Peak force infrared (PFIR) microscopy is a new infrared imaging technique developed in Dr. Xu’s lab at Lehigh University in collaboration with an industrial partner. PFIR measures the photo-thermal expansion of the sample with an atomic force microscope (AFM) operated in the peak force tapping mode or pulsed force mode. PFIR can operate in both air and fluid phase.
In PFIR, the atomic force microscope is operated in the peak force tapping mode or the pulsed force mode. Infrared laser pulses illuminate the AFM tip and the sample when they are in contact and at every other peak force tapping cycle. Consequently, the subtraction of the cantilever deflection waveforms leads to the pure response from the laser-induced photo-thermal expansion. Subsequent analysis on the photo-thermal expansions extracts mechanical signals that correspond to the infrared absorption. PFIR microscopy has two operation modes: imaging mode and spectroscopy mode. In imaging mode, the laser-induced mechanical responses are mapped when the AFM tip is scanned over the surface of the sample at a fixed infrared frequency. In spectroscopy mode, the frequency of the infrared light source is scanned at a fixed spatial location, while the photo-thermal mechanical responses are recorded.
The spatial resolution of PFIR microscopy is found to be 6 nm. Block copolymers form nanoscale phase separation. We used PFIR microscopy to reveal individual chemical compositions of PS-b-PMMA block copolymer. The spatial resolution was found to be as high as 6 nm.
PFIR microscopy provides simultaneous spectral imaging and mechanical information. The infrared image and mechanical properties (modulus and adhesion) are simultaneously collected from the same area. PFIR allows naturally correlative imaging. By placing the AFM tip at one location, PFIR microscopy also allows the collection of the infrared absorption spectrum.
In order to further improve the signal of PFIR, we also implemented a multipulse excitation configuration for PFIR. Per one peak force tapping cycle, multiple laser pulses are used to further increase the signal quality. Such an implementation is particularly good for imaging thin material, such as 2D materials.