# Electrical Switching in Thin Films of Nandi Flame Seed Cuticles

Authors: Wycliffe K. Kipnusu; Gabriel Katana; Charles M. Migwi; I. V. S. Rathore; Joshua R. Sangoro
Journal: International Journal of Polymer Science
 (2009)

---

## Abstract

Current-voltage (I-V) characteristics of Nandi flame seed cuticles (NFSCs) have been studied as a function of irradiation, annealing, and poling temperature. The cuticles showed memory and threshold switching. Threshold voltage Vth was about 5 V which is almost five times higher than Vth  observed in synthetic polymers. The threshold voltage Vth increased to 6–8 V after irradiation and annealing depending on the duration of annealing or irradiation. After switching, conductivity increased by an order of 102. In reverse bias, increase of current was observed and the memory hysteresis loop was at higher conductivity than at the time of switching. Switching effect was minimized at a poling temperature of 370 K. Formation of semiquinones and quinoid radicals from phenolic compounds may have contributed to electrical switching and hysteresis effect.


---

## Body

## 1. Introduction

Electrical switching and memory is the
basis of many potential molecular electronic devices, and has gained
considerable attention of numerous research efforts in recent years [1–3]. Aviram
and Ratner [4] described the first concrete
idea of molecular conductance switching in which the chemical structure consisted of an electron donating molecule and a
modified acceptor connected via a saturated (insulating) σ bridge. According to their
theoretical calculations, insulating molecule becomes conductive after applying
a certain threshold voltage that brings the π-levels in resonance. A range of other molecular structures
which exhibit electrical threshold switching have been reported [1] including self-assembled monolayers of phenylethynyl oligomers [5] and rotaxanes oriented between two conductors by
Langmuir-Blodgett assembly [6, 7]. Conductance switching has
also been reported in synthetic polymeric materials which consist of large
assemblies of molecules [8–12].
Whereas several
biopolymers have well-documented properties as organic semiconductors [13–19], limited
studies on electrical switching phenomenon of these biological assemblies have
been done [20, 21].Biopolymers are a rich resource for development of
novel electronic devices, since there is tremendous natural variation in their
composition [22]. Biopolymers also have the
advantage of being biodegradable and biocompatible
[23]. Plant
seed cuticles have been shown to contain cutin and suberin biopolymers [24, 25]. Cutin is
the main component (between 40% and 85%, w/w) of plant cuticles [26, 27]. Predominant
aliphatic monomers of most cutins are C16 and C18ω-hydroxylated fatty acids, typically carrying
in addition hydroxy- or epoxy groups in midchain positions. Besides aliphatic monomers,
cutin also contains glycerol and small amounts of phenolic compounds [28, 29]. Suberin occurs
at abscission zones such as chalazal region of seed coats formed during late
stages of seed maturation [30, 31]. Suberin contains both aliphatic and aromatic monomers [32, 33]. The aliphatic part
of suberin is mainly composed of C16 to C28ω-hydroxy fatty acids and C16 to C26α,ω-dioic acids. The aromatic network is a hydroxycinnamate-derived
polymer, primarily comprised of ferulic acid, N-feruloyltyramine, cinnamic
acid, p-coumaric acid, or caffeic acid [33]. Kipnusu et al.
[34] have reported that Nandi flame seed
cuticle is an insoluble biomaterial which has high-specific heats and shows
glass transition temperature comparable to glass transition of cutin reported
elsewhere [35]. Three-dimensional
structures of cutin and suberin macromolecules are not well understood, since
their constitutive moieties can vary appreciably both in specific nature and
relative abundance within macromolecular network [30]. However, Kolattukudy
[36] suggested
hypothetical models of cutin and suberin structures as shown in Figures 1 and 2, respectively.Figure 1
Models showing type of structures present in
cutin biopolyester.Figure 2
Hypothetical structure of suberin polymer.In this paper, we present
results of electrical switching properties of thin films of NFSC as a function
of annealing temperature, irradiation with laser beam, and poling temperature.
Fourier transform infrared (FT-IR) spectroscopy of NFSC is first
analyzed and compared with FT-IR of cutin and suberin. Possible composition of
monomers in NFSC is inferred from this comparison. Current-voltage (I-V) characteristics of NFSC under
different conditions are analyzed and finally a discussion of possible
electrical switching mechanism is presented.


