Out-Of-Plane Directional Charge Transfer-Assisted Chemical Enhancement In The Surface-Enhanced Raman Spectroscopy Of A Graphene Monolayer

We present an out-of-plane directional charge transfer (CT)-assisted chemical enhancement (CM) in the surface-enhanced Raman scattering (SERS) of a graphene monolayer (GM) sandwiched at an individual Au prism-Au thin film (TF) plasmonic junction. By comparing previous reports, in which the enhancement of out-of-plane phonon modes of a GM was mostly governed by z-directional local field formed at a Au nanoparticle (NP) Au TF junction, we can reveal that a CT-assisted CM along the z-direction has a pivotal role by enhancing radial breathing-like mode (RBLM) intensity at face-to-face junction types. The anticorrelation between 2D and RBLM intensity from ridge to flat domains of an Au prism indicates that a plasmonic EM effect is not a critical element at flat domain, whereas the RBLM intensity is strongly enhanced at face-to-face junction. Furthermore, the closest GM from a z-axis has a significant role based on vector-based investigation in enhancing CT-assisted CM effect between Au prism and TF. These phenomena may be interpreted by (1) introducing the coupling of the plasmon modes of a metallic nanostructure, especially interaction between dipolar and quadrupolar modes and their interference with a large contact area of flat compared to ridge from an Au prism adjacent to the dielectric GM at a face-to-face junction by hot (or ballistic) electrons (Fano-like resonance) and (2) the electrons present at flat Au faces may have a high probability to generate CT in terms of relatively large portion of homogeneous electron distribution than sphere, leading to secure a large amount of CT electrons. Moreover, (3) close to the surface-to-normal direction of GM, we can reveal that CT along the z-axis is more favorable than the tilted. These interpretations offer an opportunity to study the mechanism of CT-assisted CM in detail and for optimizing the sensitivity of nanostructures in terms of Fano resonance and the resulting high performance localized surface plasmon resonance (LSPR) sensors.