  IR spectroscopy has been widely used for chemical identification and
quantitative analysis of reactions occurring in a specific time and space
domains by measuring an average signal of the entire system1. Achieving IR
measurements with nanometer-scale spatial resolution is highly desirable to
obtain a detailed understanding of the composition, structure and function of
interfaces2-5. The challenges in IR nanoscopy yet exist owing to the small
molecular cross section and pristine optical diffraction limit. Although atomic
force microscopy (AFM) based techniques, such as scattering-type scanning
near-field optical microscopy and photothermal-induced resonance microscopy
(PTIR), can acquire IR spectroscopy in a few tens of nanometer scale
resolution6-9, IR measurements with monolayer level sensitivity remains elusive
and can only be realized under critical conditions10,11. Herein, we demonstrate
sub-10 nm spatial resolution sampling a volume of ~360 molecules with a strong
field enhancement at the sample-tip junction by implementing noble metal
substrates (Au, Ag, Pt) in photo-induced force microscopy (PiFM). This
technique shows versatility and robustness of PiFM, and is promising for
application in interfacial studies with hypersensitivity and super spatial
resolution.
