Noble and alkali adatoms on a Si(111)√3 x √3-Ag surface: a first-principles study

Using first-principles calculations, we present a comprehensive study on the atomic and electronic structures of metal adatoms (noble metals Ag, Au, Cu and alkali metals Li, Na, K) adsorbed on a Si(111)root 3 x root 3-Ag (hereafter root 3-Ag) surface. We found that adsorption of noble and alkali adatoms can induce significant structural changes in the topmost Ag layer. The most striking and interesting results are the immersion of the noble and Li adatoms into the substrate Ag layer and the finding of the most stable configurations with three adatoms incorporating into or being adsorbed on the surface dependent on their atomic radii. We also found that the almost empty two-dimensional free-electron-like band s(1) and its band folding s(1)* of the original surface band s(1) of the root 3-Ag surface split into a gap at the surface Brillouin zone (SBZ) boundary with adsorption of an adatom. The two surface bands gradually move downwards and the s(1) band is gradually filled with an increase of coverage. The s(1) band is fully occupied with the largest band gap similar to 0.25 eV between the s(1) and s(1)* bands at the critical coverage of 0.14 monolayers (ML) [three adatoms in a Si(111)root 21 x root 21-Ag (hereafter root 21-Ag) unit cell], which corresponds to the most stable adsorption phase. Although the adsorption configurations are different, both the noble and alkali adatom adsorptions give rise to similar electronic structures at low coverages, indicating a free-electron-like character of the adsorption surfaces.