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Wires and Circuits

A. Gourdon, C. Viala, N. Girdhar

Collaboration: University of Kiel, Germany (Pr. R. Berndt); University of Aarhus Denmark (Prof. F. Besenbacher), UCLA USA ( Prof. J. Gimzewski).
Duration: 1998 to now

Despite the current interest in electron transport studies of molecular-scale systems, little is known on, for instance, the influence of chemical structure, molecular conformation or molecule-electrode contact on conductance of molecular wires.
Full understanding of the parameters controlling the electron transport properties requires sub-molecular imaging by STM during the electron transport measurement as it has been recently done with carbon nanotubes between two electrodes.

Along these lines we have developed molecular landers, comprising a rigid polyaromatic main board (wire, device), maintained above a metallic surface by spacers at a distance large enough so that the electronic coupling between this molecular wire and the metal is very small. These insulating spacers are also poorly coupled to the main board and to the surface to avoid as much as possible electronic leakage. The end of the board extends beyond the spacers and can be used to connect metallic terraces.

This type of set-up has been used to measure for the first time the exponential decay in the conductance of a single molecular wire with distance. In this experiment, the lander, comprising a central anthracene core and 4 bulky 3,5-di-ter-butylphenyl spacers, was adsorbed perpendicularly to a double atomic step of a clean Cu(100) surface. One of the two terminal platforms was electronically coupled by adsorption to the upper terrace whereas the spacers lied on the lower terrace. The metallic tip of a Scanning Tunneling Microscope was employed as a second movable counterelectrode. We showed that the electrons were transported along the molecular wire by virtual resonance tunneling with an inverse decay length of 4 nm-1, in excellent agreement with theoretical calculations.
In order to improve this conductance and allow efficient electron transport over longer distances, it was found necessary to prepare a new family of landers 1-4 , showing a smaller gap, with an overall length ranging from 20.7 Å to 33.4 Å and with only 4 spacers in order to allow a closer approach of the STM tip.

This work is currently extended to the study of single molecular switches, i.e. molecules for which the conductance can be controlled by an external action. We have recently been able to fully control the conformational changes of a single molecule by STM. In this latter experiment, the tip was used to reversibly induce the rotation of an external part of the molecule, thus modifying its electronic properties and therefore realizing a conformational molecular switch. We are currently extending this study to systems in which the mobile part is incorporated within the core of the molecule so that the electrons tunnel through the rotor.

Any modification of the angle between the main boards planes and the rotor plane will have a deep impact on the longitudinal conductance of the molecule by modification of the electronic coupling between these molecule components. Therefore, the manipulation of the rotor conformation by the STM tip could lead to a switching effect. Alternatively, we have envisaged this type of device to study of the influence the inelastic part of the tunnelling current on the movement of the central rotor. As the tunnel current passes through the molecule, the internal temperature increases by inelastic absorption, which populates higher energy rotational modes. The fine analysis of the tunnel noise should show this variation of internal motion.

Along these lines, we have carried out the syntheses of several landers designed according to this architecture and shown here.

For these studies, three compounds with different types of rotors were synthesized: one with a low energy rotation barrier comprising the diethynylanthracene, one with a phenyl and one with a biphenyl moiety. Indeed, attempts to transfer 1 to the substrate by sublimation in a STM chamber showed that a large part of these molecules could not withstand the process in UHV conditions due to thermal decomposition during sublimation and/or reaction with the bare metallic surface. This led us to prepare molecules such as 2 and 3 with more robust rotors, better thermal stability on bare metallic surfaces but higher rotation barriers.

 
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References :

Synthesis of Molecular Landers, André GOURDON, EurJOC (1998) 2797-2801.

Synthesis of Polyaromatic Hydrocarbons with central rotor, Christine VIALA, Andrea SECCHI and André GOURDON, EurJOC (2002), in press.

 

 

 

 

  
 
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