▎ 摘　　要

We report transport experiments on graphene quantum dots (QDs). We focus on excited state spectra in the near vicinity of the charge neutrality point and signatures of the electron-hole crossover as a function of a perpendicular magnetic field. Coulomb blockade resonances of a 50 nm wide and 80 nm long dot are visible at all gate voltages across the transport gap ranging from hole to electron transport. The magnetic field dependence of more than 40 states as a function of the back gate voltage can be interpreted in terms of the unique evolution of the diamagnetic spectrum of a graphene dot including the formation of the E = 0 Landau level, situated in the centre of the transport gap, and marking the electron-hole crossover. Around the crossover, excited states and cotunneling features are studied for varying perpendicular magnetic field. [GRAPHICS] Schematic illustration of an etched graphene quantum dot with rough edges. The devices studied are still significantly larger than the presented one. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim