**2. Charge transfer mechanism in biopolymers**

Several biopolymers have well documented properties as organic semiconductors (Eley et al., 1977; Leszek et al., 2002; Radha & Rosen, 2003; Mallick & Sakar, 2000; Lewis & Bowen, 2007; Ashutosh & Singh 2008). DNA-based biopolymer material possesses unique optical and electromagnetic properties, including low and tunable electrical resistivity, ultralow

Charge Transport and Electrical Switching in Composite Biopolymers 227

biopolymers is a multi-step process. According to this mechanism, the overall distance between primary electron donor and final electron acceptor is split into a series of short electron transfer steps. The essential difference is the existence of bridge units (oxidized or reduced species) that function as relays system and the fact the hopping process has a weak distance dependence (Cordes & Giese, 2009).Both of the two processes can take place at the same time and have been observed by several experimental groups. Treadwaya et al. (2002) noted that DNA assemblies of different lengths, sequence, and conformation may allow

From measurements that probed changes in oxidized guanine damage yield with response to base perturbations, Armitage et al. (2004) noted that charge transfer through base-base of DNA molecules takes place through hopping via the π-π bond overlap. Tao et al. (2005) reported electron hopping and bridge-assisted superexchange charge transfer between donor and an acceptor groups in peptide systems. The charge and dipole of the peptide play an important role in the electron transfer (Amit et al., 2008). Galoppini & Fox (1996) demonstrated the effect of electric field generated by the helix dipole on electron transfer in Aib-rich α helical peptides and found out that other than the effect from secondary structure (α helix and β sheet), dipole and hydrogen bonding, the solvent also has a marked influence on the study of the electron transfer. Due to complexity of peptides, the importance of individual amino acids in controlling electron transfer is not yet understood in detail. Similar studies in proteins have concluded that electron transfer can occur across hydrogen bonds and that the rate of such transfer is greatly increased when the electron motions are strongly coupled with those of the protons (Ronald et al., 1981). While studying energy transport in biopolymers, Radha & Rossen (2003) suggested, based on the experimental results, that a soliton in biopolymers is an energy packet (similar to the "conformon" which is the packet of conformational strain on mitochondria) associated with a conformational strain localized in region much shorter than the length of a molecule. It was also noted by the same group that as the soliton (localized curvature) moves on the polymer, it could trap an electron and drag it along. This mechanism may be important in understanding charge transport in biological molecules, where curvatures abound. Studies on charge transport in ethyl cellulose- chloranil systems have also been done, (Khare et al., 2000) where the space charge limited current (SCLC) was found to be the dominant mode of electrical conduction

Mechanisms leading to charge conduction in metal-polymer-metal configuration have been the subject of intensive study in the past two decades. Much of these studies have focused on doped and undoped synthetic polymers where the commonly discussed high-field electronic conduction mechanism for various films are Fowler-Nordheim tunneling, Poole-Frenkel (P-F), Richardson-Schottky (R-S) thermionic emissions, space charge limited conduction and variable range hopping. Based on the same sample geometry it is reasoned that the mechanism mentioned above could also contribute to detectable current flow in biopolymers sandwiched between metal electrodes. These mechanisms are discussed hereunder and in section 5, experimental results based on cutin biopolymer are presented

In Fowler-Nordheim tunneling the basic idea is that quantum mechanical tunneling from the adjacent conductor into the insulator limits the current through the structure. Once the carriers have tunneled into the insulator they are free to move within the material.

and discussed in reference to the charge transport mechanism mentioned above.

tunneling, hopping, or some mixture of the two mechanisms to actually dominate.

at high field in these systems.

**2.1 Fowler-Nordheim tunneling** 

optical and microwave low loss, organic field effect transistors, organic light emitting diodes (LED) ( Hagen et al., 2006). Nonlinear optical polymer electro-optic modulators fabricated from biopolymers have demonstrated better performance compared to those made from other materials (Piel et al., 2006).Naturally occurring Guar gum biopolymer chemically modified with polyaniline exhibits electrical conductivity in the range of 1.6 × 10-2 S/cm at room temperature (Ashutosh & Singh 2008). Mallick & Sakar (2000) investigated electrical conductivity of gum arabica found in different species of Acacia babul [*Acacia Arabica*] (Boutelje, 1980) and found that its electrical properties are similar to that of synthetic conducting polymer doped with inorganic salt and are proton conducting in nature.

