学术报告|Peak Force Infrared Microscopy: super-resolution photothermal infrared imaging and spectroscopy at 6 nm spatial resolution
Xiaoji Xu established his research group since 2014 as an assistant professor in the department of chemistry at Lehigh University in Bethlehem, Pennsylvania, United States. Before at Lehigh University, he was a postdoctoral fellow at the University of Toronto, and University Colorado Boulder. He received his B.S. in Chemistry from Peking University in 2004 and Ph.D. from The University of British Columbia, Vancouver, Canada in 2009. His research focus is on chemical sensitive nanoscale imaging, ultrafast laser spectroscopy, and development of the analytical instrumentation for chemical measurement. He holds four patents on nanoscale infrared microscopy. He has published several top papers as the first/corresponding author, including Nature Physics, Nature Communications (4), Science Advances, Nano Letters and ACS Nano. Dr. Xu was selected as a Beckman Young Investigator by Arnold and Mabel Beckman Foundation in 2018. He is also a recipient of the NSF CAREER award in 2019.
Abstract:Traditional optical spectroscopy and microscopy rely on the detection of photons. They have two limitations. First, the wave nature of photons leads to the optical diffraction limit that put a boundary to the spatial resolution. Second, optical detectors are typically sensitive within a certain frequency range. It is not practical to have a unifiedandsensitive optical detector for all frequency ranges from ultraviolet to far infrared. In this presentation, we present our solution, peak force photothermal microscopy, to overcome these two limitations of microscopy and spectroscopy. In peak force photothermal microscopy, the photothermal effect of optical absorption is mechanically measured by an atomic force microscope (AFM) operated in non-resonant peak force tapping mode. The background-referencing signal detection mechanism allows direct measurement of the photothermal expansion as a result of the absorption of photons. The spatial resolution of peak force photothermal microscopy is determined by the contact area of the AFM tip rather than the wavelength of the photons, thus bypassing the optical diffraction limit. Besides probing optical absorption, peak force photothermal microscopy provides simultaneous mechanical information for multimodal characterization. With the infrared excitation, we demonstrate chemical sensitive imaging on a range of samples from the block copolymer, amyloid fibril, secondary organic aerosols, to oil shale. A spatial resolution of 6 nm is observed, and the detection limit is on the zeptomole level. With visible light excitation, we demonstrate nano-imaging of optical absorbing molecules and organic photovoltaic materials. A spatial resolution of <5 nm is observed without the need for optical detection or fluorescence emitting labels. Peak force photothermal microscopy is a generally applicable multimodal analytical surface imaging method with high spatial resolution and sensitivities。