Novel insights into nanoscale surface displacement detection in polystyrene thin films using photothermal mirror- and atomic force microscopy-mid-IR spectroscopy

Photothermal spectroscopy techniques operating at single wavelengths in the vis-NIR range have been widely used to measure optical absorption and thermal characteristics of materials owing to their high sensitivity. We introduced photothermal mirror spectroscopy employing a highly tunable mid-IR pump laser (PTM-IR) for the chemical analysis of thin film polystyrene samples on IR transparent calcium fluoride substrates. PTM-IR spectroscopy surpasses conventional PTM spectroscopy as it provides chemical specificity through molecule-specific absorption <i>via</i> the detection of the magnitude of the PTM signal as a function of the excitation wavelength. We compared the obtained spectra with those measured using atomic force microscopy-infrared spectroscopy (AFM-IR), an already well-established photothermal technique also operating in the mid-IR range, and standard Fourier-transform infrared (FT-IR) spectroscopy. Numerical simulations using finite element analysis were employed to estimate the expected increase in temperature and surface deformation induced by the laser pulse train in each photothermal technique. Excellent agreement was obtained across the studied techniques in terms of qualitative mid-IR spectra and thickness determination.