Single layer and bilayer graphene quantum dot devices

Graphene quantum devices, such as single electron transistors and quantum dots, have been a vital field of research receiving increasing attention over the past six years. Quantum dots (QDs) made from graphene have been suggested to be an interesting system for implementing spin qubits. Compared to the well-established GaAs-based devices their advantages are the smaller hyperfine interaction and spin-orbit coupling promising more favorable spin coherence times. However, while the preparation, manipulation, and read-out of single spins have been demonstrated in GaAs QDs, research on graphene QDs is still at an early stage. So far, effects like Coulomb blockade, electron-hole crossover and spin-states have been studied in graphene QDs.This thesis reports low temperature transport experiments on single-layer and bilayer graphene quantum dot devices focusing on the analysis of excited state spectra and the investigation of the relaxation dynamics of excited states. Graphene nanoribbon based charge sensors are characterized as an important tool to detectindividual charging events in close-by QDs. All devices are fabricated following the concept of width-modulated graphene nanostructures. Carving nanoribbons out of graphene sheets opens an effective band gap and offers the chance to circumvent the limitations originating from the gapless band structure in "bulk" graphene. Shaping graphene into small islands(with typical diameters between 50 and 120 nm) connected to contacts via narrow ribbons (typical widths between 30 and 50 nm) allows the confinement of electrons and the formation of QDs with gate-tunable tunneling barriers.Graphene nanoribbon-based charge detectors coupled to QDs are characterized.They allow the resolution of charging events on the QD even in regimes where the direct current is below the noise floor and excited states of the QD can be resolved by this technique as well. The detector remains operating even in adverse conditions like under the influence of magnetic fields of several Teslas or while RF pulses in the MHz regime are applied to the QD. These findings are in particular relevant for experiments addressing spin states or relaxation processes in graphene QDs. Special emphasis lies on the influence of the detector on the transport properties of the probed QD. The effects of back action and counterflow are measured and discussed.The relaxation dynamics of excited states of graphene QDs is investigated. Finite-bias spectroscopy measurements unveil the excited state spectrum of the device. Long and narrow constrictions are used as tunneling barriers which allow driving the overall tunneling rate to a few MHz. In such a regime rectangular RFpulse schemes are applied to an in-plane gate to probe the relaxation dynamics of excited state to ground state transitions. Measurements of the current averaged over a large number of pulse cycles yield an estimate of a lower bound for charge relaxation times on the order of 60 to 100 ns which is roughly a factor of 5 to 10 larger than what has been reported in III/V quantum dots. The experimental results are compared to a basic model regarding electron-phonon coupling as the dominant relaxation mechanism.A bilayer graphene double quantum dot device is characterized. The capacitive interdot coupling can be tuned systematically by a local gate. The electronic excited state spectrum features a single-particle level spacing independent on the number of charge carriers on the QDs which is in contrast to single-layer graphene.In in-plane magnetic fields a level splitting of the order of Zeeman splittings is observed