| AVS 54th International Symposium | |
| Surface Science | Tuesday Sessions |
| Session SS2-TuA |
| Session: | Excitations at Surfaces |
| Presenter: | P.E. Colavita, University of Wisconsin-Madison |
| Authors: | P.E. Colavita, University of Wisconsin-Madison B. Sun, University of Wisconsin-Madison K.-Y. Tse, University of Wisconsin-Madison R.J. Hamers, University of Wisconsin-Madison |
| Correspondent: | Click to Email |
Carbon based materials are attractive for a wide range of applications, from biomaterials to fuel cells; however their effective use often requires controlling the surface chemistry to incorporate recognition moieties or reactive centers. The high stability of carbon also makes it a challenging material to functionalize; recently, the use of ultraviolet light (254 nm) to initiate functionalization of carbon surfaces has emerged as a way to obtain carbon/organic interfaces with tailored properties. We have investigated the mechanism of covalent grafting of amorphous carbon surfaces with functional organic molecules using the photochemical reaction of terminal alkenes. Measurements comparing the reactivity of different n-alkenes bearing different terminal groups at the terminus opposite the olefin showed pronounced differences in reactivity. We characterized the rate and final coverage of the resulting organic layers using X-ray Photoelectron Spectroscopy (XPS) and Infrared Reflection Absorption Spectroscopy (IRRAS). Ultraviolet Photoelectron Spectroscopy (UPS) and photocurrent measurements suggested that the reaction involves photoelectron emission from the carbon surface into the liquid phase. Density functional calculations show a strong correlation between the electron affinity of the alkenes and the observed reactivity. The specific terminal group opposite to the olefin was found to play an important role in the stabilization of excess negative charges on the molecule, thus explaining the strong dependence of reactivity on the particular terminal group. These findings suggest that the reaction involves injection of photoelectrons into the alkene acceptor levels, leading to the formation of radical anions in the liquid phase. Finally, we demonstrate that the grafting of marginally reactive alkenes can be enhanced by seeding the surface with a small amount of good electron accepting groups. These results provide fundamental new insights into the role of electronic excitations in controlling rates and mechanisms of olefin reactions at surfaces. While demonstrated here for amorphous carbon, these results may also be significant for the UV initiated grafting of olefins on other semiconductors.