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model, the tilt always points to the center of the cluster, but the magnitude varies
from straight up at center of the cluster to some maximum tilt at the cluster edge.
Such a structure contains display distortion of the tail ordering and has been called
“micellar cluster.” Both models exhibit contributions to translational disorder beyond
that existing in the head group lattice. The micellar cluster model preserves strong
bond orientational correlations, while the first model degrades them to some extent.
Simulations have shown that the micellar model can fit the general features of the
diffraction pattern better than the first model. The precise nature of the positional
disorder, liquid-like or microcrystalline, is difficult to determine for the strongly dis-
ordered structure of complex molecules found in the LB monolayer. Especially, one
would like to know if the structure is similar to a stacked-hexatic phase (long-range
bond orientation with exponentially decaying translational correlations) or micro-
crystalline (perfect translational correlations).
Electrical behavior of LB films:
Insulating thin films in the thickness range 100-
20000 Å (100-20000 10
−10
m) have been the subject of varied interest among the
scientific community because of the potential applied significance for developing
devices, such as optical, magnetic, electronic, etc. Some of the unusual electrical
properties possessed by thin LB films that are unlike those of bulk materials led to
the pursuit of their technological applications, and, consequently, interest in thin-
film studies grew rapidly. Earlier studies did not prove to be very inspiring because
the LB films obtained always suffered from the presence of pinholes, stacking faults,
and other impurities, and hence, the results were not reproducible. It is only in the
past few decades that many sophisticated methods have become available for the
production and examination of thin films, and reproducibility of the results could
be and controlled to a greater extent. Nevertheless, the unknown nature of inherent
defects and a wide variety thin-film systems still complicate the interpretation of
many experimental data and thus hinder their use in devices.
Breakdown conduction in thin films, the major subject of investigation, has been
based on the films prepared by thermal evaporation under vacuum or similar tech-
niques. It was realized that LB films have remained less known among the investiga-
tors of this field. Their various interesting physical properties have been investigated
in the current literature.
4.6.1 S
T r u c T u r e
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The structure and orientation of the deposited amphiphile molecules have been found
to be governed by the angle of contact between the monolayer and the solid surface.
The deposited monolayers, in general, have been characterized as “X-,” “Y-,” and
“Z”-type, and their molecular arrangements can be described as follows.
In the case of X and Z-type films, the molecules are oriented in the same direc-
tion, and thus, the surface will be composed of carboxyl and methyl groups. On the
other hand, in a Y-type film, the molecules in adjacent layers will be oriented in
opposite directions, and the LB film surface will be composed of methyl groups.
Of these three types of LB films, the one that has been studied is the Y-type, in
which the monolayer transfer takes place both ways on each dipping and withdrawal
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