Tuning the Charge State of (WO3)3 Nanoclusters Deposited on MgO/Ag(001) Films

The electronic properties of tungsten oxide (WO3)3 nanoclusters deposited on epitaxial MgO films grown on Ag(001) can be tuned and modified simply by changing the thickness of the oxide film. In particular, we show, on the basis of first principle molecular dynamics calculations, that (WO3)3 clusters on ultrathin (<1 nm) or thick MgO films have different structure, charge state, and bonding mode. On the ultrathin film a net transfer of two electrons occurs from the MgO/Ag(001) film to the oxide cluster via electron tunneling through the thin MgO dielectric barrier. The resulting [(WO3)3]2– cluster stays intact and assumes a geometry similar to that of gas-phase (WO3)3. On bulk MgO or on thick films, on the contrary, electron tunneling is not possible and (WO3)3 is bound via formation of covalent bonds that lead to a strong distortion of the cluster structure. As a consequence, completely different properties and chemical reactivity are expected for (WO3)3 nanoclusters on MgO/Ag(001) as a function of the thickness of the supporting oxide film.