Final-state diffraction effects in angle-resolved photoemission at an organic-metal interface

Abstract

In this paper it is shown that angle-resolved photoemission performed using low-energy photons on an organicmetal interface allows to clearly distinguish genuine interface states from features of substrate photoelectrons diffracted by the molecular lattice. As a model system an ordered monolayer of Zn-phthalocyanine is used as a diffraction lattice to probe the electronic band structure of a Ag(110) substrate. Photoemission close to normal emission geometry reveals strongly dispersive features absent in the pristine substrate spectra. Density functional theory modeling helped identifying these as bulk sp direct transitions undergoing surface-umklapp processes. The present results establish the important role of final-state diffraction effects in photoemission experiments at organic-inorganic interfaces. Angle-resolved photoelectron spectroscopy (ARPES) is a preferred tool to study the electronic band structure of single crystals.1 When performed with low photon energy, bulk bandto-band direct transitions may dominate the spectrum.2 Energy and momentum conservation laws allow the band structure to be mapped, for instance, by changing the detection angle and photon energy and by referring to band-structure calculations. This renders the technique to be particularly adapted to probe energy regions relevant for the optical properties of solids. Recently, ARPES was successfully used to investigate the interface properties of conjugated organic nanostructures self-assembled on single-crystal surfaces, which form a new class of materials of steadily growing importance.4-7 ARPES of single-domain structures reveals interface states dispersion, thus enabling access to the electronic properties of twodimensional (2D) systems such as the quasiparticle bandwidth, their effective mass, and correlation effects. 4,5 Alternatively, Fourier-transform analysis of ARPES spectra was recently used to reconstruct the interface molecular orbital densities, including adsorbate-substrate hybridization effects. 6,7 The presence of organic-inorganic hybrid states is not the only aspect relevant to the interpretation of ARPES spectra at interfaces, though. In fact, the adsorption of ordered arrays of atoms 8 or nanostructures 9 on a surface can modify the escape conditions of substrate photoelectrons. The resulting spectra are sensibly affected and reveal the substrate electronic structure via an additional diffraction process often referred to as surface umklapp. 8,9 Within the one-step photoemission model a bulk Bloch wave is optically excited into a damped state extending into the vacuum as a time-reversed lowenergy electron diffraction (LEED) state. 1 The presence of an adsorbate-induced lattice on the substrate crystal surface promotes new LEED states and, consequently, a new set of final states outside the surface. As a result, features observed by ARPES around a given direction may actually originate from bulk direct transitions that are excited one surface reciprocal lattice vector apart.Once deposited on noble-metal substrates, sel...

Role of charge transfer, dipole-dipole interactions, and electrostatics in Fermi-level pinning at a molecular heterojunction on a metal surface

“…This can be explained by Umklapp scattering (diffraction) of the substrate photoelectrons by the molecular overlayer. [41][42][43][44][45] Additional intensity due to ML 6T is mostly visible between 1 and 4 eV BE.…”

Interpretation of valence band photoemission spectra at organic-metal interfaces

“…Interface scattering also affects the substrate sp band emission 38,40 of angle-integrated spectra. Whether this will result in distinct peaks or a modified background depends on photon energy and analyzer integration angle.…”

“…In a recent low-photon energy (7-11 eV) angle-resolved photoemission spectroscopy (ARPES) study of ZnPc/Ag(110), it was shown that direct transitions in the sp bands occurring away from the point are diffracted by the reciprocal-lattice vectors giving rise to new, intense peaks at low BE in angle-integrated spectra measured around NE. 38 At higher photon energy (e.g., 21 eV) the angular dispersion of the sp bands will reduce. 40 When using an integration angle of ±7 • , diffracted (or backfolded) sp bands are unlikely to show distinct peaks.…”

“…When atoms or molecules are adsorbed on a clean single-crystal surface the escaping conditions of substrate photoelectrons are modified. 15,[35][36][37][38] The overlayer acts as a scattering layer which is able to provide parallel momentum and thus to change the emission directions of the photoelectrons. A photoelectron whose emission direction is off NE in the clean substrate can then be found at NE in the ML spectrum.…”

“…It has to be noticed that angular dispersion from any "umklapped" state is expected in the case of well-ordered single-domain molecular superstructures. 38,40 Consequently, although angular dispersion from genuine molecule-metal mixed states may take place in these systems, the modification of the EDC has to be benchmarked with a careful test on the role of interface diffraction from photoelectrons. This can be done, for instance, by measuring the ARPES spectra of the clean substrate at an angle corresponding to an overlayer reciprocal-lattice vector (umklapp vector).…”