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Zurich scientists position individual molecules at room temperature
 Zurich,
Switzerland, January 12, 1996 -- For the first time, scientists
at IBM's Zurich Research Laboratory have succeeded in moving and
precisely positioning individual molecules at room temperature,
using the extremely fine tip of a scanning tunneling microscope
(STM). This is another important step towards being able to do a
wide range of "engineering" on the nanometer scale (one
millionth of a millimeter). It could help lead to the ultimate limits
of miniaturization and open the way to fabricating molecules with
specific properties and functions, constructing computers of ultimately
small size, and even to building minute molecular machines capable
of cleaning or repairing nano-scale electronic circuits, for example.
One key to this "nanocosmos" is the STM, which was invented
at IBM's Zurich Research Laboratory and for which its creators were
awarded the Nobel prize in Physics in 1986. The STM can be used
not only for imaging surfaces with atomic resolution but also for
positioning individual atoms and molecules. However, as STM co-inventor
and Nobel laureate Heinrich Rohrer explains, "Most atoms and
molecules tend to stick quite strongly to the surface and to the
STM tip, making it difficult to pick them up and release them in
a precisely controlled way." On the other hand, the less "sticky"
ones jitter and jump around at room temperature. Scientists at IBM's
Almaden Research Center in San Jose, California, overcame the jitter
problem by cooling the sample to –270 °C, which is nearly
absolute zero. In 1989, they were first to position individual atoms
when they wrote the letters "IBM" with 35 xenon atoms.
However, room-temperature positioning is required for broad practical
uses, such as creating chemical reactions that build functional
units from individual atoms and molecules. The first successful
room-temperature manipulation of atoms was performed in 1991 at
IBM's T.J. Watson Research Center in Yorktown Heights, New York,
using electrical pulses to pick up and release individual silicon
atoms. Most molecules, however, would be torn apart by the strong
electrical pulses required by this technique.
Zurich scientists have now succeeded in positioning individual
molecules at room temperature by purely mechanical means. The nature
of the molecules and their interaction with the surface plays an
important role: they have to stick tightly enough to remain at their
position but not so tightly that they cannot be moved. The chemical
bonds within the molecule, on the other hand, must resist being
changed or broken when the molecule is pushed by the STM tip. Zurich
scientists evaluated a wide range of molecules as possible candidates
in experiments and performed elaborate molecular mechanical simulations
in collaboration with colleagues at the French National Center for
Scientific Research (CNRS) in Toulouse. They selected an organic
molecule having a total of 173 atoms. Its core consists of a stable
ring of atoms known as a porphyrin. Porphyrins are found widely
in nature, for example as the basis of red blood cells (heme group
of hemoglobin). Four strongly but flexibly bonded hydrocarbon groups
attached vertically to the ring make the molecule, which has a diameter
of approximately 1.5 nanometer, ideal for displacement experiments:
its position and structure are easily identified by STM imaging,
and the four hydrocarbon groups act as "legs" that lift
the "body" of the molecule from the atomically flat copper
surface. Computer simulation revealed that when pushed by the STM
tip the molecule "walks" in uncorrelated steps and exhibits
exactly the desired degree of stickiness.
IBM scientists, and colleagues at the University of Cambridge,
UK, developed software that moves and positions the STM tip with
extreme precision. The same STM can also be switched to the imaging
mode by slightly increasing the distance between the tip and the
surface.
This research work is part of the "PRONANO" project sponsored
by the Swiss Federal Office of Education and Science within the
European Strategic Program for Research in Information Technology
(ESPRIT) of the European Union. The long-term goal is to create
new and complex molecular structures and to customize their specific
properties and functions. The porphyrin-based molecule selected
for these manipulation experiments has a number of potential technological
uses. For example, the single copper atom at its center can be replaced
by another metal atom having different electronic properties. This
could be exploited in principle to construct data storage devices
with densities 100,000 times higher than today's most advanced disk
drives. Another technological vision involves wires only one molecule
wide that could be used to build ultra-small electronic components.
The work at IBM's Zurich Research Laboratory was performed by James
Gimzewski, Thomas Jung, and Reto Schlittler; their colleagues are
Christian Joachim and Hao Tang of CNRS, and Mark Welland, Martin
Murrell, and Timothy Wong of the University of Cambridge. A scientific
report was published in the January 12, 1996, issue of Science (Vol.
271).
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