Quantitative Chemical Analysis at the Nanoscale Using the Photothermal Induced Resonance Technique

Photothermal induced resonance (PTIR), also known as AFM-IR, is a scanning probe technique that provides sample composition information with a lateral resolution down to 20 nm. Interest in PTIR stems from its ability to identify unknown samples at the nanoscale thanks, in first approximation, to the direct comparability of PTIR spectra with far-field infrared databases. The development of rapidly tuning quantum cascade lasers has increased the PTIR throughput considerably, making nanoscale hyperspectral imaging within a reasonable time frame possible. Consequently, a better understanding of PTIR signal generation and of the fine details of PTIR analysis has become of paramount importance for extending complex IR analysis methods developed in the far-field, e.g., for classification and hyperspectral imaging, to nanoscale PTIR spectra. Here we calculate PTIR spectra via thin-film optics, to identify subtle changes (band shifts, deviation from linear approximation, etc.) for common sample parameters in the case of PTIR with total internal reflection illumination. Results show signal intensity linearity and small band shifts as long as the sample is prepared correctly, with band shifts typically smaller than macroscale attenuated total reflection (ATR) spectroscopy. Finally, a generally applicable algorithm to retrieve the pure imaginary component of the refractive index (i.e., the chemically specific information) is provided to overcome the PTIR spectra nonlinearity.