Biomedical Engineering Reference
In-Depth Information
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
-
V
data for annealed samples of NFSC were analyzed by potting
2
JE
versus 1/
E
. 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
ln(
/
)
*
3/2
42
m
/3 (
q
=
q
ϕ
)
where
m*
is the
B
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
