Chemical Enhancement and Quenching in Single‐Molecule Tip‐Enhanced Raman Spectroscopy

Abstract

Despite intensive research in surface enhanced Raman spectroscopy (SERS), the influence mechanism of chemical effects on Raman signals remains elusive. Here, we investigate such chemical effects through tip‐enhanced Raman spectroscopy (TERS) of a single planar ZnPc molecule with varying but controlled contact environments. TERS signals are found dramatically enhanced upon making a tip–molecule point contact. A combined physico‐chemical mechanism is proposed to explain such an enhancement via the generation of a ground‐state charge‐transfer induced vertical Raman polarizability that is further enhanced by the strong vertical plasmonic field in the nanocavity. In contrast, TERS signals from ZnPc chemisorbed flatly on substrates are found strongly quenched, which is rationalized by the Raman polarizability screening effect induced by interfacial dynamic charge transfer. Our results provide deep insights into the understanding of the chemical effects in TERS/SERS enhancement and quenching.

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.…”