Carbon nanotubes are the corner stone in molecular electronics. Since their discovery, they have been used as a model system to study electronic transport at the nanoscale. Defects are known to modify the electrical resistance of single walled carbon nanotubes (SWCNTs). In particular, a small number of defects, 5-9 (created by Ar+ irradiation), brings up strong Anderson localization effects and a seemly universal curve for the resistance as a function of the number of defects [1,2].
At low bias, defect-free metallic SWCNTs are quasiballistic conductors presenting a very weak interaction between electrons and acoustic phonons. For high biases (above 0.3 eV), strong scattering of electrons with optical phonons destroys their phase coherence, resulting on a highly diffusive electronic transport characterized by a linear dependence of the resistance versus length. Recent experimental data on the evolution of the resistance with applied voltage in SWCNTs of lengths ranging from 100 nm to 6 microns confirm that behaviour for short NTs, but for long NTs (longer than 1 micron) a more complex scenario is revealed [3]. The differential conductance as a function of length first saturates and even decreases for very long nanotubes. A theoretical analysis, based on a MonteCarlo solution of the semicalssical Boltzman equation, shows that this saturation is associated with the finite number of optical phonons that one electron can excite as it travels along the nanotube. The resistance drop versus length is a consequence of the decreasing role played by the optical phonons at vey long SWCNTs, a limit in which scattering with acoustic phonons is the main source of electron drag at low and high voltages.
Finally, I will present a complete landscape of the electronic transport properties of SWCNTs by combining the different regimes, ballistic, diffusive and localized, associated with defects and with the scattering of electrons by acoustic or optical phonons.
[1] C. Gomez-Navarro et al, NATURE MATERIALS 4 (7): 534-539 JUL 2005
[2]. B.Biel, PHYSICAL REVIEW LETTERS 95 (26): Art. No. 266801 DEC 31 2005
[3]. P. Sundqvist, F.J.Garcia-Vidal, F.Flores, M.Moreno, C.Gomez.Navarro, S,Bunch and J.Gomez.Herrero, in preparation.
|