## 2. Experimental

### 2.1. Sample Preparation

FT-IR spectrometer of Hyper model 8400 series
was used to obtain IR spectra of pristine and annealed samples. Cuticle samples
cut out from a seed (Figure3) were fitted into the sample holder of the FT-IR
spectrometer.Figure 3
Thin and transparent cuticle attached to the
Nandi flame seed.Annealing
was done by placing the sample inside a temperature-controlled furnace that was
fitted inside an electrically shielded cage of Lindberg/Blue Tube Furnace
TF55035C. Samples were annealed at
various temperatures of 320 K, 350 K, and 400 K for 30 minutes each. A mechanical
mask was made by cutting circular holes of diameter 0.56 cm through a thin soft
cardboard of dimensions 3 cm ×
8 cm. Samples were mounted on each of the holes and then irradiated with He-Ne
laser beam of wavelength 632.8 nm in a dark room. Irradiation was done
separately on different samples for a time period of 10 minutes, 30 minutes, or
60 minutes each. Electrode coating on the film of pristine, preannealed, and
preirradiated samples was done by using quick drying and highly conducting flash-dry silver paint
obtained from SPI Supplies, Pa, USA. A mask of circular aperture of 0.56 cm
diameter was used while coating to ensure uniformity in sizes of the coated
regions. Circular aluminum foils of the same diameter were placed on the coated
surfaces such that samples were sandwiched between two aluminum electrodes. The same flash-dry silver
paint was used to connect thin wires onto the aluminum electrodes. A sample holder is shown in Figure4. The thickness of the films was measured by a micrometer
screw gauge and found to be 4±0.02μm.Figure 4
Sample holder. Thin wires are connected on both sides of the samples.


### 2.2. Current-Voltage (I-V) Measurement

Before measuring voltage and current, the samples in Figure4 were electrically annealed by driving high current through
them. This was done by driving current of 0.3 A through the samples for 10–15
minutes. This reduced contact resistance and improved ohmic properties of the
contact. Resistance of the contact was found to decrease considerably after it
had been electrically annealed. I-V for both annealed and irradiated samples was measured using Keithley
2400 source meter. The setup
of the sample holder shown in Figure 4 was shielded with aluminum metal cage
coupled with an electrical insulating material in order to minimize
disturbances from electrical noise, temperature fluctuation, and
electromagnetic waves. I-V data for
each treated sample was determined with cable guard on. The cable guard of the
source meter was activated in order to minimize leakage current. Step voltages
of 0.1 V (within a range of 0.1 V–1.0 V), 1.0 V (within a range of 1.0 V–10.0 V), and 5.0 V
(within a range of 10.0 V–40.0 V) were sourced for both forward and reverse biases. When measuring
current at poling temperature, a sample sandwiched between aluminum electrodes
was placed inside the Lindbarg/Blue Tube. Furnace temperature was set to a
constant value while recording current-voltage measurements.


