Abstract
Infrared photoinduced force microscopy (IR-PiFM) is a scanning probe spectroscopic technique that maps sample morphology and chemical properties on the nanometer (nm)-scale. Fabricated samples with nm periodicity such as self-assembly of block copolymer films can be chemically characterized by IR-PiFM with relative ease. Despite the success of IR-PiFM, the origin of spectroscopic contrast remains unclear, preventing the scientific community from conducting quantitative measurements. Here we experimentally investigate the contrast mechanism of IR-PiFM for recording vibrational resonances. We show that the measured spectroscopic information of a sample is directly related to the energy lost in the oscillating cantilever, which is a direct consequence of a molecule excited at its vibrational optical resonance—coined as opto-mechanical damping. The quality factor of the cantilever and the local sample polarizability can be mathematically correlated, enabling quantitative analysis. The basic theory for dissipative tip-sample interactions is introduced to model the observed opto-mechanical damping.
Highlights
Infrared photoinduced force microscopy (IR-PiFM) is a scanning probe spectroscopic technique that maps sample morphology and chemical properties on the nanometer-scale
The enhanced optical field of the scanned atomic force microscopy (AFM) probe is perturbed by the local near field generated by the excited sample, and the scattered near field is detected in the far field using an interferometer to record the image
A quantum cascade laser (QCL) is amplitude modulated at fm and focused on the tip end, and the opto-mechanical response is measured at the first mechanical eigenmode at f1
Summary
Infrared photoinduced force microscopy (IR-PiFM) is a scanning probe spectroscopic technique that maps sample morphology and chemical properties on the nanometer (nm)-scale. Photothermal-induced resonance[5,6] and peak force infrared (IR)[7] are two examples for characterizing sample chemical properties based on AFM. In these techniques, the sample thermal expansion induced by optical absorption is detected using an AFM tip in contact. Noninvasive microscopy and spectroscopy technique that has emerged recently is photoinduced force microscopy (PiFM)[8] (Fig. 1) In this method, the tip–sample optical interaction is measured with the AFM operating in non-contact mode.
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