Quantum-size effects in ultrathin Mg films

Abstract

Recent experiments on the oxidation of ultrathin Mg films have revealed the existence of a correlation between surface reactivity and quantum-size effects. Using ab initio density-functional calculations, we have investigated the electronic properties of epitaxial Mg͑0001͒ films, 5-17 atomic-layer thick, on a W͑110͒ substrate to clarify the origin of this correlation. We find that the decay length in vacuum of the thin-film local density of states at the Fermi energy exhibits a pronounced oscillatory behavior as a function of film thickness. This is expected to have a major impact on the electron transfer rate by resonant tunneling, which is believed to control the initial sticking of O 2 molecules in the oxidation process. We have also examined the atomic-scale properties of the surface, interface, and quantum-well states of the Mg͑0001͒ films on tungsten and the influence of epitaxial strain on the electronic states, in connection with the interpretation of recent photoemission spectra on these systems. In particular, we find strongly coupled surface-interface resonant states that originate from the Shockley surface states of the films. Comparison with photoemission measurements allows an unambiguous identification of the corresponding surface-interface-state splitting.

Large spin-orbit splitting in light quantum films: Al/W(110)

“…34 Moreover, recent bandstructure calculations for Mg/W͑110͒ have fully confirmed our interpretation. 35 Calculations for 5-17 ML Mg/W͑110͒ show that the split pair of states results from the interaction of the Mg surface state with surface states of the W͑110͒ that are not spin-orbit split. 35 We suggest that Al films are much better suited than Mg ones for investigating the substrate effect on the spin-orbit splitting because the additional electron in Al shifts the bands and their band gap to higher binding energy ͓Fig.…”

Quantum size effects on chemisorption properties: CO on ultrathin Cu films from first principles

“…13,14 The model predicts that all states ψ n,k || of a given subband n have the same decay length λ n ∼ 1/ √ −E n , where E n is the energy of the subband state n at k || = 0, measured relative to the vacuum level. 13,14 Hence λ is dominated by the decay length of the states at E F belonging to the subband whose energy atΓ is closest to the Fermi level. 40 With increasing width L of the film, the energy of the highest-occupied QWS atΓ (of the inverted quantum well) increases with respect to E F .…”

“…This is consistent with the predictions for λ of a particle-in-a-box model. 13,14 The model predicts that all states ψ n,k || of a given subband n have the same decay length…”

“…11,12 In the case of the Mg films, however, it was observed later on that the actual variation in the DOS per atom of the film at E F could not simply account for the order-of-magnitude change in the initial oxidation rate of the films. 13 Moreover, for the Al films, significant differences were observed between the DOS and reactivity trends as a function of film thickness. 3 In the cases of the Mg and Al films, for which the precursor adsorption mode of the O 2 molecule on the surface should be physisorption, the key parameter responsible for the changes in the initial oxidation rate was then proposed to be the decay length in vacuum, λ, of the electronic local density of states of the film at the Fermi energy.…”

“…3 In the cases of the Mg and Al films, for which the precursor adsorption mode of the O 2 molecule on the surface should be physisorption, the key parameter responsible for the changes in the initial oxidation rate was then proposed to be the decay length in vacuum, λ, of the electronic local density of states of the film at the Fermi energy. 13,14 Modifications in λ can be expected to have a direct exponential influence on the electron transfer rate by resonant tunneling, which is believed to control the initial sticking of the O 2 molecules on such surfaces. 15 The same argument, however, based on tunneling processes, does not hold in the case of CO on Cu(001), where the molecule is clearly chemisorbed and the modulated value is the chemisorption energy.…”