  We demonstrate that rectification ratios (RR) of >250 (>1000) at biases of
0.5 V (1.2 V) are achievable at the two-molecule limit for donor-acceptor
bilayers of pentacene on fullerene on Cu using scanning tunneling spectroscopy
and microscopy. Using first-principles calculations, we show that the system
behaves as a molecular Schottky diode with a tunneling transport mechanism from
semiconducting pentacene to Cu-hybridized metallic fullerene. Low-bias RRs vary
by two orders-of-magnitude at the edge of these molecular heterojunctions due
to increased Stark shifts and confinement effects.
