Page 400 - Organic Electronics in Sensors and Biotechnology

Electrophoretically Deposited Polymers for Organic Electronics 377

of organic materials such as carbon nanotubes (CNTs), polymers, etc.
on conductive substrates.
EPD of Carbon Nanotubes Since the discovery of CNT in 1991 by
72
Iijima, CNTs have been looked at extensively by researchers in vari-
ous fields such as chemistry, physics, materials science, and electrical
engineering. CNTs are unique nanostructured materials with remark-
able physical and mechanical properties. Many of the remarkable
properties of CNTs are now well established, and their exploitation in
a wide range of applications forms a major part of current research
73
and development efforts. One of the challenges is to tackle the prob-
lem of manipulating CNTs, individually or collectively, to produce a
particular arrangement needed for a given application. One very
promising technique being developed for manipulating CNTs is EPD.
For successful EPD, preparation of a stable dispersion of CNTs in a
suitable solvent is necessary. The most common strategy is the pro-
duction of an electrostatically stabilized dispersion, which in general
requires the preparation of a solvent medium in which the particles
have a high ξ potential, while keeping the ionic conductivity of the
suspensions low. The stability of CNT suspensions, determined by
ξ-potential measurements, has been studied mainly in aqueous and
ethanol-based suspensions. 74
The earliest investigations appear to be those of Du et al., who
45
explored the possibility of using EPD to deposit multiwalled CNT
(MWCNT) from ethanol/acetone suspensions on metallic substrates.
They observed strong hydrogen evolution at the cathode, leading to
a porous film of nanotubes with pore sizes ranging from 1 to 70 μm.
Thomas et al. have successfully deposited homogeneous MWCNT
46
films onto stainless-steel substrates using EPD from aqueous suspen-
sions of acid-oxidized nanotubes. No hydrogen evolution is observed
during this deposition. This result contrasts with that of Du et al. 45
and it may be attributed to the lower electric field strength used by
46
75
Thomas et al. Du and Pan have electrophoretically fabricated thin
films of MWCNT using (Mg (NO ) )6H O as electrolyte, and they also
3 2 2
reveal the application of these electrodes as super capacitors. These
MWCNT electrodes exhibit a significantly small ESR and a high spe-
cific power density. The super capacitors also show superior frequency
response, with a frequency ‘‘knee’’ more than 70 times higher than the
highest reported knee frequency for super capacitors. In addition,
this carbon nanotube thin film can act as a coating over an ordinary
current collector to decrease the contact resistance between the active
materials and the current collector for improved performance. EPD has
been used to a limited extent to deposit single-wall CNTs (SWCNTs). 49
One report describes the production of SWCNT deposits from very
dilute SWCNT suspensions in ethanol (1 mg SWCNT in 200 mL ethanol)
50
after the addition of a suitable salt (MgCl ). Other solvents investi-
2
gated for SWCNT deposition include dimethylformamide (DMF)