CPM Seminar

The adsorption of large organic molecules on surfaces is important in the emerging field of nanotechnology [1]. Molecular ordering is in general controlled by a delicate balance between intermolecular binding forces and molecule-substrate interactions. In this talk I will show how the unique resolving power of Scanning Tunneling Microscopy, together with its ability to manipulate individual atoms and molecules, can provide important new information on molecular diffusion and assembly.

I will begin by addressing the issue of surface diffusion. The motion of atomic and molecular adsorbates across surfaces can be followed directly by time-resolved `STM movies'. In the simplest picture of surface diffusion, the adsorbate jumps between nearest neighbour sites. Surprisingly, by studying the diffusion of two related aromatic molecules, DC (C₃₆H₁₈) and HtBDC (C₆₀H₆₆) on Cu(110), we find that their diffusion is dominated by so-called long jumps [2]. The HtBDC molecule has a core part identical to DC, and six additional spacer legs, which increase its diffusion coefficient by four orders of magnitude with respect to DC. These results demonstrate how molecules can be custom designed to engineer their diffusion properties.

Second, I will describe the adsorption of Lander molecules [3] (C₉₀H₉₈) on Cu (110) in the temperature range 100 - 300 K. The Lander is a prototype of a molecular wire. It is composed of a conducting backbone, and four `spacer legs' for isolation from the substrate. Manipulation experiments with the STM at low temperatures on isolated molecules adsorbed on step edges reveal that this molecule acts as a nanoscale template [3], capable of reshaping metal step edges into characteristic nanostructures. This process shows that organic molecules can be custom designed to assemble predefined structures on metal surfaces.

Finally, I will show how it is possible to order organic molecules into 1 D nanostructures by nanopatterning a clean Cu(110) surface with oxygen chemisorption. By dosing a proper amount of O₂ at 350 C we obtained a superstructure with 2 nm wide Cu troughs aligned along the [001] direction, with a periodicity of about 5 nm. When Lander molecules are deposited on this template, they preferentially adsorb on bare Cu regions. By tuning molecular coverage in a controlled manner we obtained long rows of 1 D molecular nanostructures. This type of forced self-assembly opens new possibilities for ordering organic molecules on surfaces.

[1] C. Joachim, J.K. Gimzewski and A. Aviram, Nature 408, 541 (2000).
[2] M. Schunack, T.R. Linderoth, F. Rosei et al., Phys. Rev. Lett. 88, 156102 (2002).
[3] F. Rosei et al., Organic molecules acting as templates on Metal Surfaces, Science 296, 328 (2002).

Thursday, December 12th 2002, 15:30
Ernest Rutherford Physics Building, room 114