Electronic structure of Al, Ga, In and Cu doped ZnO/Cu(111) bilayer films
The effect of doping with group-III metals (Al, Ga and In) and Cu free standing and Cu(111) supported ZnO bilayer films has been investigated computationally by using the DFT+U method including dispersion contributions. The changes in the electronic properties of doped ZnO and ZnO/Cu(111) films have been tested by adsorbing CO probe molecules. The replacement of a lattice Zn ion in a free-standing ZnO bilayer by a group-III element generates an extra electron whose distribution depends on the dopant. In particular, while the excess electron is delocalized over the conduction band for Al or Ga doping, it is localized on the dopant in the case of In. The situation is different on the supported ZnO/Cu(111) film, where the extra electron is transferred to the underlying Cu support. While the CO adsorption energy at the doped sites in the ZnO bilayer is the same as in the ZnO/Cu(111) ultrathin films, CO exhibits a larger red-shift in the unsupported ZnO bilayer. The oxidation state of Cu replacing Zn in the unsupported ZnO films is 2+, Cu(3d9) state, while it is 1+, Cu(3d10) state, in the ZnO/Cu(111) supported films where a charge transfer from the supporting Cu metal to the Cu impurity occurs. Cu doping results in a stronger interaction with CO and a large red-shift of the CO stretching frequency. In this respect, Cu doping of ZnO/Cu(111) bilayer films could have interesting consequences in gas adsorption while doping with group-III elements does not lead to major changes of the adsorption properties when the free-standing ZnO films are compared to the supported ZnO/Cu(111) counterparts.