Force Volume Atomic Force Microscopy-Infrared for Simultaneous Nanoscale Chemical and Mechanical Spectromicroscopy

Photothermal atomic force microscopy-infrared (AFM-IR) combines the nanoscale spatial resolution of AFM with the chemical identification capability of infrared spectroscopy and has thrived in various applications. Currently executed in three major AFM modes (contact, tapping, and peak force tapping) we introduce a fourth variant built upon force volume mode comprising a defined engage, hold, and retract segment in each pixel. IR laser pulsing at a probe resonance frequency during the constant-force hold segment duplicates the resonance-enhanced AFM-IR detection principle of contact mode. However, force volume AFM-IR removes the lateral forces that cause tip wear and sample damage while adding the spatial resolution of tapping AFM-IR. As demonstrated on different materials, this imaging and spectroscopy technique integrates monolayer sensitivity, sub-10 nm spatial chemical resolution, simultaneous nanomechanical property sensing, and precise force control. The ability to sweep the infrared laser repetition rate in each pixel provides additional, rich information in the form of contact resonance curves, while compensating for mechanically induced probe resonance shifts in an alternative to conventional phase-locked loop based frequency tracking. Such sweeps inherently consider the Q-factor (i.e., mechanical damping) in the AFM-IR response, a little investigated aspect. Furthermore, the probing depth can be varied by selecting different resonances recorded within a single broad frequency sweep. Switching to the surface sensitive AFM-IR detection scheme during the hold segment additionally limits the probing depth. These qualities should position force volume AFM-IR as a valuable addition to established AFM-IR modes.