and tail-to-tail conformation, the film formed, if composed of only one
kind of amphiphilic molecule, is completely centrosymmetric. Such a
film possesses no dipole moment, rendering the assembly useless for
NLO applications. However, if fatty acid spacers, such as stearic or
arachidic acid, are incorporated between the other amphiphilic species,
a net dipole moment may result. This procedure requires two separate
LB compartments for each type of molecule. The substrate is inserted
through a layer of the amphiphile of interest, moved to the other
compartment under water, and withdrawn out of the fatty acid spacer
Langmuir film. Research has shown a relatively large SHG signal for
such a polar-ordered arrangement. Conversely, X- and Z-type films are
inherently arranged in an acentric manner with a net dipole moment
due to the head-to-tail molecular conformation. In fact, studies by some
researchers have found that Z-type LB films generate a strong SHG
signal.
Despite the virtues of the LB technique, it also has its shortcomings. One
limitation is the unstable nature of the adsorbed films. In practice,
although the transfer process occurs very slowly, the layers are not
always transferred onto the substrate as would be desired. After deposition, the molecules have been shown to rearrange to more stable
conformations, which could lead to loss of the acentric order that gives
the films their NLO activity. Another disadvantage is the occurrence of
chromophore randomization. It has recently been shown that intermixing between LB layers is present. This produces structural defects
and chromophore tilting that becomes amplified when the number of
LB layers is increased. Thus, it is not surprising that experimental results
show that the SHG signal resulting from the head-to-head, polarordered (Y-type) LB films decreased with thickness (Johal et al., 1999).
Moreover, the fatty acid spacers used to make acentric alignment more
energetically favorable could lead to even more disorder in thicker LB
films. Stearic acid has been found to form two-dimensional crystalline
structures that result in a heterogeneous monolayer filled with defects.
Finally, staggered molecular structures may form due to repulsive
interaction between strong dipoles. The effect of intermolecular interactions on the order of the LB film are still sparsely understood, and
current research addresses the effects of dipolar interactions on film
formation. With the limitations of the LB technique and other aforementioned methods, the introduction of the electrostatic self-assembly
method was readily welcomed.
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
388
kind of amphiphilic molecule, is completely centrosymmetric. Such a
film possesses no dipole moment, rendering the assembly useless for
NLO applications. However, if fatty acid spacers, such as stearic or
arachidic acid, are incorporated between the other amphiphilic species,
a net dipole moment may result. This procedure requires two separate
LB compartments for each type of molecule. The substrate is inserted
through a layer of the amphiphile of interest, moved to the other
compartment under water, and withdrawn out of the fatty acid spacer
Langmuir film. Research has shown a relatively large SHG signal for
such a polar-ordered arrangement. Conversely, X- and Z-type films are
inherently arranged in an acentric manner with a net dipole moment
due to the head-to-tail molecular conformation. In fact, studies by some
researchers have found that Z-type LB films generate a strong SHG
signal.
Despite the virtues of the LB technique, it also has its shortcomings. One
limitation is the unstable nature of the adsorbed films. In practice,
although the transfer process occurs very slowly, the layers are not
always transferred onto the substrate as would be desired. After deposition, the molecules have been shown to rearrange to more stable
conformations, which could lead to loss of the acentric order that gives
the films their NLO activity. Another disadvantage is the occurrence of
chromophore randomization. It has recently been shown that intermixing between LB layers is present. This produces structural defects
and chromophore tilting that becomes amplified when the number of
LB layers is increased. Thus, it is not surprising that experimental results
show that the SHG signal resulting from the head-to-head, polarordered (Y-type) LB films decreased with thickness (Johal et al., 1999).
Moreover, the fatty acid spacers used to make acentric alignment more
energetically favorable could lead to even more disorder in thicker LB
films. Stearic acid has been found to form two-dimensional crystalline
structures that result in a heterogeneous monolayer filled with defects.
Finally, staggered molecular structures may form due to repulsive
interaction between strong dipoles. The effect of intermolecular interactions on the order of the LB film are still sparsely understood, and
current research addresses the effects of dipolar interactions on film
formation. With the limitations of the LB technique and other aforementioned methods, the introduction of the electrostatic self-assembly
method was readily welcomed.
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
388