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Positioning
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 nanoscale electronic circuits, for example.
One
key to this "nano cosmos" 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 degrees 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).
Figure
1: Result of molecule positioning
Sequence of images that illustrate the positioning of individual
molecules: a group of six molecules (1) is first disordered (2)
by cruising through the group with the STM tip. Subsequently, one
molecule after the other is pushed by the tip in a precisely controlled
manner (2 - 5). This allows a ring to be formed (6) that would not
normally be found in nature. Note that the process does not interfere
with other molecules visible at the top of the images.
Figure
2: Molecule structure / simulation of the pushing process
The four "legs" (yellow, at right) of the porphyrin-based molecule
are easily identified by STM images. Computer simulation of the
pushing process (left) revealed that the molecule would "walk" with
slip-stick-like steps of its legs.
Figure
3: Control of the Pushing Process
Thomas Jung defines the movements of the STM tip by simple point-
and-click actions with a computer mouse. The circle on the screen
(lower left) marks the STM tip and thus the starting point. The
cursor arrow marks the destination for the pushing process.
Figure
4: STM Laboratory
Reto Schlittler monitors the STM apparatus at IBM's Zurich Research
Laboratory that was used to perform the positioning experiments.
The STM itself resides within the vacuum chamber (center) to avoid
external disturbances and sample contamination.
Figure
4: IBM Scientists Involved
IBM scientists l-r James Gimzewski, Thomas Jung and Reto Schlittler
hold a model of the molecule, which has a diameter of about 1.5
millionths of a millimeter.
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