Anomalous Lifetime of Optical Phonons in Graphene at Buried Interfaces

In electrochemistry, establishing a versatile methodology to observe the microscopic structure and reaction kinetics at buried interfaces between electrodes and electrolytes has been a pivotal subject. Sum-frequency vibrational spectroscopy (SFVS) [1] has been successfully applied for such systems, but its applicability has been limited to somewhat artificial systems with transparent electrodes for the mid-IR pump pulses [2], [3]. In this study, we developed a novel time- and frequency-resolved coherent Stokes Raman spectroscopy (CSRS) system [4] and successfully measured, for the first time, the lifetime of optical phonons in single-layer graphene (SLG) on an opaque copper electrode at buried interfaces. Here we adopted an SLG-heavy water system as an ideal platform to investigate commonly used graphitic-electrode [2]. The measured lifetime of optical phonons in graphene, which is known to be strongly affected by the dynamics of the electron-hole plasma [5] and coupling to modes in the substrate [6], was found to be significantly elongated compared to that in air. In analogy with the so-called substrate effects [6], the interfacial heavy water is expected to introduce an additional cooling channel for the decay of the optical phonons in graphene, making the observed results counterintuitive.