Quantification of the Relative z‑Polarized Electromagnetic Field Contribution and Associated Investigation of Asymmetric Shape of Layer Breathing Mode from Au Nanoparticle–Graphene–Au Thin Film Junctions

Simultaneous utilization of both in-plane and out-of-plane phonon vibration of graphene sandwiched between a Au nanoparticle (NP) and Au TF junction can have a significant role in quantifying the z-polarized electromagnetic (EM) field contribution. The layer breathing mode (LBM), which involves the relative displacement of the individual graphene layers in the graphene sheet in the normal directions, can be exclusively observed at Au NP–Au TF junctions. By employing the I_LBM/I_2D value at each junction, the author can show that a nearly 50–70-times stronger z-polarized EM contribution is present at Au NP–Au TF junctions. Additionally, the author can also determine that a Au NP with a truncated shape shows a relatively ∼1.3-times higher z-polarized EM contribution than a Au NP with a spherical shape because of the higher density of the pointed part from the truncated Au NP. Furthermore, the author reveals the evolution of the asymmetric shape of the LBM peak with increasing z-polarized EM strength. The increase of the full width at half-maximum (fwhm) from the major LBM and the correlated blue shift of the minor LBM with increasing z-polarized EM contribution imply that the degree of coupling between highly localized electrons and low-energy phonons from graphene along the z axis might be reflected and that of the additional n-doped status of graphene stemming from the relatively higher electron density at truncated sites within Au NP might also be reflected. The author is currently developing this surface-enhanced-Raman-scattering- (SERS-) based nanometrology as a more facile optical characterization tool, especially for exploring out-of-plane phonon vibration of various 2D materials.