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The "Laboratoire Collisions, Agrégats et Réactivité"
(Reactivity, Aggregates and Collisions Laboratory, Université Paul
Sabatier-CNRS Toulouse) and the Fritz-Haber Institute of Berlin (Max Planck
- Germany) have initiated and interpreted chemical reactions induced under
a scanning tunneling microscope between an ammonia molecule and a copper
surface. The results of this international partnership show that the scanning
tunneling microscope enables the selective control of the dynamics of
the molecule, leading either to its translation on the surface or to its
desorption.
The active control of products in a chemical reaction through the excitation
of specific molecular vibrations is one of the major aims of selective
chemistry, made possible thanks to the scanning tunneling microscope.
This microscope consists of a metallic tip located at approximately one
nanometer from the surface of a solid. When a small difference in potential
is created between this tip and the surface, a current appears as a result
of the proximity between the two contacts, referred to as tunneling current.
This current has a lateral range of less than one tenth of a nanometer
and perfectly defines the molecule involved in the interaction. Chemical
reactions induced by the current emitted by a scanning tunneling microscope
correspond to a completely new regime in which the device only interacts
with a single molecule by using wattages on the order of a nanowatt.
The reaction selection is accomplished by acting on certain molecular
vibrations. In the case of the ammonia molecule, the results are surprising:
although the minimum energy required for the desorption of an ammonia
molecule is twice as much as the energy required for its translation,
we observed that desorption occurs at a tip-surface voltage of 0.32 Volts,
whereas translation occurs at a voltage of 0.42 Volts. In other words,
the highest energy electrons set off a reaction involving the lowest energy
input and vice versa. How is this possible?
The calculation of the energies and excitation probabilities of vibration
modes of the ammonia molecule deposited on a copper surface provide an
explanation:
- at a tip-molecule voltage of 0.42 Volts, electrons can excite the vibration
of lengths of nitrogen-hydrogen links in the ammonia molecule, but at
0.32 Volts, this vibration is not excited. In turn, this vibration induces
the translation of the molecule on the surface. At 0.42 Volts, we can
therefore effectively use the energy of the tunneling current for the
translation of the molecule.
- at a tip surface voltage of 0.32 Volts, the multiple excitation of the
"umbrella" mode of the ammonia molecule (which supplies a great
deal of energy to the molecule in the same mode) becomes the most likely
process. This vibration corresponds to the variation in the distance between
the nitrogen atom and the surface and is therefore highly correlated with
the desorption phenomenon. It thus becomes extremely likely, whereas the
energy transported by each electron is not adequate to excite the vibration
of the nitrogen-hydrogen links.
Therefore, the scanning tunneling microscope is not only a unique tool
for exploration and analysis of the nano-world, but also a sophisticated
tool for chemistry at this scale.
Reference: Nature
vol. 423, May 29, 2003, pages 525-528: "Selectivity in vibrationally
mediated single-molecule chemistry" J.I. Pascual, N. Lorente, Z.
Song, H. Conrad and H.P. Rust
Researcher
contact:
Nicolas Lorente
Laboratoire collisions, agrégats, réactivité (IRSAMC),
CNRS
Université Paul Sabatier, Toulouse, France
Tel: +33 5 61 55 60 71
Fax: +33 5 61 55 83 17
E-mail: lorente@irsamc.ups-tlse.fr
http://www.car8.ups-tlse.fr/lorente/lorente.html
Mathematics and Physical Sciences Department contact:
Frédérique Laubenheimer
Tel: +33 1 44 96 42 63
E-mail: frederique.laubenheimer@cnrs-dir.fr
Press contact :
Laetitia Louis
Tel: +33 1 44 96 49 88
E-mail: laetitia.louis@cnrs-dir.fr
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