Quadruped Molecular Anchoring to an Insulator: Functionalized Ferrocene on CaF₂ Bulk and Thin Film Surfaces

The formation of insulator-supported functional molecular structures requires a firm anchoring of the molecular building blocks to the underlying surface. With a suitable anchoring mechanism, the functionality of single molecules can be maintained and molecular reaction routes for advanced fabrication can be realized to ultimately produce a functional unit. Here, we demonstrate the anchoring of a functionalized ferrocene molecule 1,1′-ferrocenedicarboxylic acid (FDCA) to the CaF₂(111) surface. Due to the large band gap and high purity of CaF₂ crystals, as well as the presence of particularly large, defect-free terraces, CaF₂(111) is a prototypical insulator surface most suitable for the fabrication of molecular devices. Noncontact atomic force (NC-AFM) and scanning tunneling microscopy (STM) experiments performed on CaF₂ bulk and CaF₂/CaF₁/Si(111) thin film samples reveal the formation of ultrasmall molecular FDCA islands composed of only a few molecules. This molecular assembly is stable even at room temperature and at temperatures as low as 5 K. A comparison of the experimental data with results of density functional 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 that are stable at room temperature.