Molecular Sensitivity of Near-Field Vibrational Infrared Imaging

C.-F. Wang, B. Kafle, T. E. Tesema, H. Kookhaee and T. G. Habteyes

J. Phys. Chem. C 124, 21018 (2020)
Quantifying the sensitivity limit of scattering-type scanning near-field optical microscopy (s-SNOM) in vibrational infrared imaging requires assembly of molecular systems with continuous variation of height across lateral displacements, which has not been available to date. In this work, we fabricate a film of poly(4-vinylpyridine) (P4VP) with about 7° angle of elevation on gold and silicon substrates and compare the chemical contrast due to the ring stretching vibration of P4VP as a function of sample thickness. We observe that the near-field contrast starts to change at the same time as the sample height, which increases at a rate of a nanometer per 10 nm lateral displacement crossing from the bare substrates to the P4VP-coated region. Saturation of phase contrast is observed for a thicker than 100 nm sample on silicon, while it continues to increase even beyond 200 nm on gold. The presaturation regime on silicon appears to coincide with the higher net field enhancement in the sample on silicon than on gold, in spite of the higher overall scattering amplitude on gold. Although the chemical contrasts are observable as the sample thickness decreases to a sub-nanometer scale on both substrates, important distinctions are observed because of the roughness of the as-prepared gold film. The variation in the local tip–sample geometry results in signal fluctuation that creates uncertainty in the chemical contrast when the sample thickness is comparable to the roughness. The results presented here provide clarity to advance s-SNOM chemical imaging to the molecular “finger print” region of electromagnetic radiation.