Cesium Zincate

Article pubs.acs.org/JPCC

Opto-Electrical Characteristics of Poly(vinyl alcohol)/Cesium Zincate Nanodielectrics Nithin Kundachira Subramani†,‡ and Siddaramaiah*,‡ †

Postgraduate Department of Chemistry, JSS College, Mysuru-570 025, India Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysuru-570 006, India

‡

S Supporting Information *

ABSTRACT: Polymer nanocomposites for electro-optics are known to offer novel material morphologies and unique device geometries, thereby enhancing the device performance. In this study, we report the successful fabrication of one such blue-green fluorescent poly(vinyl alcohol) (PVA)/cesium zincate (Cs2ZnO2) nanocomposite by solution intercalation technique. The optoelectronic properties of prepared films were probed with the intent to establish the effect of nanointegrates on optical and electrical characteristics of particle stabilizing PVA. The optical absorbance studies revealed the UV absorbent nature of PVA/Cs2ZnO2 films exhibiting a steep UV absorption coupled with high visible transmission. The optical parameters of nanocomposite films, including absorption/extinction coefficients, optical band gap, complex refractive index (RI), and dielectric functions besides optical conductivity were evaluated, which supports the dopantdependent optical properties of PVA with a scope for band gap engineering. The dispersion and functionalization of nanofillers were characterized by FESEM. The integrated fillers induced a broad blue−green luminescence (2.88−2.58 eV) in the emission spectrum of PVA. The structural aspects were probed by FTIR studies, while charge transport properties were valued by dielectric studies. The dielectric properties (dielectric storage and dielectric loss), AC conductivity, and charge dissipation were found to increase with nanofiller content and decrease with frequency.

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INTRODUCTION Polymer nanocomposites are of particular interest in photonics and optoelectronics, owing to their excellent ability to harness unique material properties arising from the synergistic combination of opto-electrical effectiveness of the nanoinclusions, alongside admirable processability of the polymer systems. The unearthing of compounds with interesting material properties often opens up novel research pathways, eventually leading to new technology. A proper understanding of unearthed properties and their filler-dependent tuning may lead to hybrid devices with advanced properties. This is the central theme in smart material technology, which involves efforts to tune the material properties according to specific application requirements. This involves the simultaneous and perhaps synergistic tailoring of various properties, which can be nontrivial. Polymer nanocomposite technology is one such area, which utilizes the unique material properties arising from sizedependent material properties to design/develop and fabricate materials with unique physicochemical properties for specified applications. In recent years, polymer-encapsulated transparent semiconducting inorganic nanofillers have attracted a great deal of scientific and technological interest as advanced technological materials, especially in the field of photonics and optoelectronics.1−5 Polymer nanocomposites are also extensively employed in applications related to optical storage systems owing to their exceptionally high thermal stability and filler-dependent refractive index (RI).6 For applications related © XXXX American Chemical Society

to photonics and optoelectronics, these multifunctional advanced materials take advantage of filler-specific optoelectrical properties in addition to excellent matrix processability. However, for efficient harnessing of properties arising from a synergistic combination of filler−matrix interface, the size of the employed filler and its uniform distribution within the polymer matrix are the key factors to be addressed. It is well established that high dielectric polymeric materials with excellent dispersing abilities serve as excellent host matrices for metallic and semiconductor materials.7−9 Poly(vinyl alcohol) (PVA) has been widely employed in the fabrication of metallic nanocomposites due to its optical transparency and ease of processing. Moreover, the regular arrangement of −OH groups in its backbone, facilitates hydrogen bonding, which, in turn, accounts for the uniform dispersion and excellent weathering stability of embedded fillers. High optical clarity, followed by polarization response and dopant-dependent optical and electrical properties, renders PVA as a promising material for photovoltaic and optoelectronic devices.10,11 Metal oxides, owing to the wide range of morphologies in which the semiconductors have attracted considerable attention in applications relating to fabrication of nanoscale electronics, electroanalytical devices, selective Received: April 16, 2015 Revised: August 5, 2015

A

DOI: 10.1021/acs.jpcc.5b03652 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. FTIR reflectance spectra of PVA nanocomposite films with (a) 0, (b) 0.5, (c) 1.0, (d) 2.0, and (e) 4.0 wt % of Cs2ZnO2.

employing LCR meter to probe the effect of nanometal oxide addition on the dielectric properties of the PVA matrix.

