Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

Abstract

The electronic structure of CaO films of 10-60 monolayer thickness grown on Mo(001) has been investigated with synchrotron-mediated x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Upon annealing or reducing the thickness of the film, a rigid shift of the CaO bands to lower energy is revealed. This evolution is explained with a temperature-induced diffusion of Mo ions from the metal substrate to the oxide and their accumulation in the interface region of the film. The Mo substitutes divalent Ca species in the rocksalt lattice and is able to release electrons to the system. The subsequent changes in the Mo oxidation state have been followed with high-resolution XPS measurements. While near-interface Mo transfers extra electrons back to the substrate, generating an interface dipole that gives rise to the observed band shift, near-surface species are able to exchange electrons with adsorbates bound to the oxide surface. For example, exposure of O 2 results in the formation of superoxo species on the oxide surface, as revealed from STM measurements. Mo interdiffusion is therefore responsible for the pronounced donor character of the initially inert oxide, and largely modifies its adsorption and reactivity behavior.

Defects of thin CaO(001) on Mo(001): an EPR spectroscopic perspective

“…In particular, annealing above 900 K results in the formation of Mo species in the higher oxidation states ( v and vi ) preferentially located close to the surface of CaO films. 32 This is consistent with theory predicting a temperature dependent stability of different oxidation states of Mo dopants in CaO. 41 Mo-doping of CaO films was shown to alter the surface chemistry of the system.…”

“…With respect to redox processes, previous STM and photoemission experiments showed that CaO films grown on Mo(001) are doped by Mo ions which can adopt various oxidation states depending on the preparation conditions. 32 The diffusion of Mo ions into the CaO films leads to a downshift of the conduction band onset with increasing annealing temperature, an effect that decreases with increasing film thickness. 32,39 A more detailed analysis of XPS data showed the presence of Mo species in the oxidation states between II and VI.…”

“…32 The diffusion of Mo ions into the CaO films leads to a downshift of the conduction band onset with increasing annealing temperature, an effect that decreases with increasing film thickness. 32,39 A more detailed analysis of XPS data showed the presence of Mo species in the oxidation states between II and VI. In particular, annealing above 900 K results in the formation of Mo species in the higher oxidation states (V and VI) preferentially located close to the surface of CaO films.…”

“…[20][21][22][23] Among the alkaline earth oxides thin, epitaxial MgO films have been studied extensively, [24][25][26][27][28][29][30] whereas investigations on CaO films are much more limited. [31][32][33][34][35] The properties of alkaline earth oxide films being prototypical ionic oxides are known to be altered by defects such as dislocations, point defects or dopants. [36][37][38] Doping of the films with redox active aliovalent transition metal ions is another way to alter their electronic and chemical properties.…”

Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

“…The state of the dopant may be probed using XPS, whereby the depth distribution of the dopants may be analyzed using angle dependent XPS or photon energy dependent spectra via synchrotron radiation studies via an effective variation of the electron escape depth. 120,[159][160][161] Another way to bring the dopant into the film is successive evaporation of a film of a given thickness, followed by the evaporation of the dopant material and a second controlled film deposition step to bury the dopant. This allows for a controlled deposition of the dopant at a given depth below the oxide surface.…”