Nonhlanhla P. Cele^1,2, Suprakas Sinha Ray^1 and Muzi Ndwandwe^2

^1National Centre for Nano-Structured Materials, Council for Scientific and Industrial Research,1-Meiring Naude Road, Brummeria, PO Box 395, Pretoria 0001, Republic of South Africa.^2Derpartment of Physics and Engineering, University of Zululand, Private bag X 3886, Kwadlangezwa 1001, Republic of South Africa
Email: ncele@csir.co.za

Polymer nano-composites (PNCs) have recently shown the worldwide growth efforts in the fabrication of high temperature proton exchange membrane for fuel cells. In principle the nano-composites are an extreme case of composites in which case the interface interaction between two or more phases are maximised to obtain superior performance as compared to any of the bulk solid component. In PNCs, nano-meter-size particles of inorganic or organic materials are homogeneously dispersed as separate particles in a polymer matrix [1,2]. There is a wide variety of nano-particles that are blended with the Nafion membrane to generate new structures of materials to improve its properties for proton exchange membrane fuel cell (PEMFC) applications [3-5]. CNTs are considered as the most promising nano-fillers for the preparation of conducting and thermally stable polymer nano-composites, because of their excellent electrical conductivity, thermal and mechanical stability [6-9]. Nafion based nano-composite membranes were prepared with pure multi-walled carbon nano-tubes (PMWCNTs), oxidised MWCNTs (OMWCNTs) and functionalised MWCNTs (FMWCNTs) as fillers, to investigate the effect of multi-walled carbon nano-tubes on thermal stability and mechanical properties of the Nafion membranes. The results showed much improvement on thermal stability of prepared Nafion nano-composites compared to pure Nafion membrane with an addition of only 1% wt percent MWCNTs.

We report conductance fluctuation in VO2 nano-ribbons of 10 nm thickness at moderate temperatures. Synthesis of these nano-ribbons was reported elsewhere [1-4]. The fluctuations are periodic at room temperature up to the VO2 transition temperature of 70 oC. These are surprising results since dc currents are producing a.c. potential difference values in i-v characteristics of the nano-ribbons of VO2 contrary to those of normal bulk materials. Three main theories were considered in order to explain these findings (1) The LRC equivalent circuit theory (2) the Gunn effect [5] and (3) the Universal Conductance Fluctuations theories [6-15]. The first two theories failed to explain our experimental data. We have explained this anomalous behaviour by the third theory which is a manifestation of the wave nature of electrons. The wave nature of electrons has been demonstrated in many instances including the Nobel–prize–winning Davisson & Germer experiment on electron diffraction. In electronic circuits, quantum interference in metallic wires [6-8], the so-called ‘weak localization’ [9,10] and universal conductance fluctuations (UCF) [11-13] are all manifestations of this wave nature. Fluctuations originate from coherence effects for electronic wave–functions and thus the phase–coherence length, lf needs to be smaller than the momentum relaxation length lm. UCF is more profound when electrical transport is in the weak localization regime lf < lc ="M" g0="2e2/h">

Polymer materials are expected to play a major role in the development of low cost opto-electronic devices. A major advantage of polymers is that they can be mixed with other polymers or nanomaterials in solution to form composites with large area internal junctions. By tuning charge exchange and storage in these junctions one can optimise the opto-electronic properties of these composites. One class of polymer-based composites that holds much promise is polymer-nanotube composites. [1–11] However, probing these properties in detail is not trivial. On the one hand, electronic characterisation that relies on the semiconducting response of the composites cannot be used for composites with nanotube concentration above the percolation limit because the composite’s response becomes metallic. On the other hand, at low nanotube concentrations the optical response of the composites is dominated by that of the polymer making optical characterisation ideal for high nanotube concentrations. [12] In this presentation we show how a combination of electrical and optical characterisations can be used to probe the response of charge at the polymer-nanotube bulk junctions. The samples examined were prepared by mixing P3HT and single wall nanotubes (SWNTs) from dichlorobenzene solutions. Processing and measurements were performed in ambient conditions while the samples were kept at dark between measurements. Current-voltage measurements on composites reveal good dispersion of nanotubes with a percolation threshold of about 0.75%wt. Using capacitance-voltage (C-V) measurements we show that charge trapped or released from the SWNTs can be probed. By varying the measurement frequency we can also assess the time response of the polymer-nanotube junctions. The optical response of the composites was studied using spectroscopic Ellipsometry and transient photoinduced absorption measurements. With the addition of SWNTs excitonic energy levels within the polymer density of states appear to quench progressively 1 faster and always in the sub 5ps timescale. The absorption spectra also show that the addition of nanotubes influences the packing of polymer chains. By probing the response of the composites at high SWNT concentrations using optical methods and at low concentrations using C-V, our method provides a unified approach for studies of composites.

