And Applications Of Inherently Conducting Polymers Nanostructures

As ICPs approach nanodimensions a number of trends become evident which involve either the preservation of the bulk properties of the ICP and/or the enhancement of the properties of the nanomaterial such as conductivity. Some of these influences of nanostructure reported in the literature are discussed.

He et al. [144, 145] have created a conducting polymer nanojunction switch with abrupt switching characteristics that are attributed to a single nanocrystalline domain in which the individual polymer strands collectively undergo the usual insulator-conductor transition.

Nanoelectronics will be an expanding area of application for ICPs. A recent example has been illustrated for the fabrication of organic photodiodes and light emitting diodes [175]. Fullerene derivatives grafted onto soluble poly-thiophene, based upon the PEDOT system, have been utilized to form an integrated donor-acceptor polymer system. Nanopatterned electrodes were prepared by using a silicone rubber replica stamp which was used to transfer an organic resist image of an optical grating (280 nm periodicity) on top of a metal film. This film was subsequently etched to form a nanopatterned metal electrode substrate, onto which the polymer was cast forming the electronic device. Short sub-micrometer/nanoscale distances between the metallic electrodes were important due to the polymer layer having a low charge carrier mobility (exciton), emphasizing the importance of nanofabrication for these devices.

Numerous reports have discussed the fact that nano-components have inherently greater conductivity than the macroscopic analog [111]. This is not surprising given the model described by Epstein [176] and Kaiser [177] in which the bulk properties of ICPs arise from crystalline (highly conductive) nanodomains in a sea of amorphous (less conductive) material. A number of experimental approaches [178] have been used to verify the presence of these domains.

Formation of these nanodomains provides us with a unique opportunity to access these regions of high order and subsequently higher conductivity. A challenge in accessing enhanced electrical properties is that many approaches for the formation of nanoparticles require the use of an insulating surfactant. This was noted by Chan et al. [101] where removal of the insulating surfactant required for nanoparticle formation caused the conductivity to increase. Others [179] have shown that dramatic improvements in specific capacitance for polythiophene tubules can be obtained, increasing from 4 C/g polymer for a flat film up to 140 C/g polymer and 148 C/g polymer for micro- and nanotube formations, respectively.

The realization of efficient photovoltaic devices based on inherently conducting polymers also depends on the ability to control structure at the nanodimension [180, 181]. Such materials generate electron/hole pairs (excitons) upon irradiation. The ability to generate electricity from these charge pairs depends on our ability to split these charges at appropriate interfaces. Unfortunately, the lifetime of excitons is short and only those formed within 10 nm of an interface will ever reach it. The development of efficient nanostructures is therefore critical to this area.

Metallic nanoparticles exhibit well-documented phenomena of surface plasmon absorption processes where the free electrons at the surface of the nanoparticle generate a dipole moment to interact with visible wavelengths [182, 183] when the particle is too small itself to interact with light by Bragg scattering processes. It has been reported that the surface plasmon absorption can interact with an encapsulating ^-conjugated polymer when it is oxidized [98, 99]. This interaction is reported to be due to the direct electronic communication of the surface electrons of the metal nano-particle with the encapsulating polymer. The result of this interaction is a redshift of the optical plasmon absorption band with respect to a lone metal nanoparticle. This shift in absorption can be achieved by an aggregation process of the metal nanoparticle, but no such aggregation was evident implying direct electronic interaction with the polymer and metal electron.

Xia and Wang [103] elegantly demonstrated the advantages and pitfalls to be encountered as we enter the nanoworld. As particle size decreases the degree of doping and crystallinity increases, with a concomitant increase in conductivity. They also showed crystalline domain sizes ranging from 10 to 40 nm and that as size decreases lattice defects may occur causing the crystalline structure to be broken due to the smaller size.

An industrial application of polyaniline utilizing nano-dispersions of the ICP into corrosion inhibiting paint latexes has been developed whereby 70 nm particles are dispersed to form a self-organised complex of ultrafine network [184]. For a number of years it has been demonstrated that ICPs can impart corrosion resistance to metal substrates. When these nanoparticles have been melt dispersed with a loading of 1% w/v in paint, the composite material is observed to have similar metallic properties to the bulk material. This arises due to the nanoparticle forming a network structure permitting electron tunneling via a continuous conducting pathway [185]. A clear advantage of utilizing these nanodispersions is that a similar corrosion inhibitor activity is achieved to that found using larger particle sizes, requiring higher polymer loading, or coated films.

Mandal has shown that the use of polyaniline nano-components results in extraordinarily low (0.03 vol%) percolation thresholds in polyvinylalcohol [186].

Polypyrrole-iron oxide nanocomposites have been prepared for application as humidity and gas sensors [187]. Improved sensor sensitivity is achieved by increasing the surface area based upon microstructure. By this approach nanoparticles were prepared by the reaction of ferric nitrate and methoxyethanol in the presence of pyrrole to produce a nanostructured aggregate with the appropriate morphology and surface area to function as a sensor.

Recently we have used aligned carbon nanotube platforms to fabricate inherently conducting polymer based biosensors with excellent performance characteristics [170].

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