Structural, electrical and dielectric properties of CNT doped SeTe glassy alloys

Materials Chemistry and Physics 177 (2016) 455e462

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Structural, electrical and dielectric properties of CNT doped SeTe glassy alloys Mohsin Ganaie, M. Zulfequar* Department of Physics, Jamia Millia Islamia, New Delhi 110025, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 August 2015 Received in revised form 9 February 2016 Accepted 14 April 2016 Available online 23 April 2016

This paper describes the preparation of multi-walled carbon nanotube chalcogenide glasses alloys [(Se80Te20)100-x(CNT)x] (x ¼ 2 and 10) by melt quenching technique and were characterized with XRD, SEM, Raman, electrical and dielectric measurements. An XRD measurement reveals the amorphous nature of the prepared samples. The SEM and Raman study confirm the presence of CNT in SeTe alloy. The diffused prime Raman bands (G and D) have been appeared for MWCNT/SeTe glassy alloy. The current versus voltage (IeV) characteristics and DC conductivity measurement were carried out. The rapid increase in electrical conductivity by several order of magnitude from 1010 U1 cm1 to 103 U1 cm1 for 2% and 10% MWCNT/SeTe chalcogenide semiconductor were observed. Such significant increase in electrical conductivity for CNT doped glassy alloy may be due highly conducting nature of CNT. The effect is explained on the basis of conductive path which create percolation network in MWCNT/SeTe glassy alloys. The dielectric parameters were studied in the temperature range of 300 e370 K and in the frequency range of 500 Hze1 MHz. Dielectric dispersion are observed and the results are explained on the basis of dipolar type of dielectric dispersion and DC conduction losses. © 2016 Elsevier B.V. All rights reserved.

Keywords: Amorphous materials Electron microscopy (SEM) Raman spectroscopy and scattering Dielectric properties

1. Introduction In the search for novel functional materials with excellent optical and electrical properties chalcogenide amorphous semiconductor play a key role. These alloys draw a lot of attention, and various efforts has been made to tune its applications for future prospects in various field of electronic and optoelectronic devices. The chalcogenides in their glassy states become more flexible, and their property can be modified by inducing many external effects such as light, other radiations, field, particle, heat, pressure etc. Undoped chalcogenide semiconductor usually have p-type conductivity [1]. Alloying with certain additives like CNT, Te, S, Cd, Zn [2e5] has produce characteristic effect on Solid state electronics and optical devices such as solar cells, switching memory, sensors, thermal imaging, photoconductors, laser diodes, integrated fiber optics etc., [6e8]. Selenium based chalcogenide materials shows large Fermi level shift and large photovoltaic effects [9e11]. In recent years CNT doped chalcogenide semiconductors have been widely investigated for their unusual structural, physical,

electrical and mechanical properties. Depending on the material compositions and preparation methods, carbon nanotube based materials can be tuned for various applications in many fields. CNTs based alloy has become a material of Scientific, commercial, industrial and has a great potential applications in many fields, such as nano-devices, memory devices, catalyst supporters etc., [12,13]. Such nanostructures are used in dye-sensitized solar cells, as well as in charge-storage applications such as super capacitors and batteries. There properties can be engineered for better application purposes in active materials like Nano-electronics and field emission devices [14e16]. CNT in small quantity act as reinforcing fibers for alloys, which can modify the morphology of matrix by enhancing their properties. CNT when incorporated in small amount enhanced the electrical conductivity reported by A.N Upadhyay et al. and Stehlik et al. [17,18]. Surprisingly there has been no report on the CNTs-SeTe glassy semiconductor. In the present work, authors reported the structural, electrical and dielectric properties of MWCNT/SeTe glassy alloys. 2. Experimental

* Corresponding author. E-mail address: [email protected] (M. Zulfequar). http://dx.doi.org/10.1016/j.matchemphys.2016.04.053 0254-0584/© 2016 Elsevier B.V. All rights reserved.

We have made the glassy alloy of Se80Te20 by Melt-quenching technique from the element of 5 N purity. Multiwall carbon

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reveals that the sample was essentially composed of high density, long and non-uniformly distributed MWCNT. On increasing the concentration of MWCNT the density of nano tubes increases. The quantitative analysis of the elements and the stoichiometry of the samples were studied by EDX studies. Fig. 3 shows the EDX spectra of MWCNT/(Se80Te20)100-x bulk samples, it confirms the existence of Se, Te and C in MWCNT/(Se80Te20)100-x which is almost equal to their normal stoichiometry and is tabulated in Table 1.

