2D Vibrational Exciton nano-imaging as a molecular ruler of domain formation in self-assembled monolayers

Abstract

We develop precision infrared nano-imaging and -spectroscopy to probe vibrational excitons and associated vibrational wavefunction delocalization as a molecular ruler to study order and domains in molecular materials and self-assembled monolayer on the molecular scale.

Vibrational Coupling Infrared Nanocrystallography

“…Eigenmode energies will shift or split depending on the distance, magnitude, and relative orientation of infrared (IR) transition dipoles. 13,14 On one hand, this can lead to seemingly anomalous changes to lineshapes and mode energies in IR vibrational spectroscopy, 15,16 changes to Raman scattering energies or even Raman selection rules, 17−19 and potential complications in the interpretation of vibrational spectra. 20−22 On the other hand, the orientation-dependent spectral features of collective vibrations can be used as a tool to measure local molecular order, disorder, and crystallinity as established only recently.…”

“…20−22 On the other hand, the orientation-dependent spectral features of collective vibrations can be used as a tool to measure local molecular order, disorder, and crystallinity as established only recently. 13,14,23 Due to the interactions between vibrational transition dipoles, accurate interpretation of spectral features in dense molecular systems, such as liquids 21 or solids, 11,24 relies on modeling. This is commonly performed by calculating the collective eigenmodes that arise from transition dipole coupling for the vibrations of interest.…”

“…18 Recently, transition dipole interactions have been probed in small ensembles of molecules by applying the VEM to IR nanoimaging. 13,14,23 Previous VCNC studies were based on a qualitative comparison between measured and modeled vibrational lineshapes with modeling based on assumed crystal structures and transition dipole magnitudes used as fit parameters to derive nanoscale disorder. However, without knowing these transition dipole structural parameters, the underlying vibrational wave function delocalization lengths could not be quantified, leaving an incomplete picture of the accuracy and broader applicability of VCNC.…”

“…The dynamic range l max = 10−15 nm for the 2D system is much higher than l max = 5−6 nm for the 3D system, facilitating domain size measurements up to 20 molecules. 13 The corresponding effect of varying dipole moment is shown in the SI, where no variation is observed in k but Δν max increases with increasing |μ⃗ |, leading to a monotonic increase in l max with increasing |μ⃗ |.…”