Investigation of substrate crystal facet-driven self-assembly through scanning tunneling microscopy-based tip-enhanced Raman spectroscopy

Substrate crystal facet-driven self-assembly of porphyrin derivatives on metal substrates provides a versatile platform for understanding structure-property relationships at molecule-surface interfaces. In this work, we investigate the adsorption and supramolecular organization of a porphyrin derivative on Ag(100) and Ag(110) single-crystal surfaces using ultrahigh vacuum (UHV) scanning tunneling microscopy, and tip-enhanced Raman spectroscopy (TERS). On Ag(100), the molecules form nearly isotropic, close-packed two-dimensional assemblies characterized by a quasi-square unit cell, consistent with weak surface corrugation and intermolecular-interaction-dominated ordering. In contrast, adsorption on the anisotropic Ag(110) surface leads to pronounced directional self-assembly, where molecules align along the close-packed atomic rows to form rectangular unit cells and quasi-one-dimensional domains. Quantitative analysis reveals that this transition from isotropic to anisotropic packing originates from enhanced molecule-substrate interactions and direction-dependent diffusion barriers on Ag(110), which impose strong surface-templating effects. Furthermore, UHV-TERS measurements reveal distinct variations in spectral response between the two surfaces, reflecting differences in local adsorption environments. These experimental observations are further supported by phonon calculations, which capture the specific modifications of vibrational modes and provide insight into the origin of the observed spectral variations. These findings highlight the fundamental role of surface symmetry and corrugation in directing molecular organization and provide insight into the interplay between structural anisotropy and spectroscopic response at metal-organic interfaces.