Abstract by: Anu Baby

Impact of Alkali Metal-Doping on the Electronic and Optical Properties of Organic Molecules Adsorbed on Metal Surface

We study the impact of K doping of a PTCDA (sub)monolayer domain on Ag(111) to gain profound insights onto the doping-induced changes of the optical and electronic properties of the interface [1,2]. A particular advantage of the KxPTCDA/Ag(111) system is that it is known to form distinct stoichiometric phases, which allows associating the observed spectra with well-defined adsorbate structures. Unfortunately, the interpretation of the measured differential reflectance spectra is highly ambiguous, which prompted us to perform state-of-the-art density-functional theory based calculations using VASP and Quantum Espresso. The optical spectra was calculated at the IPRPA (Independent particle random phase approximation) level using Yambo software. These provide an atomistic understanding of the interfacial charge rearrangements and in conjunction with photoelectron spectra and X-ray standing wave data allow a comprehensive explanation of the interface properties. Full structural characterization is obtained by low-energy electron diffraction and scanning- tunneling microscopy experiments [2]. The presence of the K atoms is found to increase the electron density in the organic layer and in the Ag substrate; at the same time it reduces the coupling between the adsorbate and the substrate. The latter, on the one hand, is evidenced by the decreasing dispersion and hybridization of the PTCDA-derived bands in potassium-rich phases and, on the other hand, can be concluded from an increasing adsorption distance and reduced optical spectra peak widths upon K intercalation found in both theory and experiments. Results for the K doped tetraphenyldiben-zoperiflanthene (DBP) on Ag(111) will be presented as well. Undoped DBP is a donor whereas undoped PTCDA is an acceptor when adsorbed on Ag(111). Hence these two systems provide full understanding of well-defined metal-organic interface in the presence of alkali atoms both when the organic molecule has donating and/or accepting character, which will be useful as a benchmark for future studies.

References:

[1] Anu Baby, et al., ACS Nano, 11, 10495–10508 (2017).

[2] Zwick, et al. ACS Nano, 10, 2365–2374 (2016).