detection of metal ions, optoelectronics, and many such related sectors.12,13 However, for most optical applications, the polymeric matrix reinforced with highly photostable UV absorbent metal oxides are vital. One among the most promising materials is the nano-ZnO; it is an intrinsic semiconductor with a bulk band gap energy around 3.4 eV, a large exciton binding energy, possesses polar surfaces, and is biocompatible. Due to their unique properties, ZnO nanoparticles have been increasingly investigated as potential agents for applications related to solar cells, sensors, nanogenerators, and so on.14,15 Although ZnO is intrinsically an n-type semiconductor, its electrical and electro-optical properties may, however, be tuned through controlled doping. The doping of ZnO with cesium was found to enhance the electrical conductivity, in addition to optical transparency, in the visible region. Moreover, the excellent electrical and optoelectrical properties of cesium-doped zinc oxide (Cs2ZnO2), in addition to its hydrophilic nature, supports its possible utilization as an “effective additive” to fine-tune the material properties of PVA matrix. Owing to this unique combination of filler-dependent material properties, in addition to excellent processability, polymer nanocomposites are visualized as promising materials for various device applications. Particularly for photonic and optoelectronic devices, appropriate understanding of optical transitions and charge transport mechanisms are exceedingly crucial. Thus, in the present study we endeavor our contribution toward evaluating the effect of Cs2ZnO2 nanofiller incorporation on the optical and electrical behavior of PVA, aiming to understand the nature of charge transport prevalent in these materials and interlink it with the optical behaviors. The present investigation focuses on the synthesis, fabrication, and optoelectrical properties of new luminescent nanocomposites obtained by dispersing cesium-doped zinc oxide (Cs2ZnO2) nanoinclusions in a particle stabilizing PVA matrix. The influence of Cs2ZnO2 nanoparticle addition and their varying weight fractions on optical and structural characteristics of PVA films have been analyzed by ultraviolet (UV) and Fourier transform infrared spectroscopy (FT-IR) techniques. The morphological characteristics of PVA/ Cs2ZnO2 nanocomposites were analyzed by scanning electron microscopy (SEM). The electric properties of PVA/Cs2ZnO2 nanocomposites such as dielectric constant (ε′), dielectric loss (ε″), electrical conductivity (σ), real and imaginary modulus (M′, M″), and dissipation factor (tan δ) were evaluated by

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EXPERIMENTAL SECTION Materials. The materials used for the preparation of Cs2ZnO2, zinc nitrate hexahydrate (Zn(NO3)2·6H2O), cesium nitrate (CsNO3), and sugar were procured from M/s. Loba Chemie, M/s. Nice Chemicals, India and M/s. SD Fine Chemicals, India, respectively. PVA (mol wt 85−125 K) was obtained from SD Fine chemicals, India (86−89% hydrolyzed). Solution preparations and film casting were done using doublydistilled water. Synthesis of Cs2ZnO2 Nanoparticles. Cesium-doped zinc oxide nanoparticles were synthesized by solution combustion (SC) technique. The SC technique is a simple, versatile, and rapid process that permits effective synthesis of a variety of highly pure and homogeneous nanosized materials.16 Solution combustion involves an exothermic reaction initiated by heat that becomes self-sustaining within a certain time interval, resulting in fine powders.17,18 The desired nanometal oxides were synthesized with Zn(NO3)2·6H2O and CsNO3 as oxidants and commercial sugar as fuel. A total of 4.76 g cesium nitrate, 6.93 g zinc nitrate, and 2.70 g sugar were dissolved in 125 mL of distilled water. The resulting mixture was then placed on a hot plate (>100 °C), which underwent auto ignition, leaving behind fine powder. The powders were then annealed at 600 °C using a muffle furnace. Fabrication of PVA/Cs2ZnO2 Nanocomposite. PVA/ Cs2ZnO2 nanocomposite films were prepared by solution casting technique. Aqueous PVA solution (7.5 wt %) was prepared using 500 mL of distilled water under constant stirring for 2 h, with the mixture being heated to 90 °C using a water bath to prevent thermal decomposition of the polymer. At this temperature, PVA completely dissolves, resulting in a clear viscous solution. The viscous solution was brought down to room temperature and stirring was continued using mechanical stirrer at 250 rpm for 3 h to ensure homogeneous distribution. The prepared Cs2ZnO2 nanosolids were dispersed in PVA solution, and the contents were ultrasonicated at around 85 °C for 30 min to ensure uniform filler distribution in the polymeric matrix. The homogenized solution was then poured into a clean glass molds and dried at room temperature. The PVA nanocomposite films have been casted with various amounts, namely, 0.5, 1.0, 2.0, and 4.0 wt % of Cs2ZnO2. The nanocomposite films thus obtained were then post cured at B


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Figure 2. SEM photomicrographs of PVA/Cs2ZnO2 nanocomposites with (a) 0, (b) 1.0, (c) 2.0, and (d) 4.0 wt % of Cs2ZnO2.

around 3500−3000 cm−1 corresponding to the O−H stretching vibrations of PVA backbone.19−21 The peak around 1728−1704 cm−1 is due to the residual acetate groups of partially hydrolyzed PVAc. The absorption band occurred around 2924−2908 cm−1 corresponds to asymmetric stretching mode of −CH2 group and the peak at 1442−1434 cm−1 corresponds to C−H bending of −CH2 group in the PVA backbone. A band at 1258−1242 cm−1 corresponds to wagging of −CH2 group. The substantial shift in the peak position and intensities corresponding to CO (crystalline) and C−O−C stretch vibrations of PVA indicates a positive interaction between −OH groups of PVA and nanofiller. The appearance of an additional absorbance band at 580−560 cm−1 in the IR spectra of PVA/Cs2ZnO2 nanocomposite films may be attributed to metal−oxygen stretching, which in turn justifies the existence of Cs2ZnO2 in PVA matrix.22 The shift in acetyl C−O−C stretching of PVA also supports the existence of chemical interaction. Furthermore, the narrowing of the −OH stretching band with an increase in dopant concentration supports the strong tendency of −OH groups of PVA to form chargetransfer complex with metal oxide nanofillers through chelation. Field Emission Scanning Electron Microscopy Analysis. The degree of dispersion and compatibility of metallic nanofiller with PVA was assessed by the field emission scanning electron microscopy (FESEM). The FESEM photomicrographs of PVA films with (a) 0, (b) 1.0, (c) 2.0, and (d) 4.0 wt % of Cs2ZnO2 nanoparticles are shown in Figure 2. The FESEM photomicrographs of nanocomposite films exhibit increased roughness as compared to the image of plain PVA, indicating the existence of Cs2ZnO2 nanoparticles in PVA matrix. The

