Quantum size effects in chemicurrent measurements during low-temperature oxidation of Mg(0001) epilayers

Abstract

The reactivity of Mg epilayers on Si(111)-7 × 7 towards molecular oxygen is investigated as a function of the metal film thickness in the range between 7 and 45 monolayers. Quantum well and surface states are characterized with ultraviolet photoelectron spectroscopy demonstrating the epitaxial and single-crystalline structure of the Mg films. The oxidation rate is monitored during the reaction by measuring chemicurrents at 110 K in the Mg/p-Si(111) Schottky diodes due to the non-adiabatic character of at least one step in the reaction chain. For film thicknesses around 9 and 13 monolayers the chemicurrent transients demonstrate that the reaction rate is strongly enhanced by a factor of more than two. With Mg 2p core level spectroscopy, a similar enhancement can be found for the total oxygen uptake for long exposures indicating that the chemicurrent increase measures solely a quantum size effect on the reactivity and no device-related effects. The enhanced reactivity can be explained by the increased first charge transfer into the affinity level of the approaching molecule when a quantum well state appears at the Fermi level and increases the density of electronic states. A linear relationship between the photoelectron intensity at the Fermi level and the maximum chemicurrent is clearly observed. On the other hand, the surface work function and the Schottky barrier height exhibit almost no correlation with the enhanced reactivity.

Work Function Dependence of Vibrational Relaxation Probabilities: NO(v = 2) Scattering from Ultrathin Metallic Films of Ag/Au(111)

“…Third, photoelectron spectroscopy on Ag/Au(111) revealed additional quantum well electronic states (QWS) between thicknesses of 7 and 40 ML. , The energetic position and density of such states are strongly affected by the film thickness. We also note that an effect of QWS on the magnitude of chemicurrents was reported in a prior study . In our work, we observe no change in vibrational relaxation probability in this thickness range.…”

Hydrogen Oxidation-Mediated Current Discharge in Mesoporous Pt/TiO2 Nanocomposite

“…During the deposition of Mg onto Mg/p-Si(001), chemically induced current (referred to as chemicurrent) was measured and found to be exponentially attenuated with increasing Mg film thickness. 14 The attenuation length was found to be 5 nm, a typical value for hot electron transport in metal films. Upon further investigation of this system, the reactivity of the Mg film with molecular oxygen was characterized.…”

“…The attenuation length was found to be 5 nm, a typical value for hot electron transport in metal films. Upon further investigation of this system, the reactivity of the Mg film with molecular oxygen was characterized . This reactivity was found to be a function of the Mg film thickness, with chemicurrent resulting from Mg oxidation exhibiting a maximum for Mg film thickness of 9 and 13 monolayers.…”

A Stationary Reaction Current Effect in Mesoporous Pt/ZrO2 System Under H2/O2 Environment

“…Nanostructure morphology plays a crucial role on the generation of reaction current, as was evident through the Hagemann et al communiqué on Mg/p-Si(111) systems under exposure to O 2 . They found that the reaction current generated is strongly dependent upon film thickness; for thicknesses ∼ 10 monolayers, the reaction current is more than double relative to that generated by thicker films (up to 45 monolayers).…”