Adsorption of phenanthroline and its derivatives on Au (1 1 1) surface – Influence of substitution on structure and electronic properties

The adsorption of phenanthroline (PHEN), CH₃-substituted phenanthroline (PHEN-CH₃) sulfur substituted phenanthroline (S-PHEN) and CN substituted phenanthroline (CN-PHEN) on Au (1 1 1) surface was studied to understand the effect of substitution on structure and electronic properties of organic molecule/metal interface. The electronic properties, such as Bader charge analysis, electron density difference analysis were studied for the adsorbed systems with different orientations of adsorbent. Based on the calculated adsorption energy we found certain configurations, namely PHEN_VER for Phenanthroline adsorption (vertical to the surface), CH₃-PHEN_HOR for methyl substituted phenanthroline (parallel to the surface) and SV for sulfur substituted phenanthroline (parallel to the surface) were preferable orientations for adsorption of respective phenanthroline based molecules. The frontier molecular orbital patterns of the studied molecules provide further insight on why certain configurations are preferable in respective cases and how one could account the interfacial charge transport properties. We studied density of states, work function and Schottky barrier height corresponding to adsorption configuration that was identified as a preferable one. The present study highlights the importance of substitution and site of substitution on interfacial interaction and charge transport, and validates the importance of sulfur substitution on phenanthroline in phenathroline-Au (1 1 1) interaction.