**5. Electrical characterization and charge transport mechanism**

This section discusses electrical characteristics of the cuticle samples. Current-voltage (I-V) data measured as a function of annealing temperature, irradiation and pooling temperature was used in analyzing the electrical characteristics .Samples were separately annealed and irradiated before electrode coating was done. When annealing, cuticles were placed inside a temperature-controlled furnace, which was fitted inside an electrical shielded cage of a Lindberg/Blue Tube Furnace of model TF55035C. Samples were annealed at various temperatures of 320K, 350K and 400K for a constant period of 12hrs each. Irradiation of the sample was done with He-Ne laser beam of wavelength 632.8nm in a dark room each for a different period of 10minutes, 30minutes, and 60minutes. Electrode coating on the film of pristine, pre-annealed and pre-irradiated samples was done by using quick drying and highly conducting Flash-Dry silver paint obtained from SPI Supplies (USA). A mask of a circular aperture of 0.56 cm diameter was used while coating to ensure uniformity in size of coated surface. Circular aluminum foil of the same diameter was placed on freshly coated surface such that the sample was sandwiched between two aluminum electrodes. These metal-sample-metal sandwiches were left to dry at room temperature for a period of 24hrs to ensure that there was good ohmic contacts between aluminum electrode and the sample. The same Flash-Dry Silver paint was used to connect thin wires onto the aluminum electrodes. When measuring I-V at different temperatures, a sample sandwiched between aluminium electrodes was placed inside the Lindbarg/Blue Tube Furnace and temperature varied in steps of 5K between 350K and 500K at constant electric fields of 0.75V/cm, 1.50V/cm 2.25V/cm, 3.00V/cm, and 3.75V/cm.

Fig. 5(a) shows the I-V characteristics of pristine and annealed samples. These indicate clearly that there was electrical switching and memory effect in the cuticle samples. At

Charge Transport and Electrical Switching in Composite Biopolymers 235

Fig.5(c) shows I-V curves of the cuticles obtained at different poling/measurement. These curves show that electrical current increases as measurement temperature increase. This is due to thermal excitation of the trapped charges across the potential barrier. The curves also show that forward bias characteristics have two regions which are typical examples of ohmic conduction for voltages below *Vth* (OFF-state) and a space charge limited current (SCLC) for voltages above *Vth* (ON-state).*.* Increase in temperature facilitates diffusion of ions in the space charge polarization. Thermal energy may also aid in overcoming the activation barrier for orientation of polar molecules in the direction of the field. Charge carrier generation and transport in mitochondrial lipoprotein system has been investigated by electrical conductivity and the results show that increase in temperature causes a transition in conductivity where steady state conduction is correlated with chain segmental reorientations of phospholipid moiety below the transition and with an interfacial

Fig. 5. I-V curves of the pristine cuticles and cuticles treated at different conditions. (a) pristine & samples pre-annealed at 320K and 400K for 12hrs each. (b) pristine & samples pre- irradiated with laser light of wavelength 632.8nm for the duration shown in the legends.(c) pristine samples measured at different temperatures. The insets in (a), (b) and (c) show variation of threshold voltage as a function of the annealing temperature, irradiation time, and measurement temperature respectively. (d) shows combined curves with the

*Vth* rapidly decreases with the increase in measurement temperature and that switching and memory effect almost disappears at higher temperatures (370K). This is due to the fact that the *Vth* decreases and that the gap between current in the forward bias and reverse bias in the ON-state region almost closed up such that the forward bias current nearly folllows the

same path as the reverse bias current which indicates a loss of memory.

polarization process above it (Eley et al., 1977).

conditions shown in the legends.

