  Infrared spectroscopy, especially for molecular vibrations in the fingerprint
region between 600 and 1500 cm-1, is a powerful characterization method for
bulk materials. However, molecular fingerprinting at the nanoscale level still
remains a significant challenge, due to weak light-matter interaction between
micron-wavelengthed infrared light and nano-sized molecules. Here, we
demonstrate molecular fingerprinting at the nanoscale level using our specially
designed graphene plasmonic structure on CaF2 nanofilm. This structure not only
avoids the plasmon-phonon hybridization, but also provides in situ
electrically-tunable graphene plasmon covering the entire infrared fingerprint
region, which was previously unattainable. In addition, undisturbed and
highly-confined graphene plasmon offers simultaneous detection of in-plane and
out-of-plane vibrational modes with ultrahigh detection sensitivity down to the
sub-monolayer level, significantly pushing the current detection limit of
far-field mid-infrared spectroscopy. Our results provide a platform, fulfilling
the long-awaited expectation of high sensitivity and selectivity far-field
fingerprint detection of nano-scale molecules for numerous applications.
