Carbon nanotube quantum dots

Low temperature electron transport measurements on individual single wall carbon nanotubes are described in this thesis. Carbon nanotubes are small hollow cylinders made entirely out of carbon atoms. At low temperatures (below ~10 K) finite length nanotubes form quantum dots. Because of its small size a quantum dot has a discrete set of energy levels where electrons can be placed. Therefore, it resembles an atom in many aspects. The research presented here focuses on understanding the behavior of nanotube quantum dots. Until recently, we have fabricated nanotube quantum dots by just evaporating metal contacts on top of the nanotube segments. This way tunnelbarriers develop naturally at the nanotube-metal interfaces. The addition of a single electron requires a considerable energy due to the small size of the nanotube. At low temperatures this energy is absent, and the nanotube quantum dot is then in the 'Coulomb blockade' regime. Applying a bias voltage between the electrodes or changing the electrostatic potential of the quantum dot by a voltage on a nearby gate can lift the Coulomb blockade and hence allow single electron tunneling processes through the barriers. By measuring the current going through the nanotube we study the electron transport behavior of the nanotube quantum dot. We are primarily focussed on nanotube quantum dots in the 'closed' quantum dot regime. In this regime the tunnelbarriers are very opaque and the electrons are strongly confined in the quantum dot. Also, the energy width are sharply defined allowing for accurate spectroscopy measurements. In chapter 4 we report the first observation of the discrete energy spectrum of semiconducting nanotubes. Furthermore, the semiconducting nanotube quantum dot can be completely depleted from free charge carriers. Electrons or holes could be added one by one. This permits us to compare the excitation spectra for electrons and holes. We find that they are symmetric, as expected from the symmetry in the bandstructure. The energy spectra of metallic nanotubes in the closed quantum dot regime are explained in chapter 5. Using a model, which extends the 'constant interaction model' with exchange interaction effects and the orbital degeneracy of the nanotube bandstructure, the full measured energy spectrum of the nanotube quantum dot is identified and accounted for. Of recent interest in the field of nanoscience is the interplay between electrical and mechanical properties. In a theoretical study we predict that the tunneling of a single electron onto a suspended nanotube drastically modifies the quantized vibrational eigenmodes due to the electrostatic forces which bend and tension the nanotube. Measurements performed on quantum dots in freely suspended nanotube at low temperatures shows a small, harmonic excitation spectrum, which can not be identified with purely electronic excitations. We propose phonon assisted tunneling to be responsible for these excitations. Using a Franck-Condon based model, in which the phonon assisted tunneling processes are modeled as a coupling of electronic levels to underdamped quantum harmonic oscillators, we find good agreement with the measurements. Lately, the emphasis of carbon nanotube quantum dot research is shifting towards defining nanotube quantum dots with tunable barriers. In the last chapter we present a fabrication procedure for defining nanotube quantum dots with controllable barriers. This way it was possible to extend the system to form a double quantum dot. This last step was highly motivated by the possibility that double quantum dot systems could act as a solid state quantum bit. We perform transport measurements on the nanotube double quantum dot and observe the excited states of the systems. This opens up new possibilities for fundamental and applied studies on nanotube quantum dots.