around 80 °C for about 3 h. The thickness of the prepared polymer composite films varied from 0.25 to 0.30 mm. Techniques. Fourier transform infrared (FTIR) spectra of PVA/Cs2ZnO2 nanocomposite films were recorded in the attenuated total reflection (ATR) mode using JASCO 4100 spectrometer, Japan, at 2 cm−1 resolution in the wavenumber range 400−4000 cm−1. The morphological behaviors of nanocomposites were recorded by scanning electron microscopy (SEM) model ZEISS200, U.K., at an operating voltage of 10 kV. UV−visible spectra of nanocomposite films were recorded employing Schimadzu spectrophotometer, model 1800, Japan, and a photoluminescence spectrum was obtained on Hitachi F-4600 fluorescence spectrophotometer, Japan. For the electrical characterization of nanocomposites, highfrequency LCR meter (Wayne KERR model: 6500P) was employed in the frequency range 20 Hz to 5 MHz. The tests were carried out at a constant voltage of 1 V and at room temperature.

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RESULTS AND DISCUSSION Fourier Transform Infrared (FTIR) Analysis. Fourier transform infrared (FTIR) spectra of pristine PVA and its nanocomposite films were recorded in ATR mode to probe the nature of interactions between nanosized Cs2ZnO2 filler and water-soluble PVA matrix. The FTIR spectra of PVA and its nanocomposite films are shown in Figure 1. The PVA obtained as a hydrolytic derivative of polyvinyl acetate (PVAc) shows IR characteristic peaks of both −OH groups and residual acetate groups of unhydrolyzed PVAc. The IR spectrum of PVA and its nanocomposite films exhibit a strong and broad IR band C


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Cs2ZnO2 nanoparticles addition on the UV absorbance peak intensity of PVA is tabulated in Table 1.

excellent uniform dispersion of nanofillers in PVA matrix may be attributed to the presence of −OH groups in the polymeric backbone. Furthermore, the excellent adhesion and strong interfacial bonding between Cs2ZnO2 nanofillers and −OH groups of PVA matrix also accounts for the better physical interaction between matrix and filler interface, thereby offering excellent dispersion in the polymer matrix. Optical Behaviors. During the last few decades, a great deal of research emphasis has been laid on developing photoprotective materials, especially organic UV absorbing pigments embedded in a polymeric matrix.23,24 However, optically active organic UV absorbents suffer from low photostability and require protection from weathering effects. Metal oxide semiconductors having high photo stability in addition to excellent absorption in the UV region are considered to be effective optical additives.25 Thus, in the present work an attempt is made to probe the influence of dispersing Cs2ZnO2 nanoparticle on the optical properties of PVA system. Accordingly, PVA containing 0, 0.5, 1.0, 2.0, and 4.0 wt % of Cs2ZnO2 loaded films were subjected to UV− visible analysis in the transmittance, absorbance and reflectance modes. The UV spectrum of the films recorded in the absorbance mode is shown in Figure 3. As can be noticed, the

Table 1. Effect of Cs2ZnO2 Content on the UV Absorbance Peak Intensity of PVA Matrix peak intensity at different wavelength (λ) nm wt % of Cs2ZnO2 in PVA

266

279

376

0.0 0.5 1.0 2.0 4.0

0.42 0.58 0.61 0.69 0.73

0.44 0.59 0.62 0.71 0.76

0.23 0.63 0.66 0.71 0.74

Effect of Annealing Temperature on Optical Absorption. Thermal annealing is regarded as a secondary process that is expected to favor the crystal packing, which in turn enhances the degree of crystallinity resulting from structural reorganization. In recent years, polymer crystallization behavior near inorganic surfaces is being extensively studied.29 Furthermore, previous studies have revealed that exposure of polymer films to high energy radiation induces changes in the structure and, hence, optoelectrical properties of the polymeric system.30 In the present investigation, the fabricated nanocomposites were annealed at 90, 120, and 150 °C for a static period of 2 h. The effect of annealing temperature on the optical properties of PVA films was studied and the variation of optical absorption of 4% doped PVA/Cs2ZnO2 nanocomposite films is depicted in Figure 4. From the figure it is clear that the absorption in the

Figure 3. UV−visible absorbance spectra of PVA nanocomposites with (a) 0, (b) 0.5, (c) 1.0, (d) 2.0, and (e) 4.0 wt % of Cs2ZnO2.