nanotubes (length, several micro meters; 20e30 nm diameter; MWCNT content ˃ 90%). The Quartz ampoules were kept in a rocking furnace whose temperature is then raised gradually at a rate of 4  C per minute and then kept constant at 800  C for 15 h. To synthesized [(Se80Te20)100-x(CNT)x] (x ¼ 2 and 10) glassy alloy, as synthesized Se80Te20 glassy alloy is crushed into powder form and divided into three parts, two parts of Se80Te20 are mixed with 2 and 10 at wt% MWCNT and one part is kept to serve as standard materials for comparison. After then the ampoules were again sealed under the vacuum of 106 Torr and heated under the same condition up to temperature of 600  C for 15 h. The molten CNT additives are then rapidly quenched in ice cold water. The duration of mixing and shaking of the ampoules in the furnace protect the homogeneity of the materials. The quenched samples were removed by breaking the ampoules. Bulk samples in the form of pellets (diameter ~10 mm and thickness ~1 mm) were obtained by finely grinding and compressing the powder under a pressure of 5 ton. To avoid poor electrical contact, the pellets were smoothen and silver paste were applied on both sides of pellets. The bulk samples were mounted between two steel electrodes inside a metallic sample holder for electrical and dielectric measurement under a vacuum of 103 Torr. For DC conductivity measurements, a DC voltage of 1.5 V was applied across the sample and the resulting current measured by a Keithley electrometer (6514). To calculate the dielectric constant (ε0 ) and dielectric loss (ε''), the parallel capacitance and dissipation factor were measured simultaneously by using LCZ (Wayne kerr 4300 m). The temperature of the pellets were controlled by mounting a heater inside the sample and measured by calibrated chromel-alumel constantan thermocouple near to the electrodes.

Fig. 4 shows a typical Raman spectrum obtained by Raman spectrometer Renishaw (Model: Invia II), using a laser beam with an excitation wavelength of 514 nm. Raman scattering is sensitive to the degree of crystallinity in a sample, a crystalline material yields a spectrum with very sharp, intense Raman peaks, and whilst an amorphous material will show broader less intense. The band at around 180e225 cm1 might be assigned to the SeeSe bonds. Schottmiller et al. [19]. also showed that the Raman band at 216 cm1 found in SeeTe alloys was attributed to SeTe rings. The graphitic structure of CNTs shows two intensive peaks around 1314 cm1 and 1615 cm1. Generally, G-mode (TMdtangential mode) in Raman spectra corresponding to the stretching mode in graphitic plane is located around 1615 cm1. The D mode (disorder band), is a typical sign for defective graphitic structures, observed at 1314 cm1. This band indicates that there is some amorphous carbonaceous and some catalyst particles adhered to the CNTs walls [20]. We also observe one additional peak at 2615 cm1 in 10% MWCNT which is attributed to second-order Raman scattering process.

3. Results and discussion

3.3. IeV characteristics

3.1. Structural investigation

The current-voltage characteristic (IeV) of pellets at room temperature is shown in Fig. 5, which indicates that pure SeTe alloy have DC current in the range of 1010 A. On incorporation of CNT a very high value of current of order 104 A and 102 A are observed for 2% and 10% MWCNT respectively. This analysis shows that the addition of MWCNTs in SeTe alloy enhance the output current by several order of magnitude. Here conduction is not only carried by free charge carrier's electron/hole, but also carried by the formation of polarons and bipolarons. When applied voltage is increased, the formation of polarons and bipolarons increases contributing to higher values of current through the samples.

Structural study of MWCNT/SeTe were done by using X-ray analysis performed with PANalytical X0 pert X-ray diffractometer. The copper target was used as a source of X-rays with l ¼ 1.54 A (Cu Ka1). Scanning angle was in the range of 20 e80 . A scan speed of 2 /min and a chart speed of 1 cm/min were maintained. XRD pattern of the CNT doped SeTe alloy is shown in Fig. 1, the absence of any sharp peak confirms the amorphous nature of the alloys. Surface morphology of powdered sample were analyzed by scanning electron microscopes (SEM) apparatus (ZEISS Supra 40 VP) which are shown in Fig. 2. SEM is an excellent technique for investigating the morphology of MWCNT/SeTe glassy alloy. It

3.2. Raman spectra

3.4. DC conductivity measurement Measurements of the temperature dependence of DC conductivity (sdc ) were carried out through the temperature range of 300e380 K for all the samples. Fig. 6 shows the variation of DC conductivity versus 1000/T for [(Se80Te20)100-x(CNT)x] (x ¼ 0, 2 and 10) glassy alloy. The DC conductivity increases exponentially with temperature, which can be represented by well-known Arrhenius's relation [21,22].



sdc ¼ s0 exp 

Fig. 1. XRD pattern of MWCNT/(Se80Te20) bulk samples.

