Compared to the scattering scanning near-field optical microscopy (s-SNOM) technique, photothermal AFM-IR has been applied much less often to study the surface polariton waves in 2D materials. In this work, real space imaging of surface plasmon polaritons (SPPs) in monolayer graphene on silicon dioxide (SiO2) was performed for the first time using both Tapping AFM-IR and s-SNOM.
Experimental results obtained from the same graphene flake using the two techniques were compared directly and qualitative correlations were shown, and the detection mechanism of graphene SPP with Tapping AFM-IR was elucidated.
The experiments were performed on a Bruker nanoIR2-s instrument with s-SNOM and AFM-IR capabilities. Tapping AFM-IR spectra collected on the graphene flake and SiO2 substrate showed strong hybridization between graphene plasmon and SiO2 phonon modes in the 900-1200 cm-1 range. IR images recorded at 930 cm-1 using both techniques showed SPP interference fringes with maximum intensity at the edges of the graphene wedge. More detailed comparisons showed good agreement between Tapping AFM-IR and s-SNOM phase images in terms of signal intensity and fringe spacing, confirming that the same SPP mode was probed in this study. The SPP waves are launched by near-ﬁeld momentum coupling from the metallic probe tip, propagate radially, reﬂect from the graphene SiO2 edge, and interfere with the surface polariton launched at the tip. While s-SNOM detects the radiative tip-scattered light, AFM-IR detects the mechanical response of the tip-sample interaction. More specifically for this case, AFM-IR detects the photothermal expansion of the SiO2 substrate induced by the polariton energy which was efficiently coupled to the substrate through strong plasmon-phonon hybridization.
Controlling the photothermal response of surface polaritons appears to be a potentially usefully approach for spatially patterning temperature ﬁelds on nanometer length and ultrafast time scales.