Charge transport in biomolecular materials takes place mainly through two processes: Super exchange transport and hopping transport. Super exchange is a chain mediated tunneling transport. In this process electrons or holes are indirectly transferred from a donor to acceptor group through an energetically well- isolated bridge, where the bridge orbitals are only utilized as coupling media. In the hopping mechanism, the electron temporarily resides on the bridge for a short time during its passing from one redox center to the other, but in the super-exchange, the conjugated bridge only serves as a medium to pass the electron between the donor and acceptor (Tao et al., 2005). Tunnelling is a process process that decays exponentially with the length of the molecule. A simple tunneling model assumes a finite potential barrier at the metal-insulator interface. It describes free electron flow for a short distance into the sample from the metal contact. At low voltages the charge transfer is described by Simmons relations (DiBenedetto et al., 2009) but at higher applied voltages the tunneling is determined by Fowler-Nordheim process. Superexchange process can either be coherent or non-coherent. Coherent tunneling process is whereby a charge carrier moves from a donor to acceptor fast enough such that there is no dephasing by nuclear motions of the bridge (Weiss et al., 2007). Consequently, charges do not exchange energy with the molecules. However this process does not take place at significantly long distances. Incoherent superexchange on the other hand is a multi-step process in which a localized charge carrier interacts with phonons generated by thermal motion of the molecules (Singh et al., 2010)

As opposed to superexchange, hopping transport in biopolymers is a weakly distance dependent incoherent process. Generally superexchange is a short range transfer of charges in a spatial scale of a few Å while hopping transport takes place over a longer distance greater than 1nm. The exact mechanism of tunneling and hopping is not fully understood but it is known to be influenced by several factors. First, type of charge carriers in biopolymers which can either be holes, electrons or even polarons influences charge transport. Hole transfer is initiated by photo-oxidation of the donor groups attached to the terminus of the molecule whereas electron transfer occurs by chemical reduction of the acceptor group. A direct reduction of the molecule in contact with the metal electrode occurs when the voltage is applied. The interplay between donor acceptor and coupling fluctuation in biological electron transfer has also been observed (Skourtis et al., 2010). Secondly, band structure and hoping sites also influence charge transport. Although there are no band structures in biomolecules, energy gaps exist due to different hybridized electronic states. These energy gaps provide hopping sites through which charges propagate. Finally, conformation and spatial changes for the conducting state may overlap and hence create hopping sites as described by variable range hopping (Shinwari et al., 2010). Variable range hopping mainly describes transport mechanism in solid-state materials, but has also been observed in biopolymers (Mei Li et al., 2010). Similar to superexchange, electron hopping in

optical and microwave low loss, organic field effect transistors, organic light emitting diodes (LED) ( Hagen et al., 2006). Nonlinear optical polymer electro-optic modulators fabricated from biopolymers have demonstrated better performance compared to those made from other materials (Piel et al., 2006).Naturally occurring Guar gum biopolymer chemically modified with polyaniline exhibits electrical conductivity in the range of 1.6 × 10-2 S/cm at room temperature (Ashutosh & Singh 2008). Mallick & Sakar (2000) investigated electrical conductivity of gum arabica found in different species of Acacia babul [*Acacia Arabica*] (Boutelje, 1980) and found that its electrical properties are similar to that of synthetic

conducting polymer doped with inorganic salt and are proton conducting in nature.

molecules (Singh et al., 2010)

