  We demonstrate large rectification ratios (> 100) in single-molecule
junctions based on a metal-oxide cluster (polyoxometalate), using a scanning
tunneling microscope (STM) both at ambient conditions and at low temperature.
These rectification ratios are the largest ever observed in a single-molecule
junction, and in addition these junctions sustain current densities larger than
10^5 A/cm^2. By following the variation of the I-V characteristics with
tip-molecule separation we demonstrate unambiguously that rectification is due
to asymmetric coupling to the electrodes of a molecule with an asymmetric level
structure. This mechanism can be implemented in other type of molecular
junctions using both organic and inorganic molecules and provides a simple
strategy for the rational design of molecular diodes.
