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

Abstract

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.

Protruding hydrogen atoms as markers for the molecular orientation of a metallocene

“…Lateral and vertical stiffness were chosen as 0.5 and 20 N/m, respectively. FDCA molecular models in the DFT-optimised geometries (using geo 1 and geo 2 from [22], see Figure 1a,b) were arranged along the CaF 2 direction, resembling one molecular row of the superstructure that was observed in the experiments. All simulated Δf data are shown with a colour code representing a strong attractive tip-surface interaction as bright and a weak attractive or repulsive interaction as dark to allow for a direct comparison with the experimental data.…”

“…A previous analysis of the molecular orientations highlighted the presence of a minority population of FDCA molecules exhibiting the adsorption geometry geo 2 with a partly staggered molecular conformation [22]. The protruding hydrogen atoms align in this geometry along a different direction with respect to geo 1 due to the partly staggered conformation of the Cp rings.…”

“…We use Lennard-Jones parameters for CO and Xe tips, a neutral probe particle, and stiffness values of (k x , k y , k z ) = (0.5, 0.5, 20) N/m, yielding the results for CO reproduced in Figure 2f-i and for Xe in Figure 2k-n at the same heights. Both [22]). The orientations of the two geometries geo 1 and geo 2 are marked by solid green and dashed blue lines, respectively, and are included in all images with the respective orientation.…”

“…From this class of molecules we investigate the ferrocene derivate 1,1'-ferrocene dicarboxylic acid (FDCA), a ferrocene functionalised with two carboxylic acid moieties. This molecule has been analysed before on Ag(111), Cu(110), and Cu 3 N/ Cu(110) surfaces [20] as well as on the insulator surfaces calcite(104) [21] and CaF 2 (111) [22]. An eclipsed ferrocene conformation was found to be predominant on the metallic surfaces [20] and on calcite(104) [21].…”

“…(k-n) Simulated NC-AFM images for a Xe tip model positioned with the tip backbone positioned at the same heights as for the CO image calculations. (o, p) Structure of the simulation cell with each molecule in the single-molecule DFT-optimised geometry geo 1[22].…”