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Charge transport in MWCNT oriented networks

Unlike the case of individual nano-objects, in theses arrays the intertubes barriers and defects play an essential role in the electrical transport properties of the carbon nanotubes arrays. Therefore, different charge transport mechanisms can be observed in the arrays of nanotubes : metallic conductivity, variable range hopping(VRH), weak localization (WL), fluctuation induced tunneling, and furthermore, combination of various mechanisms is possible as well.

In thin layers of MWCNT the variation of the resistance as a function of temperature is negative in the whole investigated temperature range [4-300 K] (not shown here). In the range [4-60 K], the resistance follows a Mott’s law of two dimensional (2D) VRH [1] : R=R _0 exp(T/{T_0})^{1/3} . Fig.1 shows the relative magnetoresistance  \Delta R/{R_0} in the temperature range in which the VRH is observed. The minimum position of negative MR shifts to higher fields as the temperature rises. Both low-field NMR due to changing of phase between alternate hopping paths enclosing a magnetic flux (quantum interferences) [2, 3] and high-field positive MR (PMR) due to electronic orbit shrinkage are predicted for systems with hopping conductivity mechanism ; ie MR=NMP+PMR. In this model, the amplitude of the NMR declines as the temperature is rising. But in our samples, a contrario, the NMR amplitude increases as the temperature increases in the temperature range in which VRH can be responsible of the charge transport mechanism. From the other side negative MR is inherent for the systems where conductivity can be described in the frame of WL theory [4]. Therefore we made assumption that MR data are the sum of the positive and negative contribution due to MR effects in the VRH and WL regimes, respectively. The high-field positive part of MR can be approximated in the framework of Kamimura’s model for spin-dependent VRH conductivity (due to the surface defects of the NT’s) [5]. Thus, the low field PMR is calculated by using the parameters values obtained from the high field experimental PMR data. Next, the pure negative contribution to MR according our assumption is deduced by subtracting the calculated PMR from the experimental data and shown in (Fig.3). These data can be reasonably fitted by the equation which describe the 2D WL.

In conclusion, this study demonstrates the role of the surface defects of the NT’s by its spins.

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Fig.1 Relative magnetoresistance in the temperature range in which 2D VRH is observed
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Fig.2 Kamimura’s model for PMR (spin dependent VRH)
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Fig.3 NMR obtained by subtraction of the calculated PMR from the experimental results.

References : 

[1]Shklovskii B.I., Efros A.L. « Electronic properties of doped semiconductors » in Springer series in Solid-State Sciences Vol. 45, Cardona M. ed., Springer Verlag, Berlin 1984

[2]Nguyen V.L., Spivak B.A., Shklovskii B.I. Sov. Phys. JETP 62, 1021 (1986)

[3]Sivan U., Entin-Wohlman O., Imry Y. Phys. Rev. Lett. 60, 1566 (1989)

[4] Lee P., Ramakrishnan T.V. Rev. Modern Phys. 57, 287 (1985) [5] Kamimura H. « Electron-electron interactions in disordered systems » in Modern problems in condensed matter sciences Chap. 7, Vol. 10, Efros A.L.& Pollak M. ed., North Holland 1985