  Using scanning tunneling microscopy and spectroscopy, we demonstrate that the
adsorption of a minute amount of Cs on a Pb mesa grown in the quantum regime
can induce dramatic morphological changes of the mesa, characterized by the
appearance of populous monatomic-layer-high Pb nano-islands on top of the mesa.
The edges of the Pb nano-islands are decorated with Cs adatoms, and the
nano-islands preferentially nucleate and grow on the quantum mechanically
unstable regions of the mesa. Furthermore, first-principles calculations within
density functional theory show that the Pb atoms forming these nano-islands
were expelled by the adsorbed Cs atoms via a kinetically accessible place
exchange process when the Cs atoms alloyed into the top layer of the Pb mesa.
