Attenuation of Photoelectron Emission by a Single Organic Layer

Abstract

We report an in situ study of the thin-film growth of cobalt-phthalocyanine on Ag(100) surfaces using photoelectron emission microscopy (PEEM) and the Anderson method. Based on the Fowler–DuBridge theory, we were able to correlate the evolution of the mean electron yield acquired with PEEM for coverages up to two molecular layers of cobalt-phthalocyanine to the global work function changes measured with the Anderson method. For coverages above two monolayers, the transients measured with the Anderson method and those obtained with PEEM show different trends. We exploit this discrepancy to determine the inelastic mean free path of the low-energy electrons while passing through the third layer of CoPc.

Aurophilic Molecules on Surfaces. Part I. (NapNC)AuCl on Au(110)

“…Since the excited photoelectrons have to pass through the organic layer before leaving the sample surface, they are efficiently scattered after the completion of the first monolayer. Consequently, the electron yield monotonically decreases with increasing film thickness . Initially, dark spots (2D islands or platelet-like crystallites) and later elongated dark needles appear in the PEEM images indicating a Stranski–Krastanow growth .…”

“…Consequently, the electron yield monotonically decreases with increasing film thickness. 24 Initially, dark spots (2D islands or platelet-like crystallites) and later elongated dark needles appear in the PEEM images indicating a Stranski−Krastanow growth. 25 2D island formation and 3D crystallite growth result in the expected attenuation of the photoelectrons by the organic material.…”

“…Initially, dark spots (2D islands or platelet-like crystallites) and later elongated dark needles appear in the PEEM images indicating a Stranski–Krastanow growth . 2D island formation and 3D crystallite growth result in the expected attenuation of the photoelectrons by the organic material . Taking into account both effects, i.e., the changes in work function and the photoelectron attenuation, it is reasonable to assume that the maximum of the electron yield marks the completion of the monolayer.…”

Aurophilic Molecules on Surfaces. Part II. (NapNC)AuCl on Au(111)

“…The molecules in the second layer no longer change the work function but can reduce the electron yield of the photoelectron emitted from the substrate by scattering. 27 The enhanced contrast or increased σ image between Θ = 0.9 and 1.2 ML supports this scenario.…”

“…These islands in the second layer exhibit a stronger scattering for the photoelectrons excited from the substrate, which now have to pass through two densely packed layers. 27 Therefore, the 2D islands appear darker in the PEEM images. On the other hand, the sudden condensation could reduce the (supersaturated) density of the molecular 2D gas just before condensation so that less scattering of photoelectrons occurs in regions still covered by the 2D gas phase.…”