absorbance spectrum of PVA film reveal a shoulder like peak around 270−280 nm. The absorption band in this region may be assigned to carbonyl groups associated with ethylene unsaturation and serves as an indicator for the presence of conjugated double bonds of polyene.26 Incorporation of Cs2ZnO2 resulted in an additional peak centered at around 376 nm attributing to the formation of charge transfer complexes (CTCs) between Cs2ZnO2 nanoparticles and −OH groups of PVA.27 The appearance of a new peak on doping and relatively broadening of these peaks indicate a considerable interaction between incorporated filler and PVA matrix.28 Furthermore, the absorbance of PVA films increased with an increase in dopant content, exhibiting a maximum absorbance for 4 wt % doped film, however, there is not much variation in the position of absorption bands, indicating a better distribution of the nanoparticles in the PVA matrix. The observed low absorbance/high transparency in the visible range (>400 nm) and a steep absorption in the UV region of the spectrum indicate the possible applications of PVA/Cs2ZnO2 nanocomposite films as UV protective materials. The effect of

Figure 4. UV−visible absorbance spectra of 4 wt % Cs2ZnO2-doped PVA nanocomposite annealed at different temperatures for 2 h.

investigated range of wavelengths (225−750 nm) increases with an increase in annealing temperature from 90 to 150 °C. The observed increment in absorbance values with increase in temperature may, however, be attributed to an increase in grain size that is associated with high thermal annealing of thin films.31 Assessment of Absorption Edge. The absorption coefficient (α) of fabricated films was calculated from the absorbance (A) values after correction for reflection using the relation α= D

⎛ 2.303 ⎞ 2.303 I ⎟A =⎜ log d Io ⎝ d ⎠

(1) DOI: 10.1021/acs.jpcc.5b03652 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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where, √B is a proportional constant, also known as Tauc’s constant and Eopt is defined as the energy band gap between the valence band and the conduction band. The plots of (αhν)1/2 as a function of photon energy (hν) yields a straight line. The extrapolation of linear portions of these curves gives the values of optical band gaps for the nanocomposites. The variation of optical band gap of PVA nanocomposites with varying amounts of nanofiller content is depicted in Figure 6a,b, and the

where, d is the thickness of the sample. The variation of absorption coefficient as a function of investigated range of photon energies is presented in Figure 5. The value of

Figure 5. Absorption coefficient vs photon energy of PVA/Cs2ZnO2 nanocomposites.

absorption edges of the fabricated films were calculated by extrapolating the linear portions of the curves obtained from α versus hν plots to zero absorption values. The obtained values of absorption edge for PVA films with different amounts of dopant content are presented in Table 2. The absorption edge Table 2. Optical Parameters of PVA/Cs2ZnO2 Nanocomposite Films band gap (eV) wt % of Cs2ZnO2 in PVA

Urbach’s energy (Eu; eV)

absorption edge (eV)

direct

indirect

0.0 0.5 1.0 2.0 4.0

0.48 0.53 0.57 0.75 0.83

4.80 4.72 4.62 4.52 4.40

4.59 4.47 4.42 4.25 4.00

4.50 4.37 4.30 4.20 4.12

Figure 6. (a) Tauc’s plot for the determination of indirect band gap. (b) Tauc’s plot for the determination of direct band gaps.

of undressed PVA lies around 4.8 eV (around 258 nm), while for doped ones, their positions are found to be displaced toward lower energy values, exhibiting a minimum of 4.4 eV for 4 wt % Cs2ZnO2-doped PVA film. The shift in absorption edge toward lower energy or higher wavelength may be correlated to the effective dispersion of nanofillers in the PVA matrix.32 A sharp increase in absorbance is observed near the fundamental absorption edge indicating good crystalline nature of the films.33 Evaluation of Optical Band Gap (Effect of Nanofiller Content). The design and modeling of optically functional materials is guided by an important quantity termed optical energy gap (Eopt). For optical transitions caused by photons of energy greater than the energy gap of the material under investigation, hν > Eopt, the densities of both the conduction and valence extended electronic states is assumed to depend on the square root of energy leading to an absorption coefficient, α, which depends on the photon energy as αhν =

B [αhν − Eopt]

calculated values of band gap for both direct and indirect band transitions are summarized in Table 2. The dependence of (hν)1/n and photon energy (hν) was plotted for the fabricated films using different values of n, and the best fit was obtained for n = 2. This indicates that the transition energy for electrons is indirect in K-space and interactions with photons are feasible. As can be seen from Figure 6a, the value of optical energy gap for undoped PVA film is around 4.75 eV, indicating that the material is an insulator. However, introduction of nanofillers resulted in a gradual decrease in band gap energies from 4.75 to 4.37 eV, with an increase in doping (Cs2ZnO2) levels from 0 to 4 wt %. The observed variation in band gap energies with increasing dopant content may be due to the formation of charge transfer complexes between Cs2ZnO2 nanofiller and −OH groups of PVA, as supported by FTIR analysis. The interaction of nanofiller with PVA may also lead to the formation of new molecular dipoles originating from structural defects induced after doping. The presence of highly electronegative oxygen in the metal oxide facilitates the formation of