## 2.1. Sample Preparation

FT-IR spectrometer of Hyper model 8400 series
was used to obtain IR spectra of pristine and annealed samples. Cuticle samples
cut out from a seed (Figure3) were fitted into the sample holder of the FT-IR
spectrometer.Figure 3
Thin and transparent cuticle attached to the
Nandi flame seed.Annealing
was done by placing the sample inside a temperature-controlled furnace that was
fitted inside an electrically shielded cage of Lindberg/Blue Tube Furnace
TF55035C. Samples were annealed at
various temperatures of 320 K, 350 K, and 400 K for 30 minutes each. A mechanical
mask was made by cutting circular holes of diameter 0.56 cm through a thin soft
cardboard of dimensions 3 cm ×
8 cm. Samples were mounted on each of the holes and then irradiated with He-Ne
laser beam of wavelength 632.8 nm in a dark room. Irradiation was done
separately on different samples for a time period of 10 minutes, 30 minutes, or
60 minutes each. Electrode coating on the film of pristine, preannealed, and
preirradiated samples was done by using quick drying and highly conducting flash-dry silver paint
obtained from SPI Supplies, Pa, USA. A mask of circular aperture of 0.56 cm
diameter was used while coating to ensure uniformity in sizes of the coated
regions. Circular aluminum foils of the same diameter were placed on the coated
surfaces such that samples were sandwiched between two aluminum electrodes. The same flash-dry silver
paint was used to connect thin wires onto the aluminum electrodes. A sample holder is shown in Figure4. The thickness of the films was measured by a micrometer
screw gauge and found to be 4±0.02μm.Figure 4
Sample holder. Thin wires are connected on both sides of the samples.


## 2.2. Current-Voltage (I-V) Measurement

Before measuring voltage and current, the samples in Figure4 were electrically annealed by driving high current through
them. This was done by driving current of 0.3 A through the samples for 10–15
minutes. This reduced contact resistance and improved ohmic properties of the
contact. Resistance of the contact was found to decrease considerably after it
had been electrically annealed. I-V for both annealed and irradiated samples was measured using Keithley
2400 source meter. The setup
of the sample holder shown in Figure 4 was shielded with aluminum metal cage
coupled with an electrical insulating material in order to minimize
disturbances from electrical noise, temperature fluctuation, and
electromagnetic waves. I-V data for
each treated sample was determined with cable guard on. The cable guard of the
source meter was activated in order to minimize leakage current. Step voltages
of 0.1 V (within a range of 0.1 V–1.0 V), 1.0 V (within a range of 1.0 V–10.0 V), and 5.0 V
(within a range of 10.0 V–40.0 V) were sourced for both forward and reverse biases. When measuring
current at poling temperature, a sample sandwiched between aluminum electrodes
was placed inside the Lindbarg/Blue Tube. Furnace temperature was set to a
constant value while recording current-voltage measurements.