certain threshold voltage, Vth current rises rapidly by an order of 2. There are two distinct regions for the increasing voltage. At low voltages the log *I* versus log *V* plots are approximately linear with a slope of 1; while at higher voltages, above a well-defined threshold voltage Vth, the plots are again approximately linear with a slope of 2.04 ± 0.07. These plots therefore show that at low voltages, OFF-state, current follows ohms law but after switching to ON-state at higher voltages, current follows a power law dependence given by *<sup>n</sup> I V*α where 2.04 *n* = ±0.07 obtained from linear regression fitting parameters where the standard deviation was shown as 0.03 and coefficient of correlation as 0.0001. This shows that the ON-state region is governed by Space charge limited current (SCLC) controlled by single trapping level, the injecting carrier concentration dominating the thermally generated carriers. During the switching process the current increases appreciably leading to a local increase in temperature (Collines et al., 1993). The current does not follow the same path on decreasing applied electric field hence indicating that the samples exhibit memory switching that is not erased by annealing. The threshold voltage *Vth* for pristine samples is 5.0+0.5 volts. The width of *Vth* or transition voltage during switching from OFF to ON states is about 1.0 V. Inset of Fig 5 (a) shows non-uniform increase of *Vth* with the increase in annealing temperature and tends to attain a plateau at higher annealing temperature. Decrease in magnitude of the negative dielectric anisotropy during annealing is a major reason for the increase in *Vth* for the annealed samples (Katana& Muysoki, 2007). Annealing polymeric films at different temperatures causes structural changes which affects electrical conductivity. Annealing temperature increases grain size in the polymer films causing many changes in the electrical and other properties (Leszek et al., 2002). Threshold voltage *Vth* for pristine cuticles is higher than *Vth* reported for some synthetic polymers; PMMA (1.6V), PS (4.5V), Phthalocyanine (0.3V), 2,6-(2,2-bicyanovinyl) pyride (5.01V), Langmuir-Blodgett (1.0V) (Katana& Muysoki, 2007; Otternbacher et al., 1991; Xue et al., 1996; Sakai et al., 1988).

Fig. 5(b) shows I-V curves for cuticle samples that were pre-irradiated with laser light of wavelength 632.8nm for different duration of time. Just as noted for the annealed samples, I-V curves for irradiated samples shows electrical switching and memory effect with Vth that increases with the increase in irradiation time (see inset Fig. 5b). Increasing time of irradiation increases electrical conductivity. The increase in conductivity for irradiated samples can be attributed to dissociation of primary valence bonds into radicals. Dissociation of C-C and C-H bonds leads to degradation and cross linking which improves electrical conductivity (Ashour et al., 2006). Exposure of polymers to ionizing radiation produces charge carriers in terms of electron and holes which may be trapped in the polymer matrix at low temperatures (Feinheils et al., 1971). If the original conductivity is small, then the presence of these carriers produces an observable increase in conductivity of the polymer. Irradiation of polymers results in excitations of its molecules and creation of free electrons and ions that migrate through the polymer network till they are trapped. The electronic and ionic configurations created, cause changes in the electric conductivity. In the study of effect of gamma irradiation on the bovine Achilles tendon (BAT) collagen, Leszek et al. (2002) reported changes in electrical conductivity that is dose dependent. Higher concentration of free radicals generated by irradiation of collagen created charge carriers that increased electrical conductivity.

certain threshold voltage, Vth current rises rapidly by an order of 2. There are two distinct regions for the increasing voltage. At low voltages the log *I* versus log *V* plots are approximately linear with a slope of 1; while at higher voltages, above a well-defined threshold voltage Vth, the plots are again approximately linear with a slope of 2.04 ± 0.07. These plots therefore show that at low voltages, OFF-state, current follows ohms law but after switching to ON-state at higher voltages, current follows a power law dependence

where the standard deviation was shown as 0.03 and coefficient of correlation as 0.0001. This shows that the ON-state region is governed by Space charge limited current (SCLC) controlled by single trapping level, the injecting carrier concentration dominating the thermally generated carriers. During the switching process the current increases appreciably leading to a local increase in temperature (Collines et al., 1993). The current does not follow the same path on decreasing applied electric field hence indicating that the samples exhibit memory switching that is not erased by annealing. The threshold voltage *Vth* for pristine samples is 5.0+0.5 volts. The width of *Vth* or transition voltage during switching from OFF to ON states is about 1.0 V. Inset of Fig 5 (a) shows non-uniform increase of *Vth* with the increase in annealing temperature and tends to attain a plateau at higher annealing temperature. Decrease in magnitude of the negative dielectric anisotropy during annealing is a major reason for the increase in *Vth* for the annealed samples (Katana& Muysoki, 2007). Annealing polymeric films at different temperatures causes structural changes which affects electrical conductivity. Annealing temperature increases grain size in the polymer films causing many changes in the electrical and other properties (Leszek et al., 2002). Threshold voltage *Vth* for pristine cuticles is higher than *Vth* reported for some synthetic polymers; PMMA (1.6V), PS (4.5V), Phthalocyanine (0.3V), 2,6-(2,2-bicyanovinyl) pyride (5.01V), Langmuir-Blodgett (1.0V) (Katana& Muysoki, 2007; Otternbacher et al., 1991; Xue et al.,

