Giant spatially-resolved self-assembled donor-acceptor molecular heterojunctions*

Despite theoretical models predicting that rectification ratios (RR) $>$1000 should be achievable in molecular rectifiers, demonstrations of this have been rare. It has also been extremely challenging to unravel the structure-function relationships on the nanometer length scales that determine their behavior. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we show that RRs $>$1000 at biases $<$500~mV are realized in the two-molecule limit for self-assembled donor-acceptor bilayers of pentacene on C$_{60}$ on Cu. We show that the system behaves as a molecular analog to a Schottky diode due to strong electronic coupling of C$_{60}$ to the metallic substrate, and electronic transport is dominated by sequential tunneling from semiconducting pentacene to metallic C$_{60}$. Furthermore, we demonstrate the extreme sensitivity of the low-bias $I(V)$ characteristics to the molecularly-resolved structure of the heterojunction (HJ), which leads to negative differential resistance and $\sim 100\times$ variation in the rectification ratio within 2~nm of the edge of the molecular HJ.