Introduction to Nanotechnology

292 ORGANIC COMPOUNDS AND POLYMERS
appearing at wavelengths of 640, 656, and 652nm, respectively, which is the
expected shift toward lower energies (i.e., longer wavelengths) with increasing
particle size.
11.4. POLYMERS
11.4.1. Conductive Polymers
Many nanoparticles are metals, such as the structural magic number particle Au,,.
The metals under consideration in their bulk form are good conductors of electricity.
There are also polymers, called conductive polymers or organic metals, which are
good conductors of electricity, and polyacetylene is an example. Many polyaniline-
based polymers are close to silver in the galvanic series, which lists metals in
the order of their potential or ease of oxidation.
Acetylene HC-CH has the monomer
H H
-c=c-
(11.16)
corresponding to the repeat unit [-CH=CH-1,. Other examples of compounds
that produce conducting polymers are the benzene derivative aniline C6H,NH,,
which may also be written 4NH,, and the two 5-membered ring compounds pyrrole
(C4H4NH) and thiophene (C4H4S). The structure of thiophene is sketched in
Fig. 10.20, and pyrrole has the same structure with the sulfur atom S replaced by
a nitrogen atom N bonded to a hydrogen H. These molecules all have alternating
double-single chemical bonds, and hence they form polymers that are n-conjugated.
The n conjugation of the carbon bonds along the oriented polymer chains provides
pathways for the flow of conduction electrons, and hence it is responsible for the
good electrical conduction along individual polymer nanoparticles. Polarons, or
electrons surrounded by clouds of phonons, may also contribute to this intrinsic
conductivity. The overall conductivity, however, is less than this intrinsic conducti-
vity, and must take into account the particular nature of the polymer.
Wessling (2000) has proposed an explanation of the high electrical conductivity
of conductive polymers such as polyacetylene and polyaniline on the basis of their
nanostructure involving primary particles with a metallic core of diameter 8 nm
surrounded by an amorphous nonconducting layer 0.08nm thick of the same
[CZH2], composition. Figure 11.9 presents a sketch of the model proposed by
Wessling based on scanning electron microscope pictures of conductive polymers.
The individual nanoparticles are seen joined together in networks comprising 30-50
particles, with branching every 10 or so particles. Several of the nanoparticles are
pictured with their top halves removed to display the inner metallic core, and their
surrounding amorphous coating. The electrical conductivity mechanism is purely
metallic within each particle, and involves thermally activated tunneling of the