  We observe large, reversible, bias driven changes in the vibrational energies
of PCBM, based on simultaneous transport and surface-enhanced Raman
spectroscopy (SERS) measurements on PCBM-gold junctions. A combination of
linear and quadratic shifts in vibrational energies with voltage is analyzed
and compared with similar measurements involving C60-gold junctions. A
theoretical model based on density functional theory (DFT) calculations
suggests that both a vibrational Stark effect and bias-induced charging of the
junction contribute to the shifts in vibrational energies. In the PCBM case, a
linear vibrational Stark effect is observed due to the permanent electric
dipole moment of PCBM. The vibrational Stark shifts shown here for PCBM
junctions are comparable to or larger than the charging effects that dominate
in C60 junctions.

Mesoscale and Nanoscale Physics