  We propose a novel method for ultra-sensitive infrared (IR) vibrational
spectroscopy of molecules with nanoscale footprints by combining the tip
enhancement of the scattering-type scanning near-field optical microscope
(s-SNOM) and the plasmon enhancement of the breathing-mode (BM) plasmon
resonances of graphene nanodisks (GNDs). To demonstrate that, we developed a
quantitative model that is capable of computing accurately the s-SNOM signals
of nanoscale samples. With our modeling, we show that the s-SNOM tip can
effectively excite gate-tunable BM plasmonic resonances in GNDs with strong
field enhancement and sensitive dependence on the size of GND. Moreover, we
demonstrate that the intense electric field of tip-excited plasmonic BMs can
strongly enhance the IR vibrational modes of molecules. As a result, IR
vibrational signatures of individual molecular particles with sizes down to 1-2
nm can be readily observable by s-SNOM. Our study sheds light on future
ultra-sensitive IR biosensing that takes advantage of both the tip and plasmon
enhancement.
