Professor Xiaoji Xu of Lehigh University

Peak Force Infrared Microscopy and Correlative Chemical, Electrical, and Mechanical Characterization at ~10 nm Spatial Resolution

Abbe’s diffraction limit set a boundary to the spatial resolution of optical microscopy and laser spectroscopy and prevents easy access to the nanoscale. Spectroscopic characterizations and chemical identifications become difficult when their characteristic features are below 100 nm. In this presentation, I will describe a new type of atomic force microscopy (AFM) developed in my research group that achieves label-free infrared microscopy spectroscopy at < 6 nm spatial resolution. The peak force infrared (PFIR) microscopy detects the transient photothermal mechanical responses of the sample due to laser pulse illumination and measure signal that is proportional to the infrared absorption. We demonstrate chemical sensitive imaging and nano-spectroscopy with PFIR on a range of samples across many disciplines. Nanophase separations of the block copolymers are mapped by their chemical identities. Disorders of secondary structures of amyloid fibrils are revealed by PFIR spectra. Secondary organic aerosols are identified by the presence of oxidized chemical compositions at the nanoscale. Zymosan particles from yeast are imaged to reveal the peptidoglycan and embedded proteins. Chemical compositions of organic matter in the source rock of oil shale are obtained in situ. Phonon polaritons of hexagonal boron nitride are revealed by PFIR in polaritonic nanostructures. We have demonstrated a consistent spatial resolution of 6 nm and found the detection limit on the zeptomole level of infrared oscillators. In addition, PFIR also provides complimentary mechanical property mapping simultaneously with the infrared responses for correlative measurement. The types of organic matter in oil shale are found to correlate with the moduli of the surrounding inorganic matrix. Further extension of the PFIR microscopy also includes combination with simultaneous Kelvin probe force measurement on surface work functions of materials, providing correlative chemical and electrical information at < 10 nm spatial resolution. The PFIR microscopy provides a new platform for correlative measurement to decipher the relationships among compositions, structures, and functions of materials.