First-principles study of identical Nb4 clusters on the Si(0Â 0Â 1) surface

The first-principles calculations have been performed on the adsorption of tetrahedron- and rhombus-Nb4 clusters on the Si(001)-(4×2). The results show that both Nb4 configurations can be stably adsorbed on the Si(0 0 1) surface, and the adsorption of bidimensional Nb4 clusters is more stable than those of tetrahedron-Nb4. The energy barriers are very high which indicates that it is difficult for the Nb4 clusters to diffuse. The work functions of the most stable adsorption sites decrease compared with that of the clean Si(0 0 1) surface, which demonstrates that the electronic charges transfer from the Nb4 clusters to the Si(0 0 1) surface.