Formation and Surface Structures of Long-Range Ordered Self-Assembled Monolayers of 2-Mercaptopyrazine on Au(111).

The effect of solution pH on the formation and surface structure of 2-pyrazinethiolate (2-PyzS) self-assembled monolayers (SAMs) formed by the adsorption of 2-mercaptopyrazine (2-PyzSH) on Au(111) was investigated using scanning tunneling microscopy (STM) and X-ray photoelectron microscopy (XPS). Molecular-scale STM observations clearly revealed that 2-PyzS SAMs at pH 2 had a short-range ordered phase of (2√3 × √21)R30° structure with a standing-up adsorption structure. However, 2-PyzS SAMs at pH 8 had a very unique long-range ordered phase, showing a "ladder-like molecular arrangement" with bright repeating rows. This ordered phase was assigned to the (3 × √37)R43° structure, consisting of two different adsorption structures: standing-up and tilted adsorption structures. The average arial density of 2-PyzS SAMs on Au(111) at pH 8 was calculated to be 49.47 Å/molecule, which is 1.52 times more loosely packed compared to the SAMs at pH 2 with 32.55 Å/molecule. XPS measurements showed that 2-PyzS SAMs at pH 2 and pH 8 were mainly formed through chemical interactions between the sulfur anchoring group and the Au(111) substrates. The proposed structural models of packing structures for 2-PyzS SAMs on Au(111) at different pHs are well supported by the XPS results. The results of this study will provide new insights into the formation, surface structure, and molecular orientation of SAMs by N-heteroaromatic thiols with pyrazine molecular backbone on Au(111) at the molecular level.