Fig. 1: Device Structure.
a Single device: a 1,6-hexanedithiol (C6S2) SAM is bonded to evaporated Au fingers, and quantum dots (QDs) capped with oleic acid (OA) are then assembled on the SAM. SLG is draped over the entire structure to form a contacted array of double-barrier junctions in parallel. b Central device region of a single sample containing 39 individually addressable devices with different areas. Devices are created where SLG overlaps the tips of the Au electrodes (vertical lines). The left of the image shows an OL sample and the right an EBL one, where device areas are reduced further by EBL. The central horizontal electrodes are left bare for contacting the SLG. c Computer render of an EBL device showing its open (etched) edge highlighted in the foreground. In the background, the open edge of the original OL device. The open-edge length:area ratio of a device increases when EBL is used to reduce its area. d Single double-barrier structure (centre) as part of a larger array. C6S2 and OA form tunnel barriers between the QD and Au and SLG, respectively. e Energy structure of a junction aligned with the picture in (d). The Au and SLG electrodes are the source and drain, respectively. The C6S2 SAM and the QD’s OA ligands form tunnel barriers at the Au/QD and QD/SLG interfaces, respectively. The QD has a discrete energy spectrum due to its addition energy EC = e2/2C, where C is the QD’s capacitance, and its single-electron energy levels. In this example a negative source-drain bias VSD is applied across the junction. This determines the energy window directing the transport by altering the electrodes' electrochemical potentials. The available energy − eVSD has just increased enough to allow a second electron to reside in the QD. The electrons do not tunnel out quickly as the exit barrier is thicker than the entrance one, but the total probability of leaving becomes twice what it was with one electron, and hence the tunnelling current steps up by about a factor of two. Addition of extra electrons (or holes) like this is exhibited as steps in the I − V characteristic, dubbed a Coulomb staircase. f Single sample showing each device in the central region as individually addressable via bond pads on its perimeter. g False-colour SEM image of 7 OL devices (electrodes shown vertically in pink), each with a different area (0.34 ± 0.22 − 11.9 ± 1.2 μm2) arising as a result of the electrodes' relative positions spanning the optical-lithography alignment accuracy of the SLG patterning (shown in blue). A horizontal grounding electrode with no SAM is seen at the left of the image in yellow. h False-colour SEM of single EBL device (like that in c) with area 11500 ± 2300 nm2.
