Highlighting functional groups in self-assembled overlayers with specific functionalised scanning tunnelling microscopy tips

Overlayers of a fatty acid (palmitic and lauric acid) formed at the interface between a solution of this molecule in phenyloctane and the basal plane of graphite is studied by in situ scanning tunneling microscopy. The layer organise in lamellae, which are formed by a close packing arragement of molecules parallel to the graphite surface. The chemical modification of STM tips used allowed the functional group identification. Indeed, the gold tips used are functionalised by 4-mercaptobenzoic acid (4MBA) and 4-mercaptotoluen (4MT). The same functional group on sample is then 'seen' as a dark and a bright spot when imaged with 4-MBA and 4-MT modified tips, respectively. This contrast distinction is related to interactions (or not) between the carboxyl group on the sample and molecules on the tip, which can facilitates (or not) the electron tunnelling.