WITHDRAWN: Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors

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Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors

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_ Uslu d S. Demirezen a,⇑, A. Kaya b, S.A. Yerisßkin c, M. Balbasßı c, I.

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a

Department of Physics, Faculty of Sciences, Gazi University, Ankara, Turkey Department of Opticianry, Vocational School of Medical Sciences, Turgut Özal University, Ankara, Turkey Department of Chemical Engineering, Faculty of Engineering, Gazi University, Ankara, Turkey d Department of Chemistry, Faculty of Science and Arts, Gazi University, Ankara, Turkey b c

a r t i c l e

i n f o

Article history: Received 2 November 2015 Accepted 21 December 2015 Available online xxxx Keywords: Thin films Electrical properties Interface/interphase

a b s t r a c t In this study, praseodymium barium cobalt oxide nanofibers interfacial layer was sandwiched between Au and n-Si. Frequency and voltage dependence of e0 , e0 , tan d, electric modulus (M0 and M00 ) and rac of PrBaCoO nanofiber capacitor have been investigated by using impedance spectroscopy method. The obtained experimental results show that the values of e0 , e0 , tan d, M0 , M00 and rac of the PrBaCoO nanofiber capacitor are strongly dependent on frequency and applied bias voltage. The values of e0 , e00 and tan d show a steep decrease with an increase in frequency for each forward bias voltage, whereas the values of rac and the electric modulus increase with an increase in frequency. The high dispersion in e0 and e00 values at low frequencies may be attributed to the Maxwell–Wagner and space charge polarization. The high values of e0 may be due to the interfacial effects within the material, PrBaCoO nanofibers interfacial layer and electron effect. The values of M0 and M00 reach a maximum constant value corresponding to M1  1/e1 due to the relaxation process at high frequencies, but both the values of M0 and M00 approach almost zero at low frequencies. The changes in the dielectric and electrical properties with frequency can also be attributed to the existence of Nss and Rs of the capacitors. As a result, the change in the e0 , e00 , tan d, M0 , M00 and ac electric conductivity (rac) is a result of restructuring and reordering of charges at the PrBaCoO/n-Si interface under external electric field or voltage and interface polarization. Ó 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

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Introduction

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The surface stability of semiconductors in the metal–semiconductor (MS) and metal–insulator–semiconductor (MIS)-type Schottky barrier diodes (SBDs) plays a very important role. When the thickness of interfacial layer is greater than 500 Å, MIS type SBD acts as MOS type capacitors. Due to some advantage of high dielectric materials in respect of traditional low dielectric SiO2 various high dielectric materials such as HfO2, TiO2 and Bi3Ti4O12 (BTO). Recently, the research on MIS-type SBDs, capacitors and supercapacitors has been intensely stimulated due to their some advantages such as high power density and faster charge/discharge process at longer cycle life [1–10]. In response to the fossil fuel energy depletion and global warming issues, clean and renewable energy storage has been becoming a great challenge. Therefore, the

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⇑ Corresponding author. Tel.: +90 312 2021276; fax: +90 312 2122279.

energy stored in supercapacitors is currently an order of magnitude lower than that of batteries [4–6,9]. Especially, ultra/super capacitors able to store a large amount of charge which can be delivered at much higher power ratings than rechargeable batteries [4,6]. Therefore, ultracapacitors can be used in a wide range of energy capture and storage applications and are used either by themselves as the primary power source or in combination with batteries or fuel cells. Ultra capacitors are provided with a class of materials with dielectric constants greater than 105 at low frequency (<102 Hz) [4,9]. The main goals are to fabricate ultrahigh capacitors with long life time, increase of the rated voltage and increase of both operating temperature and power densities. The history of capacitors is based on 250 years ago, but the first scientific study about them was done by Faraday. Capacitors based on titanium dioxide (TiO2) were available commercially around 1926. In 1941, barium titanate with a dielectric constant was discovered which is 20 times greater than TiO2 and 250 times greater than

