Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation.

Borja Cirera,Adnan Hammud,Takashi Kumagai,Mariana Rossi,Chenfang Lin,Yair Litman,Martin Wolf,Alaa Akkoush

doi:10.1021/acs.nanolett.1c02626

Abstract

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C60-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Highlights

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions
Atomic-scale structures on metal nanostructures play a crucial role to generate atomically confined electromagnetic fields through excitation of localized surface plasmon resonance (LSPR).[4−6] quantum mechanical effects, for example, electron tunneling across the junction, have a significant impact on the gap plasmon,[7] which will be related to the enhancement mechanisms in tip-enhanced Raman spectroscopy (TERS)
Excitation, chemical interactions between molecule and metal cluster(s) can largely contribute to the Raman scattering enhancement.[8]. This chemical enhancement effect was found to be important when the molecule is fused between two metal nanoclusters,[9] which may be manifested as a dramatic change of SERS/TERS spectra in plasmonic nanojunction fused with molecules.[10,11]