DE kT

 (1)

Wheresdc , DE,s0 ,k are the DC conductivity, activation energy, pre-exponential factor and Boltzmann constant respectively. The calculated values, for pure and MWCNT/SeTe glassy alloy in the temperature range of (300e380 K) are given in Table 2. There is continuous increase in conductivity with increase of temperature, which suggests that the conduction is due to thermally assisted tunneling of charge carriers in the localized states present in the

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Fig. 2. SEM images of (a) SeTe, (b) 2% CNT/SeTe, (c) 10% CNT/SeTe bulk samples.

Fig. 3. Energy dispersive X-ray (EDX) picture (a) SeTe, (b) 2% CNT/SeTe, (c) 10% CNT/SeTe bulk samples.

Table 1 EDX data for MWCNT/(Se80Te20) bulk samples. Composition

Se80Te20 2% MWCNT/(Se80Te20) 10% MWCNT/(Se80Te20)

Observed at% with EDX Se

Te

C

82.54 32.10 28.43

17.46 7.07 7.55

e 60.82 64.02

band tails. The value of sdc increase from 1011 U1 cm1 to 103 U1 cm1 at 300 K and activation energy decrease from 0.463 to 0.0165 eV for [(Se80Te20)100-x(CNT)x] (x ¼ 0, 2 and 10) glassy alloy. Such rapid enhancement in DC conductivity is because of excellent mobility and highly conducting nature of CNT, which make a percolation path in CNT doped glassy alloys. The dynamic percolation behavior of CNT network in the alloys, which essentially examine the formation of conductive path (covalent CeC bond) inside the samples in the form of cluster of conducting CNTs [17]. In order to make a clear distinction between whether the conduction takes place in the extended states above the mobility or

by hopping in the localized states, because both these conduction mechanism occurs simultaneously. Mott [23] parameter shows that the conduction in extended states, the value of s0 for Se and Se based materials are of order 104 U1 cm1. The value of preexponential factor s0 calculated for all composition in our case are found to be less than 104 U1 cm1 which suggests that conduction mechanism is most likely in localized states. 3.5. Dielectric theory Systematic investigations on the dielectric properties of MWCNT/(Se80Te20)100-x alloys with 2% and 10% CNT concentration have been carried out in the frequency range 500 Hze1 MHz and temperature range 300e370 K. It is observed that dielectric constant (ε0 ) and dielectric loss (ε'') increase with temperature. Fig. 7 (a) and (b) shows the temperature dependence of dielectric constant (ε0 ) and dielectric loss (ε'') at various frequencies for SeTe glassy alloys. The increase in dielectric constant (ε0 ) with temperature is due to the fact that these material are covalent type in nature having dipoles which cannot orient themselves at low

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Fig. 4. Raman spectra of MWCNT/(Se80Te20) bulk samples.

Fig. 6. Log s versus 1000/T of MWCNT/(Se80Te20) bulk samples.

Fig. 5. IeV characteristic properties of MWCNT/(Se80Te20) bulk samples at 300 K.

temperature. As the temperature increases, the orientation of the dipole starts, which increase the dielectric constant (ε0 ). Fig. 8(a) and (b) shows the frequency dependence of dielectric constant (ε0 ) and dielectric loss (ε'') at various temperatures for SeTe glassy alloys. It is clear from the figures that both ε0 and ε'' decrease with increases in frequencies for all temperature range. The reason behind this is that as the frequency increases, the variation of field become rapid for the molecular dipole to follow, so their contribution to the polarizability decreases [24,25]. This type of behavior has been reported by various workers [26e31]. When the relaxation time is much higher than the frequency of the applied electric field, polarization occurs instantaneously. When the relaxation time is much lower than the frequency of the applied electric field, no polarization occurs.

According to the Debye theory [28] for viscosity dependence of relaxation time, dielectric constant (ε0 ) should increase exponentially with temperature, and this has been found true for the present sample. The above discussion indicates that the dipolar-type dielectric dispersion is occurring in the present system. However, no peak has observed in dielectric loss ε'' versus log f curve in the present samples as expected in the case of dipolar-type relaxation. Such peaks may be absent because of the wide distribution of relaxation times. However on incorporation of CNT in SeTe alloy, the value of dielectric behaviors seemed to be different than pure sample. The dielectric constant for 10% of MWCNT show increase in comparison with pure SeTe and 2% MWCNT. Large dielectric losses in the alloys have been observed in the low frequency range. The dielectric loss is substantially increased by increasing the MWCNT weight percentages in the alloys.