Fig. 5(b) shows I-V curves for cuticle samples that were pre-irradiated with laser light of wavelength 632.8nm for different duration of time. Just as noted for the annealed samples, I-V curves for irradiated samples shows electrical switching and memory effect with Vth that increases with the increase in irradiation time (see inset Fig. 5b). Increasing time of irradiation increases electrical conductivity. The increase in conductivity for irradiated samples can be attributed to dissociation of primary valence bonds into radicals. Dissociation of C-C and C-H bonds leads to degradation and cross linking which improves electrical conductivity (Ashour et al., 2006). Exposure of polymers to ionizing radiation produces charge carriers in terms of electron and holes which may be trapped in the polymer matrix at low temperatures (Feinheils et al., 1971). If the original conductivity is small, then the presence of these carriers produces an observable increase in conductivity of the polymer. Irradiation of polymers results in excitations of its molecules and creation of free electrons and ions that migrate through the polymer network till they are trapped. The electronic and ionic configurations created, cause changes in the electric conductivity. In the study of effect of gamma irradiation on the bovine Achilles tendon (BAT) collagen, Leszek et al. (2002) reported changes in electrical conductivity that is dose dependent. Higher concentration of free radicals generated by irradiation of collagen created charge carriers

where 2.04 *n* = ±0.07 obtained from linear regression fitting parameters

given by *<sup>n</sup> I V*α

1996; Sakai et al., 1988).

that increased electrical conductivity.

Fig.5(c) shows I-V curves of the cuticles obtained at different poling/measurement. These curves show that electrical current increases as measurement temperature increase. This is due to thermal excitation of the trapped charges across the potential barrier. The curves also show that forward bias characteristics have two regions which are typical examples of ohmic conduction for voltages below *Vth* (OFF-state) and a space charge limited current (SCLC) for voltages above *Vth* (ON-state).*.* Increase in temperature facilitates diffusion of ions in the space charge polarization. Thermal energy may also aid in overcoming the activation barrier for orientation of polar molecules in the direction of the field. Charge carrier generation and transport in mitochondrial lipoprotein system has been investigated by electrical conductivity and the results show that increase in temperature causes a transition in conductivity where steady state conduction is correlated with chain segmental reorientations of phospholipid moiety below the transition and with an interfacial polarization process above it (Eley et al., 1977).

Fig. 5. I-V curves of the pristine cuticles and cuticles treated at different conditions. (a) pristine & samples pre-annealed at 320K and 400K for 12hrs each. (b) pristine & samples pre- irradiated with laser light of wavelength 632.8nm for the duration shown in the legends.(c) pristine samples measured at different temperatures. The insets in (a), (b) and (c) show variation of threshold voltage as a function of the annealing temperature, irradiation time, and measurement temperature respectively. (d) shows combined curves with the conditions shown in the legends.

*Vth* rapidly decreases with the increase in measurement temperature and that switching and memory effect almost disappears at higher temperatures (370K). This is due to the fact that the *Vth* decreases and that the gap between current in the forward bias and reverse bias in the ON-state region almost closed up such that the forward bias current nearly folllows the same path as the reverse bias current which indicates a loss of memory.

