Atomic Force Microscope-based Multimodal Infrared Spectroscopy

Presenter: Professor Xiaoji Xu, Department of Chemistry, Lehigh University

Spectroscopy and microscopy with optical detection of photons have been the pillars of chemistry toolsets to study molecules and materials. However, traditional optically detected spectroscopy and microscopy are bound by Abbe’s diffraction limit on their spatial resolution. Nanometer-scale features— nano-phase separation in heterogeneous materials, clusters of nanostructures, and interfaces across material domains—are not directly resolvable in situ by linear or nonlinear visible or infrared spectroscopy. In this talk, I will present a different route of spectroscopy and microscopy that is intrinsically not bound by the diffraction limit. A sharp atomic force microscope (AFM) tip mechanically detects the photothermal response of the sample after light-matter interaction. In 2017, we invented a type of AFM-based photothermal microscopy, peak force infrared (PFIR) microscopy, that delivers ~6 nm spatial resolution. Over the past seven years, we have continuously developed the technique to enhance its signal strength and convenience of use and extend it into the aqueous phase for biological applications. We have demonstrated the PFIR on structured polymer surfaces, oil shale source rock, cell surfaces, field-collected outdoor and indoor aerosols, and perovskite photovoltaics. Recently, we have integrated PFIR microscopy with the time-domain two- dimensional infrared (2DIR) technique to bypass the diffraction limit for 2DIR. Using this new tool, we revealed the anharmonicity of vibrational modes through photothermal signals and deciphered the energy transfer pathway of hyperbolic polariton modes in hexagonal boron nitride. In the second part of this presentation, I will present the invention of the pulsed force Kelvin probe force microscopy (PF-KPFM) that delivers <10 nm spatial resolution of samples’ surface potential under ambient conditions. I will discuss its novelty over existing commercially available Kelvin probe force microscopy and present its applications to reveal spatial charge layers and local accumulation of charges on amyloid fibrils, 2D materials, and perovskite photovoltaics.