Preparation and characterization of self assembled monolayers of 2-mercaptonicotinic acid on Au(111)

Understanding the Two-Dimensional Mixing Behavior of 1-Naphthalenethiol and Octanethiol

A two-dimensional (2D) mixture in the form of a self-assembled monolayer composed of two distinct organothiol compounds was created by sequentially depositing 1-naphthalenethiol (1NT) and octanethiol (OT) on a gold surface. By varying the sequence of deposition, two mixed surface systems were created. The surface structure of the resulting mixed monolayer was characterized with Scanning Tunneling Microscopy (STM) and showed surface disorder across all investigated domains. Elemental analysis was carried out with X-ray Photoelectron Spectroscopy (XPS) and indicated that the 1NT monolayer was prone to significant oxidation. Reductive desorption (RD) was used to characterize the binding strength and electrochemical environments of the molecular components in the mixture, and confirmed disordered molecular layers. Due to the presence of oxidized species in the 1NT monolayer, 1NT was displaced by OT resulting in a novel surface structure composed of either OT or 1NT. Monolayers of OT that were exposed to a solution of 1NT resulted in disordered surface structures with a significant amount of gold vacancy islands. To date, there is no experimental phase diagram explaining the chemical behavior of two-dimensional mixtures. This study addresses the need for an experimental understanding of the phase behavior of mixed organothiol self-assembled monolayers (SAMs).

Comparative Study of the Adsorption of Thiols and Selenols on Au(111) and Au(100)

The effect of the Au crystalline plane on the adsorption of different thiols and selenols is studied via reductive desorption (RD) and X-ray photoelectron spectroscopy (XPS) measurements. Self-assembled monolayers (SAMs) using aliphatic (ATs) and aromatic thiols (ArTs) on both Au(111) and Au(100) were prepared. The electrochemical stability of these SAMs on both surfaces is evaluated by comparing the position of the RD peaks. The longer the AT chain the more stable the SAM on Au(100) when compared to Au(111). By means of XPS measurements, we determine that the binding energy (BE) of the S 2p signal corresponding to the S atoms at the thiol/Au interface, commonly assigned at 162.0 eV, shifts 0.2 eV from Au(111) to Au(100) for SAMs prepared using thiols with the C* (C atom bonded to S) in sp³ hybridization, such as ATs. However, when the thiol presents the C* with an sp² hybridization, such as in the case of ArTs, the BE remains at 162.0 eV regardless of the surface plane. Selenol-based SAMs were characterized comparatively on both Au(100) and Au(111). Our results show that selenol SAMs become even more electrochemically stable on Au(100) with respect to Au(111) than the analogue sulfur-based SAM. According to our results, we suggest that the electronic distribution around the Au-S/Se bond could be responsible for the different structural arrangements reported in the literature (gold adatoms, etc.), which should be dependent on the crystalline face (Au(hkl)-S) and the chemical nature of the environment of the adsorbates (sp³-C* vs sp²-C* and Au-SR vs Au-SeR).