(2) E


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The Journal of Physical Chemistry C hydrogen bonding with PVA, making the films highly capable of being polarized.34 The increase in concentration of Cs2ZnO2 nanofillers in PVA increases the number of molecular dipoles. Furthermore, doping with Cs2ZnO2 nanofillers may also introduce voids in the PVA matrix, creating localized sites between the highest occupied and lowest unoccupied molecular orbital energy bands, making lower energy transitions feasible. This decrease in band gap with increase in filling levels enhances the suitability of such materials for optoelectronic applications which demands band gap tunability. Effect of Annealing Temperature on Optical Band Gap. Optical band gap of nanocomposite films are found to be temperature dependent, which facilitates the tuning by varying the annealing temperature. The structural reorganization or molecular ordering is the temperature-dependent factor that affects the optical band gap. The variation of energy gap values of PVA nanocomposite films with annealing temperature is depicted in Figure 7. As can be noticed, the band gap values are

where, αo is a constant and Eu is Urbach energy, interpreted as the width of the tails of localized states corresponding to optical transition between the localized state adjacent to valence band and the extended state of the conduction band, which is found to be above the mobility edge. The width of the localized state is characterized by the slope of absorption edge, which eventually indicates the ordering of structure. It is usually believed that an increase in the concentration of the nanofillers bridges the gap separating the localized states, thereby lowering the potential barrier and facilitating the transfer of charge carriers.37 The Urbach’s energy (Eu) values of PVA nanocomposites were calculated as the reciprocal gradient of the linear portion of the plot of ln(α) versus E (Figure 8), which

Figure 7. (αhν)1/2 vs photon energy of PVA/4 wt % Cs2ZnO2 nanocomposite film annealed at different temperatures.

Figure 8. Urbach’s plot for PVA/Cs2ZnO2 nanocomposites as a function of photon energy.

found to decrease with an increase in annealing temperatures. The decrease in band gap may be a consequence of an increase in crystallite size associated with high temperature and improved crystalline structure resulting from structural reorganization of the polymer chain upon annealing. The decrease in energy gap with molecular ordering is based on effective mass approximation.35

gives a quantitative measure of defect states. Incorporation of nanometal oxides brings about structural changes in the host PVA matrix. As inferred from FTIR results, these structural changes induce localized states within the band tails of electronic states, which affect the optical and electrical transport properties of PVA matrix. The values of Eu are found to be monotonic with dopant content (Table 2) and shows a gradual increase with dopant level, indicating molecular ordering of nanocomposites leading to raise in number of charge trapping centers. Dielectric Studies and Optical Conductivity. Dielectric constant is an intrinsic material property that effects the propagation of electromagnetic radiation through matter. The rate of propagation of light through matter is, however, found to be dependent on the nature of the materials through which it propagates with rates decreasing with an increase in the values of the dielectric constant. The dielectric constant of the material is a complex quantity, which may be obtained theoretically as ε = εr + iεi. The real part (εr) of the dielectric constant of a polymer indicates the extent to which the velocity of light is reduced in a polymeric material, while the imaginary part (εi) indicates the energy absorption by the polymer from an electric field caused by dipole motion.38 The estimated values of real and imaginary parts of the dielectric constant indicate the optical loss factor that is the ratio of imaginary part

ΔEg =

2 (hν) ⎛ 1 1 ⎞ 1.76e 2 + ⎟− 2 ⎜ mh ⎠ ER 2R ⎝ me

coefficient at the photon energy below the optical gap (tail absorption) depends exponentially on the photon energy and obeys the Urbach relation36 given as

α = αoexp(E − Eu)

(3)

where, me and mh are the effective masses of electrons in the conduction band and holes in the valence band, respectively, E is the static dielectric constant of the material, and ΔEg is the change in the optical band gap. The first term in the above expression represents the localization energy and is found to be inversely dependent on the square of particle radius (R2), while the second term columbic energy has an inverse dependence. Hence, one can expect the lower energy optical transitions induced by a higher degree of molecular ordering or crystallization upon thermal annealing. Urbach Tail Analysis of PVA Nanocomposites. In case of solids, the absorption of photons could also occur for photon energies lesser than or equal to the energy gap, hν ≤ Eopt due to the presence of tail states in the forbidden gap. The absorption F

(4)


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The Journal of Physical Chemistry C to real part of the dielectric constant. The real part of the dielectric constant can be calculated by the following equation εr = n2 − k 2

(5)

The imaginary part of the dielectric constant is given by εi = 2nk

(6)

The variation of real part of dielectric constant (εr) is presented in Figure 9, and the obtained results at 376 nm are

Figure 10. Variation of optical conductivity of PVA/Cs2ZnO2 nanocomposites as a function of wavelength.

observed behavior may be due to a molecular ordering with an increase in conducting filler content.39 Effect of Cs2ZnO2 Nanoparticle Content on Refractive Index. The application of polymers in the field of optics and optoelectronics is often limited owing to their relatively narrow range of refractive indices (RIs). However, in recent years it has been established that the introduction of inorganic fillers with a wide range of RIs can result in polymeric composites with extreme RI, thereby increasing the application window of the resultant materials in the field of optoelectronics,40 and the effect is found to be maximum with nanosized inorganic fillers owing to their lesser density and higher surface area. Thus, in the present investigation, the effect of Cs2ZnO2 nanoparticle addition on the RI of the PVA films were studied and the variation is presented as a function of wavelengths in Figure 11,

Figure 9. Variation in the real part of the dielectric constant of PVA/ Cs2ZnO2 nanocomposites as a function of wavelength.