Charge Transport and Electrical Switching in Composite Biopolymers 237

surge of current with decreasing field since the model assumes that once the carriers have

 E - 1 (V - 1 cm ) 0 .01 0 0 .01 2 0.0 14 0.0 16 0 .01 8 0 .02 0 -30 -29 -28 -27 -26 -25

(a) (c )

(b) (d)

*<sup>S</sup>* ) and Poole-Frenkel coefficient (

β

Fig. 6. Fowler-Nordheim curves: (a) Forward bias at low electric field (b) Reverse bias at low electric field (c) forward bias at high electric field and (d) reverse bias at high electric field. Fig. 7. shows curves of current density versus square root of electric (*ln J versus E1/2*) in the low field for forward bias regime. These curves neither support conduction mechanism by Poole-Frenkel nor Schottky emissions which predict linear graphs of *ln J versus E1/2* with positive slopes. Fig.8 shows linear fittings of *ln J versus E1/2* for forward and reverse bias at high fields (104 -105 V/cm). The current levels in the reverse bias are higher than forward bias. This behaviour may be interpreted either in terms of Poole-Frenkel effect which is due to thermal excitation of trapped charges via field assisted lowering of trap depth or by Schottky effect which is a field lowering of interfacial barrier at the blocking electrode (Deshmukh et al. 2007). The expressions for these processes are given in Eq.(2) and (3)

> β*<sup>S</sup>* and

the slopes of plots of *lnJ versus E1/2* (Fig.8) at different temperatures are listed in Table 1. The standard deviation and coefficient of linear correlation were obtained as 0.34 and 0.005

β

24 J V1/2 m1/2 and 7.01×10-24 J V1/2 m1/2 respectively. Experimental values of

shown in Eq. (4). Using the value of static dielectric permitivity (



ln (J /E 2 )A V - 1




1 0 <sup>3</sup>

E - 1 (V - 1 cm )

0 .0 1 0 .0 2 0 .0 3 0 .0 4

β

*PF* obtained from Eq. (4) are 3.51×10-

ε

*PF* ) are related as

obtained from

) of 3.0, (determined from

β

1 0 <sup>3</sup>

 P ris tin e s a m p le A n n e a le d 3 2 0 K A n n e a le d 3 5 0 K A n n e a le d 4 0 0 K

E - 1 ( V - 1 cm )

ln (J /E 2 ) A V - 2

tunneled into the insulator they are free to move within the material.

01234

01234

1 0 <sup>3</sup>

respectively. Schottky coefficient (

dielectric spectroscopy) theoretical values of

 P ris tin e s a m p le A n n e a le d 3 2 0 K A n n e a le d 3 5 0 K A n n e a le d 4 0 0 K

E - 1 (V - 1 cm )

1 0 <sup>3</sup>



ln (J /E 2 ) A V - 2




ln (J /E 2 ) A V - 2

It is difficult to draw unambiguous conclusion from Fig.5 (d) because all the three imposed conditions affect conductivity in unique ways and also depends on duration of annealing, irradiation and the measurement temperature. However it is worthy mentioning that at measurement temperature of 320K conductivity is higher than conductivity of presitne samples at low electric field (OFF state). This observation is however reversed at higher electric fields where conductivity of pristine samples is higher.

Switching and memory behavior can be attributed to the fact that external electric field triggers embedded molecules with ridox centers hence creating some traps. Switching mechanism in these systems is by quantum interference of different propagation parts within the molecules which involve permutation of Lowest Unoccupied Molecular Orbitals (LUMO+1) and Highest Occupied Molecular Orbitals (HOMO-1)-the frontier orbitals. To get electric field induced switching effect, the relative energies of HOMO (localized on the donor group) and LUMO (localized on the acceptor group) must be permuted (Aviram et tal*.,* 1988). This switching model supposes that both the permuting orbitals are initially doubly degenerate resulting to what is referred to as frontier orbitals. In the absence of electric field HOMO-1 is localized on the acceptor group and LUMO+1 on the donor group. When the external field is applied, electron orbitals are "pulled" towards the acceptor group reducing the HOMO-LUMO gap of frontier orbitals and the switching and hybridization between HOMO-1 and LUMO+1 takes place. While the strength of the field increases, the HOMO-LUMO gap in the molecular spectrum becomes smaller and the HOMO, HOMO-1 and LUMO orbital split into the HOMO-LUMO gap and hence become delocalized. Electrical switching can also in part be explained by formation of quinoid and semiquinone structures from phenolic compounds accompanied by redox reactions. The quinoid form is planar, and is highly conjugated compared to phenyl groups. Changes in bond length and rotation of benzene ring during formation of quinoid structure results in activation barriers which are considered to be the origin of the temperature dependence conductance. Details of this explanation is found in Kipnusu et al. (2009b)

