Effect of Headgroup on the Formation and Structure of Self-Assembled Monolayers on Au(111) Derived from Adamantane Derivatives

Abstract

The formation and structure of self-assembled monolayers (SAMs) on Au(111) derived from adamantane derivatives such as 1-adamantanethiol (AD-SH), 1-adamantyl isocyanide (AD-NC), and 1-adamantyl isothiocyanate (AD-NCS) were characterized by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), contact angle (CA), and cyclic voltammetry (CV). It was found that the formation and structure of AD-SH (thiol headgroup), AD-NC (isocyanide headgroup), and AD-NCS (isothiocyanate headgroup) SAMs on Au(111) were strongly influenced by the headgroup attached to adamantane cage group. Molecular-scale STM imaging revealed that AD-SH SAMs on Au(111) prepared in 1 mM ethanolic solution at 50 oC for 1 hour had uniform ordered phase, which can be described as hexagonally close-packing (7 × 7) structure with an intermolecular distance of ~ 7 Å. AD-NC SAMs on Au(111) prepared in 1 mM ethanolic solution at 75 oC for 90 min showed (2√3 × 2√10) lattice structure, whereas AD-NC SAMs formed in 0.1 mM ethanolic solution at 75 oC for 90 min showed row structure with a row spacing of 1.25 ± 0.03 nm. In addition, AD-NC SAMs formed in DMF solution at 90 oC for 1 h can form highly ordered structure, and additional thermal annealing at 100 oC enhanced the structural quality of SAMs with long-range ordered domains. On the other hand, the surface structure of AD-NCS SAMs prepared at 50 oC for 12 h displayed ordered row structure with a row spacing of 1.66 ± 0.02 nm. XPS measurements showed strong S 2p peaks for AD-SH and AD-NCS SAMs at 162.0 (S 2p3/2) and 162.2 eV (S 2p3/2) respectively, implying that the SAMs were formed via chemical reaction between the sulfur atoms and gold surfaces. N 1s peaks for AD-NC and AD-NCS SAMs were observed at 400.3 and 400.0 eV respectively, suggesting the formation of each SAM. TDS measurements showed that the desorption behaviors of these SAMs derived from adamantane derivatives differ considerably, implying that the interaction between headgroup and Au(111) surface affect the thermal stability of SAMs. Reductive desorption peaks for AD-SH, AD-NC, and AD-NCS SAMs were observed at -876, -883, and -877 mV, respectively, which means that AD-NC SAMs have the strongest binding energy. To understand the adsorption behavior of AD-SH and AD-NC molecules on Au(111), we prepared binary SAMs by controlling various molar ratio of these molecules. As a result, we found that even though the mole fraction of AD-NC is higher than AD-SH (AD-NC:AD-SH=99:1), AD-SH SAMs were dominantly formed. This result means that the adsorption of AD-SH is much faster than that of AD-NC, which may be driven by higher adsorption affinity of thiol on Au(111) surface compared to isocyanide. From our study, we clearly revealed that the headgroup of adamantane derivatives on Au(111) strongly affects the formation, structure, thermal stability, and electrochemical behavior of SAMs. We believe that our results would provide significant information for further understanding of various surface characteristics on Au(111).