Table 3. Optical Parameters for PVA/Cs2ZnO2 Nanocomposite Films near Band Edge (376 nm) wt % of Cs2ZnO2 in PVA

n

k (10−4)

εr

σ (S cm−1)

0.0 0.5 1.0 2.0 4.0

1.72 1.87 1.90 2.19 2.21

0.52 1.26 1.28 1.34 1.53

6.12 4.21 3.21 2.49 2.32

0.82 1.78 1.83 2.18 2.27

tabulated in Table 3. The observed variation in the values of the dielectric parameter is influenced by the refractive index (n) and extinction coefficient (k). Thus, the observed trend in the values of the real and imaginary parts of the dielectric constant may be due to the formation of CTCs between the nanofiller surface and hydroxyl groups of PVA, thereby enhancing the molecular ordering of the resultant films. The PVA films with varied content of Cs2ZnO2 nanofiller were also studied for their optical conductivity (σ), since it is one of the powerful tools to evaluate the electronic states in materials. The optical conductivity of PVA/Cs2ZnO2 nanocomposite (Figure 10) was determined using the following relation αnc σ= (7) 4π

Figure 11. Refractive index dispersion of PVA/Cs2ZnO2 nanocomposite films.

and the corresponding RI values at 376 nm are also tabulated in Table 3. The obtained results clearly indicate an increase in RI values of PVA nanocomposite films with an increase in filler content. This increase of RI values of doped films in comparison to pure PVA may be explained using the predictions of Bhar and Pinto model41 developed through simulation of Lorimer’s theory. On applying this model to our investigation, the increase in RI of the composite may be attributed to the increased density of the films with an increase in filler content, which, in turn, leads to a continuous increase

The variation of calculated values of optical conductivity for PVA films with varying amounts of filler loading is addressed in Table 3. As can be seen, optical conductivity of the nanocomposite films is found to increase with an increase in dopant content, exhibiting a maximum conductivity of 2.27 Ω−1 cm−1 for PVA film containing 4 wt % of Cs2ZnO2. The G


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The Journal of Physical Chemistry C in the number of atomic refractions due to increase of the linear polarizability. The enhanced packing density of polymer composites with nanofiller inclusions may be owed to the formation of strong intermolecular bonding between the introduced nanofiller and the −OH groups of PVA through electrostatic interactions. Furthermore, RI is initially found to decrease with increase in wavelength (300−380 nm) and, finally, almost saturates for the nanocomposites, suggesting normal dispersion behavior above 400 nm, explained on the basis of dispersion theory. The computed values of extinction coefficient (k) of PVA films with varying amounts of nanofillers increases with the filler content (Figure 12). The sharp decrease

Figure 13. Fluorescence emission spectra of PVA/Cs2ZnO2 nanocomposite films containing 0, 0.5, 1.0, 2.0, and 4 wt % of Cs2ZnO2 nanoparticles. Inset: fluorescent microscopic image of PVA/4 wt % Cs2ZnO2 nanocomposite film exited at 380 nm.

possible interaction between Cs2ZnO2 nanofiller surface and −OH groups of PVA via hydrogen bonding. This observation indicates that PVA/Cs2ZnO2 nanostructured film has potential applications similar to the semiconductor nanoparticles, such as optoelectronic and sensor applications. Electrical Properties. The ever increasing demands of miniaturization in addition to increased functionality and high performance for microelectronic products has led to the development of conducting fillers incorporated polymer composites. The integrated filler may present various advantages, including improved functionality, higher component density, increased design flexibility, in addition to reduced unit cost. For embedded capacitor applications, materials with high dielectric constant (ε′) and low dielectric loss are desirable. One of the most promising materials in this regard is the nanosized inorganic conductive filler integrated polymer composites, which exhibit an exceptional increase in the dielectric constant close to the percolation threshold. Thus, in the present investigation, an attempt is made to design and develop PVA/Cs2ZnO2 nanocomposites with high dielectric constant and low dielectric loss, which is a prerequisite of materials for applications in high performance capacitors, conductive inks, and also as a polymeric gate insulator for organic thin film transistors (OTFTs).44,45 Dielectric Properties. The dielectric properties of PVA films were probed, aiming to study and understand the nature of storage and dissipation of electric energy in insulating materials. Dielectric properties of a material are complex functions of applied frequencies and are found to be dependent on material permittivity and conductance in addition to size, shape, and spatial arrangement of the constituents. Under an alternating electric field, dielectric parameter as a function of frequency is described by the complex permittivities:

Figure 12. Variation of extinction coefficient as a function of wavelength of PVA/Cs2ZnO2 nanocomposites.

