The wave-like nature of photons limits the spatial resolution of the traditional optical microscopy to Abbe’s diffraction limit at approximately half of the wavelength. In the infrared regime, such a spatial resolution is much larger than the characteristic scales of many micro/nanoscale heterogeneous materials. In this seminar, I will describe one of our approaches toward infrared imaging and spectroscopy to bring the spatial resolution to sub 10 nm. The method of peak force infrared (PFIR) microscopy utilizes temporal domain mechanical detection of the tip-enhanced infrared photothermal response of the sample with a nanoscopic atomic force microscope (AFM) tip. The infrared absorption is measured as the photothermal expansion signal during the intermittent contact between the AFM tip and the sample. The PFIR microscopy also simultaneously yields mechanical properties of the sample in parallel to the infrared absorption. We have demonstrated the imaging capability of PFIR on a wide range of materials in chemistry, physics, and material sciences, from block copolymer, amyloid fibril, cellular structures to secondary organic aerosols, oil shale source rock, and two-dimensional polaritonic materials. A spatial resolution of 6 nm is demonstrated across samples. Recently, we have upgraded the PFIR microscopy to operate within the liquid phase in a method called the liquid phase peak force infrared (LiPFIR) microscopy. We probed the hyperbolic phonon polaritons of hexagonal boron nitride submerged in water to reveal modification of its dispersion relations. Another recent progress is integrating the PFIR microscopy with the pulsed force Kelvin probe force microscopy (PF-KPFM) to achieve simultaneous chemical, mechanical, and surface potential imaging at 10 nm spatial resolution. Using this multimodal imaging tool, we have observed the charge accumulation along the boundary of perovskite photovoltaic materials as well as a correlation between the secondary structures of the amyloid fibrils and the residual electrical charges. The PFIR microscopy provides a generally applicable label-free chemical imaging tool at the nanoscale.

Multimodal chemical Nano-imaging and spectroscopy through peak force infrared microscopy

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ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.