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(a) Scanning electron microscope image of the etched sample structure. The bar has a length of 200 nm. The upper small quantum dot as the main device has a diameter of 90 nm while the bottom single electron transistor as a charge sensor has a diameter of 180 nm. The bright lines define barriers and the graphene side gate. (b) Schematic of a representative device.
(a) Conductance through the quantum dot vs the side gate voltage. (b) The example of conductance through the single electron transistor for the same parameter ranges as in panel (a). The steps in conductance have about 30% change in the total signal and are well aligned with the CB in panel (a). (c) Transconductance of the single electron transistor for the same parameters as in panel (a). The spikes and dips indicate the transitions in the charge states by addition of single electron in quantum dot. In particular, the dashed green lines show that the charge detection can allow measurement in the regime where the current through the dot is too small to be seen clearly by direct means. The vertical dashed red lines are a guide for the eyes to relate features in these graphs.
The magnitude of the SET signal as a function of the modulating pulse frequency. The dashed green line illustrates that the bandwidth of the SET device is about 800 Hz corresponding to a gain of 0.707 (−3 dB). Due to the stray capacitances, the response decreases rapidly after 800 Hz.
(a) Plot of the differential conductance of the quantum dot as a function of the bias voltage and the side gate voltage applied on the dot. From the lines parallel to the edges of Coulomb diamonds, we can identify the excited states. (b) Transconductance of the single electron transistor with the same parameters as in panel (a). Perfect matching with panel (a) and resolving more excited state spectra indicate that the single electron transistor can be used as a highly sensitive charge detector. Data in panels (a) and (b) were recorded simultaneously during a single sweep. The dashed green lines are the guide for identifying the excited states.
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