Identifying tips for intramolecular NC-AFM imaging via in situ fingerprinting.

A practical experimental strategy is proposed that could potentially enable greater control of the tip apex in non-contact atomic force microscopy experiments. It is based on a preparation of a structure of interest alongside a reference surface reconstruction on the same sample. Our proposed strategy is as follows. Spectroscopy measurements are first performed on the reference surface to identify the tip apex structure using a previously collected database of responses of different tips to this surface. Next, immediately following the tip identification protocol, the surface of interest is studied (imaging, manipulation and/or spectroscopy). The prototype system we choose is the mixed Si(111)-7×7 and Ag:Si(111)-(√3 × √3) R30° surface which can be prepared on the same sample with a controlled ratio of reactive and passivated regions. Using an "in silico" approach based on ab initio density functional calculations and a set of tips with varying chemical reactivities, we show how one can perform tip fingerprinting using the Si(111)-7×7 reference surface. Then it is found by examining the imaging of a naphthalene tetracarboxylic diimide (NTCDI) molecule adsorbed on Ag:Si(111)-(√3 × √3) R30° surface that negatively charged tips produce the best intramolecular contrast attributed to the enhancement of repulsive interactions.