Page 405 - Organic Electronics in Sensors and Biotechnology

382 Chapter Ten

agglomerate formation within the diameter range of 90 to 120 nm can be
seen at certain places. The AFM image of the composite (Fig. 10.6b) clearly
reveals uniform and homogeneous distribution of CNTs within the
polymeric matrix poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene
vinylene].

10.3 Applications of EPD

10.3.1 Photon Crystal Technology
The photon crystal technology (PCT) has attracted increasing interest
for the past few years. The current focus of research lies in the search
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for a three-dimensional full photonic band gap (PBG). The full PBG
was first observed in the microwave regime; subsequent reductions
in the wavelength were achieved. Recently, it has been developed in
the 5 to 10 μm wavelength regions by wafer fusion techniques and in
the 1.35 to 1.95 μm wavelength with silicon processing techniques. 92
The present goal is to achieve drastic reduction in the operating
wavelength range because of the enormous number of applications
of these materials when operated in the near-infrared/visible ranges
and inhibition of the spontaneous emission of lucent materials
embedded therein that can lead to a thresholdless laser. To achieve it,
ordered arrays with micron and submicron parameters are needed.
Various techniques based on the use of colloids have been developed
to construct these solid arrays. Recently, a technique using a local
electric field generated in the EPD suspension has been developed for
a novel particle assembling. This technique is called the micro-EPD
(μ-EPD) process. A microdot consisting of mono-dispersed polysty-
rene or silica spheres has been prepared, which works as photonic
crystals.
The EPD technique produces single-crystal colloidal multilayers 93
on the time scale of minutes, which is a drastic acceleration in com-
parison with the most common gravity sedimentation technique. It is
also quite simple to realize and yields three-dimensional photonic
crystals of quality comparable to or higher than that with the other
methods. Figure 10.7 shows SEM images of colloidal crystals made
from the colloidal suspension of 300 nm PS latex spheres. This tech-
nique is applicable to a wide variety of particles; however, it is neces-
sary to note that the colloids to be deposited should be tolerant to the
addition of alcohol to the dispersion. Besides this, EPD offers the pos-
sibility of patterning and impregnating the photonic crystals with
94
luminescent materials. This technique has opened the possibility of
preparation of uniform coatings of photonic crystals on curved sub-
stances such as spheres, which would be impossible by means of
gravitational or centrifugal forces. These coatings can be the basis of
unique diffraction devices. The overlap of the photonic stopband and