E-mail address: [email protected] (S. Demirezen). http://dx.doi.org/10.1016/j.rinp.2015.12.010 2211-3797/Ó 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Demirezen S et al. Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors. Results Phys (2015), http://dx.doi.org/10.1016/j.rinp.2015.12.010

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SiO2. On the other hand, ceramic capacitors dielectrics have a high dielectric constant but a low breakdown field [9]. The performance and the reliability of electronic devices are dependent on various parameters such as the surface preparations, the homogeneity of interfacial layer and barrier height between metal and semiconductor, interface traps (Dit), series resistance (Rs), doping concentration of donor or acceptor atoms, sample temperature, frequency, polarization and applied bias voltage [11–20]. Among them Dit, Rs, interfacial layer and both its homogeneities and thickness have more effect on the capacitance–voltage (C–V) and conductance–voltage (G/w–V) characteristics [21–24]. It is well known that the forward and reverse bias C–V and G/w–V measurements only at one or narrow frequencies range cannot supply information on their electrical and dielectric properties. On the other hand, when these measurements were carried out in the wide frequency range can be supplied to us enough information both on electrical and dielectric properties and conduction mechanism [25–29]. AC impedance spectroscopy method which included C–V and G/ w–V measurements, allows the determination of the real and imaginary part of complex dielectric constants (e0 , e00 ), complex electric modulus (M0 ), loss tangent (tan cd) and rac at various frequencies and voltages at room temperature. It is well known, the polarization of a dielectric is contributed by the electronic, ionic and dipolar polarization and electronic polarization occurs during very short intervals of time and the process of ionic polarization occurs in the range of seconds, while the dipole polarization requires relatively a longer time. The dielectric material can be polarized by applying an external electric field or voltage that displaces the charge carriers from their equilibrium position. According to Debye theory , dielectric permittivity of free dipoles

Table 1 The values of solution components. Solution components Cobalt acetate (g)

Barium acetate (g)

Praseodymium acetate (g)

0.5

0.5127

0.8732

oscillating in an alternating field can be described as: (a) at low angular frequencies (w), period (T = 1/2pf) is very lower than the inverse of relaxation time (1/s) and dipoles can easily follow the external ac electrical field; (b) at intermediate frequencies, as the frequency increases the dipoles begin to lag behind the held and e0 slightly decreases; (c) at the characteristic frequency where x = 1/s, e0 drops sharply (relaxation process) and (d) at quite high frequencies (T  s), the dipoles can no longer follow the field and e0  e1 (the high frequency value of e0 )[8]. Usually, electronic polarization (ae) may be observed in very high frequencies (f P 1015 Hz), atomic or ionic polarization (aa) may be observed in the frequency range of 1010–1013 Hz, while dipolar or oriental polarizations (ao) usually sensitive in the frequency range of 1 kHz–1 MHz, and interfacial polarization (ai) may be sensitive to low frequencies or few kHz. Therefore, in our sample the last two polarization types easily affected the dielectric properties. The first aim of this is to get a high capacitor. For this purpose, the high value of interfacial layer (PrBaCoO nanofibers) was sandwiched between Au and n-Si. Therefore, the frequency and voltage dependence of the main dielectric parameters such as the real and imaginary parts of complex dielectric constant (e0 , e00 ) and, dielectric modulus (M0 and M00 ), tan d, and the ac electrical conductivity (rac) of the Au/PrBaCoO nanofibers/n-Si capacitors have been investigated in detail both as function of frequency and applied bias voltage using capacitance and conductance data in the wide frequency range of 1 kHz–1 MHz. These measurements were carried out under small ac signal of 40 mV amplitude from the external pulse generator, while the dc bias voltage was swept from (2 V) to (+3 V) at room temperature. The dielectric constant value of PrBaCoO nanofibers was found at about 1050 at 1 kHz which is almost 276 times greater than the dielectric constant value of conventional SiO2, and 21 times than the dielectric constant value of TiO2.

