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Nanostructured graphene quantum dot device. (a) Schematic illustration of the tunable graphene quantum dot. (b) Scanning force microscope (SFM) image of the investigated graphene device after RIE etching and (c) after contacting the graphene structure. The minimum feature size is approximately . The dashed lines indicate the outline of the graphene areas. (d) shows a SFM cross section along a path [marked in (b)] averaged over perpendicular to the path proving the selective etch process. (e) Confocal Raman spectra recorded on the final device at the graphene island with a spot size of approximately , clearly proving the single-layer character of the investigated device. For more information on the D, G, and 2D (also called ) line please refer to Ref. 17.
Source-drain current as a function of the two barrier gate voltages and for constant bias . The dashed lines indicate transmission modulations and oscillations attributed to the graphene constrictions (horizontal and vertical lines) and to the island (diagonal line). Measurements are preformed at and .
Source-drain current through the graphene nanostructure as function of the plunger gate voltage . (a) Clear Coulomb resonances are observed on top and next to the large scale conductance modulations. (b) shows a marked close-up of (a), and in (c) the peak spacing is plotted for 18 consecutive peaks. Measurements are preformed in the dot configuration: , , and .
Coulomb diamonds in differential conductance , represented in a logarithmic color scale plot (dark regions represent low conductance). A dc bias with a small ac modulation is applied symmetrically across the dot, and the current through the dot is measured. Differential conductance has been directly measured by a lock-in amplifier. The charging energy is estimated to be from this measurements. Measurements are preformed in the dot configuration: , , and .
Source-drain current as a function of the plunger gate voltage for different bath temperatures. Note that the plunger gate sweep includes the region shown in Fig. 3(b). Measurements are preformed in the dot configuration: , , and . The different bath temperatures are indicated.
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