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Andrea Centrone1 Georg Ramer1 Jungseok Chae1 Vladimir Aksyuk1

1, National Institute of Standards and Technology, Gaithersburg, Maryland, United States

The photothermal induced resonance (PTIR)[1] is a scanning probe technique that combines AFM with IR (or visible) absorption spectroscopy, enabling material identification, molecular conformational analysis, mapping of composition and electronic bandgap at the nanoscale. In PTIR, the absorption of a laser pulse induces a rapid thermal expansion of the sample. Conventional cantilevers are too slow to track the sample thermal expansion dynamics; however, the fast sample expansion kicks the cantilever in oscillation (like a struck tuning fork), with amplitude proportional to the absorbed energy.
After a brief introduction, I will leverage PTIR to provide direct proof of ion electron migration[2] and ferroelasticity[3] (but not ferroelectricity) in organic-inorganic perovskites solar cells. As second example, I will show that the PTIR near-field mechanical detection enable the observation of dark-polaritonic modes in hexagonal boron-nitride nanostructures (hBN), for the first time.
Later, I will discuss how we revolutionize AFM (and PTIR) signal transduction by integrating cavity-optomechanics for sensing the motion of fast, nanosized/picogram scale AFM probes with unprecedented precision and bandwidth, thereby breaking the trade-off between AFM measurement precision and ability to capture transient events.[4] Applied in PTIR, the probe near-field ultralow detection noise and wide bandwidth improves the time resolution, signal-to-noise ratio and throughput by a few orders of magnitude each. Remarkably, this synergy enables a new PTIR measurement modality: capturing the previously inaccessible fast thermal-expansion response of the sample to nanosecond laser pulses, thus allowing concurrent measurement of the chemical composition and thermal conductivity, at the nanoscale.
We validate these new capabilities using polymer films and measure the intrinsic thermal conductivity (η) of metal-organic framework (MOF) individual microcrystals, a property not measurable by conventional techniques. MOFs are a class of nanoporous materials promising for catalysis, gas storage, sensing and thermoelectric applications where accurate knowledge of η is critically important. Additionally, the improved sensitivity enable measurement of nanoscale IR spectra of monolayer this sample with high signal to noise ratio (≈ 170).
We strongly believe that the radical AFM innovation enabled by nanofabrication and cavity-optomechanics is broadly-applicable and will benefit a wide range of AFM-based dynamic observations in nanoscience and biology and it greatly improve the impact of the PTIR technique in those fields.
1. Centrone, A. Annu. Rev. Anal. Chem. 2015, 8, (1), 101-126.
2. Yuan, Y., et al. J. Adv. Energy Mater. 2015, 5, (15), 1500615.
3. Strelcov, E., at al. Sci Adv 2017, 3, (4), e1602165.
4. Chae, J.; An, S.; Ramer, G.; Stavila, V.; Holland, G.; Yoon, Y.; Talin, A. A.; Allendorf, M.; Aksyuk, V. A.; Centrone, A. Nano Lett. 2017, 17, (9), 5587-5594.

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