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Self‑assembled monolayers (SAMs) control the surface functionalities of a number of materials and have a wide range of important applications. Disorders such as point defects and domain formation can disrupt the desired interfacial properties. In this work, the authors demonstrated that vibrational exciton nanoimaging using Bruker's nanoIR spectrometer with s‑SNOM can be used as a local probe of molecular disorder and domain size in 4‑nitrothiophenol (4-NTP) SAM on gold. Combined with theoretical modeling, a domain size ranging from 3 to 12 nm was determined for the 4‑NTP SAM system.
Experimentally, a series of mixed SAMs of 4‑NTP and thiophenol were prepared with varying molar ratios, which led to the formation of SAMs with controlled density and domain size of 4‑NTP. For the s‑SNOM study, the N‑O stretching mode near 1340 cm⁻¹ was used as a resonance marker, and the standing-up domains of 4‑NTP exhibited a field enhanced intensity because the metal-coated AFM tip selectively probed the out‑of‑plane component of the N‑O stretch. The s‑SNOM results showed a systematic red shift and broadening of the N‑O stretching band as the 4‑NTP was diluted by thiophenol, with the peak position observed at 1344.5 cm⁻¹ for 100% 4‑NTP and 1337.3 cm⁻¹ for 20% 4‑NTP. The red shift for the diluted 4-NTP SAM was attributed to the weaker vibrational coupling between NO₂ groups at larger separations.
The measured spectral shifts of the N‑O stretching band were theoretically modeled to give the number of coupled molecules, which was then used to determine the sizes of standing‑up domains. To do that, the vibrational wave function delocalization of neighboring nitro groups in the standing‑up phase was treated as vibrational excitons, which were formed through transition dipole coupling between the NO₂ groups under conditions of tight packing or large transition dipole moments. The modeled red shifts of vibrational excitons were in good agreement with the s‑SNOM measurements, and the calculated domain sizes were close to the results from STM studies.
Overall, this study confirms that the s‑SNOM spectral nano‑imaging is a powerful tool in local chemical imaging, with a high sensitivity to probe molecular domains of 2.8‑8.8 nm for the 4-NTP SAM in this work, which represents an average delocalization length between 7 and 22 4‑NTP molecules.