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Experimental procedure

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Au/PrBaCoO nanofibers/n-Si capacitors used in this study were fabricated on phosphor doped (n-type) Si wafer with (1 1 1) surface orientation, having 280 lm thickness, 200 diameter and 0.005 X cm resistivity. Before the fabrication of the capacitor, the n-Si wafer

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Fig. 1. Schematic cross-section of the PrBaCoO nanofibers capacitor and measurement system.

Please cite this article in press as: Demirezen S et al. Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors. Results Phys (2015), http://dx.doi.org/10.1016/j.rinp.2015.12.010

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Fig. 2. Voltage dependent (a) e0 , (b) e00 and (c) tan d characteristics of the PrBaCoO nanofibers capacitor at different frequencies.

Please cite this article in press as: Demirezen S et al. Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors. Results Phys (2015), http://dx.doi.org/10.1016/j.rinp.2015.12.010

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was degreased for 5 min. in boiling trichloroethylene, acetone and ethanol consecutively and then etched in a sequence of H2SO4 an H2O2, 20% HF, a solution of 6HNO3: 1HF: 35H2O, 20% HF. After that the wafer was rinsed thoroughly in de-ionized water of resistivity of 18 MX-cm for 10 min. After surface cleaning of n-Si wafer, immediately the high purity (99.999%) Au with a thickness of 1500 Å was thermally evaporated onto the whole backside of the n-Si wafer at a pressure of 2  106 mbar in a high vacuum metal evaporation system. To get obtain low resistivity ohmic back contact, n-Si/Au structure was annealed at about 450 °C for 30 min in flowing dry nitrogen ambient. This process served to sinter the Au on the back surface of the n-Si wafer. After that praseodymium barium cobalt oxide (PrBaCoO) nanofibers were grown in front of the n-Si wafer as follows: In order to perform PrBaCoO nanofibers, cobalt (II) acetate was supplied from Merck (Whitehouse Station, NJ). Poly (vinyl alcohol) (PVA) (Mw 85,000–124,000), barium acetate, and bismuth (III) acetate were supplied from Sigma Aldrich (St. Louis, MO). The deionized water was used in the study as a solvent. First, an aqueous PVA solution (10 pct) was prepared by dissolving the PVA powder in distilled water. It was heated at 353 K (80 °C) with stirring for 3 h, and then the temperature was reduced to room temperature. In the experiments, two hybrid polymer solutions were obtained. For each composition, the proper amounts of the bismuth (III) acetate, the barium acetate and the cobalt (II) acetate were added to 20 g of the aqueous PVA (10 pct w/w) solution at 333 K (60 °C). Each hybrid polymer solution was stirred vigorously using a magnetic stirring bar for 1 h at the same temperature. This process continued for 3 h at room temperature. Table 1 shows the values of solution components. The solution was then put into the capillary tube for the production of nanofibers via electrospinning. The electrospinning system have a plastic capillary tube, a direct current (DC) high-voltage power supply (ES 30P-20W/ DAM; Gamma High Voltage Research, Inc., Ormond Beach, FL), a metal collector, and a dosing pump. The polymer solutions were filled into a capillary tube and copper pins were connected to the power supply. The distance between the capillary tube and the metal collector was set to 15 cm and 18 kV was applied to the solution at a flow rate of 0.8 mL/h. After the formation of PrBaCoO nanofibers on n-Si wafer, electrospun samples were heat treated in vacuum atmosphere at 363 K (90 °C) nearly12 h. Finally a thin coating of the samples were achieved using high purity (99.999%) Au with a thickness of 1000 Å and 1.5 mm diameter dots were formed onto the heat treated n-Si wafer samples in high vacuum metal evaporation system at a pressure of 2  106 mbar. In this way, the fabrication process of Au/PrBaCoO nanofibers/n-Si capacitors was completed and its schematic diagram is given in Fig. 1. The metal thickness layer and the deposition rates were monitored with the help of quartz crystal thickness monitor. The interfacial layer thicknesses of PrBaCoO nanofibers were estimated to be about 5200 Å using stylus type profilometer. After the fabrication of the samples, they were mounted on a copper holder using silver paste and the electrical contact were made to the upper electrodes using thin silver coated wires with silver paste. The frequency dependence C–V and G/x–V measurements were performed in the frequency 1 kHz–1 MHz using an HP 4192 A LF impedance analyzer (5 Hz–13 MHz) and test signal of 40 mV rms. The measurement system is given in Fig. 1. All measurements were carried out with the help of a microcomputer through an IEEE-488 AC/DC converter card.

