Chemistry Reference
In-Depth Information
0
10
N (number of layers)
FIGure 4.12
Variation of [1/C] (C = capacitance) of LB films versus number of layers (
N
)
(schematic).
The dielectric anisotropy of long-chain fatty acid monolayers was analyzed. These
fatty acids were considered as being oriented in a cylinder cavity with length (L) >>
diameter (D). Considering each bond in these molecules as a polarization ellipsoid
with axial symmetry about the -C-C- bonds, the mean polarizibtlity of the bonds
was calculated.
The LB film of fatty-acid salt has been known for long time to have breakdown
strength ca. 10
6
V/cm. The earliest study of breakdown of the films under high DC
voltages were measured using a galvanometer to read the current. Using an Hg drop
as the upper electrode, no current was detected, until the magnitude of the applied
voltage across the specimen reached critical value in the case of a 30-layer LB film,
whereas in thinner films, appreciable currents were present almost from start. The
breakdown per layer was found to rise sharply at a thickness of ca. 20 layers when
its order of magnitude was ca. 10
6
V/cm. The specific resistance was found to be ca.
10
13
ohms, below the breakdown voltage. In later studies, water drop with electrolyte
was used instead of Hg, and no difference was found between the two methods. The
destructive breakdown in LB films has been found to be composed of two events: the
destructive breakdown voltage and the maximum breakdown voltage. These events
have been determined in various fatty-acid soaps in the thickness range correspond-
ing to 16-80 layers.
The destructive breakdown voltage has been reported to be independent of the
material and thickness of the film, and the maximum breakdown voltage has been
reported to vary slightly with increase film thickness. The breakdown voltage system
has been reviewed by various investigators. The breakdown mechanism in LB film
has been described in terms of a statistical model (Agarwal, 1974; Birdi, 1999). The
AC breakdown in LB film is still at a stage where it is not well understood. The study
of AC voltage characteristics in MOS structures has been found to provide much
useful information about the mechanism of certain type of devices. The LB films
of Ca-behenate have been investigated with orthophenanthroline with three stearate
chains. In these studies, the p-type silicon surfaces (metal-insulator-semiconductor
[MIS]), after etching in CP4, cleaning, and drying, were used to deposit LB films,
and were covered with aluminum in the form of two circles connected by a channel.
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