Fowler-Nordheim emission current is given in equation (1). To check for this current mechanism, experimental *I*-*<sup>V</sup>* data for annealed samples of NFSC were analyzed by potting 2 ln( / ) *J E* versus 1 /*<sup>E</sup>* . Four plots were made to represent different regime with different levels of measured current (Fig.6). Fowler-Nordheim tunneling mechanism is confirmed by

straight lines with negative slopes given by; ( ) 1/2 \* 3/2 4 2 /3 ( ) *m qq*ϕ*<sup>B</sup>* = where *m\** is the

effective mass of the tunneling charge, *q* is the electron charge, = is a reduced planck constant, and φ*B* is the barrier height expressed in eV units. Fig.6 shows that Fowler-Nodheim curves for low and high fields forward bias and low filed reverse bias were quite non-linear or had positive slopes therefore ruling out possibility of Fowler-Nordheim mechanism in these regimes. However in Fig.6 (d) the curves are relatively linear with negative slopes. This is the high fields' regime of the reverse bias where the current increased with decreasing voltage (see Fig.5). Making an assumption that *m\** equals the electron rest mass (0.511MeV), and using the slope obtained from linear fits of Fig. 6 (d),the potential barrier height at the Al/cuticle junction is found to be 11.28 eV and 1.13 eV for pure samples and samples annealed at 400K respectively. Vestweber et al. (1994) noted that if the barrier height exceeds 0.3 eV tunnel process prevail with the consequence that high anodic fields are required in order to attain high current densities. It can therefore be concluded that Fowler-Nordheim quantum mechanical tunneling was responsible for the

It is difficult to draw unambiguous conclusion from Fig.5 (d) because all the three imposed conditions affect conductivity in unique ways and also depends on duration of annealing, irradiation and the measurement temperature. However it is worthy mentioning that at measurement temperature of 320K conductivity is higher than conductivity of presitne samples at low electric field (OFF state). This observation is however reversed at higher

Switching and memory behavior can be attributed to the fact that external electric field triggers embedded molecules with ridox centers hence creating some traps. Switching mechanism in these systems is by quantum interference of different propagation parts within the molecules which involve permutation of Lowest Unoccupied Molecular Orbitals (LUMO+1) and Highest Occupied Molecular Orbitals (HOMO-1)-the frontier orbitals. To get electric field induced switching effect, the relative energies of HOMO (localized on the donor group) and LUMO (localized on the acceptor group) must be permuted (Aviram et tal*.,* 1988). This switching model supposes that both the permuting orbitals are initially doubly degenerate resulting to what is referred to as frontier orbitals. In the absence of electric field HOMO-1 is localized on the acceptor group and LUMO+1 on the donor group. When the external field is applied, electron orbitals are "pulled" towards the acceptor group reducing the HOMO-LUMO gap of frontier orbitals and the switching and hybridization between HOMO-1 and LUMO+1 takes place. While the strength of the field increases, the HOMO-LUMO gap in the molecular spectrum becomes smaller and the HOMO, HOMO-1 and LUMO orbital split into the HOMO-LUMO gap and hence become delocalized. Electrical switching can also in part be explained by formation of quinoid and semiquinone structures from phenolic compounds accompanied by redox reactions. The quinoid form is planar, and is highly conjugated compared to phenyl groups. Changes in bond length and rotation of benzene ring during formation of quinoid structure results in activation barriers which are considered to be the origin of the temperature dependence conductance. Details

