Seminars Archive
Diffusion, Anchoring and Forced Assembly of Large Organic Molecules on Metal Surfaces
Abstract
Wednesday, November 20, 2002, 15:00
Seminar Room, ground floor, Building "T"
Sincrotrone Trieste, Basovizza
Diffusion, Anchoring and Forced Assembly of Large Organic
Molecules on Metal Surfaces
Federico Rosei
(INRS-EMT, Universit du Qubec, Varennes (QC), Canada)
Abstract
The adsorption of large organic molecules on surfaces plays a
vital role for 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 our STM can provide important new information
on molecular diffusion and
assembly.
First, I will address 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 neighbor sites.
Recent results suggest this
is not necessarily true for complex organic molecules. 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, spanning multiple lattice spacings
[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. This ultimately demonstrates
how molecules can be
custom designed to engineer their diffusion properties.
Second, I will describe the adsorption of the Lander molecule
[3] (C90H98) on Cu(110) by Scanning
Tunneling Microscopy (STM) in the temperature range 100 ˆ 300 K. The
Lander has a central
polyaromatic molecular wire (conducting backbone), and four „spacer
legs‰ (3,5-di-tert-buthylphenyl)
for isolation from the substrate. Lander molecules are imaged as four
separate lobes by STM, which
may be arranged in three different conformations. Manipulation experiments
with the STM at low
temperatures on isolated Lander molecules adsorbed on step edges reveal
a restructuring of Cu steps
[3]: when removed from a step, a tooth-like nanostructure appears.
The structure‚s width is two
atomic rows, corresponding to the distance between the spacer legs
within the molecule. This process
is thermally activated: repeating the same manipulation experiments
on molecules adsorbed at low
temperatures (150 K), no restructuring of the Cu step edges is found.
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.
References.
[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).