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Result and discussion

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In this study, the dielectric properties such as dielectric constant (e0 ), dielectric loss (e00 ), loss tangent (tan d), the real and imag-

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inary parts of electric modulus (M0 and M00 ) and ac electrical conductivity (rac) of PrBaCoO nanofibers capacitor have been investigated using the impedance/admittance spectroscopy method which includes C–V and G/w–V measurements in the wide frequency range of 1 kHz–1 MHz, at room temperature. The complex permittivity (e⁄) can be defined in the following form [22–24]; 

0

00

e ¼ e  je

C G ¼ j Co xC o

ð1Þ

where e0 and e00 are the real and the imaginary part of complex permittivity, j is the imaginary root of 1, C and G are the measured capacitance and conductance of the dielectric, Co is the capacitance

Fig. 3. Frequency dependence of the (a) e0 , (b) e00 and (c) tan d for the PrBaCoO nanofibers capacitor at room temperature.

Please cite this article in press as: Demirezen S et al. Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors. Results Phys (2015), http://dx.doi.org/10.1016/j.rinp.2015.12.010

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of empty capacitor (=eoA/di) and x is the angular frequency (x = 2pf) of the applied electric field. Thus, the frequency dependent real and imaginary parts of the e* (e0 and e00 ) for various bias voltages can be calculated using the values of C and G, respectively, from the following relations [22–28]:

e0

C Cdi ¼ ¼ C o eo A

ð2Þ

e00

G Gdi ¼ ¼ xC o eo xA

ð3Þ

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Here, A is the rectifier contact area of the capacitor in cm2, di is the interfacial nanofibers layer thickness and eo is the permittivity of free space charge (e0 = 8.85  1014 F/cm). In the strong accumulation region, the maximal capacitance of the capacitor corresponds to the interfacial layer capacitance (C ac ¼ C i ¼ e0 eo A=di ). The loss tangent (tan d) can be expressed as follows [25–28]:

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e00 G tan d ¼ 0 ¼ e xC

ð4Þ 0

00

Thus, the voltage dependent of e , e and tan d values of the capacitor were obtained from Eqs. (2)–(4) for various frequencies, respectively and were given in Fig. 2(a)–(c) at room temperature. These values were found to be a strong function of frequency and voltage. This dispersion in the e0 and e00 values in the inversion and depletion regions was attributed to the surface states localized at interfacial layer/semiconductor and surface polarization. On the other hand this dispersion in the accumulation region was attributed to the series resistance (Rs). Similar results observed are quite compatible with the different materials [27–31]. It is well known, the real values of these parameters are corresponding to enough high bias voltage. Such behavior of e0 , e00 can be attributed to the Rs of devices. Therefore, in order to see the effect of the applied bias voltage e0 , e00 and tan d values were also drawn (Fig. 3(a)–(c)) in the

Fig. 4. Frequency dependence of the (a) real (M0 ) and (b) imaginary (M00 ) parts of complex electric modulus (M⁄) of the PrBaCoO nanofibers capacitor at room temperature.

