Anomalous K-point Phonons in Noble Metal/Graphene Heterostructure Activated by Localized Surface Plasmon Resonance

Abstract: Metal/graphene interface has been one of the most important research topics with regard to charge screening, charge transfer, contact resistance, and solar cells. Chemical bond formation of metal/graphene can be deduced from the defect induced D-band and its second-order mode, 2D band, measured by Raman spectroscopy, as a simple and non-destructive method. However, a phonon mode located at ~ 1350 cm-1, which is normally known as the defect-induced D-band, is intriguing for graphene deposited with noble metals (Ag, Au, and Cu). We observe anomalous K-point phonons in non-reactive noble metal/graphene heterostructure. The intensity ratio of the mid-frequency mode at ~ 1350 cm-1 over G-band (~1590 cm-1) exhibits non-linear but resonant behavior with the excitation laser wavelength and more importantly, the phonon frequency？laser energy dispersion is ~ 10-17 cm-1/eV, which is much less than the conventional range. These phonon modes of graphene at non-zero phonon wave vector (q ≠ 0) around K points are activated by localized surface plasmon resonance, and not by the defects due to chemical bond formation of metal/graphene. This hypothesis is supported by density function theory (DFT) calculations for noble metals and Cr along with the measured contact resistances.