Effect of hybrid field coupling in nanostructured surfaces on anisotropic signal detection in nanoscale infrared spectroscopic imaging methods

Abstract

Anisotropic intensity distributions on nanostructured surfaces and polarization-sensitive spectra have been observed in a number of nanoscale infrared spectroscopic imaging methods, including nano-FTIR [Bakir et al., Molecules, 2020, 25, 4295], photothermal induced resonance (PTIR) [Waeytens et al., Analyst, 2021, 146], tapping AFM-IR [Hondl et al., ACS Meas. Sci. Au, 2025, 5, 469; Luo et al., APL, 2022, 121, 23330], infrared photoinduced force microscopy (PiF-IR) [Anindo et al., JPCC, 2025, 129, 4517; Shcherbakov et al., Rev Methods Primers, 2025, 5, 1; Ali et al., Anal. Chem., 2025, 97, 23914] and peak force infrared microscopy (PFIR) [Xie et al., JPCC, 2022, 126, 8393; Anindo, JPCC, 2025]. A recent work combining modeling and experiment demonstrated that the hybrid field coupling of the IR illumination E0 with a polymer nanosphere and a metallic AFM probe is nearly as strong as the plasmonic coupling in case of a gold nanosphere [Anindo, JPCC, 2025]. For p-polarized illumination, this results in enhanced IR absorption on the surface perpendicular to the propagation of E0 which can explain the observed anisotropic intensity distribution. An additional anisotropy may be introduced by aligned surface molecules with oriented vibrational transition moments [Bakir et al., Molecules, 2020, 25, 4295; Luo, APL, 2022]. PiF-IR is strongly surface sensitive combining an unprecedented spatial resolution < 5 nm with high spectral resolution [Shcherbakov, Rev Methods Primers, 2025; Ali, Anal. Chem., 2025], which allows, for example, to visualize nanoscale chemical variation on the surface of bacteria cells affected by antimicrobial interaction [Ali, Anal. Chem., 2025]. We compare PiF-IR hyperspectra of aligned perylene Langmuir Blodgett monolayers on nanostructured and planar gold substrates and use quantum chemical calculations of the oriented vibrational oscillators to interpret the observations.

Figures (11)

• Figure 1: Experimental and computed infrared spectra of PMIS-C8. a) and b) PiF spectra of PMIS-C8 monolayer on nanostructured and on planar Au, respectively; red and blue arrows in a) mark bands showing intensity variations and no variations, respectively; c) FTIR and ATR spectra; d-e) computed stick and line broadened IR spectra: d) unpolarized line broadened spectrum (dotted line) and component spectra parallel to long axis (x) of perylene core (solid line, and stick spectrum), e) component spectra parallel to short axis (y) of perylene core and f) component spectra perpendicular (z) to perylene core.
• Figure 2: Local molecular orientation of PMIS-C8 on nanostructured Au substrate visualized using PiF-IR hyperspectral bands. a) PiF and b) simultaneously acquired topography in single frequency scan at $\nu=$ 1324 cm$^{-1}$, c) topography acquired with PiF-IR hyperscan in the same area, d) PiF contrast of selected bands in the hyperscan, e) PiF intensity correlations of characteristic PMIS-C8 bands of the hyperscan to the bands at 1694 cm$^{-1}$ (left) and 1581 cm$^{-1}$ (right); f-h) HCA analysis of hyperspectrum: f) cluster mean spectra, g) dendrogram and h) factor maps in matching colors, i,j) modeled photo-induced $E$-field on Au nanostructure: i) orientation and j) normalized field strength in sample plane.
• Figure 3: Local molecular orientation of PMIS-C8 on planar Au substrate visualized using PiF-IR hyperspectral bands. Selected areas of a) PiF and b) simultaneously acquired topography in single frequency scan at $\nu=$ 1694 cm$^{-1}$ shown in Figure S5, c) topography acquired with PiF-IR hyperscan in the same area, d) PiF contrast of selected bands in the hyperscan, e) PiF intensity correlations of characteristic PMIS-C8 bands of the hyperscan to the bands at 1694 cm$^{-1}$ (left) and 1581 cm$^{-1}$ (right); f-h) HCA analysis of hyperspectrum: f) cluster mean spectra with characteristic bands marked by red arrows, g) dendrogram and h) factor maps.
• Figure 4: Molecular orientation of PMIS-C8 compared to field effects in PMIS-C8 monolayer on nanostructured Au: a-c) PiF contrasts of a 1 $\mu$m wide area acquired at $\nu=1378$ cm$^{-1}$, $\nu=1650$ cm$^{-1}$ and $\nu=1694$ cm$^{-1}$, respectively; d,e) RGB images of cropped areas of PiF contrasts at d) 1694 cm$^{-1}$ (B), 1650 cm$^{-1}$ (R) and 1378 cm$^{-1}$ (G), and e) 1694 cm$^{-1}$ (B) and 1650 cm$^{-1}$ (R+G = yellow); f) AFM topography and g,h) gradients of topography calculated in g) horizontal ($x$) and h) vertical ($y$) direction. The blue shape in g) depicts the inclination of the scanning AFM tip. The blue arrow in h) marks the propagation $k_0$ of the incident field $E_0$. i-k) Correlation plots of PiF intensities in the three frequencies with sample topography and gradients.
• Figure 5: BAM images acquired during PMIS-C8 monolayer deposition: a) air-water interface before adding PMIS-C8 solution and b) three different positions of a PMIS-C8 monolayer formed on the water surface at a surface pressure $\Pi=20$ mN/m.