in the values of extinction coefficient (375−475 nm) is due to free carrier absorption. The low values of extinction coefficient infers that the optical loss due to absorption or scattering is very minimum and also serves as an evidence of good surface homogeneity of fabricated films. The dopant-content-dependent RI of PVA/Cs2ZnO2 nanocomposite films support their possible applications in the field of optoelectronics. Optical Emission Characteristics. Luminescent polymerinorganic nanocomposites are being extensively studied as potential materials for photonic applications, owing to their unique tunable light emission properties.42 It is well established, that polymer composite films emitting in the visible region of the spectrum are considered as a medium of great potential, as they can provide stable sources of light for displays and illumination sources. Thus, to probe the possible applications of Cs2ZnO2 nanoparticle integrated high RI and transparent PVA nanocomposite films in lighting and display devices. The fabricated films with varied concentration of nanofillers were subjected to fluorescence measurements at an excitation wavelength of 380 nm, and the results obtained are presented in Figure 13. As can be seen, the spectrum/curve for pure PVA means nonemission. However, the fluorescence spectrum of PVA samples containing Cs2ZnO2 nanoparticles exhibited broad violet−blue−green emission resulting from the super position of photoluminescence of either component. The 2.81 eV/440 nm violet emission from PVA is found to overlap with 2.33 eV/530 nm green emission from Cs2ZnO2, making the visible emission broader.43 Furthermore, the emission intensity is found to increase with an increase in the nanofiller content exhibiting emission maximum at 470 nm for 4.0 wt % Cs2ZnO2. The enhanced emission may, however, be attributed to the

ε*(ω) = ε′(ω) − ε″(ω)

(8)

where, ε′ and ε″ are real and imaginary permittivities, the representative components of energy storage and loss. Dielectric Constant (ε′). The dielectric constant (ε′) of PVA nanocomposite films was determined from obtained values of capacitance, which is a quantitative measure of electric H


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The Journal of Physical Chemistry C charge that can be stored in a capacitor, and deduced by the expression ε′ =

Ct ε0A

(9)

where, t is the thickness of the dielectric layer, ε0 is the permittivity of free space, C is the capacitance, and A is the electrode area. The variation of dielectric constant (ε′) as a function of nanofiller content is presented in Figure 14. As can

Figure 15. Dielectric loss of PVA/Cs2ZnO2 nanocomposite films as a function of Cs2ZnO2 content and frequency.

observed moderate increase in dielectric loss with an increasing in Cs2ZnO2 content may be attributed to space charge polarization mechanism prevalent in heterogeneous systems, originating from the excessive polarized interface induced by Cs2ZnO2 nanofillers. In contrast to the changes in nanofiller content, the frequency changes have a profound effect on the dielectric loss, which decreases sharply with an increase in applied frequency. The higher values of dielectric loss at relatively low frequencies may possibly be attributed to the mobile charges within the PVA matrix. Moreover, higher frequencies result in a faster field reversal, leading to a diminished charge accumulation and, hence, decreased loss factor. Loss Tangent (Tan δ). The obtained values of dielectric constant (ε′) and dielectric loss (ε″) were employed to obtain the loss tangent (Tan δ) using the relation

Figure 14. Dielectric constant of PVA/Cs2ZnO2 nanocomposite films as a function of nanofiller content and frequency.

be seen, the dielectric constant (ε′) of the composites increased with an increase in the content of embedded fillers, which directs toward the enhancement of the charge storage capacity of the polymeric matrix. The ability of the dielectric materials to store energy is attributed to the field-induced parting and aligning of electric charges/polarization. In the case of multicomponent systems, such as polymer nanocomposites, the observed increase in ε′ may be attributed to the formation of mini-capacitor networks in the PVA/Cs2ZnO2 nanocomposite films with an increase in nanofiller content. An additional validation is the Maxwell−Wagner (MW) interfacial/ space-charge polarization arising in the insulator−conductor interface, owing to the accumulation of unbound charges at the interface of the constituents.46 Furthermore, the energy storage capacity of PVA/Cs2ZnO2 nanocomposites was found to diminish significantly with an increase in applied frequencies. The observed higher values of ε′ at relatively lower frequencies may be attributed to the fact that, lower frequencies favor higher relaxation times thus enabling the successful alignment of permanent and induced dipoles to align themselves leading to enhanced polarization. However, at higher frequencies, the translating charge carriers migrating through the dielectric are impeded by and trapped at the physical barriers, thereby slowing down the charge carrier migration and decreasing the dielectric constant. A similar trend is observed in a number of other polymeric systems, verifying the fact that polar materials exhibit high values of ε′ at lower frequency regions, with the values drastically dipping with an increase in applied frequencies. Dielectric Loss (ε″). Dielectric loss is a material property, that gives an indication of energy loss in the dielectric during AC operation. Figure 15 shows the plot of dielectric loss as a function of Cs2ZnO2 content at varied frequencies. The

ε″ = ε′ tan δ

(10)

where, δ is the phase angle between the electric field and the polarization of dielectric. The variation of dissipation factor, tan δ, as a function of Cs2ZnO2 loading, is shown in Figure 16. The loss tangent for PVA/Cs2ZnO2 nanocomposite films increased

