On the surface structure of 4-mercaptopyridine on Au(111): A new dense phase

TASCA, FEDERICO; CARRO, PILAR; WILLIAMS, FEDERICO JOSÉ; SALVAREZZA, ROBERTO C.; HERRERA, SANTIAGO E.; CALVO, ERNESTO JULIO

LANGMUIR

AMER CHEMICAL SOC

Lugar: Washington; Año: 2017

0743-7463

4-Mercaptopyridine (4MPy) self assembly on Au(111) was studied by in-situ electrochemical scanning tunneling microscopy (STM) in HCLO4, cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Two sets of samples were prepared by varying the immersion time at constant concentration named short time (30 seconds) and long time (3 minutes) adsorption process. Cyclic voltammetry and XPS shows that the chemistry of the adsorbed molecules does not depends on the deposition time giving a well established self assembled monolayer on which the thiols chemisorb over the substrate. By performing a detailed in-situ potentiostatic STM study over the short time adsorption sample we demonstrate that after a cathodic desorption/re-adsorption of the self assembly, a new structure can be observed if the potential remains negative respect to the Au(111) zero charge potential (EPZC). DTF calculations were also performed and a correlation between the observed structure and a dense physisorbed state phase was found giving a surface coverage of θ=0.4 and a 5x√3 lattice configuration. Shifting the potential positive respect to the EPZC, the physisorbed state becomes unstable and a different structure is formed due to the chemisorption of the molecules driven by the electrostatic interaction. Long time adsorption experiments on the other hand shows the typical 5x√3 structure with surface coverage of θ=0.2 (chemisorbed phase) and stable over the whole potential range. The contrast between the behavior of the long time and short time immersion samples can be explained by considering the optimization of the molecular interactions over the self assembly process.