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Digging ditches in molecular strata.

Digging ditches in molecular strata

Most researchers use the scanning tunneling microscope (STM) to probe molecular and atomic landscapes and to image materials at these smallest of structural scales. But some are beginning to use the atom-tipped instrument to sculpt or even rearrange the diminutive landscapes.

One recent afternoon, Bruce Parkinson of E.I. duPont de Nemours & Co. in Wilmington, Del., used his STM to etch a three-tiered pit of squares (left) into a tin diselenide crystal -- a stack of orderly molecular sheets, each separated by slightly more than 0.6 nanometers. A million of the inscribed micropits might fill a flattened sesame seed. Each side of the smallest and deepest square, lying six molecular sheets below the surface, measures 200 nanometers. The medium-sized tier, two sheets thick, has a square hole measuring 500 nanometers per side. The largest, uppermost tier includes three sheets and has a square hole 1,000 nanometers (1 micron) to a side. To date, the smallest pit Parkinson has etched measures 25 x 25 x 1.2 nanometers.

On another afternoon, he used his STM to etch the less orderly pit shown on the right. Parkinson suggests in the Oct. 10 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY that this flaky topography might emerge from crystalline defects and impurities in the tin diselenide, which would enable underlying sheets to begin eroding before overlying layers have completely disappeared.

Because the STM tip itself rarely scratches the crystal surface, Parkinson says he suspects the etching process involves additional mechanisms, including heating and electrical field effects.
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Title Annotation:scanning tunneling microscopes to sculpt and rearrange molecules
Publication:Science News
Date:Nov 3, 1990
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