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

Abstract

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.

The Bouguer‐Beer‐Lambert Law: Shining Light on the Obscure

“…This means that the absorbance band maxima are redshifted towards the maxima of the index of refraction and that the band shapes can strongly change. [52][53][54][55] A simple way to reduce this problem is to use high index ATR crystal materials like Si and Ge and to apply high angles of incidence. Unfortunately, this means that the penetration depths and, thereby, the overall intensities are reduced and by that the sensitivity of the method.…”

Revealing the Distribution of Metal Carboxylates in Oil Paint from the Micro‐ to Nanoscale

“…[12] A key characteristic that distinguishes PTIR from other chemically-sensitive AFM techniques is its ability to probe the composition in samples even thicker than 1 µm. [13] Since the PTIR spatial resolution depends in part on the sample thermomechanical properties, the sample stratification and its thickness, [5,14] the best spatial resolution is obtained for thin (< 500 nm) samples that are vertically homogeneous. While PTIR experiments are typically carried out in contact-mode, here we employ tapping mode with a novel heterodyne detection scheme (Figure 2 A, B) where the tapping motion of the cantilever is mixed with the sample expansion due to non-linear tip-sample interactions.…”

Nanometre-scale infrared chemical imaging of organic matter in ultra-carbonaceous Antarctic micrometeorites (UCAMMs)

“…Effort has been made to overcome the diffraction limitations imposed by classical Fourier transform IR microspectroscopy analyses (Kebukawa et al 2010;Dominguez et al 2014;Yesiltas & Kebukawa 2016), and preserve the information on chemical functional groups that is complementary to conventional elemental and isotopic mapping provided by electron microscopy and secondary ion mass spectrometry (NanoSIMS). AFMIR provides the spatial resolution mapping capability and is also, if well calibrated, among the best near-field quantitative techniques (Ramer et al 2017).…”