## 3. Results
and Discussion

Figure5 shows
FT-IR spectrum of pristine NFSC. IR absorption bands of NFSC are compared with
IR bands of cutin and suberin biopolyesters found in plant cuticles. IR bands and
band assignments are listed in Table 1. Cutin and suberin IR data bands and
comments in Table 1 are based on literature [33, 37–39]. The IR spectrum of NFSC shows a wide
band at 3348cm−1 assigned to O–H stretching vibration. This band is
correlated to absorptions at 3300cm−1 and 3464cm−1 in
cutin and subrin, respectively, and is caused by presence of alcoholic and
phenolic hydroxyl groups involved in hydrogen bonds [40]. Bands
located near 2900cm−1 and 2300cm−1 resulted from C–H asymmetric
and symmetric stretching vibrations of methoxyl and methylene groups,
respectively [37, 39]. Methylene is the most
repeated structural unit in the cutin biopolyester [37].
Around 1750–1630cm−1 wavenumbers, absorption bands are caused by
stretching vibrations of carbonyl groups. Absorption around 1603cm−1 and 1513cm−1 is
assigned to the stretching of C=C bonds and the stretching of benzenoid
rings. Absorption bands in the range 1300–1210cm−1 are related to asymmetric vibration of C–O–C linkages in ester to esters or
phenolic groups. Other IR absorption band assignments of NFSC in comparison
with those of cutin and suberin are summarized in Table 1. IR bands of NFSC
between 3300–1500cm−1 result from components
of both cutin and suberin while bands around 1400–1200cm−1 are more related to those of suberin components as shown in Table 1. Types of
monomers in cutin and suberin are shown in Table 2. Figure 6 shows FT-IR
spectrum of pristine NFSC in comparison with spectrum of NFSC sample annealed
at 400 K. The spectra show that IR absorption peaks for pristine and annealed
samples occurred at nearly the same frequency. This means that annealing (400 K)
does not destroy the functional groups in the sample.Table 1
FT-IR frenquencies (cm−1) of cutin, suberin, and NFSC. *Benítez et al. [37],
Cordeiro et al. [39].
Cutin*Suberin*NFSCBand assignmentComments*330034643348ν(O–H)Alcoholics and Phenolic hydroxyl
groups292429192916νass(C–H),Aliphatic stretch,28522851Methoxyl groups,2615νs(C–H)Methylene groups173617381682νas(COO−),Ester carbonyl,1731ν(C=O)Ketone groups163016351603ν(C=C)Conjugated double bonds,15301513benzenoid rings1463δ(C–H)Aromatic ring14371427νass(C–H)1364Metoxyl groups13001319Esters linkages,1210νass(C–O–C)Phenolics,Glycosidic link.116711301157ν(C–O)Aliphatic alcohols or Aliphatic esters1100ν(C–H)Benzene ring105510501033δ(O–H),Primary and secondary alcoholsν(C–O)Table 2
Types of monomers and their % abundance in cutin and
suberin biopolyesters. Pollard et al. [30].
Monomer typeAbundance (%) and common monomersExampleCutinSuberin1–25 %1–10%C 16:0,  C18:0,
C18:1, C18:2C18:0 to C24:01–32%11–43%,C16:0, C18:1, C18:2C18:1 to C26:0Usually<5% in Arabidopsis24–45%C16:0, C18:0, C18:1, C18:2C18:1, C18:2, C16:0 to C26:00–34%0–30%C18:0 (9,10-epoxy)C18:1 (9,10 epoxy-18-dyroxy)C18:1 (9, 10-epoxy)C18:0 (9,10-epoxy-1, 18-diacid)16–92%0–2%C16:0 (10,16-dihydroxy)C18:0 (9,10, 18-trihydroxy)C18:0
(9,10,18-trihydroxy)Traces0–8%C18:0 (9,10-dihydroxy)0–8%1–6%C16:0, C18:1C18:0 to C22:00–5%0–3%C18:1C22:01–14%14–26%0-1% Ferulate0–10% Ferulate, Samller amounts of coumarate, sinapate, ceffeateFigure 5
FT-IR spectrum of pristine NFSC.Figure 6
Comparison between FT-IR spectra
of pristine and annealed (400 K) sample.Figure7(a) shows I-V characteristics of pristine
and annealed samples. I-V curves were drawn as log-log plots because in many cases drastic
conductivity changes of several orders of magnitude were observed. It is noted that at
certain threshold voltage, Vth current rises rapidly by an order of about 104. There
are two distinct regions for forward bias regime. High-impedance region, which
occurs before Vth, is referred
to as OFF state and low-impedance region, which occurs after Vth, is known as ON state. At
low voltages, logI versus logV 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 1.8±0.2. The plots 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 I=Vn,
where n=1.8±0.2 obtained from the slopes. 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. Current increased by an order of
102 after switching to ON state. It was also noted that current did
not follow the same path on decreasing applied electric field, hence indicating
that the samples exhibit electrical switching with memory. This hysteresis
effect was not erased by annealing or irradiating the samples. The threshold
voltage Vth for pristine
samples was 5±0.5 volts which is higher than Vth reported for some synthetic polymers; polymethylmethacrylate (1.6 V), polystyrene (4.5 V) [12], phthalocyanine
(0.3 V) [41], 2,6-(2,2-bicyanovinyl) pyride
(5.01 V) [42], Langmuir-Blodgett (1.0 V) [43]. The width of Vth or transition voltage during switching from OFF to ON states was about 1.0 V. All samples returned to OFF state after about 30 minutes. Figure 7(b) shows that Vth increased nonuniformly
from 5–9 V, with increase
in annealing temperature. A decrease in magnitude of the negative dielectric
