Figure 4. Highly selective detection of molecular vibrational fingerprints.

(a) The plasmon-induced vibrational mode response of the PEO molecules extracted from the extinction spectra of the graphene plasmonic resonance peak at different effective gate voltages. The change of the center frequency of each plasmon peaks is indicated by the red arrow. The Lorentz line shapes (the thin solid curves) are used to fit the peaks induced by different vibrational modes of PEO molecules. The shaded areas with different colours indicate the superposition of the fitted peak areas. (b) The enhancement factor of typical vibrational modes as a function of the distance between the mode (υ_Mode) and graphene plasmon resonance peak (υ_Res). The error bars in the plots are standard deviation from large numbers of measurements. (c) Enlarged extinction spectra for the PEO ultra-thin films with and without plasmon enhancement in the range 1,045–1,200 cm⁻¹. The pristine infrared absorption spectrum of the 8-nm PEO film without plasmon enhancement is shown as the black line at the bottom. The vertical lines indicate the positions of modes E, F and G.