Applications brief: Tip-enhanced infrared nanospectroscopy via molecular expansion force detection

Key points


  • AFM-IR collects spectra of molecular monolayers and monolayer islands with high spatial resolution
  • Comparison of AFM-IR and s-SNOM techniques
  • Achieving sub-monolayer sensitivity

Key words

s-SNOM | QCL | infrared nanospectroscopy | monolayers | microscopy | sub-wavelength optics

Authors

Feng Lu, Mingzhou Jin, and Mikhail A. Belkin

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 nanometer spatial resolution is highly desirable for applications in materials and life sciences.

At present, scattering near-field scanning optical microscopy 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.

(a) Topography of PEG monolayer islands. (b) Height of monolayer islands measured by a topographic line scan along blue arrow in (a). (c) Spectra taken at points shown in (b).
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