  Vibrational modes of molecules are fundamental properties determined by
intramolecular bonding, atomic masses, and molecular geometry, and often serve
as important channels for dissipation in nanoscale processes. Although
single-molecule junctions have been employed to manipulate electronic structure
and related functional properties of molecules, electrical control of
vibrational mode energies has remained elusive. Here we use simultaneous
transport and surface-enhanced Raman spectroscopy measurements to demonstrate
large, reversible, voltage-driven shifts of vibrational mode energies of C60
molecules in gold junctions. C60 mode energies are found to vary approximately
quadratically with bias, but in a manner inconsistent with a simple vibrational
Stark effect. Our theoretical model suggests instead that the mode shifts are a
signature of bias-driven addition of electronic charge to the molecule. These
results imply that voltage-controlled tuning of vibrational modes is a general
phenomenon at metal-molecule interfaces and is a means of achieving significant
shifts in vibrational energies relative to a pure Stark effect.
