Center for Functional Nanomaterials Seminar

Energy level alignment and geometric structure of organic chromophores at metal and oxide semiconductor surfaces play a critical role in photo-induced charge transfer and barriers to interfacial charge transport. We have performed systematic studies of a series of organic molecules at the TiO2(110) and at noble metal surfaces to understand the origins of energy level alignment and the relevant molecule-molecule and molecule-surface interactions that generate self-organization of these molecules at these surfaces. Combining scanning tunneling microscopy as a local probe of the molecular adsorption configuration with x-ray, ultraviolet and inverse photoemissions as probes of the electronic structure we have compared preparing monolayer molecular systems from sub-monolayer to monolayer coverage ("bottom-up") to forming a monolayer by desorption of a molecular multilayer ("top-down"). For each the metal systems, we find that the molecular monolayer assumes a metastable structure when prepared at room temperature, which converts to a different minimum energy state after annealing. Moreover, further annealing leads to a transition from intact molecular adsorption to dehydrogenation and subsequent rehybridization. This rehybridization is both intramolecular, with a flattening of the molecules and a measurable alteration of the electronic structure, and intermolecular leading to 2D-growth of extended covalently bound structures as shown in the figure. These results suggest it is possible to create ordered 1D or 2D covalently-bonded structures through direct chemical reactions at the surface. When adsorbed on TiO2(110), the chromophores align themselves along the bridging oxygen rows of this surface. However, the energy level alignment depends on both the surface concentration and electronegativity of the organic chromophore, potentially inducing important band bending in the oxide semiconductor.

This work was funded by NSF and DOE.


Host: Dey