Atomic scale control of hexaphenyl molecules manipulation along functionalized ultra-thin insulating layer on the Si(1 0 0) surface at low temperature (9 K).

Ultra-thin CaF2 layers are grown on the Si(1 0 0) surface by using a Knudsen cell evaporator. These epitaxial structures are studied with a low temperature (9 K) scanning tunneling microscope and used to electronically decouple hexaphenyl molecules from the Si surface. We show that the ultra-thin CaF2 layers exhibit stripe structures oriented perpendicularly to the silicon dimer rows and have a surface gap of 3.8 eV. The ultra-thin semi-insulating layers are also shown to be functionalized, since 80% of the hexaphenyl molecules adsorbed on these structures self-orients along the stripes. Numerical simulations using time-dependent density functional theory allow comparison of computed orbitals of the hexaphenyl molecule with experimental data. Finally, we show that the hexaphenyl molecules can be manipulated along or across the stripes, enabling the molecules to be arranged precisely on the insulating surface.

A rack-and-pinion device at the molecular scale.

Molecular machines, and in particular molecular motors with synthetic molecular structures and fuelled by external light, voltage or chemical conversions, have recently been reported. Most of these experiments are carried out in solution with a large ensemble of molecules and without access to one molecule at a time, a key point for future use of single molecular machines with an atomic scale precision. Therefore, to experiment on a single molecule-machine, this molecule has to be adsorbed on a surface, imaged and manipulated with the tip of a scanning tunnelling microscope (STM). A few experiments of this type have described molecular mechanisms in which a rotational movement of a single molecule is involved. However, until now, only uncontrolled rotations or indirect signatures of a rotation have been reported. In this work, we present a molecular rack-and-pinion device for which an STM tip drives a single pinion molecule at low temperature. The pinion is a 1.8-nm-diameter molecule functioning as a six-toothed wheel interlocked at the edge of a self-assembled molecular island acting as a rack. We monitor the rotation of the pinion molecule tooth by tooth along the rack by a chemical tag attached to one of its cogs.