Metal-Organic Complexes at Surfaces
Abstract: The adsorption of different metal-organic complexes at surfaces was investigated by spectroscopic techniques (x-ray photoemission spectroscopy, x-ray absorption spectroscopy, resonant photoemission spectroscopy) and by scanning tunnelling microscopy. The investigation methods are able to provide atomic scale information on the studied systems, which moreover originates from the top-most atomic layers in a sample, meaning that they are surface sensitive techniques. Since the interest is always directed towards adsorbate-substrate interfaces, surface sensitivity is essential. The spectroscopic techniques also ensure atomic and chemical specificity. The information obtained relates to the electronic and geometric structure of the adsorbates on surfaces. The systems studied are iron phthalocyanine, oxalic acid and bi-isonicotinic acid molecules on different supports (Au(111), Cu(111) and highly oriented pyrolitic graphite). A significant part of the research deals with the study of iron phthalocyanine at surfaces and its interaction with different small gas molecules such as ammonia, pyridine, carbon monoxide and nitric oxide. It is interesting to note that the iron phthalocyanine shows different coupling strength on a Au(111) substrate compared to highly oriented pyrolitic graphite. On Au(111), the adsorbate-substrate coupling is relatively strong resulting in covalent-type interactions, while on graphite the coupling is weak and of van der Waals type. The adsorption of different gases on iron phthalocyanine structures on Au(111) results in the formation of bonds between the iron atom and the gas molecule which acts as a molecular ligand. This leads to significant changes in the electronic structure of the phthalocyanine molecular network. First, a decoupling of the phthalocyanine adsorbates from the Au(111) substrate is observed and second, the spin state of the phthalocyanine molecule is significantly changed. The spin changes range from a lowering of the spin to a complete spin quench, depending on the gas molecule that is bound to the iron site. By comparing the adsorption of different molecules it is possible to show that the spin state can be tuned to different values by using different gas molecules. On the other hand the adsorption of oxalic acid on Cu(111) shows the possibility of forming an iron –oxalate coordination network on a Cu(111) surface by incorporation of iron atoms inside a previously formed oxalate network. The study of of bi-isonicotinic acid on Au(111) by resonant photoemission spectroscopy shows the possibility of femtosecond charge transfer from the substrate to the bi-isonicotinic acid molecules, while charge transfer in the opposite direction does not seem to take place.
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