Max NiederreiterMSc Student Karl-Franzens-Universität GrazInstitute of Physics – Surface Science
Integer charge transfer in organic adsorbates on metallic substrates Masters Thesis
In this work charge transfer into molecular monolayers on metal substrates is studied. The first goal was to find clear experimental evidence for the coexistence of charged and uncharged copper-phthalocyanine CuPc molecules in monolayers (ML) on silver surfaces. The first findings were made using scanning tunneling microscopy STM, where images of CuPc monolayers on Ag(111) showed two distinct molecular species, which could be identified as charged and neutral from the appearance of their orbital structure. This is further supported by Konod resonance observations with scanning tunneling spectroscopy STS. On Ag(100) larger scale images also indicated two distinct species with different contrast. These findings are complemented by the results obtained with angle resolved ultraviolet photoemission spectroscopy ARUPS. Studying the sub-monolayer growth of CuPc on Ag(100) strongly suggests the coexistence of a charged and an uncharged CuPc species within the first ML. Estimates of the fraction of charged molecules could be made from the intensities of orbital features in ARUPS and from the measured work function behaviour. STM and ARUPS both suggest the coexistence of integer charge and neutral molecules on the metal substrate. Even beyond that, three different methods of determining the ratio of charged CuPc molecules have been used, which ultimately yield similar results and therefore support each other. These findings are in contradiction to the conventional wisdom that equilibration will lead to all molecules experiencing similar charge transfer. Furthermore, displacement studies with pentacene 5A and CuPc performed with ARUPS allow a deeper understanding of the process of charge transfer and the role the electron affinity (EA) plays in it. It could clearly be shown that CuPc displaces 5A both on the metallic Ag(100) and on the dielectric MgO(100) surface. This is remarkable since the binding mechanisms on these two surfaces would generally be considered very different from each other, with hybridization on the metal and charge transfer on the dielectric being the dominant processes. This strongly suggests that indeed the EA and therefore charge transfer play a central role in the binding process of the adsorbates. This work furthers the understanding of charge transfer on metals and paves the way for understanding displacement reactions and suggests a means of controlling and predicting molecular heterostructures.