Small Fe clusters are important catalysts which are used in industry and nature. So far, their chemical reactivity was studied as a function of cluster size and mainly in the gas phase. However, a precise determination of their “active sites” – chemical most reactive locations within the clusters – was missing, so far. Within this thesis, the chemical reactivity of small Fe clusters is measured on the atomic scale by utilizing a combined scanning tunneling and atomic force microscope with CO-terminated tips operating at ultra-high vacuum and low temperature conditions.
The first part of this work deals with the artificial creation of small Fe clusters in a controlled manner: First, the lateral manipulation of single Fe adatoms on the Cu(111) surface with monoatomic metal and CO-terminated tips is compared. Second, it is demonstrated that small Fe clusters can be assembled atom by atom using CO-terminated tips while the tip’s structure is preserved. Latter finding is specifically important as CO-terminated tips are quite flexible and relatively fragile. Using CO-terminated tips allows building up Fe clusters with atomic precision as they can also be used to image the Fe clusters in-between each enlargement step with atomic resolution.
The second part of this thesis describes the interaction between the manually assembled Fe clusters and the CO-terminated tip. By approaching the CO-terminated tip towards the various Fe atoms of one cluster, it was found that the chemical interaction between a specific Fe cluster atom and the tip’s CO molecule decreases the bigger the coordination number of the Fe atom is. In this way, it was revealed that the Fe cluster corner atoms are most chemical reactive followed by the edge atoms. This finding is interpreted as a direct measurement of the chemical reactivity of small Fe clusters on the atomic scale
