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

The\nability to precisely control the electronic coupling/decoupling\nof adsorbates from surfaces is an essential goal. It is important\nfor fundamental studies not only in surface science but also in several\napplied domains including, for example, miniaturized molecular electronic\nor for the development of various devices such as nanoscale biosensors\nor photovoltaic cells. Here, we provide atomic-scale experimental\nand theoretical investigations of a semi-insulating layer grown on\na silicon surface via its epitaxy with CaF2. We show that,\nfollowing the formation of a wetting layer, the ensuing organized\nunit cells are coupled to additional physisorbed CaF2 molecules,\nperiodically located in their surroundings. This configuration shapes\nthe formation of ribbons of stripes that functionalize the semiconductor\nsurface. The obtained assembly, having a monolayer thickness, reveals\na surface gap energy of ∼3.2 eV. The adsorption of iron tetraphenylporphyrin\nmolecules on the ribbons of stripes is used to estimate the electronic\ninsulating properties of this structure via differential conductance\nmeasurements. Density functional theory (DFT) including several levels\nof complexity (annealing, DFT + U, and nonlocal van\nder Waals functionals) is employed to reproduce our experimental observations.\nOur findings offer a unique and robust template that brings an alternative\nsolution to electronic semi-insulating layers on metal surfaces such\nas NaCl. Hence, CaF2/Si­(100) ribbon of stripe structures,\nwhose lengths can reach more than 100 nm, can be used as a versatile\nsurface platform for various atomic-scale studies of molecular devices.