Electrical characterization of carbon nanotubes grown by the chemical vapor deposition method

Single-wall carbon nanotubes after a decade of research show fascinating properties with a promising prospective for possible applications. Their nanometer size and micrometer lengths make them an ideal material for research in nanotechnology. Contrary to most of the molecules, experiments in different scientific areas have been possible due to their robust, chemical inert characteristics (Chapter 1). We have successfully produced CNTs by CVD method. It has been shown that this method is well suited for the investigation of their electrical and mechanical properties. The main advantages of the CVD technique is that CNTs can be grown at specific location with flexibility to routinely implement them for different investigation purposes. Their electronic transport properties have been explored by fabricating the contacts by EBL or optical lithography in straightforward manner. In this thesis, investigation has been focused on CNT’s electronic transport properties from room to low temperature. Although their main characterization at room temperature (the gate dependance and high bias characteristics) does not depend dramatically on the contact resistance, low temperature measurement reveal the importance of the contact resistance and their correlation with the observed phenomena. We have found that problem of the contact resistance can be overcome by annealing of the devices contacted with Ti or using Pd as metallic electrodes on as grown CNT (Chapter 2). Ambipolar field-effect transistor action has been demonstrated on asgrown semiconducting SWNTs (Chapter 3). The observed ambipolar FETs can be tuned with a back-gate from p- to n-type conduction through the semiconducting gap. We have attributed high tunability of our devices to hydrogen presence during the CVD grow, which probably reduce trapped charges in a Si/SiO2 substrate, making coupling to the gate more effective. Electron and hole transport in the Coulomb blockade regime have been investigated in detail. A strong sensitivity on disorder has been observed in semiconducting SWNTs, which effectively brake the nanotube in small sections ≈ 40 nm forming multi-dot system. However, for sufficient doping i.e. by the gate voltage, localized states can be populated, where the transport occurs through an individual orbital. Moreover, doping of the tube section to higher subbands has been demonstrated. Detailed, two terminal transport measurements at low temperature have been performed on the metallic SWNT devices (Chapter 4). Spectroscopy on a SWNT QD can be performed by measuring the conductance as a function
of the gate and bias voltages. For low transparent contacts, a SWNT
behaves as a quantum dot where the transport phenomena are dominated
by the single-electron charging effect (Coulomb blockade). For high transparent
contacts, the four-fold degeneracy is observed with the Kondo effect.
We have discussed several possible shell filling scenarios in SWNTs. In detail,
the ground state of CNTs at half-filling, i.e. for N = 2 added electrons
to one shell, is analyzed. We demonstrate that this state is either the singlet
or a state for which the singlet and triplet are effectively degenerate,
allowing in the latter case for the appearance of the Kondo effect at N = 2.
For even higher transparency Fano resonances are observed. The origin of
these resonances are identified as an interference between the resonant and
non-resonant channels within a bundle of SWNTs.
Finally, as grown CNTs have been suspended with three different methods
to explore their mechanical properties (Chapter 5). We have shown that
thermal vibrations are readily observed in SEM if the suspended length of
a SWNT is sufficient (& 1 μm). Good agrement is found between thermal
vibration derived for an elastic beam in continuum mechanics, with corresponding
Young’s modulus in TPa range. However, for the SWNTs suspended
by wet etching Young’s modulus seems to be smaller (� 20GPa).
The possible interpretations are that wet etching severely damage CNT or
that the boundary conditions are different than in the cases for the growth
over predefined trenches and Si3N4 membranes. Our investigation showed
that possible implementation of CNTs as nano-electromechanical resonators
urges for careful design of the experiments with desirable control of their
physical properties (diameter).
In this thesis, we have demonstrated that transport investigation in
CNTs show pronounced quantum effects with fascinating possibilities to
explore the fundamental phenomena which has been elusive in other systems
(S-QD-S, spin injection in low dimensional systems, Luttinger liquid,
etc). In that respect, the exploration of CNTs opened a new chapter in
material science and nanotechnology, as important milestone for the future
investigation of molecular based devices.