Quadruped molecular anchoring to an insulator: FDCA on CaF2 bulk and thin film surfaces

The formation of insulator-supported functional molecular structures requires a ﰣrm anchoring of the molecular building blocks to the underlying surface. With a suit- able anchoring mechanism, the functionality of single molecules can be maintained and molecular reaction routes for advanced fabrication be realised to ultimately produce a functional unit.
Here we demonstrate anchoring of a functionalised ferrocene molecule (1,1'-ferrocene dicarboxylic acid, FDCA) to the CaF2(111) surface. Due to the large bandgap and high purity of the crystal as well as the presence of particularly large, defect-free terraces, CaF2(111) is a prototypical insulator surface most suitable for the fabrication of molec- ular devices. Non-contact atomic force (NC-AFM) and scanning tunnelling microscopy experiments performed on CaF2 bulk and CaF2/CaF1/Si(111) thin ﰣlm samples reveal the formation of ultra-small molecular FDCA islands composed of only a few molecules. This molecular assembly is stable even at room temperature and at temperatures as low as 5K. A comparison of the experimental data with the results of density func- tional theory (DFT) calculations indicates that the exceptional stability is based on a robust quadruped binding motif. This quadruped anchoring bears strong potential for creating tailored molecular structures on CaF2(111) surfaces which are stable at room temperature.