Using a scanning tunneling microscope, single adatoms can be extracted from a Si(111)7×7 surface by field evaporation, when the sample voltage is pulsed at 4 V or more in either polarity. Statistically, adatoms at the center of the 7×7 unit cell are more frequently removed than those near the corner holes, by a ratio of 1.6:1. This difference can be explained by assuming that the binding energy of center adatoms is approximately 0.01 eV less than for corner adatoms. The relationship of this result to previous observations of greater chemical reactivity at center adatom sites is discussed.

• Received 21 October 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.2040