Tip-enhanced infrared nanospectroscopy via molecular expansion force detection

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 microscopy 1 , 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. Mid-infrared spectroscopy with nanometre spatial resolution is highly desired for materials and life sciences applications. A nanoscale mid-infrared spectrometer is demonstrated that detects mechanical forces exerted by molecules on an atomic force microscope tip upon light excitation. It operates under ambient conditions with a high sensitivity and a spatial resolution of better than 25 nm.