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Pinpoint splitting of molecules.

The sharp tip of a scanning tunneling microscope's needlelike probe has proved a versatile tool for mapping a surface's microscopic ridges and hollows and for moving small clumps of atoms from place to place. Scientists can also uses this instrument to split up individual molecules lying on such surface. Phaedon Avouris of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., and his co-workers have now demonstrated that electrons emitted from a microscope's tip can transfer sufficient energy to excite and shake apart a decaborane molecule sitting on a silicon surface.

Each decaborane molecule consists of 10 boron atoms and 14 hydrogen atoms. When deposited on a silicon surface and viewed with a scanning tunneling microscope, these molecules appear as rounded, elongated protrusions about 7 angstroms across. Warming up the coated silicon slab frees these molecules, and they tend to migrate to certain irregularities in the otherwise orderly arrangement of silicon atoms at the slab's surface. The molecules break apart at these defects, and boron atoms slip into the silicon structure.

Instead of heating up the entire crystal to produce borondoped silicon, Avouris and his team selectively excite individual decaborane molecules to dope only small, specific regions of the silicon surface. They use a scanning tunneling microscope to locate the surface-hugging molecules. Then, by carefully readjusting the voltage applied to the microscope's tip, they send a pulse of electrons of just the right energy to excite a particular molecule, which dissociates.

Avouris suggests that the same procedure could be used for controlling surface chemistry on a molecular scale in a variety of situations.
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Title Annotation:use of scanning tunneling microscope
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Mar 28, 1992
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