Charge transport in biomolecular materials takes place mainly through two processes: Super exchange transport and hopping transport. Super exchange is a chain mediated tunneling transport. In this process electrons or holes are indirectly transferred from a donor to acceptor group through an energetically well- isolated bridge, where the bridge orbitals are only utilized as coupling media. In the hopping mechanism, the electron temporarily resides on the bridge for a short time during its passing from one redox center to the other, but in the super-exchange, the conjugated bridge only serves as a medium to pass the electron between the donor and acceptor (Tao et al., 2005). Tunnelling is a process process that decays exponentially with the length of the molecule. A simple tunneling model assumes a finite potential barrier at the metal-insulator interface. It describes free electron flow for a short distance into the sample from the metal contact. At low voltages the charge transfer is described by Simmons relations (DiBenedetto et al., 2009) but at higher applied voltages the tunneling is determined by Fowler-Nordheim process. Superexchange process can either be coherent or non-coherent. Coherent tunneling process is whereby a charge carrier moves from a donor to acceptor fast enough such that there is no dephasing by nuclear motions of the bridge (Weiss et al., 2007). Consequently, charges do not exchange energy with the molecules. However this process does not take place at significantly long distances. Incoherent superexchange on the other hand is a multi-step process in which a localized charge carrier interacts with phonons generated by thermal motion of the

As opposed to superexchange, hopping transport in biopolymers is a weakly distance dependent incoherent process. Generally superexchange is a short range transfer of charges in a spatial scale of a few Å while hopping transport takes place over a longer distance greater than 1nm. The exact mechanism of tunneling and hopping is not fully understood but it is known to be influenced by several factors. First, type of charge carriers in biopolymers which can either be holes, electrons or even polarons influences charge transport. Hole transfer is initiated by photo-oxidation of the donor groups attached to the terminus of the molecule whereas electron transfer occurs by chemical reduction of the acceptor group. A direct reduction of the molecule in contact with the metal electrode occurs when the voltage is applied. The interplay between donor acceptor and coupling fluctuation in biological electron transfer has also been observed (Skourtis et al., 2010). Secondly, band structure and hoping sites also influence charge transport. Although there are no band structures in biomolecules, energy gaps exist due to different hybridized electronic states. These energy gaps provide hopping sites through which charges propagate. Finally, conformation and spatial changes for the conducting state may overlap and hence create hopping sites as described by variable range hopping (Shinwari et al., 2010). Variable range hopping mainly describes transport mechanism in solid-state materials, but has also been observed in biopolymers (Mei Li et al., 2010). Similar to superexchange, electron hopping in biopolymers is a multi-step process. According to this mechanism, the overall distance between primary electron donor and final electron acceptor is split into a series of short electron transfer steps. The essential difference is the existence of bridge units (oxidized or reduced species) that function as relays system and the fact the hopping process has a weak distance dependence (Cordes & Giese, 2009).Both of the two processes can take place at the same time and have been observed by several experimental groups. Treadwaya et al. (2002) noted that DNA assemblies of different lengths, sequence, and conformation may allow tunneling, hopping, or some mixture of the two mechanisms to actually dominate.

From measurements that probed changes in oxidized guanine damage yield with response to base perturbations, Armitage et al. (2004) noted that charge transfer through base-base of DNA molecules takes place through hopping via the π-π bond overlap. Tao et al. (2005) reported electron hopping and bridge-assisted superexchange charge transfer between donor and an acceptor groups in peptide systems. The charge and dipole of the peptide play an important role in the electron transfer (Amit et al., 2008). Galoppini & Fox (1996) demonstrated the effect of electric field generated by the helix dipole on electron transfer in Aib-rich α helical peptides and found out that other than the effect from secondary structure (α helix and β sheet), dipole and hydrogen bonding, the solvent also has a marked influence on the study of the electron transfer. Due to complexity of peptides, the importance of individual amino acids in controlling electron transfer is not yet understood in detail.