Fowler-Nordheim emission current is given in equation (1). To check for this current mechanism, experimental *I*-*<sup>V</sup>* data for annealed samples of NFSC were analyzed by potting 2 ln( / ) *J E* versus 1 /*<sup>E</sup>* . Four plots were made to represent different regime with different

levels of measured current (Fig.6). Fowler-Nordheim tunneling mechanism is confirmed by

effective mass of the tunneling charge, *q* is the electron charge, = is a reduced planck

Nodheim curves for low and high fields forward bias and low filed reverse bias were quite non-linear or had positive slopes therefore ruling out possibility of Fowler-Nordheim mechanism in these regimes. However in Fig.6 (d) the curves are relatively linear with negative slopes. This is the high fields' regime of the reverse bias where the current increased with decreasing voltage (see Fig.5). Making an assumption that *m\** equals the electron rest mass (0.511MeV), and using the slope obtained from linear fits of Fig. 6 (d),the potential barrier height at the Al/cuticle junction is found to be 11.28 eV and 1.13 eV for pure samples and samples annealed at 400K respectively. Vestweber et al. (1994) noted that if the barrier height exceeds 0.3 eV tunnel process prevail with the consequence that high anodic fields are required in order to attain high current densities. It can therefore be concluded that Fowler-Nordheim quantum mechanical tunneling was responsible for the

*B* is the barrier height expressed in eV units. Fig.6 shows that Fowler-

1/2 \* 3/2 4 2 /3 ( ) *m qq*

ϕ

*<sup>B</sup>* = where *m\** is the

electric fields where conductivity of pristine samples is higher.

of this explanation is found in Kipnusu et al. (2009b)

straight lines with negative slopes given by; ( )

constant, and

φ

surge of current with decreasing field since the model assumes that once the carriers have tunneled into the insulator they are free to move within the material.

Fig. 6. Fowler-Nordheim curves: (a) Forward bias at low electric field (b) Reverse bias at low electric field (c) forward bias at high electric field and (d) reverse bias at high electric field.

Fig. 7. shows curves of current density versus square root of electric (*ln J versus E1/2*) in the low field for forward bias regime. These curves neither support conduction mechanism by Poole-Frenkel nor Schottky emissions which predict linear graphs of *ln J versus E1/2* with positive slopes. Fig.8 shows linear fittings of *ln J versus E1/2* for forward and reverse bias at high fields (104 -105 V/cm). The current levels in the reverse bias are higher than forward bias. This behaviour may be interpreted either in terms of Poole-Frenkel effect which is due to thermal excitation of trapped charges via field assisted lowering of trap depth or by Schottky effect which is a field lowering of interfacial barrier at the blocking electrode (Deshmukh et al. 2007). The expressions for these processes are given in Eq.(2) and (3) respectively. Schottky coefficient ( β *<sup>S</sup>* ) and Poole-Frenkel coefficient ( β *PF* ) are related as shown in Eq. (4). Using the value of static dielectric permitivity (ε) of 3.0, (determined from dielectric spectroscopy) theoretical values of β *<sup>S</sup>* and β *PF* obtained from Eq. (4) are 3.51×10- 24 J V1/2 m1/2 and 7.01×10-24 J V1/2 m1/2 respectively. Experimental values of β obtained from the slopes of plots of *lnJ versus E1/2* (Fig.8) at different temperatures are listed in Table 1. The standard deviation and coefficient of linear correlation were obtained as 0.34 and 0.005

Charge Transport and Electrical Switching in Composite Biopolymers 239

To analyze the effect of temperature on conductivity of the samples, Arrhenious curves

<sup>0</sup> exp

where *σ* is conductivity, *σo* the pre-exponential factor, *Ea* the activation energy, k is Boltzmann's constant, and T is temperature in Kelvin. Conductivity was obtained from the

> *I d V A*

> > β

Forward bias Reverse bias

γ

*ph* of 1013 s-1. Other Mott parameters, the hopping

where *I* is measured current, *V* is measured Voltage, *d* is the thickness of the samples (≈4.0 ×10-4 cm), and *A* is the electrode active area ( circular electrode of diameter 0.56cm was used). Initial increase in conductivity at low temperature is due to the injection of charge carriers directly from the electrodes. The increase in conductivity at selectively low field is due to the increase in the magnitude of the mean free path of the photon (Sangawar et al., 2006). At high temperatures, the increase in conductivity may be attributed to softening of the polymer which causes the injected charge carrier to move more easily into the volume of the polymer giving rise to a large current. Increased conductivity at higher temperatures

