Anchoring of dye molecules on a nickel oxide surface : an atomic force microscopy study

Abstract

The properties of metal oxides, such as NiO, can be modified when functionalized with organic molecules. This kind of organic/inorganic interfaces are nowadays highly regarded, in particular, for the design of hybrid devices such as p-type dye sensitized solar cells (DSSCs). A key parameter for optimized interfaces relies not only in the choice of the compounds but also on their adsorption properties. The latter predicates fundamental studies of surface/molecules interactions at the nanoscale. In this thesis, non-contact atomic force microscopy (nc-AFM) is first of all used at room temperature (RT) to investigate the surface of NiO(001). Following a well defined preparation process, atomically clean and flat surfaces are obtained. The adsorption of a set of different organic molecules, involving several deposition methods, on these surfaces is studied. The first strategy relies on thermal evaporation of molecules. Using this technique, two standard dye molecules, which are normally used for the design of n- and p-type DSSCs are compared. Kelvin probe force microscopy (KPFM) is used to quantify and observe the direction of the charge transfer between the metal oxide surface and the molecules, which appears to occur in opposite directions depending on the studied molecule. The second strategy used for functionalization is on-surface synthesis, where the dye molecules are assembled directly on the surface of NiO(001). RT nc-AFM as well as low temperature (LT) scanning tunnelling microscopy (STM)/AFM are used in order to follow this assembly process not only on NiO(001) but also on Au(111), highlighting that the reaction is surface independent. On both surfaces, the adsorption of the dye precursor, which occurs in a transoid geometry, as well as the formation of the corresponding metal complex, resulting in a cisoid conformation, are observed with submolecular resolution. The last sensitization strategy investigated in this work involves electro spray deposition (ESD). In order to test the good functioning of this method it is first applied to the well known C60 organic molecule, resulting in interfaces that are not accessible via other preparation methods but fulfilling the cleanliness prerequisite of nc-AFM imaging. Finally, ESD is used to study three different dye molecules and their structure is resolved with submolecular accuracy.