Chemistry Reference
In-Depth Information
4. DISCUSSION
As has been mentioned in Section 3, the surface potential of PI LB films shows an
interesting behavior. That is, the surface potential created by the application of the
voltage depends on the polarity of the applied voltage, the number of deposited
layers, and workfunction of metals. These results suggest that the main contribu-
tion to the creation of the surface potential is the excess electronic charges. Simi-
larly, the electrical breakdown voltage of PI LB films depends on the workfunc-
tion of metals used as substrates for the LB film preparation. In this section,
firstly we explain our experimental results phenomenologically, and then discuss
the possibility of charge transfer created by the application of biasing voltage.
As described in our previous papers [13, 25], very high density of electron ac-
ceptor states, which can acquire excess electrons displaced from the metal elec-
trode, exists in PI LB films. These acceptor states exist contiguous to the Fermi
energies of contacting metal electrode at the PI LB film/metal interface. Excess
electronic charges are transferred from metals to these electronic states in PI LB
films until a thermodynamic equilibrium is established at the interface. Figures 7
(a) and (b) illustrate the space charge distribution and the distribution of density
of states at the interface. As shown in Fig. 7 (a), the excess charges are injected
into PI LB films from the metal electrodes. Thus electric flux diverging from the
positive charges on the surface of metal electrodes falls on the excess charges in
PI LB films. Therefore, the surface potential V
s
built across the PI LB films is
given by [24, 26]
D
x
ρ
x
x
(
ε
)
V
=
dx
=
,
(1)
S
ε
ε
0
ε
r
0
0
r
D
x
ρ
x
dx
D
(
)
D
=
ρ
x
dx
with
x
=
0
and
Q
(
)
.
ρ
x
dx
0
(
)
0
ε
0
is dielectric permittivity of vacuum,
ε
Here,
(= 3) is the relative dielectric
constant of PI, D is the film thick
n
ess,
x
is the distance from the metal electrode,
()
r
is the space charge density,
x
is the mean location of the excess charges in-
jected from the electrodes, and
Q
is the total charge injected from the metal elec-
trodes. As described in our previous study [24, 25], most of the excess charges ex-
ist in PI LB films within a distance of 4 nm from the electrodes, and about 1 to
10% of monomer units of PI accept electrons from the metal electrodes in this re-
gion. At equilibrium, it is expected that the surface Fermi level of PI LB films and
the Fermi level of metals coincide at the interface as shown in Fig. 7 (b). There-
fore, the electronic states of PI whose electronic energy is higher than the Fermi
ρ
x
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