  Molecular electronics originally proposed that small molecules sandwiched
between electrodes would accomplish electronic functions and enable to reach
ultimate scaling. However, so far, functional molecular devices have been only
demonstrated at low frequency. Here, we demonstrate molecular diodes operating
up to 17.8 GHz. DC (direct current) and RF (radio frequency) properties were
simultaneously measured on a large array of molecular junctions composed of
gold nanocrystal electrodes, ferrocenyl undecanethiol molecules, and the tip of
an interferometric scanning microwave microscope. The present nanometer-scale
molecular diodes offer a current density increase by several orders of
magnitude compared to that of micrometer-scale molecular diodes, allowing RF
operation. The measured S11 parameters show a diode rectification ratio of 12
dB which is linked to the rectification behavior of the DC conductance. From
the RF measurements, we extrapolate a cut-off frequency of 520 GHz. A
comparison with the silicon RFSchottky diodes architecture suggests that the
RF-molecular diodes are extremely attractive for scaling and high-frequency
operation.
