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Just a Mo: Tiny Mo concentrations in a CaO film can influence the growth of adsorbed gold. On undoped films, three dimensional gold structures form (see picture, top) whereas on the doped films flat islands appear (bottom). The change results from a charge transfer from the Mo dopant to the Au atoms, a process suggested by STM measurements and DFT calculations.

The ability of Mo (Cr) impurities in a CaO (MgO) matrix to act as charge donors to adsorbed gold has been investigated by means of scanning tunneling microscopy and density functional theory. Whereas CaO(Mo) features a robust donor characteristic, as deduced from a charge-transfer-driven crossover in the Au particles' geometry in the presence of dopants, MgO(Cr) is electrically inactive. The superior performance of the CaO(Mo) system is explained by the ability of the Mo ions to evolve from a +2 oxidation state in ideal CaO to a +5 state by transferring up to three electrons to the Au adislands. Cr ions in MgO, on the other hand, are stable only in the +2 and +3 charge states and can provide a single electron at best. Since this electron is likely to be captured by cationic vacancies or morphological defects in the real oxide, no charge transfer to Au particles takes place in this case. On the basis of our findings, we have developed general rules on how to optimize the electron donor characteristics of doped oxide materials.

The use of transition metals (TM) as dopants is a versatile way to change the properties of oxides. One example is the possibility to induce charge transfer from the TM to species adsorbed on the oxide surface. In the case of wide-gap alkalineearth oxides, such as MgO and CaO, the charge transfer is determined by the respective position of the energy levels of the impurity and the adsorbate and resides in the capability of the TM to be stabilized in various oxidation states. The structural and electronic properties of Cr-and Mo-doped MgO and CaO are investigated by density functional theory (DFT) calculations. Because of limitations of DFT in the description of band gaps and energy levels alignment, we compare the results obtained with standard GGA, GGA+U, and hybrid exchange-correlation functionals. The ability of the impurity ion to transfer one electron to the adsorbed species is considered for the case of electronegative Au adatoms. The study of the thermodynamic stability of the TM ions in combination with the formation of cation vacancies shows that Mo impurities are stable in the oxide lattice as Mo 3+ or Mo 4+ but can assume also higher oxidation states and are therefore able to transfer electrons to adsorbed gold. Conversely, Cr impurities are stabilized as Cr 3+ and the high cost of further oxidation results in the absence of charge transfer to gold on both Crdoped MgO and Cr-doped CaO.

Recent experiments have demonstrated that tiny amounts of Mo impurities give rise to drastic changes in the adsorption characteristic of a wide-gap CaO(001) film. In this scanning tunneling microscopy (STM) and density functional theory paper, we elucidate the underlying mechanism by analyzing the energy levels of the Mo dopants as a function of their oxidation state and depth below the surface. We show that Mo²⁺ ions in CaO subsurface layers can be reversibly charged and discharged by inducing local band-bending effects with the STM tip. A similar charge switching is not possible for Mo species in a higher oxidation state, as their highest-occupied molecular orbitals are located well below the onset of the CaO conduction band. The easiness of charge switching in Mo²⁺ ions explains the remarkable chemical properties of doped CaO films, as it renders the material a strong electron donor to adsorbates bound to the oxide surface