Optimizing substrate characterization to grow 2D Si layers on surfaces is a major issue towards the development of synthesis techniques of the promising silicene. We have used inverse photoemission spectroscopy (IPES) to study the electronic band structure of an ordered 2D Si layer on the √ 3× √ 3-Ag/Si(111) surface (√ 3-Ag). Exploiting the large upwards band bending of the √ 3-Ag substrate, we could investigate the evolution of the unoccupied surface and interface states in most of the Si band gap. In particular, the k-dispersion of the √ 3-Ag free-electron-like S1 surface state measured by IPES, is reported for the first time. Upon deposition of ∼1 ML Si on √ 3-Ag maintained at ∼ 200 • C, the interface undergoes a metal-insulator transition with the complete disappearance of the S1 state. The latter is replaced by a higher-lying state U0 with a minimum at 1.0 eV above EF. The origin of this new state is discussed in terms of various Si 2D structures including silicene.