Electronic properties of K doped PTCDA monolayer on Ag(111)
Anu Baby1, Christian Zwick2, Marco Gruenewald2, Elisabeth Verwüster3, Oliver T. Hofmann3, Roman Forker2, Guido Fratesi4,1, Gian Paolo Brivio1, Torsten Fritz2 and Egbert Zojer3
1.Department of Materials Science, University of Milano-Bicocca, Milan, Italy
2.Institute of Solid State Physics, Friedrich Schiller University Jena, Helmholtzweg 5, 07743 Jena, Germany
3.Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria
4.Department of Physics, University of Milano, Milan, Italy
Alkali metal doped organic semiconductors have been used in numerous interesting applications ranging from superconductivity [1, 2] to hydrogen storage . We hereby investigate the K doping of well-ordered herringbone PTCDA monolayer on Ag(111) by means of theoretical techniques and the results are compared with those of the experiments to identify the structural evolution upon doping and the modified molecule-substrate interaction . The calculations are done with density functional theory (DFT) methods, using pseudopotentials, plane waves and Perdew-Burke-Ernzerhof (PBE) functional on the VASP and Quantum Espresso platforms. The experimental methods employed are: low-temperature scanning tunneling microscopy (LT-STM), scanning tunneling hydrogen microscopy (STHM) , low-energy electron diffraction (LEED), differential reflectance spectroscopy (DRS – a variation of absorption spectroscopy)  and X-ray standing wave (XSW). Tersoff-Hamann approach was employed to compute the ST[H]M images and time dependent DFT (TDDFT) method as implemented in Yambo  was used to simulate the absorption spectra. Two highly ordered and stable doping stages are obtained even without annealing named K2PTCDA and K4PTCDA as per their stoichiometry. The K atoms adsorb in the vicinity of the oxygen atoms of PTCDA in both cases as clearly seen in the STHM images. K interacts and decouples the oxygen atoms from the Ag surface. This changes the adsorption structure from that of the undoped PTCDA on Ag(111) as now the molecular backbone is bend into a small U-shape with the perylene ring closer to the Ag surface than the oxygen atoms. K is oxidized loosing its electrons to PTCDA and Ag thereby reducing the surface work function. Density of states show the LUMO which was at the Fermi level in the case of PTCDA/Ag(111) getting filled and shifting continuously to higher binding energies with K doping. As for the absorption spectra, we observe increasing intensity on the higher energy side of the spectrum with increasing doping levels.
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