Abstract:

Two-dimensional infrared (AFM-2DIR) and electronic spectroscopies resolve anharmonicity, mode coupling, and energy transfer, but far-field optical detection limits their spatial resolution to the diffraction limit and averages over heterogeneous samples. We present atomic force microscopy-based two-dimensional spectroscopy which overcomes this limit by combining a tip-enhanced femtosecond pulse sequence with local photothermal detection by the AFM tip. In AFM-2DIR, a metal-coated AFM tip confines the mid-IR pulse sequence to a nanoscale region beneath its apex. Nonradiative relaxation of the excited-state populations produces local thermal expansion, which is read out mechanically by the cantilever under peak force tapping. Sequential Fourier transforms of the resulting photothermal interferograms yield 2D spectra with the spatial precision of the AFM tip. We demonstrate vibrational anharmonicity and overtone ladder climbing of carbonyl modes in polymer thin films. We also reveal energy transfer from hyperbolic phonon polaritons to phonons, and between polariton modes, in two-dimensional materials of boron nitride. To interpret these action-detected signals, we developed a density-matrix theory in which the photothermal signal is a population-terminated fourth-order response projected onto an energy-weighted thermal-action operator. The theory explains why photothermal 2D spectra closely resemble optically detected spectra: both read out the same third-order coherence, with different readout weights. In addition, we will present the further development of AFM-2DIR with pulse shaping for faster data acquisition and better stability. 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. We will discuss the novelty of this approach 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.

Bio:

Dr. Xiaoji Xu is currently a professor in the Department of Chemistry at Stony Brook University. 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. He started as an assistant professor and then associate professor at Lehigh University before joining Stony Brook. 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.