Current
Research on Molecular Processing
Nanotechnology and surface
engineering
Attempts to control chemical processes at
the fundamental level using electronic excitations are now possible by
combining the Scanning Tunnelling Microscope (STM) with an understanding of
dynamics of electron-molecule interactions. For example, one of the most
exciting developments in the area of surface modification is the use of STM
to cleave specific bonds on surfaces and adsorbates. Many of these
unimolecular reactions are mediated by vibrational coupling of electronic
energy into vibrational motion, placing energy into specific reaction
coordinates. These “molecular surgery” techniques allow control of local
phenomena introducing the prospect of designer synthesis on the nano-scale.
As an exciting example of achieving the
goal of controlled chemical synthesis consider one of the simplest and most
well-studied reactions, oxidation of CO to CO2, an archetypal reaction in
heterogeneous catalysis and an elementary reaction that is central to
automobile emissions control, air purification, and chemical sensing. Using
a STM Ho and Hahn [Phys. Rev. Lett., 87, 166102 (2001)] gently coaxed
a CO molecule towards a pair of closely spaced oxygen atoms on a 10K silver
surface. At such low temperatures molecular vibrations are minimized
confining reactants and intermediates to specific sites on the surface. As
the CO molecule is moved to within 2 Ĺ of the oxygen atoms, the atoms and
molecule form an O-CO-O complex. Then, by applying a brief electron pulse
from the STM the complex is excited to form CO2, which desorbs from the
surface, leaving behind a lone oxygen atom (figure 1).
Figure 1. A pair of
oxygen atoms (oblong) and a CO molecule (circular) appear as distinct
features in an STM image (top left). As the species are brought closer
together (top right), the image changes until, at less than 2 Ĺ separation,
an O–CO–O complex forms (bottom left). Exciting the complex causes CO2 to
form and desorb, leaving behind a lone O atom (bottom right).
Chemical reactions may also be
induced by low lying transient anion states formed by the attachment of
electrons to the parent molecule. Such quasi-bound states form since
molecules are very ready to accept electrons into vacant molecular orbitals.
In the case of dissociative electron attachment (DEA) in many systems a 100
% selectivity with respect to the cleavage of a particular bond can be
obtained with a high cross section. This opens interesting prospects for
selective chemistry induced by electrons. For example thermal electrons with
CCl4 react to yield exclusively Cl- and CCl3. The reactive (radical) CCl3
may then be transported by the STM tip to another part of the surface where
it may react at a specified chemical site. Such Single Molecule STM
Chemistry has already been demonstrated for the iodobenzene molecule C6H5I
(figure 2)
Figure 2: Schematic
drawing of the sequence of steps by which an STM probe can (a) dissociate a
C6H5I molecule on a terrace; (b and c) draw the iodine atom away; (d) pull
one C6H5 (phenyl) molecule toward another; (e) weld them together; (f) pull
one phenyl to confirm the association. Hla et al, Physical Review Letters 85
2777 (2000).
Thus our understanding of the interaction and
manipulation of molecules by electron processing is of key importance to the
development of nano-scale processing. This in turn requires closer
co-operation (co-ordination) of research in fundamental atomic, molecular
physics and surface sciences communities
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