  We demonstrate rectification ratios ($RR$) of $\gtrsim$1000 at biases of
1.3~V in bilayers of \buck deposited on copper. Using scanning tunneling
spectroscopy and first-principles calculations, we show that the strong
coupling between \buck and the Cu(111) surface leads to the metallization of
the bottom \buck layer, while the molecular orbitals of the top \buck are
essentially unaffected. Due to this substrate-induced symmetry breaking and to
a tunneling transport mechanism, the system behaves as a hole-blocking layer,
with a spatial dependence of the onset voltage on intra-layer coordination.
Together with previous observations of strong electron-blocking character of
pentacene/\buck bilayers on Cu(111), this work further demonstrates the
potential of strongly-hybridized, \buck-coated electrodes to harness the
electrical functionality of molecular components.
