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 microscopy1–6 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. A schematic of the experimental set-up is shown in Fig. 1a. Pulses of tunable infrared light are incident on a sample. Upon optical absorption, molecules transit into an excited vibrational state. In a very short time ( 10 ps, ref. 7, much faster than the atomic force microscope (AFM) cantilever response time), the excited vibrational mode non-radiatively dissipates into molecular vibrational modes of lower energies as well as to vibrational and kinetic modes of the surrounding molecules and substrate. Because of the anharmonicity of molecular vibrations, the effective molecular volume increases (on a macroscopic scale this leads to thermal expansion), which results in a force acting on the AFM tip positioned in contact with the sample. This force leads to a small cantilever deflection and subsequent oscillation, which may be detected by the AFM position-sensitive photodetector (PSPD) and amplified by the lock-in amplifier (Fig. 1a). In the first approximation, the force on the AFM tip is linearly proportional to the absorbed optical energy. The dependence of the AFM cantilever deflection Dz on the excitation laser wavelength l is then expected to follow the molecular absorption aabs(l) after normalization to the incident laser intensity I(l):