Nanotubes and nanowires hold special positions in nanoscience. The fascination is not only because of their elegant symmetry, but also because of the theoretical simplicity and the maneuverability of the material's integration. Different from nanodot, the microscale length of nanoline offers us an easy access to low-dimensional confinement by microscale manipulation  [ 1 ], which is important to the device fabrication, and more importantly, to comprehensive applications of nanoelectronics. That's why finding new one-dimensional structures remains hot topics. Interestingly, this hot sometimes makes us feel difficult to depict appropriate relationship between nanoscience and nanolines because it seems a dilemma of the chicken or the egg, but we are clear that we will still mumble a published route for nanomaterials that it is good if it can make them with right quality, and better to get simultaneously with high yield; and best to get a semiconductor that can be doped n- or p-type. 

We have known many one-dimensional objects: the inorganic or organic, the insulator, semiconductor or metal, and the crystalline or amorphous [ 2 ]. However, due to the complexity, outstanding challenges hold in those organic species. The omnipotent potentials have been revealed for carbon nanotubes, but it is only at the primary stage to tell exactly what an organic nanotube can be competent for the leading role of the polymer nanoelectronics. 

Who can be next VIP in nanopolymers? Key successes in today's microelectronics kindly imply that the possibility is larger to start with those conducting-tunable materials. Therefore, the conjugated polymers of polyacetylene, polythiophene and polyaniline come into view as their novel properties and big potentials in polymer electronics have been demonstrated in the past three decades [ 3, 4 ]. 

For example, as a model polymer, polyaniline can be reversibly non-redox doped/de-doped via simple acid protonation/base deprotonation with little or no degradation of the polymer backbone (Figure1). The conductivity of its emeraldine base form can be changed from 10^-10   to 10^0~1 Siemens per centimeter by 'protonic-acid doping' with, for example 1 M aqueous HCl-during which the number of electrons associated with the polymer chain remain unchanged [ 3 ]; 

However, charge transport in conducting polymers, as in all metals and semiconductors, is limited by a combination of intrinsic electron-photon scattering and sample imperfection. Although metallic levels of conductivities have been identified for partially oriented and heavily doped polyacetylene, the absence of a metal-like temperature dependence suggests that the observed values are not intrinsic. [ 4 ]

Polyaniline

Hopefully, the stone came last year [ 5 ]. It's the emeraldine base form of polyaniline film doped with camphor sulphonic acid. By using of self-stabilized polymerization, Korean researchers got it with conductivity three times than that of the samples made by conventional methods [ 6 ]. They and A. J. Heeger finally proved a metal polyaniline that possesses right temperature dependence of conductivity and a metal-like plasma-frequency in the near infrared. This frequency defines a metallic optical reflectivity that can be accurately fitted by the Drude model, which is used to explain charge transport of metal through the free-electron approximation.

"In fact, the surprise is just how like an ordinary metal polyaniline turns out to be." said Richard Friend [ 7 ], "this finding pitches the metallic model against a different model, loosely referred as to as the 'polaron model'  [ 4 ], that over the past two decades has been the favoured explanation of polymer conductivity."

Obviously, the metallic transport in polyaniline as well as the fabrication details will remain a contentious issue in near future. Beyond the favoured polaron model or the modified Drude model [ 8 ], what are the intrinsic causes of the transition from polaronic conductor to simple metal? Of which does a factor deserve more recognition during synthesizing? In the route of metal polyaniline [ 6 ], the polymerization is performed in a heterogeneous biphasic system of organic and aqueous medium without any stabilizers, in terms of main modification with which the monomers and growing polymer chains act as stabilizers, resulting in enough dispersion of the organic phase within the aqueous reaction medium. Supporting with the spectroscopic evidence of X-ray diffraction, a higher weight of spontaneous order arising in the polyaniline backbone should be responsible for the metallic behavior, hinted the authors [ 5 ]. 

Up to date, because of light atom weights of C, H and O etc., we can't see the lattice structures of polymer chain by high resolution TEM. This limitation much impedes us to develop more improved synthesis, and decisively correlate ordered packing of polymer chains with performances' enhancement. Most often we prefer to believe in this correlation for low-dimensional materials while there is still hesitation. The spectroscopic evidence is not enough. Why?

The properties of bulk solid material, thus the electronic structure is sensitive to micro/mesoscopic environment (defects and doped impurities etc.) as well as macroscopic arrangement of the inorganic lattice or organic chain (the long-range order). When the dimension shrinks down to the nanometer regime, many classical concepts face the definitions' limes. One big topic is what an aggregation of atoms can be called a solid, which is very related to hot words of 'size' or 'size-selected/control', and 'shape' or 'shape-control' in nanoscience. 

For bulk polycrystalline material, the shape effect, so-called the boundary conditions of the potential field in physics, has no evident influence on many macroscopic properties. It's partially because those different functional responses, such as the mechanical, thermal and electromagnetic responses, intrinsically take effect on different scales, and many mesoscopic effects are statistically screened or neutralized due to lager-size aggregation of randomly oriented single crystallites; Moreover, some physical responses can show in combined features that have one-to-more correspondences to different featured sizes (order parameters). Therefore, a nanoscale entity may show a bulk-like property here and yet can act with low-dimensional effect there. This combination poses great difficulty on applying classical models to nano-phenomena as well as the redefining of the concepts [ 9 ]. That's why we should be very careful when we only extract the conclusions from those spectroscopic evidences that may screen true origin.

