Energetics and dynamics of unoccupied electronic states at theh−BN∕Ni(111)interface

Abstract

Adsorbed on the nickel ͑111͒ surface, hexagonal boron nitride ͑h-BN͒ forms a commensurate monolayer, which retains much of the band structure of layered bulk h-BN, specifically the wide band gap. This article explores its unoccupied band structure with femtosecond time-resolved two-photon photoelectron spectroscopy in a two-color setup, and presents spectra, dispersion, and cross-correlation measurements. Two distinct, dispersive intermediate states are observed at the origin of the surface Brillouin zone ⌫: a à -related interface state at 1.51 eV above the Fermi level, which forms the conduction band, and an image-potential state at 0.65 eV below the vacuum level. With 260 fs for the image state and 110 fs for the interface state the lifetimes of the two unoccupied states are remarkably high due to the very small overlap of the corresponding wave functions with the nickel bulk states.

Exchange-split interface state at h-BN/Ni(111)

“…We have to note that our result of the effective mass does not agree with the effective mass of 0.43±0.1m e measured by Muntwiler et al. [20] We can only speculate about the origin of this difference. In the 2PPE process, three states are involved (initial, intermediate and final state), whereas only two states (initial and final state) participate in the IPE process.…”

“…This is in good agreement with previous experimental and theoretical values. [16,20] The long-range crystallographic order of substrate and overlayer was checked with low-energy electron diffraction (LEED). In both cases, the LEED pattern showed sharp diffraction spots of hexagonal order, indicating the crystalline quality of the substrate and the BN monolayer.…”

“…For comparison, 2PPE data point at a change in binding energy of 0.15 eV. [20,37] As expected for transitions into image-potential states, the dipolar axis is perpendicular to the surface resulting in more intensity in GM 70 than in GM 35 . [38] (iii) At about 1.8 eV, a new structure A appears in the h-BN/Ni(111) spectra, which we attribute to the predicted interface state of the h-BN/Ni(111) system.…”

“…Former experiments with twophoton photoemission identified interface-related states at an energy comparable to A 2 but no clear-cut interpretation could be derived from the measurements. [20] In the following, we present a detailed investigation of structure A with the objective of extracting the different contributions of A. We recorded a series of spectra for varying angles of electron incidence along the ΓK direction of the crystal, i.e.…”

“…[16] The unoccupied part of the band structure between the Fermi energy and the vacuum level was investigated with two-photon photoemission (2PPE) by Muntwiler et al. [20] For the interface state, they determined an energetic position of 1.51 eV and an effective mass of 0.43±0.1 m e .…”