Interplay between Hydrogen Bonding and Molecule-Substrate Interactions in the Case of Terephthalic Acid Molecules on Cu(001) Surfaces

Abstract

The adsorption and self-assembling properties of terephthalic acid (TPA) molecules deposited on Cu(001) at room temperature have been systematically studied using both experimental and theoretical tools. The system forms two phases at room temperature, the metastable β-phase and the stable 3×3 one. In the case of the β phase, the low-energy electron diffraction and scanning-tunneling microscopy (STM) results indicate that the β phase has a (9 √ 2×2 √ 2)R45◦ unit cell with exactly the same molecular coverage as the 3×3 phase. In addition, the high-resolution X-ray photoelectron spectroscopy O1s spectra indicate that the irreversible β → 3 × 3 transition involves the following two processes: i) deprotonation of the complete carboxyl groups remaining in the metastable phase and ii) re-arrangement of the molecules into the 3×3 configuration. On the other hand, we explored possible molecular configurations for the β phase with different degree of deprotonation (including structures with Cu adatoms) by means of density functional theory calculations. Our theoretical results indicate the formation of strong bonds between the O atoms in carboxylates and the Cu atoms of the surface, which causes a bending of the molecules and a buckling of the first Cu layer. In the 3 × 3 phases, we show that the bending produces observable effects in the molecular STM images. We also observed that the strong interaction between the carboxylates and the Cu atoms at the step edges drives the reorientation of the surface steps along the < 100 > crystallographic directions.