"Building molecules one at a time"
> EMBARGOED FOR RELEASE: 25 NOVEMBER 1999 AT
> 14:00 ET US
> Contact: Bill Steele
> Cornell University News Service
> Building molecules one at a time
> Cornell researchers probe secrets of chemical bonding by assembling
> molecules one at a time
> Above, schematic representation of single-molecule construction
> with a scanning tunneling microscope. The tip of the STM is
> lowered over a carbon monoxide molecule adsorbed on a silver
> surface, and the voltage and current increased to attach the
> molecule to the tip. The tip is then moved over an iron atom and
> the voltage reversed to bond the molecule to the atom. In the
> resulting Fe(CO) molecule, the CO structure is observed to attach
> at an angle. A high-resolution copy of this graphic (922x930 pixels, 84K)
> is available here.
> ITHACA, N.Y. -- Researchers are interested not only in what happens when you
> combine two chemicals, but also in how the bonds between these chemicals are
> formed. That understanding might lead to better control over chemical reactions and
> perhaps even the creation of complex molecules with unusual properties.
> Cornell University physicists now have shed new light on chemical bonding by
> combining a single molecule with a single atom to form a new molecule.
> Wilson Ho, Cornell professor of physics, and graduate research assistant Hyojune Lee
> used a specially designed scanning tunneling microscope (STM) to pick up single
> molecules of carbon monoxide and graft them onto an iron atom to form molecules of
> iron carbonyl. By obtaining the new assembly's "vibrational spectrum" -- a measure of
> the energy in the bonds between atoms -- they verified that a chemical bond had truly
> been formed to produce a new molecule.
> The primary purpose of the work, reported in the
> Nov. 26, 1999, issue of the journal Science, is
> to demonstrate a technique for single molecule
> formation and to learn more about the properties
> of chemical bonds. But Lee noted that the
> technology used could have future applications in
> nanofabrication, the creation of materials and
> devices at the atomic or molecular level. "One
> implication is that since we can build up
> molecules from atoms step by step, in the future
> it might be possible to build up more
> complicated molecules from separate
> ingredients," he said.
> The researchers used an STM of exceptional
> precision to manipulate atoms of iron and
> molecules of carbon monoxide adsorbed (bonded)
> on a silver surface in a vacuum. The entire
> apparatus was cooled to a temperature of 13
> degrees Kelvin (13 degrees above absolute zero,
> or -260 degrees Celsius).
> The advanced STM built by Ho's research group
> can resolve parts of atoms and molecules and, by
> measuring current flow through a single
> molecule, return characteristic signatures of
> chemical bonds. In an STM, a metal tip is
> suspended a fraction of a nanometer (a
> nanometer is a billionth of a meter) above a
> surface and a voltage is applied between the tip and the surface. A tiny electric
> current called a "tunneling current" flows between the tip and the surface.
> The tip is scanned across the surface and at the same time raised and lowered in such
> a way as to keep the tunneling current constant. The up and down movements are
> recorded and can be translated by a computer into a relief map of the surface so
> detailed that it shows the outlines of individual atoms.
> In their recent experiment, the researchers used the STM to scan a surface and locate
> iron atoms and carbon monoxide molecules. They then lowered the tip over a CO
> molecule and increased the voltage and current flow of the instrument to pick up the
> molecule. Finally, they moved the molecule on the tip over an iron atom and reversed
> the current flow, causing the molecule to bond to the atom and form an iron carbonyl
> Fe(CO) molecule.
> The researchers then added another CO molecule to the Fe(CO), forming a molecule of
> Fe(CO)2. In subsequent images of the surface, the Fe(CO) molecule appears to have a
> small lobe on one side, indicating that the CO structure is not attached in a straight
> up-and-down fashion to the iron atom but is tilted slightly to one side. The Fe(CO)2
> molecule appears to have two lobes, indicating that the two CO structures are tilted
> in opposite directions in a "rabbit-ears" shape. Since there is no way to determine
> exactly what the angle of the tilt is from their measurements, that would be left to
> theoretical calculations, the researchers said in their paper.
> "By the image change we can predict that a new
> molecule has been formed, but we need absolute
> confirmation," Lee said. To measure the
> vibrational energy of bonds in the molecules, the
> researchers hold the STM tip at a constant height
> above a bond and vary the voltage. At certain
> voltages they will see peaks, where energy is
> absorbed by the bond. These peaks have a
> characteristic pattern, or vibrational spectrum,
> by which the type of bond can be identified. To
> confirm the presence of a bond between carbon
> monoxide and iron, the researchers compared the
> vibrational spectrum of the molecules they had
> created with the spectra of CO molecules bonded
> to the silver surface.
> In the course of their work, Ho and Lee
> discovered serendipitously that when a CO
> molecule was attached to the tip of the STM, the
> resolution of the instrument increased, enabling
> the researchers to see the lattice of silver
> atoms on the working surface. This made it
> possible to see the bonding sites on the surface
> for the different chemical species, they said.
> The paper in Science is titled "Single Bond
> Formation and Characterization with a Scanning
> Tunneling Microscope." The research was funded
> by the U.S. Department of Energy.
> FECO_Fig2.72.GIF: STM images of steps
> in the molecule-building process. (A)
> carbon monoxide molecules (small dots)
> and iron atoms on a silver surface. (B)
> A carbon monoxide molecule has been
> bonded to an iron atom to form an
> Fe(CO) molecule. (C) A second Fe(CO)
> molecule has been formed, above and to
> the right of the first, and another CO
> molecule is about to be added to the
> first. (D) A second CO molecule is
> added to form Fe(CO)2. The lobed image
> indicates that the CO structures are
> attached at an angle. A high-resolution
> copy of this photo (1021x1080 pixels, 244K)
> is available here.
> Related World Wide Web sites: The following sites provide additional information on
> this news release. Some might not be part of the Cornell University community, and
> Cornell has no control over their content or availability.
> Wilson Ho's home page: http://www.lassp.cornell.edu/lassp_data/wilsonho.html
> Related story on single-molecule vibrational spectroscopy:
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