Jan. 17, 2001 College Station -- A Texas A&M University physicist is developing a technique to identify molecules on the surfaces of metals, which could have far-reaching implications in the control of chemical reactions in operations within the chemical and petroleum industries.
"It is possible to get images of molecules today with the Scanning Tunneling Microscope (STM), but until recently, it has been impossible to identify the molecules in these images," says Glenn Agnolet, a Texas A&M physicist who is part of a team doing research on the STM at Texas A&M.
"The STM has been an enormous revolution for surface science," adds Agnolet. "Before the advent of the STM, surface scientists used probes to determine the molecule positions averaged over a large sample. With the STM, scientists can obtain local information on the atomic scale."
Like a phonograph, the STM uses a tiny needle that follows the bumps and troughs on the surface of a material, giving an image of what this surface looks like.
As the needle approaches the surface, an electrical current flows from the needle to the metallic surface. Since the current depends on the distance, the current variations reflect the topography of the surface.
"The surface of a material is like a landscape made of mountains and valleys," Agnolet says. "The STM can detect these changes of elevation and make a topography of the surface."
Although molecules are revealed by the change of topography, until recently scientists have been unable to identify the molecules.
"With the STM, you get these beautiful pictures," says Agnolet, "but the question remains: 'What am I really looking at?'"
In an attempt to identify molecules, Agnolet is using Self-Assembled Tunnel Junctions (SATJs), a device developed by Stephen Gregory, a low temperature physicist at the University of Oregon.
An SATJ is made of two long, fine metallic wires, with a layer of neon in between. the first metallic wire plays the role of the STM's needle, the second wire the role of the STM's surface, and the neon layer acts to keep the two surfaces apart.
If a molecule sits between the two metal surfaces, then, as the electrons flow from one metal to the other, they can cause the molecule to vibrate.
"Every molecule has vibrational modes that can be used to distinguish it from other molecules," says Agnolet.
From a study of acetylene molecules on platinum, Agnolet found that the coupling between the molecule and the electrons is ten times larger than expected. By understanding the mechanisms of this effect, he hopes to be able to identify more complex molecules.
To improve his measurements, Agnolet is now studying undesirable effects due to imperfections and defects on the wires.
"The biggest problem is that we do not see the same thing every time," says Agnolet. "So we are trying to find the dominant behavior as opposed to materials problems. We take lots of data, and we try to sift out of this data the underlying features that we are interested in."
Agnolet has published his results in the journals Applied Physics Letters and Physica.
"The ultimate goal is to be able to take a metallic surface, to add molecules to it, and then to be able to find those molecules on the surface with an STM type of instrument," Agnolet says.
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