anisotropy during annealing is a major reason for the increase in Vth for the annealed samples
[41].(a)I-V Switching characteristics for pristine and annealed samples. (b) Variation of threshold voltage Vth with annealing temperature.
(a)(b)Figure8(a) shows that irradiated samples
have similar I-V switching properties
as annealed samples. The curves show electrical switching and memory
effect with Vth that
increased (from 5 to 8 V) with the increase in irradiation time. It is also
noted that irradiation increased electrical conductivity especially in the
reverse bias. It also increased the gap between OFF state and ON state currents
in the forward bias. Increase in conductivity for irradiated samples can be
attributed to dissociation of primary valence bonds into radicals. The
dissociation of C–C and C–H bonds leads to degradation and cross-linking which
improves electrical conductivity [44]. Exposure of polymers to ionizing
radiation produces current carriers in terms of electron and holes which may be
trapped in the polymer matrix at low temperatures [45].
If the original conductivity is small, then the presence of these carriers
produces an observable increase in the background conductivity of the polymer.
Irradiation of polymers also 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.
Figure 8(b) shows that Vth increased almost linearly with the
increase in irradiation time. Since irradiating time is associated with
an increase in irradiation dose [46],
increasing duration of irradiation could have led to an increase in the number
of chain ends acting as trapping sites which increased Vth.(a)I-V switching characteristics for irradiated
samples. (b) Variation of threshold voltage Vth with irradiation time.
(a)(b)Figure9 shows I-V curves of NFSC obtained at different
poling/measurement temperature. These curves show that electrical current
increases with the increase in measurement temperature. This is due to thermal
excitation of the trapped charges across the potential barrier. The curves also
show that forward bias characteristics for 320 K and 350 K have two distinct
regions, the ON and OFF states which are typical of electrical switching
process. It is also noted that Vth decreases with the increase in measurement temperature and that switching and
memory effects
almost disappeared at higher temperatures (370 K). Increased
temperature facilitates diffusion of ions in the space charge
polarization. Thermal energy may also aid in overcoming activation barrier for
orientation of polar molecules in the direction of the field [47].Figure 9
I-V switching characteristics at
different poling temperatures.Although
several examples of conductance switching have been investigated extensively, switching
mechanism in most cases is unknown and is the subject of some controversy [5]. Several possible mechanisms for the conductance
transition observed in NFSC were considered. This resulted in the identification
of a possible mechanism that is in agreement with the available electrical conductance
data and possible structure of this system. The first mechanism considered was
the quantum interference of different
propagation parts within the molecules which involve permutation of highest
occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals
(LUMOs) [48].This can be explained by considering the potential
barrier distinction between the metal contact regions. Under the positive bias,
the electrons are easier to inject from the electrode into the molecule and are
more difficult to transfer from the molecule to the opposite electrode, thus
resulting in the charging
of the molecule. In this bias, charging (charging energy) in the molecule moves
the HOMO up with respect to Fermi level, whereas under negative bias, the
deficit of electrons in the molecule moves the HOMO down [49]. Consequently, it can be reasonably concluded that
a smaller (larger) positive (negative) bias is necessary to drive the HOMO into
the HOMO-LUMO gap, hence allowing large currents to flow. This could also
result from permutation of frontier orbitals, HOMO−1, and LUMO+1. 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” toward the
acceptor group reducing the HOMO-LUMO gap of frontier orbitals and the
switching and hybridization between HOMO−1 and LUMO+1 take place. As the
strength of the field increases, the HOMO-LUMO gap in the molecular spectrum
becomes smaller and the HOMO, HOMO−1, and LUMO orbitals split into the
HOMO-LUMO gap, and hence become delocalized. Figure 10 shows a schematic
diagram of the switching effect under an electric field. However, this
explanation does not account for the formation of hysteresis and the return of
the samples to OFF state within 30 minutes.Figure 10
Schematic diagram for the switching effect
under an electric field.Another possible
mechanism considered for electrical switching in NFSC is nondifferential
resistance (NDR) which results from the injection of electrons from electrodes
leading to build up of space charges across the metal electrodes and formation
of high-field domains (or an accumulation layer due to dipole formation) [49, 50]. Formation
