Heating and cooling in current-carrying molecular junctions
is
a crucial issue in molecular electronics. The microscopic mechanism
involves complex factors such as energy inputs, molecular properties,
electrode materials, and molecule–electrode coupling. To gain
an in-depth understanding, it is a desired experiment to assess vibrational
population that represents the energy distribution stored within the
molecule. Here, we demonstrate the direct observation of vibrational
heating in a single C60 molecule by means of tip-enhanced
Raman spectroscopy (TERS). The heating of respective vibrational modes
is monitored by anti-Stokes Raman scattering in the TERS spectra.
The precise control of the gap distance in the single-molecule junction
allows us to reveal a qualitatively different heating mechanism in
distinct electron transport regimes, namely, the tunneling and single-molecule
point contact (SMPC) regimes. Strong Joule heating via inelastic electron–vibration scattering occurs in the SMPC
regime, whereas optical heating is predominant in the tunneling regime.
The strong Joule heating at the SMPC also leads to a pronounced red
shift of the Raman peak position and line width broadening. Furthermore,
by examining the SMPC with several types of contact surfaces, we show
that the heating efficiency is related to the current density at the
SMPC and the vibrational dissipation channels into the electrode.