Chemical modification of scanning tunneling microscopy tips for identification of functional groups in self-assembled monolayers

Since 30 years, scanning tunneling microscopy (STM) allows individual atoms or molecules to be observed. Amongst others, it is particularly well adapted to image molecular organizations at liquid/solid interfaces as it is already successful in imaging physisorbed self-assembled monolayer (SAM) structures adsorbed on graphite, highlighting a variety of physical, chemical, dynamical and electronic phenonema. In this context, another important aspect of the molecular layers investigation with STM is its ability to discriminate functional groups from the molecule backbone. However, while functional groups such as amines or thiols can be easily distinguished from the molecule skeleton in STM images, others (ester or carboxyl groups for example) can not. During the last decade, several research groups focused on the STM tip chemical modification in order to change contrast of functional groups in STM images. In this minireview, we describe the STM tip modification phenomenon and its effect on STM tunneling current. The different STM tip modification performed are listed and their application to the identification of various functional groups within SAM at the liquid/graphite interface detailed.