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AFM-IR
AFM-IR 〔A. Dazzi, R. Prazeres, F. Glotin and J. M. Ortega, Local infrared microspectroscopy with subwavelength spatial resolution with an atomic force microscope tip used as a photothermal sensor, ''Optics Letters'', 30 (18), 2388-2390 (2005)〕〔A. Dazzi, F. Glotin and J.M. Ortega, Subwavelength infrared spectromicroscopy using an AFM as a local absorption sensor, ''Infrared Physics and Technology'', 49 (September 2006), 113-121 (2006).〕〔A. Dazzi, Sub-100-Nanometer Infrared Spectroscopy and Imaging Based on a Near-Field Photothermal Technique (PTIR), in ''Biomedical Vibrational Spectroscopy'', edited by J. Kneipp and P. Lasch (Wiley, Hoboken, NJ, USA, 2008), pp. 291-312.〕〔A. Dazzi, C. B. Prater, Q. Hu, D. B. Chase, J. F. Rabolt and C. Marcott, AFM-IR: combining atomic force microscopy and infrared spectroscopy for nanoscale chemical characterization, ''Applied Spectroscopy'', 66 (12), 1365-1384 (December 2012).〕〔A. Dazzi, R. Prazeres, F. Glotin and J. M. Ortega, Analysis of nano-chemical mapping performed by an AFM-based ("AFMIR") acousto-optic technique, ''Ultramicroscopy'', 107 (12), 1194-1200 (November 2007)〕 refers to atomic force microscope (AFM) based infrared (IR) spectroscopy. AFM-IR is a technique for chemical analysis of samples at nanoscale spatial resolution. AFM-IR is related to techniques including Tip-Enhanced Raman Spectroscopy (TERS) and scanning near-field optical microscopy (SNOM) and other methods of vibrational analysis with scanning probe microscopy. The AFM-IR technique uses a sharp tip of an AFM probe to measure the absorption of infrared light by a sample. Recording the amount of IR absorption as a function of wavelength or wavenumber creates nanoscale IR absorption spectra, which can be used to chemically characterize and even identify unknown materials.〔T. Eby, U. Gundusharma, M. Lo, K. Sahagian, C. Marcott and K. Kjoller, Reverse engineering of polymeric multilayers using AFM-based nanoscale IR spectroscopy and thermal analysis, ''Spectroscopy Europe'', 24 (3), 18-21 (June 13, 2012).〕 Recording the IR absorption as a function of position can be used to create chemical composition maps that show the spatial distribution of different chemical components. AFM-IR can overcome the diffraction limit that limits the spatial resolution of conventional infrared microscopy and microspectroscopy to the scale of several microns. AFM-IR can achieve spatial resolution down to around 20 nm, limited in some case only by the sharpness of the AFM probe tip and sensitivity down to the scale of molecular monolayers.〔F. Lu, M. Jin and M. A. Belkin, Tip-enhanced infrared nanospectroscopy via molecular expansion force detection, ''Nature Photonics'', 8 (4), 307-312 (January 19, 2014).〕
==History==
The earliest measurements combining AFM with IR spectroscopy were performed by Hammiche et al.〔A. Hammiche, H. M. Pollock, M. Reading, M. Claybourn, P. H. Turner and K. Jewkes, Photothermal FT-IR Spectroscopy: A Step Towards FT-IR Microscopy at a Resolution Better Than the Diffraction Limit, ''Applied Spectroscopy'', 53 (7), 810-815 (July 1999).〕 in the UK (dubbed "photothermal microspectroscopy") and separately by Anderson at JPL.〔M. S. Anderson, Infrared Spectroscopy with an Atomic Force Microscope, ''Applied Spectroscopy'', 54 (3), 349-352 (2000).〕 Both groups used a conventional FTIR as the infrared source and focused this beam onto a sample near the tip of an AFM. Hammiche et al. detected the absorbed IR radiation using temperature sensitive AFM probes developed originally for microthermal analysis to detect the temperature rise resulting from IR radiation. Anderson used conventional AFM cantilever probes to detect thermal expansion of the sample resulting from IR absorption. Both of these approaches had limited spatial resolution, however, due to thermal diffusion, i.e. the spreading of heat away from the region where the IR light was absorbed. The thermal diffusion length, i.e. the distance the heat spreads, is inversely proportional to the root of the modulation frequency. Thus in practice, the spatial resolution achieved by the FTIR-AFM approaches was around 1 μm or more, due to the low modulation frequencies used in these approaches, which in turn allowed heat to diffuse over larger distances during the measurement time.
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