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Investigation of Single Functional Molecules on Metal Surfaces by Scanning Tunneling Microscopy
| Content Provider | Semantic Scholar |
|---|---|
| Author | Mielke, Johannes |
| Copyright Year | 2013 |
| Abstract | In this thesis, two classes of functional molecules were studied with a scanning tunneling microscope (STM) on metal surfaces. The first molecule is tetra-phenyl-porphyrin (TPP). On Au(111) it adopts two distinct states, which differ in their apparent height in STM images. Their tunneling spectra, which reflect their local electronic structure around the Fermi energy, are similar but shifted in energy with respect to each other. Reversible switching is possible between these two states and the switching process can be activated thermally at room temperature or with voltage pulses from the STM tip at low temperatures of about 5 K. By analyzing the switching behavior at room temperature, manipulation experiments at low temperature and comparison with density functional theory (DFT) calculations, it could be shown that for both molecular states, the porphyin molecule is in a saddle shape conformation, but that underneath the bright state a gold adatom is present. This adatom causes the changed STM contrast and the spectral shift. The switching process at room temperature was investigated by imaging the same sample area repeatedly. From the statistical analysis of such an image series, information about correlated processes in the molecular layer can be extracted, which is not possible from single images. A hopping of the bright molecular state could be observed, corresponding to a hopping of the adatom underneath, as well as time correlated switching events, where many molecules switch simultaneously. To assess the influence of the intermolecular bonding, the results from close-packed islands were compared to covalently bonded dimers of TPP molecules. It was found that either side of the dimers is still capable of adopting the two states and that the switching process is very similar to the one of the monomers. No “communication” between the two sides of the dimer could be found in terms of their switching properties. The second class of studied molecules are so-called nanocars. They contain four wheels, spherical molecular groups which, when rolling on the surface, could restrict the movement of the car to one dimension on the surface, giving a directionality to the motion of the car. Furthermore, they contain a molecular motor, which is in principle capable of transforming light and thermal energy into a unidirectional rotation and could drive the car forward. The first version of the car was equipped with p-carborane wheels and was successfully deposited on various coinage metal surfaces. It was not possible to activate the rotation of the motor with light nor with voltage pulses from the STM tip. Comparing STM images of the same molecule before and after the illumination did not show any directed motion or change in the appearance of the motor. As it was suspected that the p-carborane wheels bind too strongly to the metal surfaces and thereby quench the rotation of the motor, they were replaced by adamantane wheels in the second version of the car, in the hope that different wheels might make it work. However, also in this second version, the molecular motor could not be activated by illumination or voltage pulses applied with the STM tip. |
| File Format | PDF HTM / HTML |
| DOI | 10.17169/refubium-17938 |
| Alternate Webpage(s) | https://refubium.fu-berlin.de/bitstream/handle/fub188/13740/Mielke_J_Doktorarbeit.pdf?isAllowed=y&save=y&sequence=1 |
| Alternate Webpage(s) | https://doi.org/10.17169/refubium-17938 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
