Phthalocyanines have been studied for long time due to their many possible applications in both optical and electronic devices. A better understanding of the adsorption of phthalocyanine, at the atomic scale, on different substrates provides insight in the molecule-molecule and molecule-substrate interaction for different systems, possibly influencing the functionality of the overlayers. In addition, doping of such systems results in altered electronic structure and new characteristics, of interest for different applications.
Here Scanning Tunneling Microscopy (STM) studies have been undertaken to characterize monolayer (ML) structures of metal-free phthalocyanine (H2Pc) on HOPG and Au(111). In addition the H2Pc monolayer on Au(111) has been doped by Rubidium (Rb).
H2Pc adsorbed on HOPG have been found to form dense-packed ordered structures after annealing to about 400 °C. The molecules adsorb with the molecular plane oriented parallel to the substrate with an adsorption unit cell of squared symmetry. Molecular arrangements for sub-monolayer and monolayer coverages have been investigated and the high resolution of our investigations have permitted to image single molecule orientation. In addition, depending on the bias voltage, different molecular electronic states have been probed and their identification has been possible due to the very good agreement between the experimental data and the performed Density Functional Theory (DFT) single molecule simulations.
In the case of H2Pc monolayer on Au (111) the results have shown the molecules adsorbed onto the herringbone reconstructed Au surface, with the molecular plane parallel to the substrate surface. The ML has thereafter been doped by Rb, to investigate the influence of the alkali metal atom on the monolayer adsorption structure. The excellent resolution of our STM images has revealed that the initial molecular adsorption unit cell of square symmetry become strongly modified by the Rb doping, resulting in a new H2Pc-Rb hexagonal pattern.
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