Bonding and orientational ordering of long-chain carboxylic acids on Cu(111):: Investigations using X-ray absorption spectroscopy

The adsorption of various carboxylic acids with different chain lengths on Cu(111) has been investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). To systematically study the influence of the chain length, we used three different carboxylic acids in the range of 1-30 carbon atoms, namely, formic acid (HCOOH), hexanoic acid (C5H11COOH), and triacontanoic acid (C29H59COOH). Films with coverages ranging from the sub-monolayer to the multilayer regime are prepared by organic molecular beam epitaxy. The adlayers were characterized in situ by XPS and NEXAFS directly elucidating the nature of the chemical bond to the substrate and the degree of orientational order in the film. Highly oriented organic monolayers could be fabricated for all investigated carboxylic acids on the Cu(111) surface, but their adsorption geometries differ strongly. Whereas in the saturated monolayer the O-C-O plane of formic acid is significantly tilted, the alkyl chain and the O-C-O plane in hexanoic acid are oriented almost normal to the surface. For triacontanoic acid, a coverage driven reorientation of the molecular chain from flat lying to upright standing takes place upon completion of the first layer. In the saturated monolayer, the investigated carboxylic acids interact via two equivalent oxygen atoms, i.e., they form a bidentate carboxylate bonding to the copper surface.