Joule Heating in Single-Molecule Point Contacts Studied by Tip-Enhanced Raman Spectroscopy

Abstract

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.

Local heating and Raman thermometry in a single molecule

“…Among these approaches, Raman spectroscopy has received extensive interest as a remote calorimetric technique based on the examination of temperature-dependent peak widths, positions, or intensities of distinct vibrational modes (23)(24)(25)(26). In particular, the ratio between the anti-Stokes and Stokes Raman intensities for each vibrational mode of a molecular system is directly related to the population of the corresponding vibration or phonon mode, providing a unique approach to evaluate how far the system is away from a global thermal equilibrium (27)(28)(29)(30)(31)(32)(33)(34). In this context, single-molecule tip-enhanced Raman spectroscopy (TERS) offers a great potential for probing the local thermal property of a single molecule because of its advantage in achieving sensitive Raman fingerprints with simultaneous ultrahigh spatial resolution down to the angstrom level (35)(36)(37).…”

Materials Nanoarchitectonics for Advanced Physics Research

“…This has become clear due to the development of nanotechnology since the later period of the 20th century. Nanotechnology has enabled direct observation of atomic and molecular structures, [8,9] their dynamic behaviors, [10,11] and evaluation of physical properties at the nanoscopic level, [12,13] which has dramatically advanced our understanding of nano-level functions and properties. As the next stage of development, it is important to apply nano-related knowledge to material chemistry and related physics.…”

Resonant Tip-Enhanced Raman Spectroscopy of a Single-Molecule Kondo System

“…Here, we perform UHV-TERS experiments at an open-shell molecule, which provides an ideal playground for revealing the complex interplay among the charge, spin, and electronic states of a molecule in a nanocavity and its impact on the optical properties. It has been shown that a controlled atomic contact between the STM tip and molecules leads to a dramatic rise of Raman intensity, − which is a clear indication of enhancement beyond the electromagnetic field intensification. Therefore, we select a highly tunable single-molecule break-junction system that exhibits a reversible transition between a mixed-valence state and a singly charged radical upon decoupling from a metal substrate. − We manipulate a single perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecule on Ag(111) while simultaneously measuring the electronic spin state and the Raman spectra in the chemical enhancement mode.…”