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 used to manipulate electronic structure and related functional properties of molecules, electrical control of vibrational mode energies has remained elusive. Here we fabricate gold bowtie structures with nanometer inter- electrode spacing using controllable electromigration. Those gold nanostructures support highly localized plasmons and have proven to be suitable SERS substrates with single-molecule sensitivity, which enable the study of molecular vibrational and electronic physics.  We use simultaneous transport and surface-enhanced Raman spectroscopy measurement to demonstrate large, reversible, voltage-driven shifts of vibrational mode energies of C60molecules in gold junctions. C60mode energies are found to vary approximately quadratically with bias, but in a manner inconsistent with a simple vibrational Stark effect. Our theoretical model instead suggests 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.

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Ms. Yajing Li received her B.S. in Physics from Tsinghua University, China in 2010. She was admitted to Rice University and got her M.S. in Physics in 2013. She is now PhD candidate advised by Professor Natelson, whose group focuses on condensed matter physics, particular electronic, magnetic and optics at nanoscale. She’s been working on Surface enhance Raman spectroscopy on molecular scale gold junctions, which combines the simultaneous electronic transport and SERS measurement on single molecules. Her recent work “Voltage tuning of vibrational mode energies in single-molecule junctions” was published in Proc. Nat. Acad. Sci. US.