320 5.65 ± 0.12 3.17± 0.12

350 6.31 ± 0.14 3.56± 0.12

370 3.44± 0.04 0.93± 0.02

from equations 6 to 10 using the slope and intercept values of the plots in inset (b ) of Fig. 9

distance *R* and average hopping energy *W* are determined from Eq. (9) and Eq. (10) respectively. Mott parameters from this calculation are listed in Table 2. Table 3 shows the variations of the Mott parameters with temperature in our samples. It is evident from Table 3 that γ*R >* 1 and *W > kT,* which agrees with Mott's condition for variable range hopping. It can therefore be concluded that the main conduction mechanism in NFSC is the variable

σ

cm,3.00kV/cm, and 3.75kV/cm) were plotted (Fig. 9) using the Arrhenius Eq. (11);

σ σ

could also be due to thermionic emission across the barrier potential.

obtained from experimental data

with negative slope (Fig.9, inset b). The Mott parameters- *Td*,

Variable range hopping mechanism predicts linear dependence of 1/2 ln( )

υ

Temperature (K) Experimental

vs 103/T) for different polarizing fields (0.75kV/cm, 1.50kV/cm, 2.25kV/

*Ea kT*

⎛ ⎞ <sup>−</sup> <sup>=</sup> ⎜ ⎟ ⎝ ⎠ (11)

= (12)

× 10-23 (Jm1/2 V1/2) values

σ

and *N(EF)-* are determined

*T* versus *T-1/4*

( logσ

Eq. (12);

Table 1. Values of

range hopping.

β

and assuming a phonon frequency (( )

respectively. The large discrepancy in experimental values of β listed in Table 1 and theoretical values of β *<sup>S</sup>* and β *PF* leads to a conclusion that current transport mechanism in our samples governing the high field at a temperature range of 320K-370K cannot be explained in terms of Shottky or Poole-Frenkel emission.

Fig. 7. Semi logarithmic plots of *ln J* versus *E ½* for the low field of 225-2500 V/cm at a temperature range of 320K-400K

Fig. 8. Semi logarithmic plots of *ln J* versus *E ½* for the high field of 3.2×104- 9.8×105V/cm in forward and reverse biases at temperature range of 320K-400K

our samples governing the high field at a temperature range of 320K-370K cannot be

Fig. 7. Semi logarithmic plots of *ln J* versus *E ½* for the low field of 225-2500 V/cm at a

Fig. 8. Semi logarithmic plots of *ln J* versus *E ½* for the high field of 3.2×104- 9.8×105V/cm in

forward and reverse biases at temperature range of 320K-400K

β

*PF* leads to a conclusion that current transport mechanism in

listed in Table 1 and

respectively. The large discrepancy in experimental values of

explained in terms of Shottky or Poole-Frenkel emission.

β *<sup>S</sup>* and β

theoretical values of

temperature range of 320K-400K

To analyze the effect of temperature on conductivity of the samples, Arrhenious curves ( logσ vs 103/T) for different polarizing fields (0.75kV/cm, 1.50kV/cm, 2.25kV/ cm,3.00kV/cm, and 3.75kV/cm) were plotted (Fig. 9) using the Arrhenius Eq. (11);

$$
\sigma = \sigma\_0 \exp\left(\frac{-E\_a}{kT}\right) \tag{11}
$$

where *σ* is conductivity, *σo* the pre-exponential factor, *Ea* the activation energy, k is Boltzmann's constant, and T is temperature in Kelvin. Conductivity was obtained from the Eq. (12);

$$
\sigma = \frac{I}{V} \frac{d}{A} \tag{12}
$$

where *I* is measured current, *V* is measured Voltage, *d* is the thickness of the samples (≈4.0 ×10-4 cm), and *A* is the electrode active area ( circular electrode of diameter 0.56cm was used). Initial increase in conductivity at low temperature is due to the injection of charge carriers directly from the electrodes. The increase in conductivity at selectively low field is due to the increase in the magnitude of the mean free path of the photon (Sangawar et al., 2006). At high temperatures, the increase in conductivity may be attributed to softening of the polymer which causes the injected charge carrier to move more easily into the volume of the polymer giving rise to a large current. Increased conductivity at higher temperatures could also be due to thermionic emission across the barrier potential.


