The fabrication of sub-10 nm planar electrodes and their use for a molecule-based transistor.

The typical size of a single molecule is of the order of a few nanometres; for this reason metal electrodes separated by a nanometre-scale channel are required to make molecule-based electronic devices. In this work, we report on the fabrication of metallic (Cr/Au, Ti/Au or Ti/Pt) arrow-shaped electrodes on Si/SiO2 substrates, with tip separation between 100 and less than 10 nm. They can be used to implement two and three terminal molecular devices, just by connecting them by the chosen molecules and adding an Ag electrode on the back of the Si substrate. Electron beam lithography (EBL) allowed us to obtain electrodes with separation around 40 nm. In order to reduce the tip separation down to 20 nm, before the EBL process, we used a defocused e-beam to brush the poly(methylmethacrylate) (PMMA) layer for a short time (from 10 to 40 s). The smallest gap between the electrodes (less than 10 nm) was obtained with standard EBL and lift-off followed by Au electroplating deposition. The fabricated devices were inspected by plan-view scanning electron microscopy (SEM) and electrically tested by I/V measurements in the range ±2 V. Open-circuit characteristics gave very low currents (in the range −10 to 10 pA) and a resistance ≈1 T. As a typical example, we demonstrate a field effect transistor (FET) based on a deoxyguanosine derivative (a DNA base) placed between the fabricated planar electrodes by room temperature (RT) cast deposition. The FET device tested at RT and ambient pressure exhibited a maximum voltage gain as high as 0.76.