Electronic states of two-dimensional adatom gas and nanocluster array: Na on Si(111)7×7

Electronic structure evolution of Na adsorbates on Si(111)7×7 has been investigated by angle-resolved photoemission spectroscopy, which was recently reported to transit from a two-dimensional adatom gas state to a nanocluster array as Na coverage increases [Wu, Phys. Rev. Lett. 91, 126101 (2003)]. The distinct electronic states being characteristic to the adatom gas and the nanoclusters are clearly resolved. Both phases are semiconducting and the nanocluster-induced state has a higher binding energy than that due to the mobile adatoms. The adatom-induced state shifts systematically to a higher binding energy as the density of the adatoms increases in the gas phase. This is apparently incompatible with the localized bond formation between an adatom and Si atom(s). The effective delocalization of the bond charge through the dynamic motion of adatoms is proposed.