Atomic structure of a thallium nanodot lattice formed on the Si(1 1 1)-7 × 7 surface

Using synchrotron X-ray diffraction we have determined the atomic arrangement of a two-dimensional lattice of thallium (Tl) nanodots self-assembled on the Si(1 1 1)-7 × 7 surface at room temperature. By analyzing a set of in-plane reflections as well as crystal truncation rods, we find that the atomic structure of a single Tl nanodot is best described by a dynamical model where nine Tl atoms are arranged 0.91 ± 0.19 Å above the substrate Si adatoms with three central atoms slightly bulging over the surrounding six atoms by about 0.18 Å. Our model further suggests that the Tl atoms are quite mobile within the attractive basins of the frozen substrate 7 × 7 surface. The mobile nature of Tl atoms naturally explains the blurred scanning tunnelling microscope images and may also provide a clue to understand the intriguing metallic feature of the Tl nanodots.