of high-field regions leads to elongation of low-field regions till the
interface separating the high- and low-field regions lies on the same potential
and is perpendicular to the current flow direction [51].
Electron injection into molecules
sandwiched between metal electrodes was originally proposed to explain NDR [50] but was criticized due to the requirement for a counterion to neutralize
the resulting space charge. The mechanism would also generate a negative space
charge in the molecules, which cannot be compensated easily. As more charge is
injected, space charge potential will increase, and the voltage required for
charge injection also increases. While space charge effects may not be
favorable for electrical switching, they do not appear to prevent switching a
minority of molecules in the absence of counterions [5].The most
likely source of conductance switching in NFSC is the formation of quinoid and semiquinone
structures from phenolic compounds present in plant cuticles [52, 53] and which are presumably present in NFSC. It
has been observed that electrical switching molecules are likely to have one or
more phenyl rings and hemiquinone structures [5, 54]. Organic conductors are assemblies of molecules
whose HOMO orbitals are partially filled [55]. This allows for easy proton-electron transfer
within molecules which incidentally may cause conductance switching. Aviram et
al. [54] suggested that electron-proton motion
within hemiquinones molecules
that comprised of catechol and o-quinone, molecules between two contacts may switch the
molecules to low-impedance (ON) state due to the formation of semiquinones-free
radicals. When an electron is injected into the molecules from the metal
contact, it is gained by an electron acceptor (o-quinone) molecule. An electron
donor (catechol) molecule then transfers the electron to the opposite contact
thus allowing flow of charges. The dipole, thus, setup would be rapidly
cancelled by one-proton motion from the positively charged catechol to the
negatively charged quinone group, giving rise to two neutral semiquinones which
are free radicals responsible for the onset of large current. The threshold switching occurs when
the fields become sufficient for proton motion and level matching for the
electron motion. Since conduction in cutin involves protons [56], conductance switching in NFSC is feasible under
this mechanism. However, hysteresis effect is better explained by considering
the formation of quinoid structures. Ferulic acids, which are compounds of
phenolic nature with some ionizable caboxylic and hydroxyl groups, can act as a
charge network forming pathways for electron-proton transfer. An electron
transfer into a phenolic compound may result in the formation of quinoid structure with many
resonance forms. The quinoid form is planar, and is highly conjugated compared
to phenyl groups. As the electric field increases, quinoid structures become delocalized
over the entire structure. This is the basis for the change of conductance
state from OFF to ON state. In addition to increased conjugation and planarity,
a decrease in HOMO-LUMO gap in the quinoid structures occurs in all forms of “switching”
molecules [5]. Benzene ring rotation and bond
length changes during the formation of quinoid result in activation barriers which are presumably
the origin of the temperature dependence conductance. Activation barrier
associated with the structural rearrangement is responsible not only for the
activation barrier but also for the switching hysteresis. The quinoid form is
apparently the higher energy form in the absence of an electric field, but is the
favored form in a high field [5]. Reorganization
energy is sufficiently large that the quinoid form survives after the field is
removed, at least for several minutes to several hours [5]. High conductance high field of the reverse regime may also be
due NDR and Fowler-Nordheim field emission process which involves tunneling of
free carriers in the bulk of the material [56].


## 4. Conclusion

I-V characteristics of Nandi flame seed
cuticle biomaterial have shown reproducible switching and memory properties.
The switching and memory effects are explained in terms of formation of
semiquinones and quinoids structures from phenolic compounds due to redox
processes. The switched samples lost memory within 10–30 minutes. Threshold
voltage Vth for pristine
samples was about 5 V. Vth increased to about 6–8 V after
irradiation with laser light of wavelength 632.8 nm. Annealing also increased Vth by the same magnitude.
Hysteresis effect in the I-V curves
is due to the presence of quinoid radicals. At a poling temperature of 370 K,
both switching and memory effects almost disappeared. Current in the OFF state
was ohmic but after switching to ON state, current was governed by space charge-limited
current (SCLC). These results have shown that NFSC has electrically switchable
molecules which can be explored further for their application in gas sensing
and electrical switching circuits.



---

*Source: 830270-2009-03-16.xml*