Biomedical Engineering Reference
In-Depth Information
certain threshold voltage, V
th
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 V
th
, 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
n
I
α where
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
V
th
for pristine
samples is 5.0+0.5 volts. The width of
V
th
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
V
th
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
V
th
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
V
th
for pristine cuticles is higher than
V
th
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 V
th
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.
2.04