Similar studies in proteins have concluded that electron transfer can occur across hydrogen bonds and that the rate of such transfer is greatly increased when the electron motions are strongly coupled with those of the protons (Ronald et al., 1981). While studying energy transport in biopolymers, Radha & Rossen (2003) suggested, based on the experimental results, that a soliton in biopolymers is an energy packet (similar to the "conformon" which is the packet of conformational strain on mitochondria) associated with a conformational strain localized in region much shorter than the length of a molecule. It was also noted by the same group that as the soliton (localized curvature) moves on the polymer, it could trap an electron and drag it along. This mechanism may be important in understanding charge transport in biological molecules, where curvatures abound. Studies on charge transport in ethyl cellulose- chloranil systems have also been done, (Khare et al., 2000) where the space charge limited current (SCLC) was found to be the dominant mode of electrical conduction at high field in these systems.

Mechanisms leading to charge conduction in metal-polymer-metal configuration have been the subject of intensive study in the past two decades. Much of these studies have focused on doped and undoped synthetic polymers where the commonly discussed high-field electronic conduction mechanism for various films are Fowler-Nordheim tunneling, Poole-Frenkel (P-F), Richardson-Schottky (R-S) thermionic emissions, space charge limited conduction and variable range hopping. Based on the same sample geometry it is reasoned that the mechanism mentioned above could also contribute to detectable current flow in biopolymers sandwiched between metal electrodes. These mechanisms are discussed hereunder and in section 5, experimental results based on cutin biopolymer are presented and discussed in reference to the charge transport mechanism mentioned above.

#### **2.1 Fowler-Nordheim tunneling**

In Fowler-Nordheim tunneling the basic idea is that quantum mechanical tunneling from the adjacent conductor into the insulator limits the current through the structure. Once the carriers have tunneled into the insulator they are free to move within the material.

Charge Transport and Electrical Switching in Composite Biopolymers 229

carriers causes a field gradient, which limits the current density and the mechanism is then referred to as space charge limited current. Starting from the basic Gauss's law in onedimension, assuming that the insulator contains no free carriers if no current flows the

where *J* is current density, *ε* is relative permittivity,*µ* is charge mobility, *V* is applied voltage and *d* is electrode spacing. Space charge limited current results from the fact that when the injected carrier concentration exceeds the thermal carrier concentration, the electric field in the sample becomes very non-uniform, and the current no longer follows Ohm's law.

Charge conduction in semiconducting polymers is thought to take place by hopping of charge carriers in an energetically disordered landscape of hopping sites (Meisel et al., 2006). The variable-range hopping (VRH) conduction mechanism originally proposed by Mott for amorphous semiconductors (Mott & Davis, 1979) assuming a phonon-assisted hopping process has also been observed in conducting polymers and their composites at low temperature (Ghosh et al., 2001; Luthra et al., 2003; Singh et al*.*, 2003). Bulk conductivity of conducting polymers depends upon several factors, such as the structure, number and nature of charge carriers, and their transport along and between the polymer chains and across the morphological barriers (Long et al., 2003). 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. Eq. (6) gives the DC

1/4

⎛ ⎞ = −⎜ ⎟ ⎝ ⎠ (6)

= (8)

(7)

<sup>0</sup> exp[ ] *Td T T*

2 1/2 ( )

γ

3

*kN* γ

Two other Mott parameters, the variable range hopping distance (R*VRH*) and hopping

( )

*EF*

*ph EF q v N*

<sup>⎡</sup> <sup>⎤</sup> <sup>=</sup> <sup>⎢</sup> <sup>⎥</sup> <sup>⎣</sup> <sup>⎦</sup>

and *q* is the electron charge, k is the Boltzmann's constant, *vph* is the typical phonon frequency obtained from the Debye temperature ( ≈1013 Hz), γ is the decay length of the localized wave function near the Fermi level and *N(E*F*)* is the density of states at the Fermi level. The characteristic Mott temperature *Td*, as shown in Eq.(8) corresponds to the hopping barrier for charge carriers (also known as the pseudo-activation energy) and measures the

0 is given by Eq.(7)

σ

<sup>0</sup> 1/2 2(8 )

π *k*

18.11 *<sup>d</sup>*

*T*

activation energy (W) are given by Eq. (9) and (10) respectively

σ

σ

σ

2 3 9 8 *<sup>V</sup> <sup>J</sup> <sup>d</sup>* εμ

= (5)

expression for the space charge limited current can be obtained as shown in Eq. (5)

**2.4 Variable-range hopping mechanism** 

conductivity based on the VRH conduction model.

where the pre- exponential factor

degree of disorder present in the system.

