Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy

Abstract

Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter or substitute organic molecules or metallic substrates offers the opportunity to modify and optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control molecular interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence x-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg2 surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg2 surface alloy [Phys. Rev. Lett. 117, 096805 (2016)] . In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local σ-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules.

Modification of the charge and magnetic order of a low dimensional ferromagnet by molecule-surface bonding

“…The signal of the Dy 4f states is significantly attenuated and is replaced by a spectral broad feature located in the energy region between 1.0 eV and 2.5 eV. Such a featureless valence band structure of PTCDA is very surprising compared to recent findings on noble metal surfaces [28][29][30] and heavy-metal noble metal surface alloys 14,31,32 .The absence of any-well-defined molecular features in the valence band structure is more commonly observed for the adsorption of planar or small organic molecules on transition metal surfaces 33,34 . We therefore attribute the very broad molecular spectral density to the extremely strong chemical interaction between the π-orbitals of PTCDA and the Dy-Ag surface alloy states.…”