Visualizing Size-Dependent Vibrational and Excited-State Evolution by Tip-Enhanced Raman Imaging

Tip-enhanced Raman scattering (TERS) enables real-space mapping of molecular vibrations and electronic excitations with subnanometer resolution, offering a direct route to track the molecular-size-dependent evolution of these properties. Here, using the hexagonal n-coronene series as a model system, we show that vibrationally resolved nonresonant TERS images of the ring-breathing mode can characterize a pronounced size-dependent redistribution where the spatial vibration migrates outward and becomes progressively concentrated at the molecular periphery as the molecule grows. This vibrational evolution is quantitatively captured by an envelope-based model. In contrast, resonant TERS imaging identifies two low-lying excited states with size-independent and distinct spatial characters across the series, including a delocalized lowest excitation and a persistently edge-localized second excitation. These findings serve as good references for future TERS experiments on visualizing size-driven property evolution, deepening our understanding of the crossover from molecule-like to graphene-like nanodisk behavior as π-conjugation expands.