Determination of the current is based on the Wentzel, Kramers and Brillouin (WKB) approximation from which Eq. (1) is obtained.

$$J\_{\rm FN} = \mathcal{X}\_{\rm FN} E^2 \exp\left[ -\frac{4}{3} \frac{\sqrt{2m^\*}}{q\hbar} \frac{\left(q\wp\_\mathbf{B}\right)^{3/2}}{E} \right] \tag{1}$$

where *JFN* is the current density according to Fowler -Nordheim, χ*FN* is the Fowler Nordheim constant, *E* is the electric field, *m\** is the effective mass of the tunneling charge, = is a reduced plancks constant, *q* is the electron charge and φ*B* is the potential barrier height at the conductor/insulator interface. To check for this current mechanism, experimental *I*-*V* characteristics are typically plotted as <sup>2</sup> ln( / ) *J E* vs 1 /*E* , a so-called Fowler-Nordheim plot. Provided the effective mass of the insulator is known, one can fit the experimental data to a straight line yielding a value for the barrier height.

#### **2.2 Field emission process**

Whereas Fowler-Nordheim tunneling implies that carriers are free to move through the insulator, it cannot be the case where defects or traps are present in an insulator. The traps restrict the current flow because of a capture and emission process. The two field emission charge transport process that occur when insulators are sandwiched between metal electrodes are Poole-Frenkel and Schottky emission process. Thermionic (schottky) emission assumes that an electron from the contact can be injected into the dielectric once it has acquired sufficient thermal energy to cross into the maximum potential (resulting from the superposition of the external and the image-charge potential). If the sample has structural defects, the defects act as trapping sites for the electrons. Thermally traped charges will contribute to current density according to Poole-Frenkel emission. They are generally observed in both organic and inorganic semiconducting materials. Poole-Frenkel effect is due to thermal excitation of trapped charges via field assisted lowering of trap depth while Schottky effect is a field lowering of interfacial barrier at the blocking electrode. Expression for Poole-Frenkel and Schottky effects are given in Eq. (2) and (3) respectively.

$$J\_{\rm PF} = J\_{\rm PFO} \exp[\left(\mathcal{J}\_{\rm PF} E^{1/2}\right) / kT] \tag{2}$$

$$J\_S = J\_{SO} \exp[\left(\beta\_S E^{1/2}\right)/kT] \tag{3}$$

*JSO* and *JPFO* are pre-exponential factors, β *<sup>S</sup>* is the Schottky coefficient, β *PF* is the Poole-Frenkel coefficient, and *E* is the electric field. The theoretical values of Schottky and Poole-Frenkel coefficient are related by Eq.(4):

$$
\beta\_S = \left( e^3 \;/\; 4\,\pi\varepsilon\varepsilon\_0 \right) = \beta\_{\rm PF} \;/\; 2 \tag{4}
$$

where *q* is electron charge, ε is relative permittivity, ε*<sup>0</sup>* permitibity in free space

#### **2.3 Space charge limited current**

For structures where carriers can easily enter the insulator and freely move through the insulator, the resulting charge flow densities are much higher than predicted by Fowler-Nordheim tunneling and Poole-Frenkel mechanism. The high density of these charged carriers causes a field gradient, which limits the current density and the mechanism is then referred to as space charge limited current. Starting from the basic Gauss's law in onedimension, assuming that the insulator contains no free carriers if no current flows the expression for the space charge limited current can be obtained as shown in Eq. (5)

$$J = \frac{9\varepsilon\mu V^2}{8d^3} \tag{5}$$

where *J* is current density, *ε* is relative permittivity,*µ* is charge mobility, *V* is applied voltage and *d* is electrode spacing. Space charge limited current results from the fact that when the injected carrier concentration exceeds the thermal carrier concentration, the electric field in the sample becomes very non-uniform, and the current no longer follows Ohm's law.

