Professor Xiaoji Xu

Professor Xiaoji Xu
Department of Chemistry
Lehigh University 

Atomic Force Microscope-based Multimodal Infrared Spectroscopy

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.

Xiaoji Xu

Dr. Xiaoji Xu is currently an associate professor in the Department of Chemistry at Lehigh University in Bethlehem, Pennsylvania. Prior to independent research career, he was a postdoctoral fellow at the University of Toronto with Dr. Gilbert C. Walker. He received his B.S. from Peking University in 2004 and Ph.D. from The University of British Columbia in 2009. His current research focuses are on nanoscale chemical imaging, laser spectroscopy, and innovations in scanning probe microscopy for chemical and material measurement. He has published more than 60 peer-reviewed articles and holds several licensed patents on AFM-based infrared microscopy. He is a senior member of the National Academy of Inventors. He was selected as a Beckman Young Investigator in 2018 and Sloan Research Fellow in 2020. He is also a recipient of the NSF CAREER in 2019, Camille Dreyfus Teacher-Scholar Award in 2021, and Richard van Duyne Award in 2025.

Host: Professor Renee Frontiera