Tip-enhanced infrared nanospectroscopy via molecular expansion force detection

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Abstract

Mid-infrared absorption spectroscopy in the molecular fingerprint region is widely used for chemical identification and quantitative analysis employing infrared absorption spectra databases. The ability to perform mid-infrared spectroscopy with nanometre spatial resolution is highly desirable for applications in materials and life sciences. At present, scattering near-field scanning optical microscopy1,2,3,4,5,6 is considered to be the most sensitive technique for nanoscale mid-infrared spectroscopy under ambient conditions. Here, we demonstrate that nanoscale mid-infrared spectra can be obtained with comparable or higher sensitivity by detecting mechanical forces exerted by molecules on the atomic force microscope tip on light excitation. The mechanical approach to mid-infrared nanospectroscopy results in a simple optical set-up that, unlike scattering near-field scanning optical microscopy, requires no cryogenically cooled mid-infrared detectors, is easy to align, and is not affected by sample scattering.

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Figure 1: Description of experiment.
Figure 2: Photoexpansion spectra of self-assembled monolayers on gold.
Figure 3: Demonstration of spatial resolution.
Figure 4: Sample heating and expansion.

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Acknowledgements

The authors acknowledge financial support from the Robert A. Welch Foundation (grant no. F-1705) and the US Department of Energy STTR program. Sample fabrication was carried out in the Microelectronics Research Center at the University of Texas at Austin, which is a member of the National Nanotechnology Infrastructure Network (NNIN). The authors thank C. Prater, V. Yakovlev and F. Lagugné-Labarthet for discussions.

Author information

M.A.B. conceived and designed the experiments. F.L. built the experimental set-up. F.L. and M.J. performed the experiments. All authors analysed the data and wrote the paper.

Correspondence to Mikhail A. Belkin.

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Competing interests

M.A.B. and F.L. are co-authors of US patent application no. 13/307,464, ‘High frequency deflection measurement of IR absorption’. M.J. declares no competing financial interests.

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Lu, F., Jin, M. & Belkin, M. Tip-enhanced infrared nanospectroscopy via molecular expansion force detection. Nature Photon 8, 307–312 (2014) doi:10.1038/nphoton.2013.373

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