Two-Dimensional Functionalized Ultrathin Semi-Insulating CaF₂ Layer on the Si(100) Surface at a Low Temperature for Molecular Electronic Decoupling

The ability to precisely control the electronic coupling/decoupling of adsorbates from surfaces is an essential goal. It is important for fundamental studies not only in surface science but also in several applied domains including, for example, miniaturized molecular electronic or for the development of various devices such as nanoscale biosensors or photovoltaic cells. Here, we provide atomic-scale experimental and theoretical investigations of a semi-insulating layer grown on a silicon surface via its epitaxy with CaF₂. We show that, following the formation of a wetting layer, the ensuing organized unit cells are coupled to additional physisorbed CaF₂ molecules, periodically located in their surroundings. This configuration shapes the formation of ribbons of stripes that functionalize the semiconductor surface. The obtained assembly, having a monolayer thickness, reveals a surface gap energy of ∼3.2 eV. The adsorption of iron tetraphenylporphyrin molecules on the ribbons of stripes is used to estimate the electronic insulating properties of this structure via differential conductance measurements. Density functional theory (DFT) including several levels of complexity (annealing, DFT + U, and nonlocal van der Waals functionals) is employed to reproduce our experimental observations. Our findings offer a unique and robust template that brings an alternative solution to electronic semi-insulating layers on metal surfaces such as NaCl. Hence, CaF₂/Si­(100) ribbon of stripe structures, whose lengths can reach more than 100 nm, can be used as a versatile surface platform for various atomic-scale studies of molecular devices.