Figure 16. Tan δ of PVA/Cs2ZnO2 nanocomposite films as a function of Cs2ZnO2 content and frequency. I


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The Journal of Physical Chemistry C with an increase in filler loading. The increase in filler content may lead to a decrease in interparticle distance, which eventually results in a reduction in the charge trapping sites at the interface. Electric Modulus. The conducting nature of materials in the frequency domain is more conveniently interpreted in terms of conductivity relaxation time (τ), using the representation of electrical modulus, M* = (1/ε*). As can be seen, electric modulus is defined as the inverse quantity of complex permittivity and may successfully be employed to probe the dielectric response of PVA/Cs2ZnO2 nanocomposites. The observed values of real part of electric modulus (M′) for PVA/Cs2ZnO2 nanocomposites is found to increase with frequency, which suggest that the measured dielectric properties are free from electrode polarization (EP) effect. Furthermore, the imaginary part of the electric modulus (M″; Figure 17) suggests that the recorded relaxation processes are

Figure 18. AC conductivity of PVA/Cs2ZnO2 nanocomposite films as a function of Cs2ZnO2 content and frequency.

increase in frequencies is a common phenomenon exhibited by disorder solids. The observed trend is in accordance to “ac universal law” and serves as strong evidence for charge migration via hopping mechanism. The application of higher frequencies allows the electron to get sufficiently energized and hop from one conducting cluster to another, leading to a sharp increase in electrical conduction with an increase in the frequency of applied field.

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CONCLUSIONS The Cs2ZnO2 nanofillers were successfully integrated into a transparent, particle-stabilizing PVA matrix by eco-friendly solution intercalation technique. The FTIR studies of fabricated films indicate the presence of Cs2ZnO2 nanoinclusion in PVA matrix and reveal a positive interaction between filler and matrix through hydrogen bonding. The UV−visible absorption studies unveil a steep absorption in the near UV region coupled with high transparency in the visible spectrum, supporting the possible application of PVA/Cs2ZnO2 films in UV shields. The energy requirements for optical transitions were found to decrease with an increase in filler content exhibiting a minimum of 4 eV for PVA/4 wt % Cs2ZnO2 film. The integrated fillers brought about an increase in the RI and optical dielectric constants of PVA films in addition to novel blue−green fluorescence (2.48−3.1 eV) with emission intensity increasing with an increase in filler content. The dielectric constant and dielectric loss of PVA/Cs2ZnO2 nanocomposites increase with the nanofiller content, while it decreases with an increase in applied frequencies. However, the AC conductivities of fabricated films increase with increasing filler loading and frequency while the dissipation factor increases with nanoinclusions and decreases with frequency. These results, therefore, suggest an approach to fabricate PVA/Cs2ZnO2 nanocomposites with a desired combination of electrical and optical properties to suit specific device operating environments. The obtained results also support the fact that the interaction of Cs2ZnO2 with PVA may provide ways of obtaining polymer nanocomposites with unique or enhanced optical and electronic properties. Thus, by suitably integrating different amounts of Cs2ZnO2 into a transparent matrix, new applications may be realized without losing the benefits offered by polymers in terms of processing, scalability, and mechanical flexibility.

Figure 17. Imaginary part of electrical modulus (M″) of PVA/ Cs2ZnO2 nanocomposite films as a function of nanofiller content and frequency.

owed to the integrated nanoinclusions in the PVA matrix. The existence of interfaces in the PVA/Cs2ZnO2 nanocomposites gives rise to space-charge (interfacial) polarization leading to the Maxwell−Wagner−Sillars (MWS) effect. AC Conductivity. Electrical conductivity of PVA/Cs2ZnO2 nanocomposite films were probed to study and understand the nature of charge transport prevalent in these heterogeneous systems. AC conductivity of PVA nanocomposite films (σac) is a frequency-dependent material property that arises from the movement of charge carriers through the polymeric matrix. The measured values of conductance were employed to determine the AC conductivity using the relation47 Gs (11) A where d is the film thickness, A is the effective cross-sectional area, and Gs is measured conductance. Figure 18 exhibits a moderate increase in electrical conduction with the filler composition owing to the electronic interactions in PVA/ Cs2ZnO2 nanocomposite films that induces a continuous conducting channel that increases with an increase in filler content. Furthermore, the integration of Cs2ZnO2 nanofillers bridges the gap separating the localized states and lowers the potential barrier, thereby facilitating the transfer of charge carriers. The observed increase in conductivities with an σac = d

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ASSOCIATED CONTENT

S Supporting Information *

UV−visible transmittance spectrum of PVA films containing varied concentrations of cesium zincate nano fillers (Figure S1), annealation effects on UV−visible transmittance intensities (Figure S2), and refractive indices (Figure S3) of PVA/Cs2ZnO2 nanocomposite films. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03652.

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(PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge University Grants Commission (UGC), Govt. Of India, and Vision Group of Science and Technology (VGST), Govt. Of Karnataka, for financial assistance.

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Cesium Zincate

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