#### **2.4 Variable-range hopping mechanism**

228 Biomaterials – Physics and Chemistry

Determination of the current is based on the Wentzel, Kramers and Brillouin (WKB)

exp <sup>3</sup>

where *JFN* is the current density according to Fowler -Nordheim, χ*FN* is the Fowler Nordheim constant, *E* is the electric field, *m\** is the effective mass of the tunneling charge,

height at the conductor/insulator interface. To check for this current mechanism, experimental *I*-*V* characteristics are typically plotted as <sup>2</sup> ln( / ) *J E* vs 1 /*E* , a so-called Fowler-Nordheim plot. Provided the effective mass of the insulator is known, one can fit the

Whereas Fowler-Nordheim tunneling implies that carriers are free to move through the insulator, it cannot be the case where defects or traps are present in an insulator. The traps restrict the current flow because of a capture and emission process. The two field emission charge transport process that occur when insulators are sandwiched between metal electrodes are Poole-Frenkel and Schottky emission process. Thermionic (schottky) emission assumes that an electron from the contact can be injected into the dielectric once it has acquired sufficient thermal energy to cross into the maximum potential (resulting from the superposition of the external and the image-charge potential). If the sample has structural defects, the defects act as trapping sites for the electrons. Thermally traped charges will contribute to current density according to Poole-Frenkel emission. They are generally observed in both organic and inorganic semiconducting materials. Poole-Frenkel effect is due to thermal excitation of trapped charges via field assisted lowering of trap depth while Schottky effect is a field lowering of interfacial barrier at the blocking electrode. Expression

*<sup>m</sup> <sup>q</sup> J E q E*

*FN FN*

= is a reduced plancks constant, *q* is the electron charge and

experimental data to a straight line yielding a value for the barrier height.

for Poole-Frenkel and Schottky effects are given in Eq. (2) and (3) respectively.

1/2 exp[( ) / ] *PF PFO PF J J* = β

1/2 exp[( ) / ] *S SO S J J E kT* = β

Frenkel coefficient, and *E* is the electric field. The theoretical values of Schottky and Poole-

β

( ) <sup>3</sup> *S P* /4 /2 <sup>0</sup> *<sup>F</sup>*

is relative permittivity,

 πεε

For structures where carriers can easily enter the insulator and freely move through the insulator, the resulting charge flow densities are much higher than predicted by Fowler-Nordheim tunneling and Poole-Frenkel mechanism. The high density of these charged

 β

ε

β

ε

χ

( )3/2 \* <sup>2</sup> 4 2

⎡ ⎤

*B*

<sup>=</sup> ⎢− <sup>⎥</sup> <sup>⎢</sup> <sup>⎥</sup> <sup>⎣</sup> <sup>⎦</sup> <sup>=</sup> (1)

φ

*E kT* (2)

β

*PF* is the Poole-

*<sup>S</sup>* is the Schottky coefficient,

= *e* = (4)

*<sup>0</sup>* permitibity in free space

(3)

*B* is the potential barrier

ϕ

approximation from which Eq. (1) is obtained.

**2.2 Field emission process** 

*JSO* and *JPFO* are pre-exponential factors,

Frenkel coefficient are related by Eq.(4):

where *q* is electron charge,

**2.3 Space charge limited current** 

Charge conduction in semiconducting polymers is thought to take place by hopping of charge carriers in an energetically disordered landscape of hopping sites (Meisel et al., 2006). The variable-range hopping (VRH) conduction mechanism originally proposed by Mott for amorphous semiconductors (Mott & Davis, 1979) assuming a phonon-assisted hopping process has also been observed in conducting polymers and their composites at low temperature (Ghosh et al., 2001; Luthra et al., 2003; Singh et al*.*, 2003). Bulk conductivity of conducting polymers depends upon several factors, such as the structure, number and nature of charge carriers, and their transport along and between the polymer chains and across the morphological barriers (Long et al., 2003). 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. Eq. (6) gives the DC conductivity based on the VRH conduction model.

