Device characterization. (a) Scanning force microscopy of the graphene quantum dot device. The overall chemical potential of the device is tuned by a global back gate, where as the right side gate (RG) is used for local asymmetric tuning. The extension of the dot is around 140 nm with 75 nm wide and 25 nm long constrictions. The white dashed lines delineating the quantum dot perimeter are added for clarity. (b) Measurement of the source (S)-drain (D) conductance for varying back gate voltage showing a transport gap from around -5 to 3 V (V
b = 200 μ V). (c) Coulomb diamond measurements in the gap showing a charging energy of around 4.5 meV. This energy is lower than what has been measured in an other dot of similar size (Ref. ), most likely because of the increased coupling to the leads. The arrows point to faint lines outside the diamonds. The extracted energy difference of around 1 meV is a reasonable addition energy for excited states. Note that for the measurement in (c), in addition to the BG the right side gate was changed according to V
rg = -0.57·V
bg -1.59 V.