Please cite this article in press as: Demirezen S et al. Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors. Results Phys (2015), http://dx.doi.org/10.1016/j.rinp.2015.12.010

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voltage range of 1–3 V by 0.5 V steps. As can be seen in Fig. 2(a), the obtained value of dielectric constant (e0 ) of PrBaCoO nanofibers at 1 kHz is almost greater than 276 times from the dielectric constant value of conventional SiO2, 21 times greater than TiO2 and 5.25 times greater than ferro-electric (Bi3Ti4O12) interfacial layer. These results confirmed that the high value of capacitance can be obtained using appropriate high-dielectric materials between metal and semiconductor such as a high dielectric TiO2, Bi3Ti4O12 (BTO) instead of conventional low-dielectric constant interfacial insulator layer such as SiO2, SnO2 and Si3N4. In other words, in order to get a high/ultra-capacitor, it is important to increase the dielectric constant of interfacial layer (e0 ). In this way, using a high dielectric interfacial layer, the capacitance value of metal–oxide-s emiconductor (MOS) capacitor can be considerably increased and it can be used in a wide range of charges/energy capture and storage applications. The use of high dielectric interfacial layer does not only prevent inter-diffusion between metal and semiconductor, it also alleviates the electric field reduction issue in these devices. Experimental results show that e0 , e00 and tan d values decrease with a increase in frequency almost as exponential. Also, the decrease in e0 with the increasing frequency may be attributed to the decreasing polarization with the increasing frequency since Nss cannot follow the ac signal at high frequencies and then reaches a constant value due to the fact that beyond a certain frequency of external field the electron hopping cannot follow the alternative field [2,23]. Additionally, the dispersion in e0 and e00 especially at low frequencies can be attributed to Maxwell–Wagner [30] and space charge polarization [14,32]. As shown in Fig. 2(c), the value of tan d shows a peak behavior depending on e0 and e00 values. This peak behavior of tan d depends on a lot of parameters such as interface states density, series resistance, and the thickness of interfacial insulator layer [2]. High values of e0 may be due to the interfacial effects within the material, PrBaCoO nanofibers interfacial layer and electron effect [30–33]. The high values of e00 are also due to the motion of free charge carrier within the material. For the analysis of dielectric properties, the complex impedance (Z⁄) and complex electric modulus (M⁄) formalisms were discussed by various authors [27–30,34]. The analysis of the electric modulus as a function of frequency allows the detection of the presence of relaxation processes in materials. Also, this analysis of the complex permittivity (e⁄) data within the Z⁄ formalism (Z⁄ = 1/Y⁄ = 1/ixCoe⁄) is commonly used to separate the bulk and the surface phenomena and to determine the bulk dc conductivity of the material [34]. The complex impedance or the complex permittivity (e⁄ = 1/M⁄) data were transformed into the M⁄ formalism using the following relation [27–30]:

M  ¼ jxC o Z  ¼

1 

e

e0

00

¼ M0 þ jM ¼

e þe 02

002

þj

e00

e þ e002 02

Experimental results show that the values of M0 and M00 were found to be a strong function of applied bias voltage and frequency. In addition, to see the effect of applied bias voltage M0 and M00 values were also drawn (Fig. 5(a) and (b)) in the voltage range of

Fig. 5. (a) The real M0 and (b) the imaginary M00 parts of M⁄ vs frequency of the PrBaCoO nanofibers capacitor, for various applied bias voltages at room temperature.

ð5Þ

The real (M0 ) and the imaginary component (M00 ) of M⁄ were calculated from e0 and e00 by using Eq. (5). Fig. 4(a) and (b) shows the voltage-dependent values of the M0 and M00 curves of the PrBaCoO nanofiber capacitor in the frequency range of 1 kHz–1 MHz. As can be seen in Fig. 4(b) M00 –V plots have two distinctive peaks which are corresponding to the inversion and depletion regions, respectively. These two peak behavior in the M00 –V plots indicated that the PrBaCoO nanofiber capacitor exhibits relaxation phenomena [21,23,32]. On the other hand, first and second peaks, both, disappeared at high frequencies especially negative bias region because at low frequencies, almost all charges at interface states/traps can easily follow an external ac signal and they cause excess capacitance and conductance (Cex and Gex/x) to measured capacitance and conductance (Cm and Gm/w). In addition, the amount of tracking interface states decreases with the increasing frequencies and cannot contribute to the Cm and Gm/w values.

