title: Na adsorption on the Si111-(7 x 7) surface: from two-dimensional gas to nanocluster array.
abstract: We have systematically investigated Na adsorption on the Si(111)-(7 x 7) surface at room temperature using scanning tunneling microscopy (STM). Below the critical coverage of 0.08 monolayer, we find intriguing contrast modulation instead of localized Na adsorbates, coupled with streaky noise in the STM images, which is accompanied by monotonic work function drop. Above the critical coverage, Na clusters emerge and form a self-assembled array. Combined with first-principles theoretical simulations, we conclude that the Na atoms on the (7 x 7) surface are, while strongly bound ( approximately 2.2 eV) to the surface, highly mobile in "basins" around the Si rest atoms, forming a two-dimensional gas phase at the initial coverage, and that the cluster at the higher coverage consists of six Na atoms together with three Si adatoms.
authors.affiliation: Institute for Materials Research, Tohoku University, Sendai 980-855, Japan.
authors.forename: Kehui
authors.initials: K
authors.lastname: Wu
authors.forename: Y
authors.initials: Y
authors.lastname: Fujikawa
authors.forename: T
authors.initials: T
authors.lastname: Nagao
authors.forename: Y
authors.initials: Y
authors.lastname: Hasegawa
authors.forename: K S
authors.initials: KS
authors.lastname: Nakayama
authors.forename: Q K
authors.initials: QK
authors.lastname: Xue
authors.forename: E G
authors.initials: EG
authors.lastname: Wang
authors.forename: T
authors.initials: T
authors.lastname: Briere
authors.forename: V
authors.initials: V
authors.lastname: Kumar
authors.forename: Y
authors.initials: Y
authors.lastname: Kawazoe
authors.forename: S B
authors.initials: SB
authors.lastname: Zhang
authors.forename: T
authors.initials: T
authors.lastname: Sakurai