$$
\sigma = \frac{\sigma\_0}{\sqrt{T}} \exp[-\left(\frac{T\_d}{T}\right)^{1/4}] \tag{6}
$$

where the pre- exponential factor σ0 is given by Eq.(7)

$$
\sigma\_0 = \frac{q^2 v\_{ph}}{2(8\pi k)^{1/2}} \left[\frac{N\_{\rm (EF)}}{\gamma}\right]^{1/2} \tag{7}
$$

and *q* is the electron charge, k is the Boltzmann's constant, *vph* is the typical phonon frequency obtained from the Debye temperature ( ≈1013 Hz), γ is the decay length of the localized wave function near the Fermi level and *N(E*F*)* is the density of states at the Fermi level. The characteristic Mott temperature *Td*, as shown in Eq.(8) corresponds to the hopping barrier for charge carriers (also known as the pseudo-activation energy) and measures the degree of disorder present in the system.

$$T\_d = 18.11 \frac{\text{y}^3}{k \text{N}\_{\text{(EF)}}} \tag{8}$$

Two other Mott parameters, the variable range hopping distance (R*VRH*) and hopping activation energy (W) are given by Eq. (9) and (10) respectively

Charge Transport and Electrical Switching in Composite Biopolymers 231

general terms, built up of space charges trapped within the sample due to presence of defects create a field which is large enough to cause flow of mobile charge carriers. This phenomenon is sometimes called coulomb blockade (Tang et al., 2005). At high electric fields, charges are injected by Fowler-Nordheim tunnelling and subsequently trapped. As a result, electrostatic barrier character of the structure is modified and so is its resistance. The most insightful switching mechanism in biomolecules is the redox process and the formation of charge transfer complex through donor-acceptor coupling. Aviram et al. (1988) suggested that electron- proton motion within hemiquinones molecules that comprised of catechol and o-quinone, molecules between two contacts switch the molecules to low impedance (ON) state due to the formation of semiquinones fee radicals. When an electron is injected into the molecules from the metal contact, it is gained by an electron acceptor molecule. An electron donor molecule then transfers the electron to the opposite contact

The present chapter discusses structural characteristics (by use of Fourier Transform Infra-Red spectroscopy and Atomic Force Microscopy). Electrical conduction in cuticular membranes of Nandi flame (*Spathodea campanulata*, P. Beav) seeds hereafter referred just as cuticles. Fig. 1 shows the cuticle also presented still attached to the seed. The cuticles are thin (about 2 µm),

Fig.2 shows the Fourier Transform Infra Red (FTIR) spectroscopy of the pristine cuticle. The samples were first annealed at 350K for 12 hrs before measurement. The wide band at 3348 cm-1 which has been observed in many other cuticular membranes (Bykov, 2008) is assigned to O-H stretching vibration. It is caused by presence of alcoholic and phenolic hydroxyl groups involved in hydrogen bonds. Methylene is the most repeated structural unit in the cutin biopolyester (Jose, et al., 2004) and these shows up in the spectra band around 2300 cm-1. The band at 2916 cm is assigned to C-H asymmetric and symmetric stretching vibrations of methoxyl groups. Absorption around 1604 cm-1 and 1427 cm-1 are assigned to the stretching of C*=*C bonds and the stretching of benzenoid rings. Absorption bands in the range 1300-1150 cm-1 are related to asymmetric vibration of C-O-C linkages in ester to esters or phenolic groups. Fig. 3 show the infra red (IR) spectra of cuticle compared with the

translucent and very light. They are adapted to wind dispersion of the seeds.

Fig. 1. Thin and translucent cuticle attached to the Nandi flame seed

thus allowing flow of charge.

**4. Structural characterization** 

spectra of other biopolymers.

