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 structu re (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). To sys tematically study the influence of the chain length, we used three differen t carboxylic acids in the range of 1-30 carbon atoms, namely, formic acid ( HCOOH), hexanoic acid (C5H11COOH), and triacontanoic acid (C29H59COOH). Fil ms with coverages ranging from the sub-monolayer to the multilayer regime a re prepared by organic molecular beam epitaxy. The adlayers were characteri zed in situ by XPS and NEXAFS directly elucidating the nature of the chemic al bond to the substrate and the degree of orientational order in the film. Highly oriented organic monolayers could be fabricated for all investigate d carboxylic acids on the Cu(111) surface, but their adsorption geometries differ strongly. Whereas in the saturated monolayer the O-C-O plane of form ic acid is significantly tilted, the alkyl chain and the O-C-O plane in hex anoic acid are oriented almost normal to the surface. For triacontanoic aci d, a coverage driven reorientation of the molecular chain from flat lying t o upright standing takes place upon completion of the first layer. In the s aturated monolayer, the investigated carboxylic acids interact via two equi valent oxygen atoms, i.e., they form a bidentate carboxylate bonding to the copper surface.