Effect of immersion time on the structural order of the self-assembled monolayers of terphenylthiols on the gold surface: Coexistence of different molecular adlayer phases

Abstract

By the example of terphenylthiolate (TPT) films on Au(111), we show by using a high-resolution scanning tunneling microscope (STM) that a long immersion time at room temperature results in the formation of distinct polymorphism. At least three different structural phases namely, α, β, and γ are found to coexist on the gold surface. Two of these phases (α and β) have not been previously detected in the STM images obtained for TPT monolayers prepared for shorter exposure times. The α phase is characterized by the commensurate (4×25)R9.7° structure, with an area per molecule of 34.7 Å2. Whereas the structure of the β phase can be described either by a semi-rectangular (25×213)R7°unit cell or an oblique (25×217)R23° unit cell with a molecular footprint of 33.4 Å2. The third γ phase can be assigned either as a (2×8)or (23×2)structure with a molecular area of 28.8 Å2. The γ phase was found to contain a high-density of translational domains. Unlike shorter immersion times, our results showed that prolonged exposure of the gold substrate to a TPT thiol solution (190 h) leads to the formation of loosely packed monolayers where molecules are largely tilted downward away from surface normal, contrasting with the fully saturated monolayers in which the (23×3)R30°superstructure with an average areal molecular density of 21.6 Å2 was observed in previous studies but, unexpectedly, it is absent in our current results.