Two-DimensionalFunctionalized Ultrathin Semi-InsulatingCaF 2 Layer on the Si(100) Surface at a Low Temperaturefor Molecular Electronic Decoupling
2020
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.
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