Table 1. Values of β obtained from experimental data

Variable range hopping mechanism predicts linear dependence of 1/2 ln( ) σ*T* versus *T-1/4* with negative slope (Fig.9, inset b). The Mott parameters- *Td*, γ and *N(EF)-* are determined from equations 6 to 10 using the slope and intercept values of the plots in inset (b ) of Fig. 9 and assuming a phonon frequency (( ) υ*ph* of 1013 s-1. Other Mott parameters, the hopping distance *R* and average hopping energy *W* are determined from Eq. (9) and Eq. (10) respectively. Mott parameters from this calculation are listed in Table 2. Table 3 shows the variations of the Mott parameters with temperature in our samples. It is evident from Table 3 that γ*R >* 1 and *W > kT,* which agrees with Mott's condition for variable range hopping. It can therefore be concluded that the main conduction mechanism in NFSC is the variable range hopping.

Charge Transport and Electrical Switching in Composite Biopolymers 241

that the charges are highly localized. Table 3 also shows that when the temperature decreases, the average hopping energy *W* decreases and the average hopping distance *R*  increases, supporting the fact that when the phonon energy is insufficient (low temperature), carriers will tend to hop larger distances in order to locate in sites which are

Current-Voltage characteristics of the cuticles, as a function of irradiation, annealing, and temperature, show electrical switching with memory effect. The threshold voltage increases with irradiation time and annealing temperature but it decreases with increase in measurement temperature. The threshold voltage of the annealed and irradiated samples ranges between 6-8 volts. Electrrical conduction in the OFF state follows Ohms' law but changes to space charge limited current after switching to ON state. A combination of Fowler-Nordheim field emission process and redox processes are responsible for electrical switching of the samples. Conduction at low temperatures takes place by variable range hopping mechanism. Since this biomaterial is biodegradable and is also considered to be biocompatible and immunologically inert, it has high potential in biomedical applications. It can be used in making contact eye lenses, scaffolds in tissue engineering, and in controlled release of drugs. Most notably, due to its switching properties, its use in the design of

Amit, P.;Watson, R; Lund, P; Xing, Y.; Burke, K.; Yufan H,; Borguet, E.; Achim, C; &

Composed of Thymine Nucleotides. *J.Phys. Chem*., Vol. 112, pp. 7233-7240 Armitage N., Briman M., & Gruner M. (2004). charge transfer and charge transport on the

Ashour, H., Saad, M., & Ibrahim, M. (2006). Electrical Conductivity for Irradiated, Grafted

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γ

*R*>1 which shows

Degree of localization of the carriers in the trap states is indicated by

energetically closer than their nearest neighbours.

biosensors utilizing ion channels is very feasible.

29( 2), pp. 351-362.

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wTimber).p234

**6. Conclusion** 

**7. References** 


Table 2. Mott parameters at temperature range of (320-440K)


Table 3. Variation of Mott parameters at temperature range of 300-450K

Fig. 9. Arrhenius plots showing variation of σ *vs 1/T* at different electric fields. Inset (a) average Arrhenius plot showing activation energy at low and intermediate temperature ranges. Inset (b) plot of ln *(*σ*T ½)* versus (*T -1/4*) within a temperature of 350K-440K and average electric field of 2.25Kv/cm.

Degree of localization of the carriers in the trap states is indicated by γ*R*>1 which shows that the charges are highly localized. Table 3 also shows that when the temperature decreases, the average hopping energy *W* decreases and the average hopping distance *R*  increases, supporting the fact that when the phonon energy is insufficient (low temperature), carriers will tend to hop larger distances in order to locate in sites which are energetically closer than their nearest neighbours.
