Two Damping Mechanisms in the Chemically Enhanced Raman Scattering on Graphene: DFT Study

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Abstract
In the search for new nanoscale spectroscopic capabilities, we simulated Raman scattering at the interface between a physisorbed organic molecule tetracyanoquinodimethane (TCNQ) and graphene. The strength of the electronic coupling across the interface was modulated by applying external electric biases. It was expected that a stronger coupling would lead to a stronger Raman signal from TCNQ, as it was previously observed on metals and semiconductors. To a great surprise, the results of this work show an opposite effect: a stronger electronic coupling with graphene quenches the Raman activity of TCNQ. The quenching is attributed to the coexistence of two damping mechanisms. One is related to the orbital relaxation or the response of the environment to the electron perturbed by the external electric field (electron damping). Another mechanism is the damping of molecular vibrations caused by the dynamic interfacial charge transfer driven by the vibrations (phonon damping). While the charge transfer in electronic excitations is commonly considered as a mechanism for the chemical enhancement in surface-enhanced Raman spectroscopy (SERS), on the level of vibrations it acts as damping.
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  • Bowling Green State University

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