Citation:

 Robert Vajtai^1*, Géza Tóth^2, Krisztián Kordás^2, Xiaohong An^3, Pulickel M. Ajayan^1
^1 Department of Mechanical Engineering & Materials Science, Rice University, Houston, TX 77005 USA
^2 Microelectronics and Materials Physics Laboratories, Department of Electrical and Information Engineering, and EMPART research group of Infotech Oulu, P.O. Box 4500, FIN-90014 University of Oulu, Finland
^3 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY
*Robert.Vajtai@rice.edu

Carbon nanotubes attracted large-scale scientific interest and their properties are well-studied for the cases when theoretical model work, and at the same time growth routes and proof of the concept applications were demonstrated (see e.g. Ref. 1). In this talk I briefly summarize our latest result on the most important parameters of multiwalled carbon nanotube growth via the floating catalyst Ferrocene-Xylene route applied earlier with success to create large CNT structures [2]. We investigated the kinetics [3] of the process both experimentally and theoretically and optimized the parameters for carbon nanotube length and also for their quality. These studies were used to reach macroscopic carbon nanotube structures with unique properties optimized to use them as synergistic units. In the main part of the talk I focus on characterization of the structures and their recent applications. Aligned carbon nanotube forests grown with different methods showed wide range of density depending on growth parameters; the physical properties of these films, e.g. compressibility, optical absorbance, thermal and electrical conductivity are unparalleled. To demonstrate the usefulness of these properties I will cite laboratory level applications. First a chip cooler setup [4], made of aligned multiwalled carbon nanotube forest will be presented, where the cooling performance of the device is comparable to a copper cooler having similar geometry; however, the carbon nanotube cooler is much lighter, mechanically stronger and it has more potential for further optimization. Another family of application is printing carbon nanotubes from different kind of “inks” [5-6]. The most interesting feature of this use is the fact that different coverage of the carbon nanotube film results in either low resistance Ohmic (for high coverage) or a nonlinear (for low coverage) behavior which latter one can be driven by gate voltage [6]. Via controlled amount of materials printed on the multi-micrometer scale the method can prepare complete electronic circuits with active elements and wires made of the same carbon nanotube ink without requiring any expensive pre-selection of semi-conductive and metallic tubes. These applications, together with several other, shortly mentioned ones, outdraw the possibilities that large scale, organized carbon nanotube structures inherently infer.

Video Content Length 30:09 Copyright © 2008 Vajtai et al

References

[1] P.M. Ajayan, Chemical Reviews 99, 1787 (1999).
[2] B.Q. Wei, R. Vajtai, Y. Jung, J. Ward, R. Zhang, G. Ramanath and P.M. Ajayan, Nature 416, 495 (2002).
[3] N. Halonen, K. Kordás, G. Tóth, T. Mustonen, J. Mäklin, J. Vähäkangas, P. M. Ajayan and R. Vajtai, J. Phys. Chem. C 112, 6723 (2008).
[4] K. Kordás, G. Tóth, P. Moilanen, M. Kumpumäki, J. Vähäkangas, A. Uusimäki, R. Vajtai, and P. M. Ajayan, Appl. Phys. Lett. 90, 123105 (2007).
[5] K. Kordás, T. Mustonen, G. Tóth, H. Jantunen, M. Lajunen, C. Soldano, S. Talapatra, S. Kar, R. Vajtai and P. M. Ajayan, Small 2, 1021 (2006).
[6] T. Mustonen, J. Mäklin, K. Kordás, N. Halonen, G. Tóth, J. Vähäkangas, H. Jantunen, S. Kar, P. M. Ajayan, R. Vajtai, P. Helistö and H. Seppä, Phys. Rev. B 77, 125430 (2008).

Citation:

R. Vajtai, G. Toth, K. Kordas, X.H. An, and P. M. Ajayan, OAtube Nanotechnology 1, 1006 (2008). http://www.oanano.org/oatube/$nanotechnology/2008/10/02/rvajtai

Video Content Length 15:36 Copyright: © 2008 Pohl et al

Andreas Othonos^1* , Emmanouil Lioudakis^1 A. G. Nassiopoulou^2