantum physics of dielectric functions. This is because of the complex dielectric, ε*, explains the material electronic response to an electromagnetic wave. However, it is tough to anticipate whether the structure of the band is a direct or an indirect sort, as mentioned previously [31], from the Tauc's equation. The imaginary part, ε′ ′, in the complex dielectric, ε*, can represent the real transition through the unoccupied and occ states (i.e. ψkc and ψkυ wave functions, respectively) using:

ε′ ′ =

4π 2e 2 m2ω2V

∑ υ, c , k

|< ψkυ | → |ψkc >|δ (Eψkc − Eψkυ − pi

hω ) 2π

(3)

It is clear from Eq. (3) that there is a correlation between the dielectric loss, ε′ ′, and the material band (Eψkc − Eψkυ ). The complex functions of the dielectric, which are linked to measurable parameters, like the extinction coefficient and refractive index, can be computed through easy equations. The optical dielectric loss against the photon 262

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Fig. 11. (a &b): (a) The dielectric permittivity ε′ and (b) dielectric loss ε″ versus frequency for different CPVA polymeric composite films.

Fig. 10. (a &b): (a) The output and (b) the normalized power by two sources of laser with wavelengths of 632.8 and 533 nm for the CPVA polymeric films as a function on CuCl2-concentrations.

be described due to the interaction influence between the doping and the chains of PVA. Similar behavior was also observed for ε″ as its values were strongly influenced by the doping concentration at different frequencies (the loss decreases with increasing the wt.% of CuCl2), as seen in Fig. 11b. These results suggest that conducting structural entities related to CuCl2 molecules operate effectively under various frequencies applied. The electrical conductivity of the materials represents the link between the macroscopic measurement and the ion movements in the microstructure. Therefore, the AC electrical response was studied to (1) determine the responsibility for the process of conduction, (2) obtain more information about the electrical properties of the investigated samples. Over a wide region of frequencies, the DC electrical conductivity (σDC), the total AC electrical conductivities (σtotal), and the AC electrical conductivities (σAC) for all CPVA polymeric films were tested according to the equations:

2.3.7. Dielectric and AC electrical conductivity analysis of CuCl2 doped PVA polymeric films It is well known that the internal structure and consequently the polarizations that are induced due to the effect of an external AC electric field, provide an obvious role in the dielectric behavior of materials [35]. Therefore, significant information about the physiochemistry characteristic of the dielectric and electrical can be obtained from the complex permittivity. As the polarization type in the dielectric materials has much influenced by the applied frequencies, Fig. 11(a&b) showed the frequency dependence of real part dielectric permittivityε′and dielectric loss ε″for pure and doped PVA composite films, respectively. Their values were estimated with the help of the expressions:

ε′ =

C (F ). X (m) εo (F . m−1). A (m2)

(4)

σDC = ε″ = tan δ × ε′

(5)

X RA

σtotal . AC =

where all symbols here are well recognized [36]. It is clear that ε′(Fig. 11a) and ε" (Fig. 11b) are depending on both the CuCl2 contents in the composite films and the applied frequency. At the lower frequencies, ε″for all samples are gradually decreased with small wt.% of doping level. This decreasing may be due to the effect of the DC electrical conductivity which causes a decrease in polarization. However, at high frequencies, their values are increased which show the contributions of the dipolar and atomic polarization. Moreover, the magnitude of ε′ decreased with raising the wt.% of CuCl2 in PVA matrix, that it can

(6)

X ZA

(7)

σtotal . AC = σAC + σDC

(8)

σAC = σtotal − σDC

(9)

Bω s

(10)

σAC =

where Z is the modulus impedance, R is the measured resistance, B is constant, s is the frequency exponent, ω is the angular frequency and A is the effective area of the samples in m2. The result shows that the σAC 263

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Fig. 12. Frequency dependence of electrical conductivity, σac , measured at RT for CPVA polymeric films with various CuCl2 content. Inset: The variation of the frequency exponent (s) with CuCl2 contents.

spectra present a plateau regime and increased with increasing the measured frequencies from 8.05 × 10−6 S m−1 and 6.18 × 10−8 S m−1 to 0.037 S m−1 and 0.0045 S m−1 for pure PVA and CPVA5 studied samples, respectively, as well as they follow the Jonscher's power law [37]. This explained the reduction in the degree order (crystallinity) of the CPVA composite films. The inset of Fig. (12) presents the exponent frequency (s), that deduced from the fitting data, versus the wt.% of CuCl2 contents in PVA. It is clear, from the inset of Fig. (12), that the value of s > 1, which correlated to the ionic conduction mechanism. A similar result was reported in a new approach of the universal dynamic power-law inside the disorder materials, ionic crystal, ion-conducting glasses [38]. Fig. 13. (a &b): (a) I–V and (b) ln I-ln V plots for different concentrations of CPVA polymeric composite films.

