Peak Force Infrared (PFIR) Microscopy

PFIR microscopy is action-based microscopy with mechanical detection of photothermal expansion by atomic force microscope (AFM) operated in the peak force tapping mode. It falls under the umbrella of the family of photothermal AFM-IR techniques. Our group at Lehigh University invented PFIR microscopy in 2016-2017.

The underlying mechanism for PFIR (and AFM-IR in general) is to leverage the high force sensitivity of an AFM cantilever to measure the mechanical effect of IR-specific photothermal expansion of the sample. The contact area of the AFM tip can be smaller than the tip radius in PFIR, thus the measurement is achieved from an area much smaller than the optical diffraction limit--a route for super-resolution microscopy.

The unique trait of PFIR among AFM-IR techniques is the AFM tip is predictably in contact and detached from the sample surface under feedback loop control. The sample surface integrity is well preserved. As the cantilever resonances behave differently when in contact or detached from the sample, no residual photothermal signals are carried over from previous tip-sample interactions, thus ensuring no signal spillover between adjacent image pixels and achieving high spatial resolution as high as 6 nm.

The PFIR microscopy has another intrinsic advantage inherited from the peak force tapping mode. Under weak IR illumination, the mechanical properties of modulus and adhesion are simultaneously collected with the IR images, without separate scans. As the peak force tapping mode works well in the fluid, the PFIR microscopy also works in the fluid phase, including water.

The details of the PFIR microscopy are described in our recent Chemical Society Review article.

The photothermal responses of the sample are typically extracted from the cantilever response using a data acquisition card, processed in real-time, and used as the PFIR signal. This approach achieves label-free chemical imaging at 6 nm spatial resolution, far surpassing the diffraction limit of conventional optical methods.

One main limitation of the regular PFIR microscopy is its relatively weak signal. The peak force tapping cycle of an AFM is usually a few kHz (1kHz up to 8 kHz), which means that the tip and sample are in contact a few thousand times per second. At each tip-sample contact, a laser pulse is emitted to excite the photothermal expansion of the sample, and the cantilever responses are extracted to be used as a signal.

However, for samples with low photothermal responses or IR absorption, the PFIR signals are often weak.

The implementation of the multi-pulse PFIR is to illuminate multiple laser pulses per peak force tapping cycle. So more photothermal responses are generated per unit time. The resulting method improves the signal quality of PFIR microscopy.

The details of multi-pulse PFIR microscopy are described in our paper Advanced Optical Materials 1901084 (2019).