Origin of p(2 x 1) phase on Si(001) by noncontact atomic force microscopy at 5 k.

The controversial issue of the origin of the p(2 x 1) reconstruction of the Si(001) surface observed in recent low temperature scanning tunneling microscopy experiments is clarified here using 5 K noncontact atomic force microscopy. The c(4 x 2) phase is observed at separations corresponding to weak tip-surface interactions, confirming that it is the ground state of the surface. At larger frequency shifts the p(2 x 1) phase of symmetric dimers is observed. By studying the interaction of a reactive Si tip with the c(4 x 2) Si(001) surface using an ab initio method, we find that the observed change in the surface reconstruction is an apparent effect caused by tip induced dimer flipping resulting in a modification of the surface structure and appearance of the p(2 x 1) phase in the image. Using an appropriate scanning protocol, one can manipulate the surface reconstruction at will, which has significance in nanotechnology.