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Table 2 Electrical and dielectric parameter at frequency of 500 Hz and at temperature of 300 K for MWCNT/(Se80Te20) bulk samples. Sample

Current (I) at 1 V

sdc (U1 cm1)

DE (eV)

s0 (U1 cm1)

ε0

ε''

ε'' (dc)

Wm (eV)

m

SeTe 2%CNT 10%CNT

5.5  1010 A 1.1  104 A 2.5  102 A

9.1  1011 1.3  105 1.0  103

0.463 0.138 0.0165

4.9  103 3.1  103 1.9  103

16.6 5.75 30.95

0.42 1262 8368

0.325 5.1  104 3.5  106

1.37 0.36 0.34

0.23 0.89 0.94

Fig. 7. Temperature dependence of dielectric constant ε0 and dielectric loss ε'' at various frequencies for SeTe bulk samples.

Fig. 8. Frequency dependence of dielectric constant ε0 and dielectric loss ε'' at various for SeTe bulk samples.

This feature is especially very pronounced in the low frequency region (lower than 10 KHz). The dielectric loss becomes several hundred in the low frequency region; these results show that the

alloys containing MWCNTs have a strong dielectric absorption. The increased dielectric loss is related to the increase in electrical conductivity, which is due to the effect of highly conducting nature

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Fig. 9. Temperature dependence of dielectric constant ε0 and dielectric loss ε'' at various frequencies for 2%CNT/SeTe bulk samples.

of MWCNTs. The strong dielectric loss of the alloys indicates that the addition of MWCNTs increases the concentration of the mobile charges in the alloys. These results are shown in Figs. 9 and 10. The frequency dependence of ε'' is also represented by a wellknown relation ε'' ¼ Bum [32e34]. Where B is a constant. Fig. 11 confirms this behavior for all samples, where lnε'' vs lnu at a fixed temperature of 300 K are found to be straight line. The value power m is calculated from the slope of these line and are found to be negative. The temperature dependence of this exponent m is shown in Fig. 12, the value of m decreases with increase of

temperature according to Guintini et al. equation given by Ref. [35].

m ¼ 4pkT=W

m

(2)

Where Wm is maximum barrier height, values of Wm can be calculated from the Eq. (2) for all samples at fixed temperature of 300 K and are given in Table 2. From the above analysis, it seems that the paired defect states behave as a dipole in (MWCNT)/(Se80Te20)100-x. The present results are in good agreement with the theory of hopping of charge carriers

Fig. 10. Temperature dependence of dielectric constant ε0 and dielectric loss ε'' at various frequencies for 10%CNT/SeTe bulk samples.

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Fig. 11. Variation of ln ε'' with ln u at a fixed temperature 300 K for MWCNT/(Se80Te20) bulk samples.

461

over a potential barrier between charged defects states (Dþ and D) as suggested by Elliot's [27]. Each pair of sites Dþ and D is assume to form a dipole, which has a relaxation time dependent on its activation energy. Thus the dielectric dispersion is dipolar type in nature which is found in the present samples. DC conduction loss is another important parameter to investigate the losses in these alloys. Due to large increase in the DC conductivity for 2% and 10% MWCNT, DC conduction losses dominate over other losses. To study the origin of dielectric loss, the DC conduction loss is calculated using the relation ε'' (dc) ¼ sdc/uε0 [36e39]. The calculated values of ε'' (dc) are shown in Table 2. Fig. 13(a) and (b) shows the comparative study of DC conduction loss and the observed loss at fixed frequency 500 Hz for pure SeTe and 2% MWCNT. The obtained value of ε'' (dc) is small when compared to observed dielectric loss for SeTe. However for 2% and 10% CNT, ε'' (dc) loss increase abruptly over observed loss, which is due to increase in DC conductivity. Schottmiller et al. [19], have reported that in glassy Se, about 40% of atoms have ring like structure and 60% are bounded as polymeric chains. The addition of CNT in SeeTe glassy system increases the SeeTe-CNT polymeric chain concentration. Thus the increase in DC conductivity is due to decrease of SeeTe-CNT ring structure which means decreasing the disorderness.

4. Conclusion

Fig. 12. Temperature dependence of parameter “m” for MWCNT/(Se80Te20) bulk samples.

We have successfully prepared the MWCNT/(Se80Te20) glassy alloys by Melt-quenching technique. XRD confirm the amorphous nature of the prepared samples. SEM and Raman study show the incorporation of CNT in SeTe glassy alloys. IeV characteristics measure shows that despite of low level of CNT concentration, there is large change in conductivity which is expected to offer remarkable advantage for solid state battery application. Dielectric measurement shows that DC conduction losses dominate over other losses for 2% and 10% MWCNT/(Se80Te20) which is due to increase in DC conductivity of the alloy.

Fig. 13. (a) and (b) observed dielectric loss ε00 and calculated dielectric loss ε00 (dc) for pure SeTe and 2%MWCNT/SeTe bulk samples.

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Acknowledgment I am highly thankful to the University Grants Commission (UGC), New Delhi (India) (UGC-RF/RPS/RO/JMI/2014) for providing me financial support in the form of BSR scholarship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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