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Electrophoretically Deposited Polymers for Organic Electronics 375

by the electrophoretic technique. Highly ordered structures such as
three-dimensional colloidal crystals (opals) 56–58 and ordered two-
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dimensional films of a binary mixture of colloids have been described
in the literature. The driving force for self-assembly in these systems
has been investigated 54, 56, 59–61 and is generally ascribed to electrohy-
drodynamic flow. 62–63 There are less-frequent reports of electropho-
retically deposited films of particles with dimensions in the low
nanometer range. Bailey et al. have shown unordered films deposited
by means of electrophoresis on substrates that were prepatterned
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using micro contact printing. Another example is from Gao et al., 65
who deposited CdTe nanoparticles on prepatterned indium tin oxide
(ITO) electrodes. Inverse opals made by EPD of small particles into
voids left between an ordered multilayer films of large particles have
been shown by Gu et al. 58
Generally, the assembled nanoparticulate films do not exhibit sig-
nificant ordering. A number of factors make deposition of ordered
films of nanoparticles and their investigation more challenging, as
opposed to their micron-sized counterparts. First, for aqueous sus-
pensions of nanocolloids, the thickness of the double layer is often
comparable to the particle size, giving rise to considerable interpar-
ticle repulsion. Second, brownian motion for small particles is more
important than for larger particles, thereby inhibiting ordering of the
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film. Furthermore, electroosmotic flow arising from electrophoretic
motion of ions in the aqueous liquid near charged surfaces, including
the substrate, interferes with the well-defined motion of the charged
particles. Kooij et al. have reported nanocolloidal gold particle
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deposition from an aqueous benzoate/benzoic acid solution at metal-
coated glass substrates, in the presence of an externally applied elec-
tric field. The spatial distribution of nanoparticles deposited in an
applied field exhibits a higher degree of order compared to the ran-
dom, irreversibly deposited nanocolloids at chemically functional-
ized surfaces. They have also explained electrohydrodynamic forces
and capillary forces as the deriving forces for the ordering of the
nanoparticles. Figure 10.4 shows the SEM images of nanocolloidal
gold films deposited in the presence of external electric field.
EPD has been used to synthesize nickel-alumina, functionally
graded materials from NiO, and alumina suspensions in ethanol by
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Nagarajan et al. Functionally graded materials (FGMs) are compos-
ites with gradual transition of microstructure and/or composition.
The importance of the gradual transition is to increase the strength of
the bond between composites of dissimilar materials; e.g., ceramic/
metal interfacial coherence can be increased by continuous gradation
rather than a sharp discontinuity. Sarkar et al. first obtained the EPD
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of functionally graded materials. Milczarek and Ciszewski have
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used the electrophoretic technique for the easy and rapid deposition
of Ni(II) and Co(II) phthalocyanines. Metal phthalocyanines (MPcs)
are macrocyclic complexes and have been known to be excellent