**4.1 Fourier transform infra Red (FTIR) spectroscopy** 

$$R\_{VRH} = \left[\frac{9}{8\pi\gamma kT N\_{\text{(EF)}}}\right]^{1/4} \tag{9}$$

$$\mathcal{W} = \frac{3}{4\pi R^3 N\_{\text{(EF)}}} \tag{10}$$

#### **3. Electrical switching mechanism in bioploymers**

Biomolecules often have sensitive bio-active sites that can change under external stimuli such as temperature, light, electrical signals, PH and chemical/biochemical reactions of their environs. Such switchable biomolecules are of tremendous usefulness in diverse areas including biological, medical and bio-electronic technology. Most research groups in this field are interested in investigating new class of switchable biological systems albeit the field is still at its infancy stage. Chu et al. (2010) reported electro-switchable oligopetides as a function of surface potential. Oligolysine peptides exhibit protonated amino side chain at PH-7 providing the basis of switching ''ON'' and ''OFF'' of the biological activity on the surface upon application of negative potential. Switching initiated by PH changes has been observed in other biomolecules and biopolymers (Zimmermann et al., 2006). Biomolecular motors of actomyosin experience rapid and reversible on-off switching by thermal activation (Mihajlovi et al., 2004). The most optimistic approach of integrating photo switchable biomolecules into opto-electronic devices is provided by highly photo sensitive bacteriorhodopsin. This molecule has shown remarkable photo sensitive switching of its electrical properties that mimic conventional Gate transistors (Roy et al., 2010; Pandey, 2006; Qun et al., 2004). Bottom-up approach toward building optical nano-electronic devices is also feasible with the discoveries of switchable photoconductivity in even the smallest structures such as quantum dots of cross-linked ligands (Lilly et al., 2011).

Electrical switching in biomolecules has wider applications in electronic industry. However, a lucid understanding of microscopic switching mechanism in these biological systems is still an outstanding challenge. In most investigations probing electrical switching in organic molecules, an external electrical signal is applied to the sample sandwiched between metal electrodes. Many materials have been reported to show hysteretic impedance switching where a system in its high impedance state (OFF) is switched by a threshold voltage into a low impedance state (ON) and remains in the ON state even with the reversal of applied voltage. This phenomenon is also known as resistive switching. Switching mechanism depend on whether the contribution comes from thermal, electronic, or ionic effects (Waser et al., 2007). Resistive switching is generally dependent on number of mobile charges, their mobility and the average electric field. In the case where the current is highly localized within a small sample area, filamental conduction take place (Scott et al., 2007). This simply involves formation of metallic bridge connecting the two electrodes. Filamental conduction accounts for negative differential resistance (NDR) model which is the basis of many molecular switching processes (Ren et al., 2010). Although this model was originally applied to inorganic materials, it can also explain resistive switching in organic samples (Tseng et al., 2006). The model assumes a trap- controlled channel where tunnelling take place in between chains of metallic islands. Similarly a decrease in electron transport channels and weak coupling between electrodes and the contact molecule causes NDR switching behaviour. Electric field induced switching mechanism is common in literature (Waser et al., 2007). In general terms, built up of space charges trapped within the sample due to presence of defects create a field which is large enough to cause flow of mobile charge carriers. This phenomenon is sometimes called coulomb blockade (Tang et al., 2005). At high electric fields, charges are injected by Fowler-Nordheim tunnelling and subsequently trapped. As a result, electrostatic barrier character of the structure is modified and so is its resistance.

The most insightful switching mechanism in biomolecules is the redox process and the formation of charge transfer complex through donor-acceptor coupling. Aviram et al. (1988) suggested that electron- proton motion within hemiquinones molecules that comprised of catechol and o-quinone, molecules between two contacts switch the molecules to low impedance (ON) state due to the formation of semiquinones fee radicals. When an electron is injected into the molecules from the metal contact, it is gained by an electron acceptor molecule. An electron donor molecule then transfers the electron to the opposite contact thus allowing flow of charge.

The present chapter discusses structural characteristics (by use of Fourier Transform Infra-Red spectroscopy and Atomic Force Microscopy). Electrical conduction in cuticular membranes of Nandi flame (*Spathodea campanulata*, P. Beav) seeds hereafter referred just as cuticles. Fig. 1 shows the cuticle also presented still attached to the seed. The cuticles are thin (about 2 µm), translucent and very light. They are adapted to wind dispersion of the seeds.
