Adsorption of Dodecanethiol on Cu(110):  Structural Ordering upon Thiolate Formation

The adsorption of dodecanethiol [CH3(CH2)11SH] films on Cu(110) by vapor deposition under ultrahigh vacuum conditions has been studied by means of thermal desorption spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction with a special emphasis on the structural changes accompanying the transition from a physisorbed monolayer to a chemisorbed saturation structure. Adsorption at 110 K leads to the formation of an ordered physisorbed layer with flat-lying thiol molecules. Upon room-temperature deposition, initially an ordered pinstripe phase is formed which may be a molecular double layer. This layer transforms with time into a stable saturation structure of upright-tilted thiolates in a local c(2 × 2) arrangement that exhibits a long-range c(12 × 16) modulation, attributed to a Moiré pattern. The XPS measurements show that the room-temperature saturation structure contains a fraction of sulfide species formed by partial decomposition and desorption of alkyl chains. At 400 K, the thiolate monolayer desorbs dissociatively, eventually resulting in a p(5 × 2) sulfur structure.