2.3.8. Non-linear I–V characteristic curves of CuCl2-doped PVA polymeric films One of the functional compensation elements in electronic circuits, to protect the other elements from excessive transient biasing or for optimal operating, is a voltage-dependent resistor (varistor). Fig. 13(a & b) showed the typical nonlinear current (I) – voltage (V), and lnI- lnV characteristic curves, respectively, for all as-prepared polymeric composite films. The PVA film has a linear relation. However, for doped PVA samples, the non-linear behavior was observed. In addition, at low biasing, the I - V relation is linear and obey the Ohm's law. Above the 100 V, the resistance decreases rapidly, and the conduction follows the empirical power law (I α Vp). The exponent P was estimated from the ln I – ln V slope of each part in the curves of pure PVA and CPVA films, in order to determine the degree of nonlinearity and responsibility for conduction mechanism. It is observed that there are three different regions existed and indicated as the region (I), (II), and (III). The values of P in the region (I) are close to unity (Table 1), i.e. follow the Ohm's law, where the thermally activated carriers are dominated rather than doping or injected one [39]. On the other hand, there is an exponential increase of the electrical current (P larger than 3) in the region (II), which indicate the transport mechanism that can be related to the current of space charge limiting (SCLC) resulting from the exponential distribution of the localized traps in the energy band gap. However, most of the transports in the region (III) are completely occupied with the injected carriers [40]. As the conduction followed by the production of a field impeding further carriers due to the SC accumulation near the electrode. This behavior shows that the PVA flexible films doped with CuCl2 can be used as a varistor to protect the damaging of the electronic elements in the electronic circuits. A similar performance was founded at high wt.% of rare-earth ions (such as La3+- and Er3+) doped PVA films [ [41,42]].

3. Conclusions Flexible and low-cost thick films for optical limiting and varistor devices made from CuCl2-doped PVA by a facile, famous and cheap casting method. XRD and FTIR studies provide a decrement in the degree of crystallinity by increasing the wt.% of CuCl2 salt from 0.037 wt % to 3.7 wt%. The shift in the temperature of the glass transition of the DTA curve with raising the CuCl2 content is due to the increment of steric and branching effects in PVA molecules. The doping concentration plays a key role in the absorbance and the energy gap values of the as-prepared films. Due to the absorption property of the samples within the UV–vis region, the PVA films with a high level of Cu2+-ions can be used as a promising material for excellent UV–Vis optical limiting and UV-protector. Moreover, the dielectric constant, dielectric loss and AC electrical conductivity of the PVA films are also influenced by the doping level. The magnitude of σac follows the power law of Jonscher's. lnI- lnV characteristic curves have three distinguished regions. The values of P in the first region (I) follow the Ohm's law (close to unity), and larger than 3 in the second region (II) due to SCLC, while it decreases again to less than 2 in the region (III). This behavior is similar to the characteristic of varistor materials. So, the PVA film with a high fraction of CuCl2 can be considered as a promising sample for different applications, as low-cost UV protection, UV–Vis optical limiting, and an electronic varistor device.

Conflicts of interest The authors declare that there is no conflict of interest in the current 264

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article. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

[10] [11] [12] [13] [14]

Authors states that

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1 There is no conflict of interest in the current article 2 The work described in the present article has not been published previously 3 It is not under consideration/submitted for publication elsewhere. 4 Its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out. 5 That, if accepted, it will not be published elsewhere including electronically in the same form, in English or in any other language, without the written consent of the copyright-holder 6 All authors have checked and approved the final version of the manuscript and have agreed to the submission 7 The article is the authors' original work, hasn't received prior publication and isn't under consideration for publication elsewhere.

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Acknowledgment

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The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through the research group's program under grant number R.G.P.2/13/39.

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