Fig. 6. Frequency dependent ac electrical conductivity of the PrBaCoO nanofibers capacitor for different bias voltages at room temperature.

Please cite this article in press as: Demirezen S et al. Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors. Results Phys (2015), http://dx.doi.org/10.1016/j.rinp.2015.12.010

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1–3 V by 0.5 V steps. As shown in Fig. 5(a) and (b), both the values of M0 and M00 increase with an increase in frequency. In addition, the values of M0 and M00 reach a maximum constant value corresponding to M1  1/e1 due to the relaxation process. Contrary to the high frequencies, both the values of M0 and M00 approach almost

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zero at low frequencies. Similar results were also reported in the

331

literature [14,15,35]. Such behavior dispersion in complex dielectric constant and electric modulus can be attributed to the fact that dielectric relaxation mechanisms were sensitive to frequency rather than applied voltage in these regions. The frequency-dependent ac electrical conductivity (rac ) of the PrBaCoO nanofiber capacitor, was obtained from the following equation [27,30,31] for various applied bias voltages at room temperature and are given in Fig. 6.

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rac

  d ¼ xC tan d ¼ e00 xe0 A

ð6Þ

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As can be seen in Fig. 6, while the value of rac increases with the increasing frequency, the values of e00 and tan d decrease. This rac contributes only to the dielectric loss, which becomes infinite at zero frequency and is not important at high frequencies. As the frequency increases, rac increases because the polarization decreases with the increasing frequency. An increase in the rac also leads to an increase of the eddy current which in turn increases the energy loss tan d. This result is also compatible with the literature, where it is suggested that the increase in rac with the increasing frequency is attributed to the gradual decrease in series resistance (Rs) [27– 30]. As a result, the change in the e0 , e00 , tan d, M0 , M00 and ac electric conductivity (rac) is a result of restructuring and reordering of charges at the PrBaCoO/n-Si interface under external electric field or voltage and interface polarization.

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Conclusion

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Frequency and voltage dependence of e0 , e0 , tan d, electric modulus (M0 and M00 ) and rac of PrBaCoO nanofiber capacitor have been investigated using impedance spectroscopy method (C–V–f and G– V–f measurements) at room temperature. C–V and G/w–V measurements were carried out in the wide frequency and applied bias voltage ranges of 1 kHz–1 MHz and (2 V)–(3 V), respectively. The obtained experimental results show that the values of e0 , e0 , tan d, M0 , M00 and rac of the PrBaCoO nanofiber capacitor are strongly dependent on both the frequency and applied bias voltage. From the analysis of the experimental results, it was concluded that the values of e0 , e00 and tan d show a steep decrease with the increasing frequency for each forward bias voltage, whereas the values of rac and the electric modulus increase with the increasing frequency. The dispersion in e0 and e00 values at

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low frequencies is considerably high at a low frequency which may be attributed to the Maxwell–Wagner and space charge polarization. The large values of e0 may be due to the interfacial effects within the material, PrBaCoO nanofibers interfacial layer and electron effect. On the other hand, the large values of e00 are also due to the motion of free charge carrier within the material. The values of M0 and M00 reach a maximum constant value corresponding to M1  1/e1 due to the relaxation process at high frequencies, but both the values of M0 and M00 approach almost zero at low frequencies. The changes in the dielectric and electrical properties with frequency can also be attributed to the existence of Nss and Rs of the capacitors. As a result, the change in the e0 , e00 , tan d, M0 , M00 and ac electric conductivity (rac) is a result of restructuring and reordering of charges at the PrBaCoO/n-Si interface under external electric field or voltage and interface polarization.

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Please cite this article in press as: Demirezen S et al. Frequency and voltage dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofibers capacitors. Results Phys (2015), http://dx.doi.org/10.1016/j.rinp.2015.12.010

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