However, before getting an image of polymer chains with atomic resolution, we have no choice but to rely on this phenomenological deduction. We hope that it's reasonable in most cases. In general, as expected, the physics of low-dimensional systems provides additional guidance for property tailoring. For example, possibly due to better packing and alignment of the polymer chain, the template-synthesized polypyrrole fiber has higher conductivity comparing with its bulk counterpart [ 10 ]. And there are also so many polyaniline mesostructures and conceptual devices that tell the stories with similar implications on order [ 11 ].

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Figure 2
[left] TEM images of the textured polyaniline plates (Fig.2a & b) elucidate the fine structures of two typical structures revealed by SEM (Fig.2c). It is evident that the ladderlike structure (a) is the base of formation for the gridlike one (b). The inset in (b) shows a typical gridlike structure. c) SEM image of the gridlike plate.
[right] Possible mechanism of the self-organization during polymerization

  
Reaction conditions: [Aniline] = 0.10 M, [FeCl3] = 0.20 M , [HCl] = 1.0 M ; hydrothermal reaction at 120 ^oC for 60 hours, followed with 5 min water cooling. 
Scale bars: 1 μm, the inset in (b) has the same scale. 

Credit:Scidea Art 2007 Source: ScideaNews.com

Figure 3 Polyaniline Nanotubes

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On my own experiences, I prefer to believe the microstructural explanation of the metallic polyaniline, i.e., the conductivity increasing with higher order. Recently, we reported some novel polyaniline mesostructures made through hydrothermal method [ 12 ]. The samples show evident changing in PL intensity and conductivity, and therefore the electronic structure, is sensitive to the microstructures, namely, can be tuned by the processing conditions with fine controlling of reaction rate. Among the samples with different morphologies, there is a surprise of the grid-like polyaniline nanoplate (Figure 2). The TEM observation reveals its fine structure consisting of the parallel nanowires arrays in different layer. We do not know why polyaniline there prefers to dominantly directional growth, but some clues indicate that the 'seeding' plates with polarized edges, developing in the early term of the synthesis, should be attributed to this ordered growth. Therefore, polyaniline-depositing on a charged substrate can be a right direction to further improve the self-organization during polymerization.

This deduction may be supported by our recent work on polyaniline nanotubes that published as the cover story in 6 February 2007 issue of Advanced Materials [13, Figure3]. We use manganese oxide as the physical template and the chemical oxidative initiator for the aniline polymerization. The template can be removed after the reaction, as manganese oxide is reduced into soluble Mn²⁺ ions [Figure4]. Many polyaniline mesostructures, such as nanotubes, spherical tube brushes, and double-shell nanotubes, can be fabricated using this method. 

In our synthesis, cryptomelane-phase manganese oxide nanowires [ 14 ], β-manganese oxide nanotubes [ 15 ], and β- and ε-manganese oxide nanowires [ 16 ] were used as templates for polyaniline nanotubes; the templates had to be washed thoroughly before use.Manganese oxide has a variety of applications, for examples, mild oxidizing catalysis, ion-exchange processes, magnetic applications, and field of electrochemistry such as cathode materials in alkaline and rechargeable batteries. There exist many effective routes for precisely controlled synthesizing of manganese oxide mesostructures, such as helices, colloidal crystals, three–dimensional structures, nanowires and nanotubes, with several crystallographic forms of α-, β-, γ- and ε-type [ 14-19 ].

Thereby, our method that using manganese oxide nanolines as reactive templates gives a big chance for the fabrication of polyaniline nanotubes. Moreover, as a 'side-effect', and also because of the importance of polyaniline, our method suggests that further attention should be imposed on exploring novel manganese oxide mesostructures as well as the improved synthesis with better size- and shape/morphology-control.

Furthermore, the method may be suitable for making nanotubes of other conductive/conjugated polymers of polypyrrole, and insulative polymers of polyacrylonitrile. This is because the oxidant potential of MnO2 (1.23 V) is higher than the half-wave potentials of their polymerization reaction.

It should be pointed here that the quality of polyaniline nanotubes from the template nanowires of cryptomelane-phase manganese oxide is very high. To my best knowledge, it's the best one among all the reported polyaniline nanotubes. Note that the larger diameter of the well-faced cryptomelane-phase manganese oxide nanowires provide enough smooth surfaces for the slow growth of polyaniline. Indeed these outer faces can act as the substrates used in conventional chemical vapor deposition (CVD). The underlying mechanism can be of diffusive nature and bears much resemblance to growing SiO2 film by thermal oxidation of Si substrate. I would like to call this diffusion-controlled procedure as chemical liquid deposition (CLD).

Do these findings along with many polyaniline mesostructures and conceptual devices suggest that we should start a new chapter on full polymer-based electronics with polyaniline nanotubes and films, in terms of a possibility in which the comprehensive modification of conductivity can be gotten from insulator, semiconductor to metal in nanopolymer, which is just similar to what we have done well for inorganic carbon nanotube?

It may be in time. □ 

Acknowledgment