Low-energy scale excitations in the spectral function of organic monolayer systems

Abstract

Using high-resolution photoemission spectroscopy we demonstrate that the electronic structure of several organic monolayer systems, in particular 1,4,5,8-naphthalene tetracarboxylic dianhydride and Copper-phtalocyanine on Ag(111), is characterized by a peculiar excitation feature right at the Fermi level. This feature displays a strong temperature dependence and is immediatly connected to the binding energy of the molecular states, determined by the coupling between the molecule and the substrate. At low temperatures, the line-width of this feature, appearing on top of the partly occupied lowest unoccupied molecular orbital of the free molecule, amounts to only ≈ 25 meV, representing an unusually small energy scale for electronic excitations in these systems. We discuss possible origins, related e.g. to many-body excitations in the organic-metal adsorbate system, in particular a generalized Kondo scenario based on the single impurity Anderson model. For more than twenty years there has been a thorough investigation on π-conjugated organic molecules, which have shown to be suitable for the application in organic electronic devices [1][2][3]. These molecules often form long-range ordered films on single-crystalline metal substrates, allowing a systematic and fundamental study by various surface sensitive techniques. In particular from electron spectroscopy methods, as photoemission spectroscopy (PES), inverse photoemission (IPES), and x-ray absorption (XAS), deep insight into many important features of the electronic properties of condensed films and their interfaces has been achieved [4][5][6][7][8]. The latter are of particular importance since they crucially determine the properties of possible devices. Additional microscopic and spectroscopic information has been obtained by use of scanning tunneling microscopy (STM), leading to complementary information about the relation between geometrical and electronic structure [9][10][11][12].Usually, even the spectra of highly ordered films show features with a line-width of several hundreds of meV, mostly determined by vibronic excitations within the adsorbed molecule [7,13,14]. Therefore, even experiments with modern high-resolution photoemission spectrometers for VUV photoemission (UPS) display only features which are about two orders of magnitude larger than the most narrow peaks in other solid state or surface systems [15]. Local spectroscopic measurements by STM, on the other hand, show narrow features in the tunneling conductivity measurements through a single molecule [10,[16][17][18][19], which have been attributed to a possible Kondo like process within the charge transport through * present address: SOLEIL L'Orme des Merisiers, Saint-Aubin -BP 48, 91192 GIF-sur-YVETTE CEDEX FRANCE SOLEIL, Paris, France the adsorbed molecule. However, neither a direct evidence of a local magnetic moment, necessary for the Kondo effect, nor an immediate experimental observation of the spectral function does exist yet.Here we report about a high-resolution photoemission study o...

Interpretation of valence band photoemission spectra at organic-metal interfaces

“…As an example, the filling of the lowest unoccupied molecular orbital (LUMO) level is detected as the appearance of new features at low binding energy in the EDC. 1,4,5,[7][8][9][10] Such evidence is generally interpreted as charge transfer from the substrate to the molecules and can be corroborated by first-principles calculations. 11 More generally, when the molecules are adsorbed on noble-metal substrates, studying the binding energy (BE) and line shape of frontier orbital levels is particularly useful in determining the molecule-substrate interaction.…”

“…[1][2][3][4][5][6] The bonding at the interface can be studied by looking at the modification of the physical and chemical properties of the two constituents: the molecule and the substrate. Ultraviolet photoelectron spectroscopy (UPS) from the valence band is often used since it can map the density of states (DOS) of the sample surface through the energy distribution curve (EDC).…”

Tailoring metal–organic hybrid interfaces: heteromolecular structures with varying stoichiometry on Ag(111)

“…For that reason, the adsorption of different prototype molecules on(111)-oriented noble metal surfaces has been investigated over the last decades [1][2][3][4][5][6][7][8][9]. These particular surfaces are highly interesting since the interaction between the adsorbed molecules and the substrate is of similar importance as intermolecular interactions.…”

Energy level alignment at molecule-metal interfaces from an optimally tuned range-separated hybrid functional

“…Similar to PTCDA on Ag(111), it also features a LUMO peak at E F , as evidenced in UPS measurements. [140][141][142] We relax the NTCDA-Ag(111) geometry with the PBE + vdW surf method 122 as implemented in VASP, using 3 layers of Ag(111) and a 4 × 4 surface unit cell as the substrate. The level alignment calculations are carried out using a 4 × 4 × 1 k-mesh (2 × 2 × 1 for the Fock exchange contribution).…”